The contribution of mango fruit (Mangifera indica L.) to human nutrition and health

2.1 Carbohydrates

Ripe mango fruit are rich in sugars (glucose, fructose, sucrose), starch, pectins, cellulose and hemicellulose (Belloet al., 2007). Ripe mango fruit pulp is about 15% soluble sugars, which contribute to the sweet flavor and the structure of the fruit. Simple sugars favor obesity and obesity-related diseases, and therefore, the high level intake of particular forms of mango is not advisable in certain age groups such as children because several studies have demonstrated the pro-obesity effects of certain types of fruit attributed to the content of glucose and fructose, the main precursors for inducing fatty acids by hepatic de novo lipogenesis, which may increase the levels of hepatic and circulatory triglycerides, as well as the circulating levels of very-low- and low-density lipoproteins (LDL), thereby resulting in increases in adipose tissue and obesity (Samuel, 2011; Tappy and Le, 2010). These carbohydrates contribute to nutrition and fruit texture, while dietary fiber has some health benefits. Dietary fiber represents 1.6 to 2.6% of the edible portion of Tommy Atkins, Haden, Kent and Keitt mangoes (USDA, 2018). They have nutritional properties based on their ability of hydration and gel formation. Partially soluble fibers have beneficial physiological effects on intestinal motility, weight and volume of the bolus, and intestinal transit time that contribute to the removal of bile acids and reduce cholesterol level in blood; likewise, reduce the risk of cardiovascular diseases and disorders of the colon (Wollowski and Pool-Zobel, 2001).

2.2 Minerals

Mango fruit provide calcium, copper, iron, magnesium, manganese, phosphorus, potassium, selenium, sodium, and zinc to the human diet (Table 2), according to the Recommended Daily Allowance (RDA) (National Research Council (US), 1989). Mango seeds and peels also contain significant amounts, in descending order, of calcium, potassium, magnesium, sodium, iron, manganese, zinc, and copper (USDA, 2018; Yahia et al., 2011).

2.3 Proteins and amino acids

Protein content of mango fruit is low, ranging between 0.5 and 5.5% depending on the cultivar and several other physiological and cultural factors (Saleem-Dar et al., 2016; Yahia et al., 2011). Amino acids (Table 3) content in mango fruit was reported to vary depending on cultivars, maturity, and ripeness stages (Hall et al., 1980; Yahia et al., 2011). Alanine, arginine, glycine, isoleucine, leucine, and serine were the predominant amino acids found in fully ripe mango fruit, with all other amino acids present in only trace amounts (Tharanathan et al., 2016).

2.4 Organic acids

The organic acids in mango fruit are primarily citrate and malate (Matheyambath et al., 2016; Yahia et al., 2011), with citric acid being found at levels ranging from 0.13 to 0.71% FW, as the importnat contributor to acidity (Tharanathan et al., 2016). These and other organic acids are also important for fruit respiratory activity (Vallarino and Osorio, 2019).

2.5 Lipids and fatty acids

Mango fruit pulp contains very low lipid content, but the seeds and the peels contain relatively high quantities of fatty acids (Table 4). Fatty acids are reported to contribute 0.8 to 1.36% of the edible portion of mango fruit, and fruit ripening favors an increase in unsaturated fatty acids (Deshpande et al., 2016).

2.6 Pigments

Mango fruit color is a very important index of maturation, ripeness, and quality (Brecht and Yahia, 2009; Yahia, 2011). The fruit peel of the different mango cultivars is green before maturation, and characterized with different colors on maturation and ripening of different cultivars including green, yellow, orange, and some cultivars also develop a red blush color. The color of the mesocarp (pulp) of all the cultivars is uniform, where it starts as white before maturation and turns into yellow and orange as the maturation and ripening process advances.

2.7 Phenolic compounds

Phenolic compounds protect plants from different types of stress such as ultraviolet radiation, infections, and reactive oxygen species (ROS), among others (Mathembayath et al., 2016). These secondary metabolites are also essential components of the human diet, with their importance being due both to biological properties and health benefits (Brglez et al., 2016). Flavonoids, stilbenes, and lignans are abundant in plants. Flavonoids comprise about 60% of the dietary phenolics (Ramos, 2007). Current research is focused on the antioxidative, anti-inflammatory, and anti-carcinogenic properties of phenolics. The major phenolics in mango fruit in terms of both quantity and contribution to fruit antioxidative capacity are anthocyanins, the flavonoids catechin, kaempferol, quercetin, and rhamnetin, tannic acid, and the xanthone mangiferin (Masibo and Qian, 2008).

2.8 Vitamins

Water-soluble and fat-soluble vitamins have been reported in Haden, Keitt, Kent, and Tommy Atkins mango fruit, with vitamins C and A being the predominant vitamins (Table 5). Mango consumption can satisfy the daily requirements of these vitamins in people of all age groups (WHO/FAO, 2003).

2.9 Volatile compounds

Volatile compounds are low molecular weight (<400 Da) compounds present in small quantities of 50 ppm or less, very diverse with a wide range of functional groups, occurring as free forms and as glycosidic compounds, have high vapor pressures, and several of them are key components of the characteristic fruit aroma (Li et al., 2017). They can be alcohols, aldehydes, esters, volatile fatty acids, ketones, lactones, and degradation products of carotenoids and phenols (Lebrun et al., 2008; Li et al., 2017; Pino et al., 2005). Mango fruit aroma volatiles in different cultivars and tissues at different stages of maturity are distributed heterogeneously, quantitatively, and qualitatively (Andrade et al., 2000; Lalel et al., 2003b). The importance of most volatile secondary compounds to human health has not been fully investigated.

5.1 General absorption processes for carotenoids and phenolics

The first step for absorption of carotenoids and phenolics is their release from the food matrix. This is achieved by mechanical actions (e.g., chewing, movements of the gastrointestinal tract, etc.), chemical actions (e.g., gastric HCl and Na₂CO₃ of pancreatic secretions), or enzymatic actions in the gastrointestinal tract (e.g., digestive enzymes secreted by mouth, stomach, pancreas, and intestine (Yahia et al., 2018a).

In order to be absorbed, carotenoids must be solubilized in droplets of dietary fat dispersed in the aqueous contents of the stomach. The size of these droplets is reduced at the duodenum due to the emulsifying effect of the bile (Yahia et al., 2018a). The reduction of the lipid droplet size favors the hydrolysis of triglycerides by pancreatic lipase at the lipid droplet surface and the formation of micelles, which are structured with the products of lipid digestion, bile salts, and cholesterol (Cervantes-Paz et al., 2016b). This lipolysis also favors the transference of carotenoids from the oil droplets to the micelles. Micellarization of carotenoids during digestion is largely recognized as a key step for their subsequent absorption. Enterocytes, or intestinal absorptive cells, are able to absorb the micellarized carotenoids (Cervantes-Paz et al., 2017). Small quantities of carotenoids are incorporated into the enterocytes by passive diffusion; however, this mechanism is quickly saturated under high concentrations of carotenoids and an active mechanism is activated for further absorption. Active absorption of carotenoids is performed by several protein transporters (e.g. SR-BI, CD36, and NPC1L1). In the enterocyte, the carotenoids are packed in chylomicrons, which are secreted into the blood system where they are exposed to the action of lipoprotein lipases leading to the formation of chylomicrons remnants, and these carotenoid-containing structures are taken up by the liver. Carotenoids are exported from the liver to different tissues by lipoproteins, especially by LDL, HDL and VLDL (Yahia et al., 2018a).

Absorption of phenolic compounds is relatively simple compared to carotenoid absorption. Phenolics diffuse from the chyme to the epithelial cells found in the stomach and small intestine, entering passively into the portal blood circulation. Unabsorbed phenolic compounds can also be metabolized by the gut microbiota, leading to the production of human health-related compounds secreted by the microorganisms, and absorbed in the large intestine (Crozier et al., 2010).

5.2 Bioaccesibility and bioavailability of mango carotenoids

The first step in the carotenoid absorption process involves the release of carotenoids from the food matrix. The natural location of carotenoids in mango cells in a lipid-dissolved form makes them more easily releasable during the digestion than the crystalline forms of some carotenoids found in tomato or carrot. Disruption of the mango tissue structure improves carotenoid release during digestion. Low et al. (2015) showed that the release of β-carotene and xanthophylls (mainly violaxanthin isomers) from Kensington Pride mango fruit flesh during in vitro digestion increased with the intensity of mastication (particle sizes of 0.075 to 2.8 mm). The release of β-carotene after gastric versus intestinal digestion was ∼ 8 to 10 and 20 to 32%, respectively, while the release of xanthophylls after these digestive stages varied between 5 and 18 and 35 to 55%, depending on the mastication intensity. Treatment of Ataulfo mango paste by ultrasound significantly increased the release of β-cryptoxanthin, β-carotene, and lutein after in vitro gastric digestion. However, digestion reactions of untreated samples showed a higher release of these carotenoids during the intestinal phase of digestion (Mercado-Mercado et al., 2018). Overall, xanthophylls are more easily released in vitro from mango tissues than from carotenes (Low et al., 2015).

Micellarization of mango carotenoids during digestion is determined by many factors. Veda et al. (2007) observed that the content of β-carotene in six varieties of mango varied between 0.55 and 3.21 mg 100 g⁻¹, with 24.5 to 39.1% of this compound being micellarized during in vitro digestion. Badami and Mallika mangoes showed the highest concentration and micellarization of β-carotene. These differences might be attributed to differences in the concentration of carotenoids or differences in fiber content, because low carotenoids doses are more efficiently absorbed than high doses while fibers compromise lipid emulsification, lipolysis, and micelle formation (Cervantes-Paz et al., 2016a, 2017; Yahia and Ornelas-Paz, 2010). In plant foods with low fiber content, the varietal differences in carotenoid micellarization have been related to carotenoid concentration in the food, but mango pectin seems to also compromise the micellarization of carotenoids (Ornelas-Paz et al., 2008b). Fruit ripeness stage is another factor that determines the micellarization efficiency of mango carotenoids. Ornelas-Paz et al. (2008a) demonstrated that 4.5 to 6.9% of β-carotene in Ataulfo mango is micellarized during in vitro digestion, and increased with fruit ripening. This effect of ripening was attributed to the ripening-mediated softening of mango, indicating a similar positive effect of mango processing on carotenoid micellarization.

Treatment of mango paste with ultrasound improved micellarization by 44–46% (Mercado-Mercado et al., 2018). Pulsed electric fields (35 kV cm⁻¹, 4 ms bipolar pulses, 200 Hz, 1800 ms) increased the micellarization of neoxanthin and cis-violaxanthin by 79% in a fruit juice mixture including mango, as compared with the untreated beverage (Rodríguez-Roque et al., 2016). Increased carotenoid micellarization have been observed for juice mixtures containing mango after treatment with pulsed electric fields (Buniowska et al., 2017).

Carotenoids are non-polar compounds, requiring the presence of dietary fat for their micellarization and subsequent absorption (Victoria-Campos et al., 2013). Studies have been conducted to investigate the impact of dietary fat on bioaccessibility and bioavailability of mango carotenoids. The addition of fat and protein to the digestion reaction increased the micellarization of mango β-carotene from 26 to 231%, as compared to digestions without fat and protein (Ornelas-Paz et al., 2008a). Veda et al. (2007) observed that digestion reactions of mango that included milk had 11 to 12% increased bioaccessibility of β-carotene. The milk protein has an emulsifying effect during digestion, improving lipolysis, which favors carotenoid micellarization, and improves absorption (Yahia and Ornelas-Paz, 2010).

The type of fat also influences the micellization efficiency of mango carotenoids. Liu et al. (2016) observed that the micellarization of carotenoids increased by emulsifying mango pulp with either medium or long chain triacylglycerols before digestion, with the micellarization efficiency of carotenoids without pre-emulsification or emulsified with medium or long chain triacylglycerols being 30, 54, and 85%, respectively. The effect of fat type on carotenoid micellarization has also been related to fat digestibility and alteration of micelle/chylomicron size (Yahia and Ornelas-Paz, 2010). The fat type also determines the particle size in the digestive media of mango, with medium or long chain fatty acids decreasing particle size from a range of 10 μm to greater than 1 μm in the oral phase, to 0.2 μm in the gastric phase, and finally to 0.13 μm in the intestinal phase. This reduction of particle size caused a hydrolysis of 80 to 100% of triglycerides during digestion, as compared with a non-emulsified test meal, where lipolysis did not occur (Liu et al., 2016).

Several studies have demonstrated that the interaction between fat and fibers determine the micellarization of carotenoids under digestive conditions. This interaction depends on the type and quantity of fat and fibers, although little is known on this in mango fruit. Ornelas et al. (2008a) demonstrated that micellarization in vitro of β-carotene from a pill decreased 10 to 14% in the presence of pectin from slightly ripe mango (cv. Ataulfo) and 3 to 28% with pectin from fully ripe mango, depending on pectin concentration in the digestion reaction. Ripening significantly alters the properties of mango pectin.

Only a minor fraction of micellarized carotenoids reaches the enterocytes. In early studies on bioavailability of mango carotenoids it was hypothesized that the consumption of mangoes was implicated in the absence of symptoms of vitamin A deficiency in Senegalese children (Carlier et al., 1992). Yuyama et al. (1991) observed that rats fed 4 IU of retinyl palmitate per g diet or 4 IU of vitamin A from mango fruit per g diet or 4 IU of vitamin A from peach palm fruit per g diet caused similar levels (28 to 32 μg) of vitamin A in plasma, although rats fed peach palm fruit stored more vitamin A in the liver than those fed with mango fruit.

Ornelas-Paz et al. (2010a) demonstrated that feeding Wistar rats with Ataulfo mango along with soybean oil caused vitamin A accumulation in the liver that was 37% higher than that of rats fed with carrots. They also demonstrated that soybean oil improved the bioavailability of mango carotenoids. Mango and carrots provided the same quantity of β-carotene to the rats. This study is very relevant because carrots have high concentrations of two provitamin A carotenoids (α- and β-carotenes) while mangoes only contain significant quantity of one provitamin A carotenoid (β-carotene), and carrots are the most important source of provitamin A carotenoids in the human diet. Ornelas-Paz et al. (2008a) found that 17% of micellarized β-carotene in mango digestions can be incorporated by intestinal cells in culture. Of course, these kinds of measurements do not represent the bioavailability of this compound because carotenoids can be secreted by intestinal cells into the digestive medium or even returned from the blood to the intestinal content again (Yahia & Ornelas-Paz, 2010). Further human studies are needed to understand the potential impact of mango consumption on circulating carotenoids, as has been done for other fruits like tomatoes.

5.3 Bioaccesibility and bioavailability of mango phenolic compounds

The release of phenolics from mango pulp during digestion depends on the ripeness stage of the fruit, because ripening alters the structure of mango tissue. Quirós-Sauceda et al. (2019) observed that although the concentration of phenolic compounds decreased in mango pulp (cv. Ataulfo) during ripening, the release of these compounds increased in the digestive media (in vitro) as ripening progressed, accounting for 26 to 40 and ∼ 30 to 45% of the phenolic content of mango pulp in the gastric and intestinal media, respectively. The release of phenolics from mango pulp during digestion may be differentially impacted by gastric and intestinal phases of digestion with the intestinal phase causing a higher release of phenolic compounds than the gastric phase, as reported for example for phenolic compounds from commercially available mango juices (Attri et al., 2017). Chen et al. (2014) also observed that phenolics in the chyme slightly decreased during the gastric phase of in vitro digestion of mango (cvs. Hainan and Shuixian) pulp while the intestinal phase caused the opposite effect.

The increase of phenolic compounds in the digestion medium has mainly been attributed to Na₂CO₃ in digestive secretions, which favors the release of phenolic compounds from Na₂CO₃-soluble pectin rich in phenolic compounds, as demonstrated for other fruits (Epriliati and Ginjom, 2012). These increases, measured commonly as the content of total phenols, not only suggest a release of phenolic compounds, but also their metabolism, as reported for example for anthocyanins after digestion, making it difficult to estimate the absorption efficiency of mango phenols.

The pH of the gastric medium plays important roles in the release of food phenolic compounds in the digestive medium and their subsequent metabolism. This is particularly true in the case of mango fruit because the most abundant phenolics in mango fruit, gallic acid and gallotannins, experience structural modifications during the digestion (Barnes et al., 2016). The gallotannins are polymers of gallic acid that are hydrolyzed to gallic acid and other metabolites during the gastrointestinal digestion. The gastric pH is responsible for the reduction of the molecular weight of gallotannins to species ranging from 4-O-galloylglucose to 8-O-galloylglucose. The enzymatic modification of gallotannins in the gastrointestinal tract seems to have minor relevance (Barnes et al., 2020).

Overall, the structure of phenolic compounds can be modified during the entire digestion process, including the oral phase, and the final metabolites are generally phenolic acids already contained in the food, making it difficult to estimate the bioavailability of the original compounds (Kim et al., 2021). The gallic acid is mainly released from penta-O-galloylglucose or trigalloyl moieties from 11- or 12-O-galloylglucose (Krook and Hagerman, 2012). The depolymerization of gallotannins involves both m-depside and galloyl-glucose bond hydrolysis (Barnes et al., 2016; 2020). This effect of digestive pH makes it difficult to estimate the absorption efficiency of the parent compounds. While there may be other factors that play a role in the release and digestive metabolism of phenolics in plant foods, the impact of such factors has received little attention in the case of mango.

A minor quantity of the phenolic compounds released during digestion is bio-accessible. Quirós-Sauceda et al. (2019) demonstrated that only ∼ 10 to 11.5% of mango phenols released during in vitro digestion were considered to be bio-accessible as determined by permeation into a cellulose dialysis bag (12 to 14 kDa). This absorption process mainly includes passive diffusion, and in some cases, active mechanisms can take place in the stomach and intestine, according to studies on other fruits (Crozier et al., 2010).

Plant tissues contain some water-soluble flavonoids that can be converted into lipid-compatible molecular complexes called phytosomes, which follow the same absorption pathway as lipid-soluble phytochemicals like carotenoids (Kidd and Head, 2005). Liu et al. (2016) observed that some phenolic compounds are micellarized during mango digestion, showing bioaccessibility values 1.5 times higher than those of carotenoids.

There is a dearth of available information regarding the influence of mango consumption on the concentration of phenolics in human fluids. Concentrations of catechol-O-sulfate, 4-O-methylgallic acid, 4-O-methylgallic acid-3-O-sulfate, pyrogallol-O-sulfate, and methylpyrogallol-O-sulfate in human plasma were 9520, 2790, 6030, 5990, and 4020 μg L^-1, respectively, following consumption of 400 g of Ataulfo mango pulp (Barnes et al., 2020). Quirós-Sauceda et al. (2017) detected ferulic, gallic, gentisic, and protocatechuic acids in mango pulp and juice and observed that the consumption of these foods caused increases in the concentration of these compounds in plasma, reaching maximum concentrations of 7.9–8.7, 49.7–109, 11.8–12.2, 30.8–34.5 ng mL⁻¹ after 2.3 to 4.4 h, suggesting that the absorption of these compounds took place in the intestine. Quirós-Sauceda et al. (2017) also noted that urinary excretion of chlorogenic, ferulic, gallic, sinapic, vanillic, and p-coumaric acids and pyrogallol increased after 8 to 24 h from mango consumption. Barnes et al. (2016) detected pyrogallol-1-O-glucuronide, 4-O-methyl-gallic acid, 4-O-methylgallic acid-3-O-sulfate, O-methylpyrogallol-O-sulfate, pyrogallol-O-sulfate, deoxypyrogallol-O-sulfate, and O-methylpyrogallol-O-sulfate in human urine after mango consumption. They hypothesized that due to their rapid appearance in urine 6 h after mango consumption the compounds 4-O-methyl-gallic acid and 4-O-methylgallic acid-3-O-sulfate were probably absorbed in the small intestine. The other metabolites were conjugates of pyrogallol, which are absent in mango pulp; however, those compounds can be produced from gallic acid via the action of decarboxylase enzymes in the microbiota in the colon, where gallic acid can be absorbed and then conjugated by hepatic and renal enzymes prior to elimination in urine (Kim et al., 2021). Barnes et al. (2016) demonstrated that gallic acid metabolites are not detectable in the plasma after mango consumption. This might be a consequence of the efficiency of the release of phenolics during digestion (food matrix effect) mediated by ripening or other factors, because several gallic acid metabolites have been observed in plasma after mango consumption (Pimpao et al., 2015).

The absorbed phenolic compounds may be transformed into other compounds as a result of conjugation reactions such as glucuronation, methylation, and sulphation catalyzed by Phase I and II enzymes in the small intestine and liver (Kim et al., 2021). However, some compounds are excreted into the small intestine via bile and reabsorbed by enterohepatic circulation, although the most important clearance pathway is through the kidney and urine excretion (Burton-Freeman et al., 2017).

The phenolic compounds that are not absorbed can be metabolized by microbiota in the large intestine and the resulting metabolites absorbed there (Epriliati and Ginjom, 2012; Kim et al., 2021). In rats, Kim et al. (2018) observed that mango juice altered the composition of gut microbiota, increasing the population of L. plantarum, L. lactis, and Clostridium butyrium, which synthesized tannase and gallic acid decarboxylase that promoted the hydrolysis of gallotannins to gallic acid and then to pyrogallol. These microorganisms can also produce bioavailable short chain fatty acids (SCFA; propionic, butyric, isobutyric, valeric, isovaleric, etc.) from mango phenolic compounds and fiber. Thus, the SCFA might be considered as bioactive compounds from mango. In the same way, Ojo et al. (2016) found that feeding a high fat diet supplemented with freeze-dried ripe Tommy Atkins mango pulp to C57BL/6 mice for 12 weeks resulted in high levels of acetic and butyric acids in the feces along with increases in the beneficial gut bacteria Bifidobacteria and Akkermansia.

Hernández-Maldonado et al. (2019) tested an in vitro gastrointestinal and colonic fermentation model using mango-based fruit bars as a dietary fiber and phenolic compounds source. They reported that the bio-accessibility of phenolic compounds was 53.78%, and during colonic fermentation these compounds were mainly converted into hydroxyphenolic acids and acetic acid followed by propionic and butyric acids as the main SCFA after 48 h. Finally, the unabsorbed phenolic compounds and microbial metabolites other than SCFA may be excreted in feces. Moreover, this in vitro experiment showed that mango-based fruit bars could also modify colonic microbiota through increasing carbohydrate metabolism that also could prevent metabolic dysbiosis (Gutiérrez-Sarmiento et al., 2020).

In a study performed using lean and obese human subjects, Barnes et al. (2019) showed that supplementing the participants’ diet with 400 g d^-1 of Ataulfo mango pulp for 6 weeks increased gallotannin-metabolites in urine, however, only lean participants increased fecal SCFA (butyric and valeric acids). Besides, obese participants had increased levels of Lactococcus lactis and decreased populations of Clostridium leptum and Bacteroides thetaiotaomicron, bacterial species that are associated with obesity. Mango intake positively modulates fecal microbial composition by significantly increasing the presence of Lactobacillus spp. and fecal butyric acid (Kim et al., 2020). This is particularly important because inflammatory bowel disease is a major risk factor for gastrointestinal neoplasias such as colorectal cancer, and knowledge of the positive effects of mango consumption on fecal microbial composition may contribute to the development of novel therapies (Kim et al., 2020).

6.1 Antioxidant activity of carotenoids

Foods containing carotenoids, such as mango, in which β-carotene is the prevalent type of carotenes in the pulp, possess potent antioxidant properties. β-carotene contributes to the antioxidant capacity of mango by direct and indirect mechanisms. It can directly neutralize free radicals such as peroxyl (ROO^.), hydroxyl (^.OH), singlet oxygen (¹O₂), and superoxide (O₂^.-) radicals (Birben et al., 2012), resulting in 5,8-carotene endoperoxides (Choe and Min, 2009). This occurs via the donation of a hydrogen to produce a carotene radical, which at low oxygen concentrations can react with another ROO^. leading to non-radical carotene peroxides (Chen et al., 2011). By binding to the Antioxidant Response Element (ARE) the transcription factor Nrf2 activates indirect antioxidant mechanisms. This is necessary for the induction of phase II enzymes including glutathione S-transferases (GSTs), NAD(P)H:quinone oxidoreductase (NQO1) and thioredoxin (Tanaka et a., 2012).

Anilakumar et al. (2003) reported that male Wistar rats that were fed a diet supplemented with 10% mango for 12 weeks presented reduced cytotoxic effects induced by dimethyldrazine via hepatic increase of vitamin A and optimized activities of hepatic catalase and GST activities.

In another study using human dermal fibroblasts (FEK4), it was observed that after UVA exposure, β-carotene specifically exerted its antioxidant effect by suppressing the up-regulation of Haem oxygenase-1 gene expression in a dose-dependent manner (Trekli et al., 2003), which is consistent with the direct antioxidant mechanism described for this carotenoid via singlet oxygen quenching. As explained by Pavan et al. (2006), this Haem oxygenase-1 gene expression can be activated by β-carotene and retinoids via the nuclear retinoic acid receptors, RAR and RXR, after binding to retinoic acid-responsive elements (RARE) in the promoter regions. However, as shown by Obermuller-Jevic et al. (1999), the activation of retinoid signaling via RARs and RXRs might not be involved because the Haem-oxygenase-1 can be expressed at low concentrations of β-carotene (0.2 μM) in response to UVA treatment of human skin fibroblasts.

In contrast, high in vitro concentrations (<10 μM) of β-carotene favor oxidation by promoting production of reactive oxygen species (ROS) and oxidized glutathione (Palozza et al., 2003). Paolini et al. (2001) showed that this effect is due to activation of NF-kB and upregulation of c-myc expression, causing inhibition of cell growth in human leukemia and colon adenocarcinoma cell lines and induction of apoptosis; however, these effects were blocked by treatment with α-tocopherol and N-acetylcysteine. Moreover, α-tocopherol and N-acetylcysteine reduced upregulated expression of Phase I enzymes and ROS overproduction in intestines, kidneys, livers, and lungs of rats fed diets supplemented with β-carotene (Paolini et al., 2001).

If β-carotene induces oxidative stress in cells, it follows that oxidative breakdown products of β-carotene will be produced. One such products are the β-apo-carotenals, which can serve as precursors of vitamin A via the enzyme β-carotene 15,15′-monooxygenase (CMO1) (Tanaka et al., 2012). The β-apo-carotenals comprise a group of oxidized compounds that have been found in smokers. Therefore, β-carotene supplementation is not indicated for chemoprevention of lung cancer because cigarette smoking causes retinoid signaling to be suppressed via reduced RAPβ gene expression and activation of the AP-1 transcription factor (Yu et al, 2015), part of the mitogenic signaling pathway of insulin growth factor type 1 (IGF-1) among others (Angel and Karin 1991).

The AP-1 complex comprises the protein families Jun (c-Jun, JunB and JunD) and Fos (c-Fos, FosB, Fra-1 and Fra-2), which exist as homodimers (Jun/Jun) or heterodimers (Jun/Fos) (Albanese et al., 1995). As described by Palozza et al. (2010), these components of the AP-1 complex can be induced by mitogenic stimuli and tumor-promoting agents, binding to the AP-1 site or TPA response element (TRE). The TRE is found on the promoter of many genes that are related to cell proliferation such as cyclin-D. The AP-1 complex proteins can also be part of the ARE transcription complex, which raises the possibility expressed by Palozza et al. (2010) that retinoic acid reduces growth factor-induced stimulation of AP-1 transcriptional activity by altering the composition of AP-1 complexes that bind to DNA.

Taken together these mechanisms may partially explain why high doses of β-carotene did not protect against lung cancer in several intervention trials (Omenn et al., 1996). Supplementation with high doses of β-carotene along with inhalation of tobacco smoke induce 3- to 4-fold increases in expression of c-Jun and c-Fos genes, leading to proliferation response in lung tissue and squamous metaplasia, along with increased cell proliferation marker and proliferation of cell nuclear antigens. Despite this evidence related to lung cancer, dietary carotenoids have been shown in numerous studies to prevent cancer and the mechanism is thought to involve induction of cell death via the oxidative stress caused by cytochrome c release from mitochondria, which leads to apoptosis, a way to kill other types of cancer cells (Tanaka et al., 2012).

6.2 Antioxidant activity of phenolics

Catechin and epicatechin react directly with H₂O₂ and prevent the Fenton reaction between Fe²⁺ and H₂O₂ (Masibo and Qian, 2008). Another mango flavonoid with significant reported antioxidant activity is quercetin, which has the ability to sequester ROS such as O₂^.-, NO^., and other reactive nitrogen species, a feature that has been attributed by Boots et al. (2008) to the catechol motif in its B-ring, and the hydroxyl group at position 3 of the C-ring.

The antioxidant capacity of mangiferin, present in mango leaves and bark, and to a lesser extent in the fruit pulp, has been widely demonstrated in different studies such as its capacity to inhibit the Fenton reactions and lipidic peroxidation through the formation of a stable complex with Fe³⁺ (Benard and Chi, 2015). The antioxidant capacity of mangiferin is comparable to the neutralization of ROS, such as ROO^., O₂^.-, H₂O₂, and^.OH by ascorbic acid (Benard and Chi, 2015). Antioxidant activity of mangiferin has also been demonstrated using the 2,2′-azinobis-(3-ethylbenzothiazolin-6-sulfonic acid (ABTS) and DPPH assays (Tang et al., 2004). Mangiferin showed in vitro values of 0.61, 1.67, and 3.69 µmol of Trolox, using DPPH, ABTS, and Oxygen Radical Absorbance Capacity (ORAC) assays, respectively (Malherbe et al., 2014). Mangiferin has also been shown to protect against oxidative stress in Wistar rats and in healthy older adults (Pardo-Andreu et al., 2008).

Among the phenolic acids in mango fruit with reported antioxidant activity are gallic and ellagic acids. Gallic acid is capable of inhibiting lipid peroxidation and neutralizing ROS such as^.OH, O₂^.-, and ROO^., as well as other oxidizers like H₂O₂ and HClO (Badhani et al., 2015), which is partially attributed to the hydroxyl groups in the ortho position. The antioxidant activity of ellagic acid measured by the DPPH assay has been shown to inhibit lipid peroxidation and enhance SOD, catalase, and glutathione peroxidase (GPx) activities in lung fibroblasts (V79-4) of Chinese hamsters (Han et al., 2006).

Zapata-Londoño et al. (2020) reported that mango juice (cv. Azúcar) consumption for 26 days did not significantly affect plasma antioxidant capacity and biomarkers of oxidative stress [lipid peroxidation, total glutathione, and 8-hydroxy-guanosine (8-OHdG)] in healthy adults with dietary habits that favor colorectal cancer risk. In contrast, Pardo-Andreu et al. (2006) reported that 30 days of consumption of 14.67 µmol L^-1 of Vimang, an extract from mango bark that is rich in phenolic acids and esters, flavan-3-ols, and mainly mangiferin (10 µM), decreased TBARS levels. In this study, the effect of the extract consumption on total glutathione was also determined, but no differences were observed after taking the extract (Pardo-Andreu et al., 2008). Differences between these studies may be attributed to the quality of dietary habits of the participants, such as the low fruits and vegetables consumption by the participants in the study of Zapata-Londoño et al. (2020), which is associated with increased levels of lipid peroxidation and oxidative damage to DNA (Suwimol et al., 2012), indicating that the ability of these foods to protect against oxidative stress is due, in part, to their phenolic compounds. Plasma antioxidant capacity is directly associated with consumption of foods rich in phenolic compounds, which was evidenced in a study by Wang et al. (2012) reporting that a high consumption of foods rich in antioxidants led to an improvement in plasma antioxidant capacity.

On the other hand, considering that mangiferin’s antioxidant capacity in vitro and in vivo has been shown in different studies (Márquez et al., 2012; Pardo-Andreu et al., 2006), the plasma content of mangiferin following consumption of mango juice was evaluated by Zapata-Londoño et al. (2020) who found higher values of mangiferin in participants (38.64 ± 6.75 ng mL⁻¹), more than those reported by Hou et al. (2012) after the administration of mangiferin (0.9 g) via the oral route to healthy individuals. These differences may be attributed to the accumulation of mangiferin present in mango juice consumed daily and because absorption of mangiferin may be greater when it is in this matrix.

Studies performed in humans and rats have shown that mango consumption increases antioxidant capacity in plasma or serum (García-Solís et al., 2008; Robles-Sánchez et al, 2011). Robles-Sanchez et al. (2011) found that healthy individuals who consumed peeled and sliced Ataulfo mangoes for 30 days had increased plasma antioxidant capacity as measured by ORAC and ABTS assays compared to the controls.

6.3 Immunomodulation activity

One of the observed effects of carotenoids and polyphenols of mango is their anti-inflammatory effect on epithelial cells such as Caco-2 in the gastrointestinal tract (Biehler et al., 2011), HT-29 cells (Ribeiro et al., 2006), human gastric epithelial adenocarcinoma (AGS) (Jang et al., 2012), ulcerative colitis (Kim et al., 2016), breast cancer cells (Arbizu-Berrocal et al., 2019), obesity (Gomes Natal et al., 2016; 2017), and in juvenile upper respiratory tract (Anaya-Loyola et al., 2020). The inflammation is initiated by an irritant agent or infection and involves movement of fluid containing macrophages and leukocytes into extravascular tissue, then tissue regeneration or repair involving cell proliferation. A respiratory burst is characteristic of this process, which is associated with ROS overproduction, oxidative stress, and damage to lipid membranes, producing lipid peroxidation and activation of the arachidonic pathway, further producing ecoisanoids that are able to induce cell proliferation. Oxidative stress also damages DNA and proteins with modification of structure and function (Hussain et al., 2003), a condition that can lead to chronic diseases like cancer. As outlined by Reuter et al. (2010), the affected cells produce proinflammatory cytokines (TNF-α, IL-1β), chemokines, and prostaglandin E2 (PGE2) because of activation of cyclooxygenase 2 (COX-2), inducible nitric oxide synthase (iNOS), and activation of the transcription factors NF-κB and Nrf2 that have been associated with inflammation and oxidative stress responses (Fig. 1). A clinical trial performed with 10 adult patients with inflammatory bowel disease, Crohn disease, and ulcerative colitis, showed that supplementation with 200–400 g d^-1 of ripe Keitt mango pulp for 8 weeks improved their clinical condition, including simple Clinical Colitis Activity Index, decreased plasma pro-inflammatory cytokines related to neutrophil-induced inflammation (i.e., Il-8, growth regulated oncogene, ranulocyte macrophage colony-stimulating factor) (Kim et al., 2020).

Close

Fig. 1

Possible mechanisms of the effects of mango carotenoids and phenolic compounds. ARE: antioxidant response element; AP1-P: activator protein 1 phosphorylated; COX-2: cyclooxygenase-2; DNA: deoxyribonucleic acid; IL1b: interleukin 1-b; iNOS: inducible nitric oxide-synthase; MAPKs: mitogen-activated protein kinases; ERK: extracellular signal-regulated kinase; ERK: extracellular signal-regulated kinase phosphorylated; GSTs: glutathione-S-transferases; JNK: c-Jun N-terminal kinase; JNK-P: c-Jun N-terminal kinase phosphorylated; NR: nuclear receptor; NF-kB: nuclear factor kappa-B; Nrf2: nuclear factor erythroid 2-related factor 2; NQO1: NAD(P)H quinone oxidoreductase 1; P38: mitogen-activated protein kinase p38; P38-P: mitogen-activated protein kinase p38 phosphorylated; RAR: retinoic acid receptor; RARE: retinoic acid response element; RXR: retinoid X receptor; PPARs: peroxisome proliferator-activated receptors; RE: Response element; ROS: reactive oxygen species; SOD: superoxide dismutase; TNFa: tumor necrosis factor-alpha; UV: Ultraviolet.

Export to PPT

As described in Fig. 1, carotenoids (including β-carotene) and phenolic compounds (phenolic acids and polyphenols) from mango may act directly as antioxidants from the effects of UV radiation, but can also interact with a wide group of cell molecules such as nuclear transcription factors (NR: nuclear receptors, Nrf2, RAR, RXR, NF-κB, MAPKs, PPARs) and MAPKs enzymes, able to modulate through the RARE, the antioxidant response element (ARE), the transcription of genes involved in cellular defense such as antioxidant enzymes (SOD, GSTs, NQO1), genes involved in the inflammatory system such as the cytokines TNFα and IL1β, the pro-inflammatory enzymes (COX-2, iNOS) for producing compounds such PGE-2 by COX-2, genes involved in apoptosis, glucose and fatty acid metabolism, and genes participating in cell survival and/or proliferation.

6.4 Anti-inflammatory activity of mango carotenoids

It has been shown in in vitro studies that β-carotene (2 to 20 μM) is able to significantly reduce ROS production and activation of the NF-κB in AGS cells after Helicobacter pylori stimulation (Bouayed and Bohn, 2010; Jang et al., 2009), including down-regulated iNOS and COX-2 expression, and production of PGE2 (Jang et al., 2009; Palozza et al., 2003). PGE2 plays an important role in the process by activating PGE2 receptors, which increase β-catenin nuclear accumulation and transcriptional activity (Castellone et al., 2005). The PGE2 also activates signal transduction pathways that favor growth of malignant tissue by enhancing cellular proliferation, promoting angiogenesis, inhibiting apoptosis, stimulating invasion or migration, and suppressing immune responses (Wang and Dubois, 2006). Corrales-Bernal et al. (2016) showed that consumption of Azúcar mango juice during the 8 weeks following AOM injections to induce colorectal cancer, significantly reduced PGE2 levels compared with the control group, a result correlated with aberrant crypt foci (ACF), indicating mango consumption might contribute to control of carcinogenesis in colonic mucosa.

The effect of carotenoids on Mitogen-activated protein kinases pathway (MAPK) has been investigated in other studies. Oxidative stress stimuli, IL-1β, and TNF-α activate these serine/threonine kinases. The three MAPK-pathways that have been characterized are the p38 kinases, the c-Jun N-terminal kinases (JNK1, JNK2, and JNK3), and the extracellular signal-regulated kinases (ERK1/ERK2) (Johnson and Lapadat, 2002). It has been observed that β-carotene inactivates the p38 and ERK pathways by decreasing phosphorylation, but this effect was not found for JNK in the cytosol of AGS cells (Jang et al., 2009).

Reducing biomarkers of inflammation that has been identified in cancer chemoprevention studies, such as TNFα and IL1β, has been addressed in several animal studies by providing foods containing carotenoids (Bousserouel et al., 2011). Proinflammatory cytokines TNFα and IL1 (α and β) are considered tumor promoting factors that act via stimulation of chemokine expression and promotion of ROS production in epithelial cells (Babbar and Casero, 2006). IL1β has been shown as the cause of inflammation and induction of pro-inflammatory gene expression. Pro-inflammatory cytokine levels in the blood of Balb/c mice injected with azoxymethane (AOM) then provided with 0.3% w/v of mango juice for 8 weeks were only slightly, but not significantly different compared with the control group (AOM + water) (Corrales-Bernal et al., 2016). The TNFα levels in the experimental group were undetectable, and IL6 was not different in treated and control animals. Elevated IL1β levels induced by AOM soon after initiation of colon carcinogenesis and diminished IL1β with mango juice consumption were novel results of this study. Choi et al. (2006) reported similar results when rats with colon carcinogenesis induced using AOM were given 0.5% β-carotene for 33 weeks, with the β-carotene failing to reduce inflammatory markers such as COX-2. Although the reasons for these results are unexplained, the approach of using in vivo models and administering water-soluble preparations of carotenoids (i.e., mango juice) is interesting.

In spite these promising results, when effects of carotenoids are evaluated by intake of carotenoid-rich foods such as mango, there are prevailing confounding factors, including the presence of other components of the food matrix such as dietary fiber, minerals, and vitamins. However, carotenoids and micronutrients provided as supplements may not result in the same synergistic effects as when they are in the food matrix, overdosing is a possibility, and bioavailability may be altered (Kaulmann and Bohn, 2014).

The effect of carotenoid supplementation on factors involved in the NF-κB pathway has been analyzed in a few studies, but foods rich in lycopene (not found in mango) have received the most attention. However, one example using β-carotene was the study of Vaisman et al. (2006) in which the subjects received enteral nutrition enriched with carotenoid (2 µg kcal⁻¹), resulting in reduced expression of lymphocyte NF-κB after 3 months compared with the control. On the other hand, daily consumption for 4 weeks of either 2 or 8 servings of carotenoid-containing fruits and vegetables had no effect on TNF-α, IL-12, or C-reactive protein serum levels, but consumption of greater than 8 daily servings did reduce C-reactive protein serum levels (Watzl et al. 2011).

6.5 Anti-inflammatory activity of mango phenolics

The anti-inflammatory activity of mango phenolic compounds like gallic acid and gallotannins was reported by Kim et al. (2016) to inhibit the nuclear factor kappa-B (NF-κB) in experimental models of ulcerative colitis employing an enriched fraction of Keitt mango polyphenolic extracts. They examined the miR-126/PI3K/AKT/mTOR signaling pathway in in vitro lipopolysaccharide (LPS)-treated CCD-18Co cells and observed that a mango extract with 10 mg GAE L^-1 suppressed NF-κB, p-NF-κB, PI3K (p85β), HIF-1α, p70S6K1, and RPS6 protein (Kim et al., 2016). They also observed that a mango beverage protected rats against dextran sodium sulfate (DSS)-induced colonic inflammation (47%) and significantly reduced expression of iNOS, the proinflammatory cytokines TNF-α, IL-1β, as well as PI3K, AKT, and mTOR. Similar results were obtained in TNF-α treated breast non-cancer cells MCF-12A and breast cancer cells MDA-MB231 exposed to a polyphenol-rich extract of Keitt mango pulp (Arbizu-Berrocal et al., 2019). These findings suggest that mango phenols have potential as anti-inflammatory therapeutic agents in the prevention of breast cancer and as cancer-specific cytotoxic treatments of breast cancer without inducing cytotoxicity in non-cancer cells.

Inflammation is not only associated with infectious processes or cancer, but also with obesity modulated by adipokines. When Gomes Natal et al. (2016; 2017) evaluated the effect of consumption of Ubá mango juices by obese Wistar rats (35 mL d^-1) for 7 days they noted that the juice prepared from pulp + peel had higher concentrations of phenolic compounds, mangiferin, and ascorbic acid than juice prepared from pulp alone. Nevertheless, both mango juices reduced obesity indicators such as total body weight, body weight gain, visceral fat, and adiposity index (Gomes Natal et al., 2016). In addition, they observed reduced expression of peroxisome proliferator-activated receptor gamma (PPAR-γ), a transcription factor of adipogenic genes, and increased blood levels of HDL-cholesterol while blood glucose and reducing total cholesterol, fatty acid synthase expression, and TNF-α were decreased (Gomes Natal et al., 2016). Moreover, both Ubá mango juice, pulp, and pulp + peel, reduced seric resistin levels and hepatic steatosis in obese rats (Gomes Natal et al., 2017). Thus, taken together, their results indicate that Ubá mango has potential as a functional food able to reduce the metabolic risk for obesity that is associated with adiposity and inflammation.

Anaya-Loyola et al. (2020) proposed that a mango juice-by-product (JBP) containing peel and pulp remanent may reduce symptoms related to gastrointestinal and upper respiratory acute infections in healthy school-age children (6 to 8 years old) via modulation of immune-related proteins due to its high content of dietary fiber (1.1 g) and 278.6 mg extractable polyphenols (mono-to-hepta galloyl hexosides and mangiferin) and 7.8 mg of hydrolyzable polyphenols (ellagic and gallic acid) per portion (2 g d^-1) supplemented for 2 months. The JBP showed beneficial effects on gastrointestinal and upper respiratory tract, as most of the children did not present gastrointestinal and respiratory infections at the end of the study. These effects might be attributed to the antimicrobial and antiviral activities associated with the high content of phenols such as gallic acid and gallotannins (Lee et al., 2016b). In addition, these effects in children were associated by Anaya-Loyola et al. (2020) with the increase in serum levels of immune-related proteins, plasminogen activator inhibitor (PAI-I), which confers protection against yersiniosis by promoting the innate immune system; macrophage inflammatory protein (MIP-1α), which promotes the ability of cytotoxic T lymphocytes to protect against a secondary viral challenge; and MIP-1β, which promotes IgA and IgE antibodies. However, in this study, the serum levels of these immunoglobulins were not significantly affected by mango JBP supplementation. On the other hand, decreases in levels of IgG, macrophage migration inhibitory factor (MIF), and osteopontin were the result of a lower number of repeated infections and less production of pro-inflammatory cytokines as a result of inflammatory process in organs such as the gastrointestinal tract (Anaya-Loyola et al., 2020).

6.6 Antiproliferative action of mango phytochemicals

Epidemiological evidence has shown inverse association between intake (food or supplements) of phytochemicals (phenolic compounds and carotenoids) and cancer risk, because the phytochemicals are able to regulate cellular differentiation and growth cell (Yu et al., 2015). Studies on the mechanisms of action of the phytochemicals present in fruits and vegetables in cancer have allowed their classification according to their ability to block the initiation stage of carcinogenesis (cancer-blocking agents) or to suppress (cancer-suppressing agents) the proliferative capacity of preneoplastic lesions in the stages of tumor promotion and progression (Surh, 2003; Yahia, 2009; Yahia et al., 2018b). Phenolic compounds are blocking agents that inhibit carcinogens by protecting DNA from nitrogen reactive intermediates and ROS attack because of their scavenging potentials shown for quercetin (Bub et al., 2003) and gallic acid (Lauricella et al., 2019). Flavonoids also induce carcinogen detoxifying systems, alter metabolism of procarcinogens in favor of conjugation and excretion of reactive metabolites, and can interact with the aryl hydrocarbon receptor as agonists or antagonists depending on structure and cell context, which influences the expression of drug metabolizing enzymes such as cytochromes P450 (Romagnolo and Selmin, 2012) as well as induction of phase II enzymes, and may facilitate the elimination of certain carcinogens or their reactive intermediates (Zhang et al., 2003). On the other hand, the suppressing mechanisms induced by polyphenols include suppression or elimination of tumor cells by interfering with cell cycle regulation, signal transduction pathways, transcriptional regulation, inhibition of cyclooxygenase activity, suppression of oncogenes and tumor formation, and induction of apoptosis of cancer cells (Surh, 2003). Many flavonoids or combinations of flavonoids induce G2/M arrest in CaCo-2 and SW620 human colon carcinoma cells (Dorai and Aggarwal, 2004). Another suppressing mechanism considered is the inhibitory effect on arachidonic acid metabolism leading to production of pro-inflammatory or mitogenic metabolites including prostaglandins and ROS (Corrales-Bernal et al., 2016).

Over the past few years, research has focused on demonstrating that the polyphenol mangiferin presents antiproliferative activity. Mangiferin has been reported by Li et al. (2016) to reduce the proliferation of human prostate cancer PC3 cells in a concentration- and time-dependent manner. In addition, mangiferin promotes apoptosis and induces caspase-3 activity of human prostate cancer PC3 cells. Mangiferin treatment significantly reduces the expression levels of Bcl-2 and improves miR-182 expression in PC3 cells (Li et al., 2016). Mangiferin in CNE2 cells, a nasopharyngeal carcinoma cell line, induced apoptosis, and inhibited cell proliferation via cell cycle arrest in phase G2/M in a dose-dependent manner (Pan et al., 2014). Moreover, mangiferin reduced antiapoptotic protein Bcl-2 and its associated mRNA and increased proapoptotic protein Bax in CNE2 cells (Pan et al., 2014).

In rhabdomyosarcoma (RD) cells, mangiferin caused cell death in a dose-dependent manner (Padma et al., 2015). It was found that the IC50 value was 70 µM and its cytotoxic effect increased depending on the dose and induced the loss of lactate dehydrogenase, the release of nitric oxide, the increase in intracellular production of ROS and intracellular calcium level, and the decrease in the antioxidant enzymes catalase, SOD, glutathione-s-transferase, and glutathione with decreasing the mitochondrial membrane potential in DR cells (Padma et al., 2015).

In in vivo animal models, the chemopreventive activity of mangiferin has been tested in ovarian adenocarcinoma cell line xenograft (OVCAR3) in nude BALB/c female mice. Xenograft tumor volume decreased significantly with increasing mangiferin dose during a 14-d treatment, and xenograft tumor volume decreased in mice treated with mangiferin plus cisplatin, suggesting that mangiferin increased the sensitivity of ovarian cells to cisplatin (Zou et al., 2017). Mangiferin has also been reported to inhibit in vivo growth of fibrosarcoma in Swiss mice (Derese et al., 2017).

The way in which carotenoids are thought to prevent development of certain types of cancers is by modulating the RARβ gene that codes for a potential tumor suppressor (Pavan et al., 2006). Contrary to this anticancer effect of β-carotene, this carotenoid and its oxidative metabolite, β-apo-14′-carotenoic acid, down-regulate RARβ in bronchial epithelial cells in the presence of smoke-borne carcinogens by affecting the transactivation of the RARβ promoter, and consequently the transcriptional activation of anti-proliferative or pro-apoptotic genes necessary for the elimination of neoplastic and preneoplastic cells with irreparable alterations (Prakash et al., 2004).

The anti-carcinogenic action of mango has been studied from in vitro cultures of cancer cells and in vivo animal models in the early stages of carcinogenesis (Boateng et al., 2007; Corrales-Bernal et al., 2014a; 2016). Cell growth of human fibroblasts (Stivala et al., 2000) and human colon cancer cell lines (COLO 320 HSR, LS-174, HT-29 and WiDr) (Palozza et al., 2002), as well as some prostate cancer cell lines (PC-3, DU 145 and LNCaP) (Williams et al., 2000) have been shown to be inhibited by β-carotene independently of p53 and/or p21WAF1 status or gene expression with the arrest of cell cycle in G2/M phase and the induction of apoptosis. These inhibitory effects occur in parallel to the accumulation of carotenoids inside the cells and their conversion into retinol, leading to a reduced expression of cyclin A, a key regulator of G2/M progression without modification of p21 or p27, two cyclin kinase inhibitors. In relation to the apoptosis that was observed in Bcl-2, Bcl-xL, and Bax, a similar effect was observed in SW480 adenocarcinoma cell line after treatment with carotenoid and polyphenol extracts from Ataulfo and Haden mangoes, respectively. This was achieved by ∼ 2-fold increase in levels of the enzyme Caspase-8 and 1.3- and 1.7-fold increases in expression of the pro-apoptotic genes BAX and BIM, respectively; and by increases in the cell cycle regulators P21 and PKMYT1 of ∼ 1.3- and 1.4-fold, respectively, compared to untreated cells. In addition, polyphenols from Ataulfo mango (10 mg GAE L^-1) induced cell cycle arrest in the G2/M phase within 24 h and this was associated with increased expression of PKMYT1, a gene that has been shown to negatively regulate the G2/M cell cycle transition (Noratto et al., 2010).

In addition, there is important evidence to support the relationship between treatment with whole mango juices, extracts, and bioactive phytochemicals from mango fruit and the modulation of many molecular pathways to prevent the in vitro development and progression of cancer as well as in murine cancer models. The effect of whole and fractions of mango juice was evaluated using HL-60 cells that showed cell cycle arrest in the G0/G1 phase after treatment (Percival et al., 2006). Furthermore, it has been shown that aqueous extract of ‘Ataulfo’ mango has inhibitory effect on viability of breast cancer cells MCF-7 (García-Solís et al., 2009). In addition, mango inhibited neoplastic transformation of BALB/3T3 cells in a dose-dependent manner (Percival et al., 2006). Similarly, exposure to mango peel ethanol extracts (125–1000 µg mL⁻¹) inhibited proliferation in human cervical cancer HeLa cells, human gastric cancer AGS cells, and human hepatocarcinoma HepG2 cells in a dose-dependent manner (Kim et al., 2010).

In a preclinical model of colorectal cancer induced by AOM using mango juice from Azúcar mango (0.3% to 1.25% w/v) offered daily for 10 weeks to Balb/c mice, Corrales-Bernal et al. (2014a) reported that the formation of ACF in the colon was inhibited by more than 60% in mice receiving 0.3% (p = 0.05) before and after AOM injection when compared with controls receiving water. A similar result (p = 0.013) was obtained when mango juice was given for 8 weeks just after AOM injection. Furthermore, the analysis of crypt multiplicity in ACF by Boateng et al. (2007) indicated that consumption of 5% w/w of mango juice highly inhibited ACF formation (by 83.3%).

6.7 Anti-diabetic effects and mechanisms of mango phytochemicals

Type 2 diabetes mellitus (T2DM) is a complex, multifactorial, systemic chronic disease characterized by hyperglycemia and other metabolic disorders. It is generally accepted that the main pathophysiological contributors to T2DM are insulin resistance (IR) and β-cell dysfunction (Sangeetha et al., 2017). As the available hypoglycemic drugs have adverse side effects, there is an increasing interest in identifying natural sources and/or natural bioactive compounds as therapeutics to treat T2DM and its complications (Shukri et al., 2011). The study of the antidiabetic properties of natural products focuses on the effects involved in carbohydrate digestion, glucose absorption, glucose uptake by adipose tissue and skeletal muscles, synthesis of sorbitol, formation of advanced glycation end products (AGEs), gluconeogenesis/glycogenolysis, and synthesis and secretion of insulin (Alam et al., 2019; Sangeetha et al., 2017). The role of mango on the preventive and adjuvant effects in the treatment of T2DM and its complications have been studied in different ways, by clinical studies, studies in experimentation animals, and in vitro studies.

One of the main concerns about the consumption of mango pulp is that it could have a hyperglycemic effect in patients with T2DM due to its high content of sugars (sucrose, glucose, and fructose). However, several studies have shown that the glycemic response to mango in healthy subjects and patients with T2DM is low with respect to the response to the consumption of glucose or other fruits such as banana, sapote, rambutan, durian, and pineapple (Elizondo-Montemayor et al., 2015; Redfern et al., 2017). A clinical trial carried out in healthy patients shows that fresh Tommy Atkins mango puree has a low glycemic index (GI; Elizondo-Montemayor et al., 2015). The GI is a quantitative measure that determines the increase in blood glucose that occurs following consumption of 50 g of the food carbohydrate equivalents compared with the consumption of the same quantity of glucose. A GI ≤ 55 is considered low, ≥70 is considered high, and the GI described for fresh ripe mango puree is 42.7 ± 19.5. Furthermore, Elizondo-Montemayor et al. (2015) reported that high hydrostatic pressure (HHP) processed mango puree had significantly reduced GI (32.7 ± 13.4). Consumption of low GI foods has been associated with improvements in both body weight and lipid profiles. The mechanism by which HHP processing reduces mango GI is unknown, but it has been postulated that an increase in the viscosity and alcohol insoluble residues solubility could delay the absorption of carbohydrates. The effect of mango fruit powder obtained from unripe Kili-Mooku mangoes, called Careless^TM, on glucose metabolism and microcirculation in healthy subjects was studied in a double-blind, randomized placebo-controlled trial (Buchwald-Werner et al., 2017). Either 100 or 300 mg of fruit powder was administered daily for 4 weeks with no modification observed in glucose tolerance, glycosylated hemoglobin (HbA1c), and microcirculation.

The streptozotocin (STZ) induction model of diabetes mellitus (DM) in rats is widely used to study the antidiabetic effects of natural products. Streptozotocin is a toxic compound that acutely destroys pancreatic β-cells, leading to the development of severe hyperglycemia because of the resulting lack of insulin. This model is particularly useful for studying the hyperglycemic effects and complications of DM such as neuropathy, nephropathy, retinopathy, and cataractogenesis.

Mangiferin has been shown to reduce diabetic nephropathy in STZ-diabetic rats (Li et al., 2010). Mangiferin in STZ-diabetic rats significantly inhibited 24 h albuminuria excretion, glomerular extracellular matrix expansion, and accumulation and transforming growth factor-β1 (TGF-β1) overexpression in glomeruli. Moreover, serum-AGEs, lipoperoxidation, and sorbitol concentration in erythrocytes were significantly decreased by mangiferin (Li et al., 2010). On the other hand, mangiferin also increased both serum SOD and GPx activities and creatinine clearance rate in STZ-diabetic rats (Li et al., 2010). Moreover, in mesangial cells, mangiferin inhibited high-glucose-induced cellular proliferation and also reduced collagen IV overexpression induced by AGEs (Li et al., 2010).

An 8-week high-fat/high fructose diet (HFFD) followed by subdiabetogeneic STZ was used to explore the antidiabetogenic effect of mangiferin. This model is useful to induce IR and β-cell dysfunction and their consequences such as hyperglycemia, depletion in liver glycogen, dyslipidemia, and obesity (Saleh et al., 2014). When 20 mg/Kg i.p. mangiferin was applied for 28 days, insulin sensitivity was improved in HFFD-STZ-diabetic rats (Saleh et al., 2014). Serum fasting glucose and lipid profile were improved and the serum tumor necrosis factor-α (TNF-α) was reduced, whereas serum adiponectin was increased. TNF-α is an adipokine related to development of IR, whereas adiponectin is an adipokine that increases insulin sensitivity. The effects of mangiferin in HFFD-STZ-diabetic rats were similar to those of the standard insulin sensitizer rosiglitazone (Saleh et al., 2014).

Supplementing the high-fat (HF) diet of mice with 1% freeze-dried mango pulp for 8 weeks improved glucose tolerance, with reduced adiposity and lowered insulin resistance in the HF group compared to the hypolipidemic drug fenofibrate and the hypoglycemic drug rosiglitazone (Lucas et al., 2011).

The preclinical model of Gestational Diabetes Mellitus (GDM) in mice has been used to show that the protective effect of mangiferin at 50 mg kg ^-1 d^-1 for 18 gestational days is due to suppression of inflammation, oxidative stress, and endoplasmic reticulum (ER) stress (Sha et al., 2019) because glucose intolerance, insulin resistance, and β-cell dysfunction that contribute to the processes associated with GDM are favored by oxidative stress. The ER regulates protein synthesis and post-translational protein modifications; thus, ER perturbation affects the biological function of proteins resulting in misfolded proteins (Roca-Rodríguez et al., 2014). Therapeutic application of mangiferin was demonstrated to reduce GDM symptoms by rescuing insulin level, decreasing blood glucose, total cholesterol, triacylglycerides and low-density lipoproteins, increasing antioxidant factors (SOD, GPx, and catalase; Roca-Rodríguez et al., 2014). Mangiferin decreased, but did not totally prevent, inflammation [IL-6, monocyte-chemoattractant-protein-1 (MCP-1) e IL-8] and ER stress (GRP78, p-IRE1α, p-eIF2α, and p-PERK) in GDM mouse placenta; the latter may be attributed to poor absorption and low bioavailability of mangiferin.

The mechanisms of the effects of mango fruit (pulp, peel, and seed) on DM are at different physiopathological levels because of very diverse compounds present in different preparations or extracts of mango fruit. Some of these hypoglycemic and hypolipidemic effects observed with mango fruit and the vegetative plant tissues are similar to those observed using pharmacological options, explained from the action of antagonists for peroxisome proliferator-activated receptors (PPARs), which are able to control glucose and lipid metabolism (Wang, 2010). The PPARs comprise a family of three members: α, β/δ, and γ. The PPAR-α activation by ligands regulates the genes involved in the catabolism of fatty acids (β-oxidation, fatty acid synthesis, ketogenesis, and lipoprotein metabolism) (Wang, 2010; Zhang et al., 2015). Upregulation of glucose uptake explains how PPAR-γ improves insulin sensitivity in adipose tissue and skeletal muscle (Wang, 2010). Finally, the PPAR β/δ modulates fatty acid metabolism and regulates endogenous antioxidant levels in the heart as well as regulating mitochondrial biogenesis, cardiomyocyte apoptosis, glucose metabolism and the insulin signaling pathway, and myocardial inflammatory responses induced by lipids (Palomer et al., 2016).

This information suggests that bioactive mango compounds could induce control of glucose and lipid metabolism by acting as PPARs agonists, but the mechanism by which mango fruit and the plant bioactive compounds lower blood glucose levels is unknown. Aderibigbe et al. (1999) reported that an aqueous extract of mango leaf demonstrating hypoglycemic activity did so by stimulating pancreatic β-cells to secrete insulin and reduce intestinal absorption of glucose.

Thus, whether mango compounds could act similarly to other PPAR agonists is not clear at present, and future studies are necessary to examine how mango consumption leads to these effects. Further research is needed to investigate if supplementing the diet with fresh mango containing these bioactive component(s) will have these same benefits.

6.8 Anti-hypertensive effects and mechanisms of mango phytonutrients

Globally, hypertension is one of the most important non-transmissible diseases and it is the most preventable risk factor for all-cause morbidity and mortality (Saiz et al., 2020). Hypertension is also the major risk factor of cardiovascular disease (heart failure, chronic kidney disease, myocardial infarction, peripheral vascular disease, stroke) and it is associated with persistently hypertension in systemic arteries (Saiz et al., 2020). Around 90 to 95% of the patients with hypertension have essential or primary hypertension that has a multifactorial gene-environment etiology (Oparil et al., 2018).

One of the factors involved in the development of hypertension is hyperuricemia (Oparil et al., 2018; Yang et al., 2018). Hyperuricemia involves impairment of nitric oxide (NO) synthesis and subsequent endothelial dysfunction (Yang et al., 2018). The liposoluble free radical NO is involved in several biological processes such as vasodilatation, angiogenesis, immune function, leukocyte-endothelial interactions, platelet aggregation, long-term synaptic transmission, memory, and reproduction (Becerril et al., 2019; Lee et al., 2016a). The three NO synthase (NOS) isoforms, neuronal (nNOS), endothelial (eNOS), and inducible (iNOS), are responsible for NO synthesis. In normal physiological conditions, nNOS and eNOS are both constitutively expressed, whereas iNOS is stimulated by inflammatory cytokines. iNOS promotes excessive NO synthesis that causes oxidative stress and cellular damage via accumulation of peroxynitrites (ONOO^–) (Becerril et al., 2019; Lee et al., 2016a). Elevated blood pressure is generally related to reduced eNOS activity and excessive abnormal iNOS expression seems to be related to progression of vascular dysfunction (Lee et al., 2016a).

There are some studies related to NO synthesis and hypertension in which the antihypertensive effect of mangiferin was explored. Yang et al. (2018) showed in rats with hypertension induced by hyperuricemia that mangiferin administered intragastrically (120 mg kg⁻¹) significantly decreased both serum uric acid (UA) and systolic blood pressure (SBP) between 8 and 12 weeks. The reduction of SBP in hyperuricemic rats by mangiferin was associated with increased serum NO and decreased serum C-reactive protein (CRP). Besides, mangiferin increased eNOS, reduced CRP and intercellular adhesion molecule-1 (ICAM-1), and decreased formation of ONOO^– in aortic segments. Moreover, in vitro experiments performed in cultured human umbilical vein endothelial cells (HUVECs) treated with UA yielded results consistent with the observed results in vivo. Mangiferin in HUVECs treated with UA reduced protein and mRNA expression of CRP, reduced mRNA expression of ICAM-1, increased mRNA for eNOS, increased formation of NO and reduced the formation of ONOO^– (Yang et al., 2018). Moreover, an in vitro study performed with digested mango fruit powder obtained from unripe Careless^TM mangoes in HUEVCs showed 23, 42, and 60% activation of eNOS for doses of 300, 1,500, and 3,000 µg mL⁻¹, respectively (Gerstgrasser et al., 2016).

On the other hand, the antihypertensive effects of mangiferin alone were studied using vascular smooth muscle cells and mesenteric resistance arteries from normotensive (NT) and spontaneously hypertensive (HT) rats (Beltrán et al., 2004). The effect of 1 ng mL⁻¹ interleukin-1β on COX-2 and iNOS protein expression in NT and HT rats was inhibited by 0.025 mg mL⁻¹ mangiferin, but noradrenaline-induced contraction was not affected. Taken together, the results suggest that mangiferin may have an important role to play in decreasing hypertension through regulation of NO synthesis and improvement of endothelial function.