A Critical Review on Polyphenols and Health Benefits of Black Soybeans

1Polyphenols are phytochemicals, found largely in fruits, vegetables, tea, coffee, chocolates, legumes, cereals, and beverages. There are over 8000 polyphenols identified in nature and their main functions are as antioxidant. They protect our body from free radical damage and defense against UV radiation or aggression by pathogens. In the last two decades, there has been more interest in the potential health benefits of dietary polyphenols as antioxidant. The average 100 grams fresh weight of fruits (grapes, apple, pear, cherries, and berries) contain up to 300 mg of polyphenols. Typically, a cup of tea or coffee or a glass of red wine contains more than 100 mg of polyphenols. In addition, cereals, vegetables, dry legumes and chocolate also contribute to the polyphenolic intake and thereby protect our body from chronic diseases. Dietary polyphenols are the subject of enhancing scientific interest due to their possible beneficial effects on human health. They are usually provided to the food as color, flavor, bitter, and astringent, and maintain stability from oxidation. Several epidemiological studies and associated meta-analyses strongly showed that the consumption of these polyphenols offered better protection against chronic diseases such as cancers, cardiovascular diseases, cerebrovascular diseases, diabetes, ageing and neurodegenerative diseases.

2Polyphenols are divided into four different categories based on the presence of number of phenolic groups and structural elements. Food usually contains complex polyphenols, predominantly found in the outer layers of the plants.

3Flavonoids: Have a potential effect on radical scavenging and inflammatory reactions. They are predominantly found in fruits, vegetables, legumes, red wine, and green tea. They are further divided into a number of subgroups namely, flavones, flavonols, flavanones, isoflavones, anthocyanidins, chalcones, and catechins.

4Stilbenes: Found in product of graphs, red wine, and peanuts. Resveratrol is the most well-known compound among the group.

5Lignans: Found in seeds like flax, linseed, legumes, cereals, grains, fruits, algae, and certain vegetables.

6Phenolic acids: Found in coffee, tea, cinnamon, blueberries, kiwis, plums, apples, and cherries and have two subgroups, namely hydroxybenzoic acids, and hydroxycinnamic acids.

7In the plant, polyphenols protect from UV radiation, pathogens, oxidative stress, and harsh climatic conditions. In the human body, polyphenols are antioxidants, and have diverse biological properties such as anti-diabetic, anticancer, anti-inflammatory, cardioprotective, osteoprotective, neuroprotective, antiasthmatic, antihypertensive, antiageing, antiseptic, cerebrovascular protection, cholesterol lowering, hepatoprotective, antifungal, antibacterial and antiviral properties (Figure 1).

8Soybeans contain various colors of seed coat including black, yellow, green, and brown. It is due to the presence of anthocyanins, chlorophyll, and other pigments. Black soybeans (BSB) have been widely consumed as food and as material for Oriental medicine for hundreds of years in Asia. The black pigmentation is due to accumulation of anthocyanins in the epidermis palisade layer of the seed coat. Different anthocyanins in BSB have been identified, including cyanidin-3-glucoside, delphinidin-3-glucoside, and pelargonidin-3-Glucoside. BSB is an excellent dietary source for disease prevention and health promotion.

9In the last two decades, isoflavones and proteins are the primary health beneficial components in BSB that have received attention. Nevertheless, there are insufficient data to explain the health benefits exclusive to BSB. They have potentially active phytochemicals such as isoflavones, sterols, phytic acid, saponins, and phenolics, which are potentially effective for human health and prevention of various chronic diseases. Research showed that BSB has the greatest antioxidant properties compared to other colored soybeans. The characteristic antioxidant potential is due the presence of phenolics, which is mainly distributed in the seed coat. In the seed coat, around 20 phenolic compounds, predominantly six anthocyanins, are greatly (13–50 times) distributed in several BSB varieties, which helps to reduce the risk of chronic diseases such as metabolic disorders and cancers. These varieties have the potential to be used in functional foods and food colorant development. The predominant quantity of anthocyanins provides a black color to the seed coat and showed to have potent antioxidant properties, which is mainly responsible for health-promoting effects of BSB. In addition to the anthocyanins, BSB contain other phenolics such as tannins, isoflavones and phenolic acids.

10BSB has a high content of protein (32–43.6%). In addition to protein, BSB contains carbohydrates (31.7–31.85%), lipids (15.5–24.7%), water (5.6–11.5%), minerals (calcium, phosphorous, magnesium, potassium, sodium, selenium, etc.) and vitamins (Vitamin E, B complex, etc.). The BSB lipid composition consists of 86% unsaturated fatty acids, especially linoleic (6.48–11.6%), linolenic (0.72–2.16%) and oleic acids (3.15–8.82%), making it beneficial to human health. Soybean is characterized by the most digestible proteins, lysine and methionine. However, it is limited by sulfur amino acids and tryptophan.

11Anthocyanins are water-soluble natural pigments that belong to flavonoids, a larger subgroup of polyphenols, and widely distributed in BSB and shown to provide numerous health benefits. Major anthocyanins have been isolated and identified from the seed coat of BSB such as cyanidin 3-O-β-d-glucoside, delphinidin-3-O-β-d-glucoside, pelargonidin-3-O-glucoside, and petunidin 3-O-β-d-glucoside. The minor anthocyanins such as catechincyanidin-3-O-glucoside, delphinidin-3-O-galactoside, cyanidin-3-O-galactoside, and peonidin-3-O-glucoside have also been isolated and identified based on the fragmentation patterns of high-performance liquid chromatography–diode array detector–electrospray ionization/mass spectrometry analysis. 5,7,3′,4′-Tetrahydroxyflav-2-en-3-ol 3-O-β-d-glucoside have also been isolated from immature BSB. The structure of anthocyanins-rich BSB is depicted in Figure 2.

12Anthocyanins rich BSB has potential health benefit as complementary medicine and utilized in various formulation implied for antioxidant, anti-inflammatory, nephroprotective, antidiabetic, anticancer, anti-infertility, anti-obesity, anti-arthritic, neuroprotective, antihyperlipidemic, anti-cataract and wound healing properties. Detailed information on dose range, route of administration, the model used, negative controls, and other pharmacological results based on the experimental research study in vivo and in vitro, according to the appropriate title depicted, are presented in Table 1.

13Some of the health benefits of anthocyanins rich BSB are listed below.

14BSB has high content of proteins and fibers. It has an enormous amount of minerals such as calcium, phosphorous magnesium, iron, manganese, copper, and zinc, which contribute to maintain and stabilize the bone and its strength. The study showed that the consumption of BSB has a definite protective effect on bone loss in postmenopausal osteoporosis study model and thereby BSB inhibits bone turnover and prevent bone resorption. This study confirmed that the intake of BSB can be used to prevent bone loss in estrogen deficiency animal studies. In addition, the observational and acute clinical trial studies also suggested that the isoflavone-genistein reduces bone loss and enhances bone mineral density in osteopenic postmenopausal women.

15Owing to the beneficial health effects on the human cardiovascular system, BSB has been a focus of intensive research. BSB contains a low concentration of sodium, which helps to maintain blood pressure at a normal range. A recent epidemiological study also showed that the anthocyanins rich BSB reduce the risk of cardiovascular diseases and maintain the blood pressure in the affected individuals. In addition, BSB has potent inhibitory activity on collagen-induced platelet aggregation and reduce cardiovascular risk, and thereby improves blood circulation. BSB has enormous quantities of fiber, potassium, folic acid, pyridoxal phosphate, and phytonutrients (quercetin and saponins) and lack of cholesterol, which supports to reduce cardiovascular complications. The fiber in BSB helps to lower total cholesterol (TC), LDL-cholesterol (LDL-C) in the blood and liver that reduce the risk of heart disease. It inhibits oxidative stress in postmenopausal women by increasing antioxidant activity and improving lipid profiles.

16The consumption of BSB may reduce the risk of coronary heart diseases. Recent researches have shown that BSB inhibit the effect of low density lipoprotein oxidation, inhibit TNF-alpha-induced vascular cell adhesion molecule-1 (VCAM), intracellular adhesion molecule-1 (ICAM), and cyclooxygenase-2 levels. Further, anthocyanins protected myocardial injury from ischemia-reperfusion in rats. Thus, anthocyanins from BSB seed coat benefit for pathological conditions like coronary heart diseases.

17BSB is known to be rich in anthocyanins, and they have been consumed since ancient times for their beneficial effects on health. It has been reported that BSB seed coat may ameliorate obesity and insulin resistance. BSB has more fibers, which have a vital role to reduce the blood sugar. Notably, one cup of BSB contributes around 15 g of fibers. In addition, the BSB seed coat extract contains polyphenol-rich food material consisting of 9.2% cyanidin 3-glucoside, 6.2% catechins, 39.8% procyanidins, and others. These compounds remarkably prevent obesity and diabetes by enhancing energy expenditure and suppressing inflammation.

18Several studies showed that anthocyanins rich BSB have been shown to inhibit cancer cell growth by suppressing oxidative stress and inflammatory responses. Anthocyanin-rich BSB seed coat extract possibly reduces the development of tumors in various organs such as intestines, breast, prostate, stomach, ovary, endometrium, and liver. In addition to the anthocyanins, saponins also prevent cancer cells from proliferation and spreading throughout the body. BSB is high in folic acid, which plays a vital role in DNA synthesis and repair, thus BSB prevents the formation of cancer cells from DNA mutations.

19BSB contain high quantity of fibers, which enhance satiety and decrease appetite, making an individual feel full for longer time, and thereby reduce the overall calorie intake. Many studies have suggested that the consumption of anthocyanins rich BSB reduces the risk of adipogenic activity, obesity, fatty acid content in subcutaneous and overall mortality while promoting a healthy complexion, visceral fat, increased energy, and overall reduce the body weight.

20Anthocyanins rich BSB may have antibacterial, anti-fungal, and anti-viral properties. The extract from BSB produced significant growth reductions of food borne pathogens such as Escherichia coli, Salmonella typhimurium and Campylobacter jejuni in broth-cultures as well as on chicken skin. A previous study also showed an isolated monomeric protein (molecular mass of 25 kDa), containing N-terminal sequence, which resembles a segment of chitin synthase. The protein, named glysojanin, demonstrated a potent antifungal activity against Fusarium oxysporum and Mycosphaerella arachidicola and antiviral potential against HIV-1, human adenovirus type 1 and coxsackievirus B1.

21Black soybeans have popularly been utilized as a food and medicinal material for a long time with low price. Anthocyanins have antioxidant effect and can be useful for the treatment of diabetes, cardiovascular disorders, cancers, etc. Although the exact mechanism by which anthocyanins prevent the expression of adhesion molecules remains to be elucidated, they can be used as good materials to modulate or prevent such chronic diseases. In any case, more support for such properties/dynamic constituents has been acquired from cellular and molecular studies, while clinical studies are as yet inadequate. Since animal research does not generally interpret human circumstances, additional clinical studies are justified for comprehending the full interpretation of the effects of Anthocyanins in BSB for human disease prevention. Subsequently, futures far-reaching clinical studies are required to warrant the therapeutic convenience of anthocyanins in BSB. Furthermore, highlighting the synergistic multi-component effects of BSB on biological functions would be a recommendation for further studies, as well as studies of the mechanism of action and new biomarkers to prove the effectiveness of BSB bioactive compounds in preventing and treating several symptoms and/or pathologies.