Soy Bioactive Components in Functional Perspective: A Review

Abstract

Functional foods are the focus of attraction for well-being. The healthy living of Asians may be attributed to a diet enriched with soy isoflavones viz. genistein and daidzein which fills the gap of proteinaceous diet in vegetarians. Several factors, such as administration, dosage, metabolism, ingestion of other pharmacological substances, type of estrogen receptors, and presence or absence of endogenous estrogen, affect the activities and bioavailability of isoflavones. The mode of therapeutic action could be either through the stimulation of estrogen receptors or by the non-involvement of estrogen receptors. Soy, due to its isoflavones content, serves as a balanced and remedial substitute in combating various lifestyle disorders, like cancer prevention, by interruption of the breakdown of extracellular matrix that surrounds growing vessels and tumors, lowering of lipid and blood insulin levels by regulating lipid and glucose metabolism, alleviation of menopausal symptoms, and osteoprotective effects by modulating estrogen receptors, thus acting as a safer switch to hormone replacement therapy, anti-hypertensive effect which may also contribute to cardioprotective, anti-oxidative effect, regulation of cognitive functions, and many others. Therefore, it could be regarded as a valuable therapeutics. However, since soy isoflavones also act as endocrine disruptors, they also possess some negative effects. Concerns have been raised in relation to thyroid function abnormality. In view of the previously mentioned facts an attempt has been made to review the literature available on both beneficial as well as deleterious role of isoflavones, soy derived bioactive compounds.

Keywords:

• Soy isoflavones
• Genistein
• Daidzein
• Diabetes
• Cancer
• Menopause
• Osteoporosis

INTRODUCTION

Nutrition is pivotal for the survival, health, and biological development of human beings. Currently, more emphasis is laid on the food, which is nutrient dense and one such noteworthy example is “Soybean” (Glycine Max).^[1] It is reckoned to be a high source of proteins in Asian countries and could be considered a suitable substitute for meat and dairy products.^[2,3] Soybeans have numerous nutritional advantages with respect to macronutrients as well as micronutrients content over other food legumes, summarized in Table 1. Soybean acts as an ideal food candidate because of its inherent health promoting compounds like omega-3-fatty acids, lectins, trypsin inhibitors, peptides, saponins, phytates, phytosterols, and isoflavones particularly genistein, daidzein, and glycitein^[4,5] (Fig. 1). Because of this, soy-based foods could serve as a therapeutic in combating various lifestyle disorders like diabetes, obesity, osteoporosis, and cardiovascular diseases.^[6–8] A study revealed that intake of food enriched with soy fiber resulted in the loss of body weight and lipid content which aids in preventing the symptoms of obesity, hyperlipidemia, and hypertension.^[9] In addition, casein fed normocaloric diet when replaced with normocaloric soy flour in rats improved antioxidant defences by increasing the activity of super oxide dismutase (SOD), catalase, and other enzymes.^[10] A recent study revealed the potential role of soy isoflavones (SI) in the maintenance of intestinal barrier functions in weaned piglets when challenged with lipopolysaccharides.^[11] The present review summarizes the mode of action of isoflavones; nutritional importance of soy, its isoflavones; and some of their potential health benefits reported until now.

TABLE 1 Nutritive value of soybeans as compared to other legumes

Legumes	Protein (%)	Fat (%)	Carbohydrate (%)	Calcium (mg/100 g)	Phosphorus (mg/100 g)	Iron(mg/100 g)	Magnesium (mg/100 g)	Sodium (mg/100 g)	Vitamin A (μg)	Thiamine (mg/100 g)	Riboflavin (mg/100 g)	Niacin(mg/100 g)	References
Soybean	40	20	20–30	226	546	8.5	236	27.9	426	73	39	3.2	[246]
Chickpea	4.9–29.6	4.99	60.1–61.7	105	168	2.89	48	7	1	0.116	0.063	0.526	[247]
Peas	21.2–32.9	0.2	14.45	43	108	1.47	33	5	38	0.266	0.132	2.09	[248]
Pigeonpea	18.8–28.5	2.19	57.3–58.7	130	367	5.23	183	17	—	0.643	0.187	2.965	[249]
Green gram	20.8–33.1	2.14	53.3–61.2	132	367	6.74	189	—	—	0.621	0.233	2.251	[250]
Lentil	20.4–30.5	1.17	59.7–61.0	56	451	7.54	122	6	—	0.87	0.211	2.605	[251]

FIGURE 1 Structures of the primary isoflavones in soybean.^[245]

Therapeutic Mechanism of Action of Isoflavones

Isoflavones classified as phytestrogens show their activity by interacting with enzymes and estrogen receptors (ERs) which result in the formation of complex. The complex then stimulates ER-positive cell growth which alters ER structure and affects transcription. The non-genomic effects that do not include ERs include inhibition of tyrosine kinase, induction of cancer cell differentiation, influencing DNA topoisomerase activities, suppression of angiogenesis and antioxidative effects of isoflavones^[12,13] (Fig. 2). Phytestrogens affect the enzymes activity essential for hormone conversions and reduces the risk of cancer by lowering the biological activity of sex hormones.^[14]

FIGURE 2 Mechanism of action of soy isoflavones.

Several factors, such as administration, dosage, metabolism, and the ingestion of other pharmacological substances, affect the activities and the bioavailability of phytoestrogens.^[15] The effectiveness of phytestrogens also depends upon target tissue, concentration, number, and type of ER and the presence or absence of endogenous estrogens.^[16] In a pharmacokinetic study, isoflavones concentration were compared in healthy subjects distributed over three meals, where the supplementation was in the form of tablets containing SI (144 and 288 mg/day) and soy foods containing 96 mg/day isoflavones. Although none of the interventions had any side effects, an increase was observed in serum concentration of isoflavones in subjects consuming soy foods relative to isoflavone supplements.^[17] Also, Szkudelska and Nogowski^[18] gathered the data about the versatile perspectives of genistein to be a potent health promoting compound by inducing hormonal (insulin, leptin, thyroid, adrenocorticotropic hormone, cortisol, and corticosterone) and metabolic changes at a cellular level.

SI

Isoflavones are a group of plant-derived phenolic compounds which belong to the subclasses of the flavonoid family.^[19,20] SI contain 12 different isoforms out of which genestein and daidzein with their respective β-glycosides viz. genestin and daidzin are the primary isoflavones present in soybeans with much lower amounts of glycitein and its glycoside glycitin. Also, it was reported in in vivo, as well as in vitro, studies that after ingestion of isoflavones, the human intestinal microflora converts daidzein into several different products^[12,21] which have been known to exert beneficial effects like induction of estrogenic effects and exhibiting antioxidant and free radical scavenging ability in a better way than the parent molecule.^[22–24]

Isoflavones and Its Potential Effect

Isoflavones have been proposed to be an active component responsible for the beneficial effects of soy foods and appear to work in conjunction with the proteins and other bioactive compounds to protect against various lifestyle disorders.^[25–31] The pros and cons of these disorders remains the focus of discussion (Fig. 3).

FIGURE 3 Potential health beneficial effects of soy isoflavones.

Isoflavones and Diabetes

Type-2 diabetes (T2D) is a looming epidemic worldwide, characterized by insulin resistance and loss of pancreatic mass, influencing almost all major sections of society. Soybean proteins display a pivotal role in diabetes which tends to reduce blood insulin levels.^[32] Soy, a rich source of bioactive compounds, seems to exert anti-diabetic effect by interaction with ER receptors^[33,34] considered to be the modulators of lipid and glucose metabolism.^[35,36] Getek et al.^[37] compiled various animal and clinical trials, designed to analyze the active role of genistein and daidzein components on pancreatic beta cells in the regulation of insulin-dependent diabetes and the various factors which are indirectly linked in its development. Various studies regarding anti-diabetic effects of soy and its isoflavones are listed in Table 2.

TABLE 2 Anti-diabetic effects of soy and its isoflavones

S. No.	Type of supplement	Study model	Outcomes	References
1.	Soy phytoestrogens	Human patients	Alteration of insulin resistance and glycemic control	[38]
2.	Soy nuts	Human patients	Improvement in glycemic control and insulin resistance	[39]
3.	Soy isoflavones	Diabetic rats	Improvement of blood glucose, lipid metabolism, and antioxidant enzyme activities	[40]
4.	Soy isoflavones (Diadzein)	In-vitro (Muscle derived cell line of L6 myoblasts)	Suppression in rise of glucose levels	[41]
5.	Soy phytochemical extract	In-vitro (Rabbit intestinal brush border membrane vesicles)	Inhibition of intestinal glucose uptake and protection against glucose induced oxidation	[42]
6.	Soy protein isolate+Soy isoflavones	Animal model	Insulin degradation thus controlling insulin action	[43]
7.	Soy isoflavones (genestein & diadzein)	Animal model	Improvement in insulin resistance	[44]
8.	Soy isoflavones	Animal model	Relieving postmenopausal symptoms viz. insulin resistance	[45]
9.	Soy isoflavone and its metabolite	Nested-case control study	Decreased risk of T2D	[46]

Apart from these, Wagner et al.^[47] assessed that isoflavones intervention increased insulin response to glucose challenge, decreased peroxisome proliferator-activated receptors (PPARs) expression (nuclear receptor proteins regulating the expression of genes) and plasma adiponectin concentration, respectively, in a dose dependent manner in male monkeys. Moreover, the administration of SI increased the adenosine monophosphate protein kinase (AMPK) activation which is characterized as a master regulator for both lipid and glucose metabolism and attenuated the risk of T2D.^[48] Moreover, 30 premenopausal women were given soybean flour fortified bread as a dietary intervention (120 g for 6 weeks) prepared by replacing 30% of wheat flour with soybean flour to see the protective effect of soy intake on the development of diabetes but no significant effect on metabolic profile was observed.^[49]

Isoflavones and Cancer

One-third of all cancers are thought to be related to dietary factors, these include types of food, preparation methods, portion size, variety, and overall caloric balance (Cancer Risk Factors, http://training.seer.cancer.gov/disease/cancer/risk.html).^[50] Hence, identifying the bioactive compounds that can prevent or delay cancer progression may have a positive impact on the quality of life.^[51,52] Although soybean has numerous phytochemicals, but most of the data points toward genistein for their hypothesized anticancerous effects.^[53] One of the mechanisms proposed is the significant impairment of angiogenesis through inhibition of cell proliferation ^[54,55] and interruption of the breakdown of extracellular matrix that surrounds growing vessels and tumors.^[56] SI particularly genistein displayed weak estrogenic activity which could hamper cancer cell proliferation by binding to ER isoforms;^[57] activation of PPARs;^[58] epigenetic and genome-wide effects;^[59,60] and induction of apoptosis.^[61,62] Saif and co-workers reviewed that genistein alleviated the problem of pancreatic cancer, since it modulates key regulatory proteins, which was further endorsed in animal models when administered in combination with a drug named gemcitabine.^[63] Various studies regarding anti-cancerous effects of soy and its isoflavones are listed in Table 3.

TABLE 3 Anti-cancerous effects of soy and its isoflavones

Sr. No.	Type of supplement	Study model	Outcomes	References
1.	Soy isoflavones (genistein and diadzein)	Human patients	Protection against PC	[64]
2.	Soy isoflavones	Post-menopausal women	Lowering of the risk of breast cancer	[65]
3.	Soy foods and isoflavones	In vitro as well as in vivo studies including humans	Decreased risk of breast cancer	[66]
4.	Soy isoflavones	Human patients	Alleviation of PC	[67]
5.	Soy isoflavones	Premenopausal women	Decreased risk of breast cancer	[68]
6.	Genistein	Comparison between diets of Asian and western population	Lower rates of breast cancer among Asians as compared to western population	[69]
7.	Soy and its phytoestrogens	In vitro and in vivo animal model	Lower risk of breast cancer in premenopausal women as compared to post-menopausal women	[70]
8.	Soy isoflavones	Human patients	Safety of isoflavones against breast cancer	[71]
9.	Soy and its isoflavones	In vivo animal and human studies	Early intake may reduce the risk of breast cancer	[72]
10.	Genistein	Animal model	Early intake may modulate the genesis of mammary tumour	[73]
11.	Equol	Human intervention studies	Reduction in the risk of cancer and other degenerative diseases	[74]
12.	Diadzein sulfate	In vitro studies	Anti-tumourogenic effect	[75]
13.	Soy and its isoflavones viz. genistein, diadzein, glycetein, and equol	In vivo human and animal studies	Lower risk of PC	[76]
14.	Soy isoflavonoids	Human intervention studies	Protection against genesis of PC	[77-79]
15.	Soy isoflavones	Human patients	Alleviation of adverse effects in PC patients	[80]
16.	Genistein + cabazitaxel (drug)	In vitro and in vivo experimental model	Regulation in the onset of metastatic castration-resistant PC (mcrpc)	[81]
17.	Genistein and diadzein	In vitro (human prostate cancer cell lines)	Down regulation of growth factors involved in proliferation of new blood vessels in tumors	[82]
18.	Genistein	Human patients	Reduction in the risk of prostate cancer by decreasing serum prostate specific antigen (psa) levels	[83]
19.	Soy isoflavones	Human intervention studies	Lowers the risk of castration resistant PC	[84]
20.	Non-fermented soy foods containing soy isoflavones	Human patients	Protective effect against stomach cancer	[85]
21.	Soybean/tofu	Korean men and women	Lowers the risk of gastric cancer	[86]
22.	Soy isoflavones	Human population (North-West China)	Reduced risk of oesophageal cancer	[87]
23.	Structurally modified genistein	In vitro and in vivo studies	As a therapy for cancer prevention	[88]
24.	Genestein	In-vivo study	Anti-cancer effects against in ER(-) breast cancer	[89]
25.	Soy isoflavone	Meta-analysis study	Lower risk of breast cancer in pre and post-menopausal women	[90]
26.	Genistein	Animal model study	Suppression of the tumor growth and metastatic transition	[91]
27.	Soy isoflavones	Epidimeological study among Japanese women	Reduction in the risk of breast cancer	[92]
28.	Soy and Soy isoflavones	Meta-analysis study	Reduction in the risk of prostate cancer	[93]
29.	Soy isoflavones and its metabolite	Human patients	Inverse relation between prostate cancer incidence and soy isoflavones intake	[94]

Although there are a good number of studies establishing the beneficial role of isoflavones on various cancers,^[95–98] there are still some contradictions. The exposure of genistein in Ishikawa cells (immortalized uterine human endometrial adenocarcinoma cell line) as a model system caused uterotrophic effects by binding to glucocorticoid receptors, therefore, altering the role of glucocorticoids which are responsible to exert anti-inflammatory actions within the immune system.^[99] Moreover, the in vitro effect of equol was reported in the progression of cancer in estrogen receptor negative (ER-) metastatic breast cancer (BC) cells (MDA-MB-435 and Hs578t), through the up-regulation of c-Myc (regulator gene that plays a role in cell cycle progression, apoptosis, and cellular transformation) transcription leading to translation of eukaryotic protein synthesis initiation factor eIF4GI, which may specifically direct the synthesis of internal ribosome entry sites (IRES) containing messenger RNA (mRNAs) that induce cell survival and proliferation.^[100] Soy containing various concentrations of isoflavones when fed at high levels to MTB-IGFIR transgenic mice (mice developing mammary tumours by the over expression of type I insulin-like growth factor receptor) promoted the development of mammary tumors which appears to be associated with increased expression of Areg (marker of ER signaling).^[101] A meta-analysis study by Hamilton-Reeves et al.,^[102] suggested that neither soy foods nor its isoflavones have any significant impact on the reproductive hormone (testosterone) levels in men with the risk of prostate cancer (PC). A 6-month intervention of mixed SI had no effect in reducing breast epithelial proliferation in healthy, high risk adult western women (n = 126).^[103] Furthermore, soy protein and isoflavones stimulated the growth of mammary tumour in ovariectomized mice implanted with estrogen-sensitive BC cells.^[104] Also, long-term exposure to genestein induced, as well as promoted, the growth of tumors in the human BC xenograft model.^[105] A recent study revealed that SI stimulated BC and further enhanced bone micro-metastasis in mice model studies.^[106] Also, no protective association was observed between the intake of soy food plus isoflavones and the incidence of endometrial cancer among Japanese women.^[107]

Estrogenic/Anti-Estrogenic Effect

Phytestrogens in the diet may have a role in modulating hormone-related diseases because of their structural similarity to the estrogens like 17b-estradiol and diethylstilbestrol. Due to this phenomenon, SI endeavor weak anti and/or estrogenic effects through alternative signaling pathways.^[108] Pino et al.,^[109] manifested good estrogenic activity of SI when administered in post-menopausal women by demonstrating an increased level of a sex hormone binding globulin (SHBG) responsible for binding to gonadal hormones. Recent studies have summarized that isoflavones could be targeted as a safer switch for hormone replacement therapy (HRT) in menopausal women.^[110,111] In ambience, supplementation of approximately 54 mg isoflavones for a period of 8 weeks in post-menopausal women decreased the levels of follicular stimulating hormone (FSH) and luteinizing hormone (LH).^[112] Alekel et al.^[113] elucidated the excellent compliance of isoflavones intake in post-menopausal women but found no evidence of treatment effects on endometrial thickness or circulating hormone concentrations. Health risks can be controlled due to the presence of SI which produces weak estrogenic effect as studied in Indian women.^[114] Isoflavones are reported to exert a balancing effect on reproductive hormones and endothelial functions in the women who are at the end of their menarche^[115] but, did not offer any benefit to post-menopausal women.^[116] However, its supplementation before the start of menopausal symptoms indicated by hot flushes, alleviated the condition.^[117] A dietary supplementation (90 mg of isoflavones) in post-menopausal women improved vaginal dryness, but failed to exert estrogenic action.^[118] The supplementation of SI for a period of 16 weeks in the form of both soy milk (71.6 mg isoflavones derived from 706 mL soymilk/d) and isoflavones (70 mg isoflavones + 706 mL cow milk/d) enhanced the immunity in post-menopausal women by modulating B cells and protection against DNA damage.^[119] Recently, a patent has been granted for development of the formulation composed of phytestrogens that includes genestein, daidzein, equol, IBSO03569, and combination thereof, involved in alleviating menopausal symptoms.^[120] However, soy protein containing 52 mg genistein, 41 mg daidzein, and 6 mg glycetein did not exert marked effects on the health status, life satisfaction, and depression of post-menopausal women.^[121]

Despite many positive health outcomes, SI has certain ramifications as it behaves like an endocrine disrupting chemical (EDC). It mimics the action of endocrine disruptors which results in the alteration of fertility by disrupting several aspects of reproduction.^[122] Perinatal exposure of genistein in CD1 male mice lead to life-long effects on the differentiation of brain circuits and behaviors like anxiety, aggression, and sexual behaviors by altering the neuronal nitric oxide synthetase (nNOS) enzyme implicated in the regulation of these behaviors.^[123] Similarly, exposure of genistein (Gen1, Gen10, and Gen100 mg/kg body weight) to female Sprague dawley rats resulted in detrimental effects on the potential of male reproductive system by causing disruption of the hormonal function, leading to infertility.^[124] Administration of SI to rats during infant stage posed serious complications on the development of female reproductive system by the activation of various apoptotic signaling pathways which was further supported by a decrease in serum estradiol levels and increase in both ovarian follicle atresia and corpus luteum number.^[125]

Isoflavones and Hypertension

High blood pressure ranked as the third most important risk factor exerts a substantial burden on the healthcare system resulting in a premature death worldwide. Hypertension revolves around angiotensin-I converting enzyme (ACE; kinases II peptidyldipeptide hydrolase, EC 3.4.15.1) which is a key factor for blood pressure regulation.^[126] Margatan et al.^[127] reported ACE-inhibitory activity of soy protein isolates (SPI) in restraining the problem of hypertension in an in vitro model. Substituting soy nuts for a non-soy protein improved blood pressure in hypertensive women as well as normotensive post-menopausal women which may also confer cardio-protective effect.^[128] Genistein enhances endothelial nitric oxide synthase (eNOS) gene transcription and protein synthesis in primary human vascular endothelial cells which have vasculoprotective effects and further supplementation of it (0–2 gm/kg for 6 weeks) alleviated hypertension in spontaneously hypertensive rats (SHR).^[129] In vivo administration of genistein (1 mg/kg/day) in combination with dimethylsulfoxide lowered blood pressure, restored ACE leading to nitric oxide (NO) synthesis via protein kinase C beta type enzyme (PKC-β II).^[130] Moreover, genistein restores the NO mediated vascular relaxation in ovariectomized and chronically hypoxic rats^[131,132] and reduces the plasma endothelin-1 levels in healthy post-menopausal women.^[133] Greater reduction in blood pressure, inflammatory markers, and adhesion molecules was observed among equol producing post-menopausal women upon supplementation 25 g of soy protein and 101 mg of isoflavones.^[134] Furthermore, in vitro trials showed that vasodilation effect of equol on uterine smooth muscles was mediated through antagonistic action for receptor-dependent calcium ion channel.^[135]

Isoflavones and Anti-Oxidation

Oxidative stress is considered as the most critical factor in the development of chronic diseases, caused by an imbalance of reactive oxygen species (ROS)/reactive nitrogen species (RNS). There is increasingly stronger evidence that the soy phytochemicals and their metabolites play a major role in the prevention of chronic diseases that arises due to oxidative stress and, therefore, could be considered as a potential functional food components.^[136,137] Soybean seed extract and fresh soymilk have been reported to exert the anti-inflammatory and anti-oxidative properties which might be due to the presence of many functional components.^[138,139] Fermented soy permeate (FSP) containing two active compounds viz. SI and alpha-galactooligosaccharides (α-GOS) administered orally (0.1 g/day) to male streptozotocin-induced diabetic wistar rats (n = 30, FSP-supplemented group = 10) exhibited antioxidant and anti-inflammatory properties.^[140] Recently, it is reported that genistein (10 mg/kg) prevents nitrosative skin injury by inhibiting peroxynitrite (ONOO-) formation which further leads to tissue protection, decreased lipid peroxide and nitrotyrosine formation and lowers the catalase activity by upregulating the cell proliferation factors (Ki67 and PCNA).^[141] Genistein has been shown to inhibit hydrogen peroxide production and increases the activity of antioxidant enzymes thus preventing cancer both in vitro (HL-60—Human promyelocytic leukemia cells) as well as in vivo (mouse skin tumorigenesis model).^[142] In vitro exposure to genistein reduces the oxidizability and suppresses the formation of lipid peroxides and thiobarbituric acid reactive substances (TBARS).^[143] Supplementation of genistein rich soy food to sprague-dawley rats exerted potent effect on serum lipids and lipoprotein oxidation.^[144] Similarly, consumption of soy protein decreased lipid peroxidation and lowered the probability of atherosclerosis in rabbits.^[145] Soy phytochemical extract (SPE) containing genistin and daidzin was found to protect against glucose-induced oxidation of low density lipoprotein (LDL), in vitro.^[146] Benign prostatic hyperplasia (BPH) is the pathological condition that arises with aging in men and is mediated by the conversion of the principle androgen, testosterone, to its more potent metabolite, 5α-dihydrotestosterone (5α-DHT). In the concern of the previously mentioned study, administration of equol has shown to be of pharmacological importance by acting as selective androgen modulator (SAM) to improve the prostate health.^[147] Although genistein, has been shown to exert antioxidative effect^[148] but, a recent study elucidated that administration of genistein at high concentration, exerts pro-oxidant effect and increases oxidative stress in the primary muscle cells by enhancing ROS production in a 5-lipoxygenase-dependent manner.^[149] However, sulfation of genistein masking its hydroxyl group, resulted in decreased antioxidant property.^[150] Furthermore, effect of SI on the activity of erythrocyte antioxidant enzyme revealed that its supplementation does not possess antioxidant activity in post-menopausal women.^[151] Mittal et al.^[152] evaluated the antioxidant potential of isoflavones (75 mg/day) in oophorectomized menopausal women (n = 43) and inferred that none of oxidative enzymes were altered significantly and strengthened the concept that cardio-protective mechanism of isoflavones is due to some other reason apart from antioxidant pathway.

Isoflavones and Anti-Atherosclerosis

Based on some clinical studies, the Food and Drug Administration^[153] authorized a health claim stating that intake of 25 g of soy protein per day may lower total and LDL cholesterol and thus, approved labeling of foods as soy enriched to be heart healthy. Among other components of soy, isoflavones are gaining high recognition due to their anti-atherogenic health effect.^[154] Cell culture and animal trials revealed constructive effect of isoflavones in decreasing the problem of atherosclerosis.^[155] Data suggested that isoflavones alleviated the level of LDL cholesterol by inhibiting its oxidation and augments high density lipoprotein (HDL) in the body.^[156] Furthermore, Gil-Izquierdo et al.^[157] suggested the biological activity of isoflavones and its metabolites in alleviating the cardiovascular problems from an epidemiological, clinical, and omics point of view. Various studies regarding anti-atherosclerosis effects of soy and its isoflavones are enlisted in Table 4. Besides this, a study by Mangano et al.^[169] elucidated that intervention of soy protein and isoflavones was successful in reducing inflammatory markers (IL-6) but did not improved serum lipid which are considered to be major risk factor in coronary heart disease (CHD). Moreover, Wang et al.^[170] put forward a hypothesis that SI, particularly genistein, is involved in Kawasaki disease pathogenesis, associated with children 5-years old or younger in which the blood vessels get inflamed throughout the body, especially in the heart.

TABLE 4 Anti-atherosclerosis effects of soy and its isoflavones

S. No.	Type of supplement	Study model	Outcomes	References
1.	Soy protein isolate	Human patients	Alleviation of risk of CVD	[158]
2.	Black soybeans+fenofibrate	Human patients	Anti-hyperlipidemic effect	[159]
3.	Soy isoflavones	Human patients with ischaemic stroke	Prevention of secondary CVD	[160]
4.	Soy isoflavone (genestein)	In-vitro (human coronary artery endothelial cell)	Anti-atherogenic and cardioprotective effect	[161]
5.	High isoflavone soybeans	Animal model	Atheroprotective benefits	[162]
6.	Diadzein metabolite viz. equol	In-vitro study	Beneficial effect on hemostatic system	[163]
7.	Soy isoflavones	Meta-analysis study	Reduction in LDL cholesterol	[164]
8.	Soy isoflavone (genestein)	Animal model	Regeneration in ischaemic tissues	[165]
9.	Soy isoflavones (genestein)	In-vitro model	Reduction in risk factors of CVD	[166]
10.	Soybean purified extract containing genestein and diadzein	Animal model	Improved cardiometabolic risk factors	[167]
11.	Soy isoflavones	Human patients	Improved lipid profiles, thereby lowering the risk of CVD	[168]

Isoflavones and Osteoporosis

Osteoporosis is a systemic skeletal disease characterized by the impairment in the bone architecture particularly among elder persons and women. HRT has been the mainstay in osteoporosis prevention but, it has been followed by various health problems particularly cardiac diseases.^[171] Soy may find possibility to relieve the symptoms and incident of osteoporosis^[172–175] and also referred as selective ER modulators.^[176] Recently, a meta-analysis study in ovariectomized rats revealed protective effects of phytestrogens against bone loss, likely due to the inhibition of bone resorption.^[177] Furthermore, isoflavones may modulate the calcium metabolism and trigger bone sparing effect in the body.^[178] Epidemiological studies evinced an inverse relation between the intake of soy rich foods and risk of osteoporosis with post-menopausal women,^[179] as well as osteoporotic model in small animals.^[180] Moreover, it was reported that high intake of soy (>62.64 mg/day) was associated with lower incidences of bone mineral density (BMD) among BC survivor patients.^[181] In vivo studies proved that the collaboration of mild exercise in combination with isoflavone administration was much more profitable as compared to either treatment alone^[182,183] and also depend on the individual’s intestinal flora for equol production.^[184] As the women enter in post-menopausal phase they become more prone to bone demineralization and fat accumulation which, is subsequently promoted by age factor. In this regard, phytohormones served as a promising tool by providing dual benefits, (1) modulate the balance between adipogenesis and osteogenesis (2) generates less side-effects, therefore, preventing from fragile fractures.^[185] Prevention of bone resorption or inhibition of osteoporotic effect may be due to insulin-like growth factor 1 (IGF-1) mRNA in presence of isoflavones or its similarity with ipriflavone respectively; studies revealed.^[186–188] A recent study elucidated that soybean oil (15% w/w) have osteoprotective effect in ovariectomized rats which restored minerals, antioxidant enzymes, antioxidant biomarkers, oxidative stress markers, inflammatory indices, and white blood cells (WBCs) count altered due to the deficiency in the estrogen hormone.^[189] Moreover, isoflavone administration could ameliorate the bone turnover resorption markers more efficiently in equol producing pre- as well as post-menopausal women and, therefore, exert anabolic effect on the bones.^[190] Relation between intake of isoflavones and BMD at the lumbar spine (LS) and femoral neck (FN) in different racial/ethnic groups viz. African American, Caucasian, Chinese, and Japanese women revealed that higher isoflavone intake is associated with higher peak FN-BMD but, also with greater rates of LS and FN-BMD loss during the menopausal transition, whereas for other races, there was no relation established.^[191] In contrary to previous studies, SI (135.5 mg/day) does not affect bone resorption in post-menopausal women^[192] and SPI supplemented with isoflavones offered no benefit on BMD and lipid profiles in early post-menopausal women.^[193] In addition, perinatal exposure to daidzein does not confer a positive effect on BMD, whereas pre-pubertal exposure of it protects against a decline in BMD without having any adverse effects on sexual development, in vivo.^[194]

Isoflavones and Anti-Fibrosis

Pulmonary fibrosis (PF) is a serious respiratory problem and ROS is one of the factors responsible in mediating the progression of PF which activates matrix metallo proteinases (MMP), transforming growth factor beta (TGFb), proteinase activated receptor 2 (PAR2), and apoptosis.^[195] Daidzein, a soy isoflavone exhibited anti-fibrotic effect in rats against bleomycin induced PF by down regulating the expression of MMP and PAR2 and upregulating the expression of TGFb.^[196] Recently, Zhao et al.^[197] elucidated that daidzein stimulates skin collagen synthesis and inhibited collagen degradation by activating the TGF-β/smad signal pathway both in vitro as well as in vivo.

Isoflavones and Renal Function

The bioactive phytochemicals in soybean manifested complementary effects on renal function by improving renal flow, reducing proteinuria, renal histological damage, and resultantly retarding the development of kidney disease.^[198–201] SI acted as an effective chemopreventive agent against KBrO₃ (potent nephrotoxic agent) mediated renal oxidative stress and subsequent cell proliferation response in wistar rats by significant reduction in levels of tumor promoter markers, scavenging ROS, induction of anti-oxidant enzyme and detoxification through induction of phase II pathways involving enzymes such as glutathione S-transferase and quinone reductase.^[202] Moreover, administration of soy protein delayed an early stage of kidney disease progression after 1st and 3rd week by reducing renal fibrosis and cyst growth in male Han:SPRD-cy rats.^[203] However, feeding of whole soy and purified daidzein to pre-hypertensive women showed no significant effect on their renal functions.^[204]

Isoflavones and Thyroid

Concerns have been expressed that SI intake adversely affect thyroid function.^[28,205] Besides numerous beneficial effects, SI possess an ability to alter thyroid cell events (referred as goitrogens); either by interfering with an enzyme called thyroid peroxidase which forms iodine-tyrosine complex responsible for production of thyroid hormones, or interferes with activity of sodium-iodine symporter protein which is responsible for the uptake of iodine in the thyroid cells.^[206,207] This was further supported by the in vitro as well as in vivo studies.^[208,209] The negative impact of soya was also well described in human subjects and animals.^[210–213] In the in vitro study, influence of soya isoflavones on thyroid hormone binding and distribution, in particular on the binding of thyroxine (T4) to its distributor proteins, i.e., transthyretin (TTR) in human and rat serum as well as in human cerebrospinal fluid (CSF) underlined that there was a potent competition between T4 and SI for binding to TTR in serum and CSF. As a consequence, there is an altered availability in thyroid hormone concentration which disturbs the hormonal networks during development.^[214] Also, many studies revealed that there is a direct relationship between soy consumption during infancy and autoimmune thyroid disease (ATD) development.^[215,216] Tran et al.^[217] elucidated anti-thyroid effect of genestein in rat thyrocytes by increasing iodine requirement and inducing the production of P40 which might be responsible for the higher incidence of ATD.

OTHER EFFECTS

In addition to the versatile role of SI in previously mentioned commonly occurring disorders, they are also seen as a future therapeutic component in the following less studied health complications.

Mucopolysaccharidosis (MPS)

MPS is a metabolic disorder arising due to malfunctioning of lysosomal enzymes, thereby resulting in the accumulation of glycosaminoglycans (GAGs) and it is classified into different types according to the deficient enzyme. Studies reported that genistein, was effectively involved in the inhibition of GAG accumulation.^[218,219] Moreover, Grynkiewicz and co-workers has been granted a patent for the prophylactic treatment of SI against MPS.^[220] Beyond its effectiveness, there is need to validate its safety. Negretto et al.^[221] investigated the in vitro effect of genistein on DNA damage for it safety in leukocytes from MPS IVA patients (with deficiency of enzyme galactose-6-sulfate sulfatase), where the increment in doses of genistein ranging from 10–50 μM was parallel to higher DNA damage. On the contrary, Kim et al. showed that high oral doses of genestein appear to be safe in MPS patients.^[222] Therefore, there is a need to establish the best doses that allow GAG decrease and clinical outcomes without significant DNA damage.

Rheumatoid Arthritis (RA)

RA is an autoimmune, systemic inflammatory disorder which affects many tissues and organs particularly flexible joints and if not adequately treated then it may lead to loss of function and mobility. Because of the limited success of disease-modifying anti-rheumatic drugs, Li et al.^[223] explored by both in vitro as well as in vivo studies that genistein due to its pain relief and joint protection properties possesses a potential as an anti-rheumatic drugs with high efficiency and less toxicity. In vivo studies showed significant decrease in the occurrence of RA and modulation of immune response on the administration of soy protein and genestien in Sprague dawley rats.^[224−227] In ambience to this, soy protein and its isoflavones restored paraoxonase and arylesterase activity and decreased malondialdehyde levels in arthritic rats.^[228]

Neuroprotection

Alzheimer’s disease (AD) is a degenerative neurological disease which worsens as it progresses mostly in old-aged persons. SI particularly aglycones and equol inhibit the amyloid-β peptide deposits which are the primary reason for the neuronal dysfunction in AD.^[229] Learning and memory ability was enhanced by intragastric pre-treatment with SI which alleviated β-amyloid 1-42 induced inflammatory response by significantly reducing inflammatory cytokines, down regulating Toll like Receptor-4 (TLR-4) and NF-ƙB expression in male wistar rats.^[230] Intraperitoneal administration of high soy diet at a concentration of 10 mg/kg exhibited antioxidant and neuroprotective effects to prevent cell damage caused by cerebral ischemia (a condition where there is insufficient blood flow to the brain to meet metabolic demand) in male Sprague dawley rats (n = 24).^[231] Oral administration of genistein (1 and 10 mg/kg/day) in male Sprague dawley rats reported to exert improved cognitive functions in glucose loaded rats at the time of memory consolidation.^[232] Lephart et al.^[233] were granted a patent on role of equol in mitigating a neuropsychiatric or neurodegenerative disease or disorder in a subject. Although many studies described about the positive aspect of SI in improving cognitive functions;^[234,235] however, there are still some contradictory studies.^[236,237] Ahmad et al.^[238] compared the neuroprotective effects of SI and tempeh isoflavones (TI) and led to the conclusion that TI were more potent in ameliorating the problem of amnesia and neuroinflammation at high doses by curbing various signaling pathways and enzyme activity. Moreover, an in vitro study showed that genestein content in soy product (tofu) resulted in enhanced accumulation of amyloid beta peptide which culminated to cognitive impairment.^[239] Therefore, isoflavones could also play a versatile role in the regulation of cognitive functions.

Isoflavones and Lactose Intolerance (LI)

LI refers to a syndrome which results in one or more symptoms including abdominal bloating, cramps, flatulence, diarrhea, and vomiting due to the enzyme lactase insufficiency which is the key enzyme in the lactose metabolism. The major contributor toward this disorder is the dairy foods due to high lactose content, therefore; soy-based infant formula may serve as an alternative.^[240,241] Safety of soy-based infant formulas (SIF) containing isoflavones: the clinical evidence). Recently, Vandenplas et al.^[242] reviewed the safety of SIF as an alternative to the cows’ milk-based formulas (CMF) and inferred that the patterns of growth, bone health, metabolic, reproductive, endocrine, immune, and neurological functions were observed to be similar in both supplementations. On the contrary, a study reported that overdosage of SI in infants may culminate to long-term effects of which are unknown.^[243,244] Although, studies related to beneficial role of soy-based supplements in improving the metabolic status of patients suffering from LI are limited, but attempts are needed to traverse clinical studies to conclusively prove its role.

CONCLUSIONS

Isoflavones are the group of plant derived phenolic compounds which belong to subclasses of the flavonoid family. Soy derived isoflavones, particularly genistein and daidzein, have nutritional and therapeutic value due to their estrogenic as well as anti-estrogenic effect. Therefore, SI is the most sought molecules and could serve as potential bioactive compounds in sustaining healthy living. In recent years, an increasing number of studies have endorsed that patients with various lifestyle disorders would derive clear benefits from increased intake of SI. Women who are unwilling or unable to take estrogen replacement therapy should consider soy-protein intake to reduce their risk of osteoporosis, menopausal symptoms and atherosclerotic cardiovascular disease. Although, the volume of information available especially in recent years overweighs in favor of functional role played by SI, still there is need to critically evaluate these attributes to elucidate the mechanisms responsible for their potential health effects through well-designed studies. There is a further scope to explore their role in less studied health disorders viz. MPS, LI, RA, and neuroprotection.

ACKNOWLEDGMENT

The authors acknowledge the support from the Director at the National Dairy Research Institute (NDRI) in Karnal, India. The authors declare that they have no competing interests.