Soy

Soy (Glycine max): A Comprehensive Reference

1. Identity, Botanical Classification, and Common Forms

Nomenclature and Taxonomy

Soybeans (Glycine max (L.) Merrill) are among the six most produced crops in the world, and their processed products are widely used for human diet and animal feed. The plant is also referred to in older botanical literature by synonymous names: Glycine max L. Merr., Glycine hispida Moench Maxim, and Phaseolus max L., belonging to the legume family (Fabaceae).

Soybean (Glycine max) is a leguminous crop, differing from other grain legumes because of the presence of high amounts of beneficial components like proteins, fibers, phytosterols, and isoflavones. Due to its high oil content and quality proteins, the soybean has been identified as an excellent source of food across the world.

Common Forms and Preparations

Phytoestrogens are present in high amounts in soy products such as beans and tofu. Soy-based products, including soy milk and tofu, are among the most valuable sources of phytoestrogens as they may be easily incorporated into the human diet. Soy is commercially available and consumed in numerous forms, including:

• Whole soybeans (edamame): consumed as a whole food, boiled or steamed.
• Tofu (bean curd): produced by coagulating soymilk and pressing the resulting curds.
• Soymilk: a beverage prepared by soaking and grinding soybeans in water.
• Tempeh: a fermented whole-soybean product with a firm texture, originating in Indonesia.
• Miso: a fermented soybean paste used as a seasoning, particularly in Japanese cuisine.
• Soy sauce: a fermented liquid condiment. The origin of soy sauce is rooted in a fermented soybean paste which was used to preserve food and also as a condiment to enhance flavor.
• Soy protein isolate: a highly refined powdered form used in supplements, infant formulas, and food manufacturing.
• Soy isoflavone extracts/supplements: concentrated preparations standardized for genistein, daidzein, and/or glycitein content, available in capsule and tablet form.
• Soy flour: ground dried soybeans used in baking and food fortification.

2. Macronutrient and Phytochemical Composition

Macronutrients

Soybean is an important source of proteins; contains polyunsaturated fats, fiber, and vitamins; and does not contain starch, saturated fats, or cholesterol. Raw soybeans are 20% fat, including saturated fat (3%), monounsaturated fat (4%), and polyunsaturated fat, mainly as linoleic acid. The majority of soybean carbohydrates can be classed as belonging to dietary fiber.

Isoflavones

Soybeans are the main source of isoflavones, a subclass of polyphenols with a high added value. Isoflavone is a selective estrogen receptor (ER) modulator, with main constituents of genistein, daidzein, and glycitein. Nine types of glycoside isoflavones and three types of aglycoside isoflavones are reported in soybean. Isoflavones mainly occur in plants in the form of biologically inactive glycosides (e.g., genistin and daidzin).

Phytoestrogens are described as natural compounds from plants characterized by their molecular structure and size, resembling estrogen, especially estradiol, and showing estrogenic and/or antiestrogenic activities. Isoflavones have been linked to estradiol since these compounds show similarity of structure to estradiol as a human hormone. Although isoflavones and estradiol structures show several differences, the moiety of phenol groups enables interaction with estrogen receptors (ER) and activated this receptor accordingly.

Phytosterols

The major sterols found in soybeans are sitosterol and stigmasterol. Other minor components are campesterol, stigmasterol, Δ7-avenasterol, and brassicasterol. These phytosterols have a cholesterol-lowering effect by interfering with the absorption of cholesterol in the blood serum.

Within soybean oil or the lipid portion of the seed, four phytosterols are found: stigmasterol, sitosterol, campesterol, and brassicasterol, accounting for about 2.5% of the lipid fraction.

Other Bioactive Constituents

Soy contains the phytoestrogen coumestans. Soybeans are also a significant source of mammalian lignan precursor secoisolariciresinol containing 13–273 µg/100 g dry weight. Soybean oil also contains α-, γ-, and δ–tocopherols (vitamin E), which act as antioxidants.

3. Historical and Traditional Use

China and East Asia

This humble legume, initially cultivated in the eastern regions of ancient China, would gradually transform into one of humanity's most versatile and vital food sources. Asian populations have consumed foods made from soybeans for centuries.

Early references to fermented soybean paste can be found in the Han dynasty, listed on bamboo slips found in the Han Tomb 1 at the Ma-wang-dui archaeological site located near Changsha, China. The word 'jiang' is also found in the silk manuscript discovered in Han Tomb 3. The term 'jiang' was initially used to refer to a wide variety of pastes produced from meat, vegetables, seafood and grain, but by the time of the Western Han was more specifically used to refer to a fermented paste made from soybeans.

The soybean's prominence in classical Chinese medicine is illustrated by its inclusion in the Bencao Gangmu (Great Pharmacopoeia) of Li Shizhen. The Bencao Gangmu was completed in 1578 but not published until 1596. It describes almost 2,000 animal, vegetable, and mineral drugs and gives over 8,000 prescriptions. A rich source of information, it is still very useful today. All foods mentioned are considered as medicines, based on the ancient Chinese saying: "Food and medicine have the same origin." Soybeans and soyfoods are discussed in two chapters of this work.

Western Adoption

In the West, certain subpopulations, specifically Seventh-day Adventists and vegetarians, have used soyfoods for approximately 100 years, although the quintessential soyfood tofu was first introduced on a large scale to the general U.S. population in the early 1970s. Health-conscious and ecologically-minded consumers were particularly attracted to soy at that time since it was perceived as a source of high-quality protein, low in saturated fat, that was more efficiently produced compared to animal sources of protein.

In the early 20th century, American farmers initially viewed soybeans primarily as a rotation crop, valuing its ability to fix nitrogen in their soil between primary crop cycles. The real turning point came during World War II, when soybean meal emerged as a crucial protein source for livestock due to disruptions in traditional feed supply chains.

Soy extract products have been widely used as an alternative treatment, targeting menopausal symptoms in patients who cannot or have concerns related to and are unwilling to take menopausal hormone therapy (MHT).

4. Key Constituents and Mechanisms of Action

Estrogenic and Antiestrogenic Mechanisms

The biological activity of isoflavones has been well reported. Isoflavones, known as phytoestrogens, are biologically active compounds with weak estrogen activity. Phytoestrogens are heterocyclic phenolic compounds with a structure similar to that of estrogenic steroids (mammalian endogenous estrogens). Therefore, these compounds show estrogen-like activity or weak antiestrogen-like properties.

There are two types of ERs: ERα, which is present in the breast, uterus, and vagina, and ERβ, which is present in the bone and urogenital tract. Isoflavones bind preferentially to ERβ over ERα, which helps explain their tissue-selective effects. Because most naturally occurring phytoestrogens act as selective estrogen receptor modulators (SERMs), which do not necessarily act as direct agonists of estrogen receptors, normal consumption of foods that contain these phytoestrogens should not provide sufficient amounts to elicit a physiological response in humans.

Equol: An Important Metabolite

Equol is a soy isoflavone metabolite that can be produced by intestinal bacteria. It is lipophilic and resembles natural oestrogens with an affinity to oestrogen receptors. In humans, not everyone can produce equol from gut metabolism. It is postulated that equol producers benefit more than non-equol producers for all the endocrine-related effects. The major product of daidzein microbial metabolism is equol.

Antioxidant Activity

A previous study demonstrated that dietary supplementation with soy-derived isoflavones reduced the in vitro oxidation susceptibility of low-density lipoprotein (LDL).

Cholesterol-Modulating Mechanisms

Phytosterols in soybeans have a cholesterol-lowering effect by interfering with the absorption of cholesterol in the blood serum. Additionally, soy protein itself appears to modulate hepatic cholesterol synthesis and LDL receptor expression, independent of isoflavone content, though the precise mechanism in humans remains under investigation.

Bone Effects

Isoflavones contained in soy stimulate the protein synthesis and the expression of alkaline phosphatases in osteoblasts. Additionally, food enriched with isoflavones prevented the reduction of bone mineral density (BMD) in ovariectomized rats or mice. Soy isoflavones have received considerable attention due to their estrogen-like properties on certain tissues such as bone, leading some investigators to refer to them as naturally occurring selective estrogen receptor modulators (SERMs).

5. Scientific Evidence by Area of Use

5.1 Cardiovascular Health and Cholesterol

In human clinical intervention trials, soy product consumption reduced levels of total cholesterol and low-density lipoprotein cholesterol (LDL-C). In October 1999, the U.S. Food and Drug Administration approved a health claim for the relationship between consumption of soy protein and reduced risk of coronary heart disease.

Dietary soy protein has been shown to have several beneficial effects on cardiovascular health. The best-documented effect is on plasma lipid and lipoprotein concentrations, with reductions of approximately 10% in LDL cholesterol concentrations (somewhat greater for individuals with high pretreatment LDL cholesterol concentrations) and small increases in HDL cholesterol concentrations.

Isoflavone-containing soy protein consumption has been linked to reduced levels of LDL cholesterol in hypercholesterolemic patients. This effect is increased with the concomitant administration of isoflavones, and seems to be also complemented by the isoflavone capacity to restore the endothelial function in patients with weak and moderate endothelial dysfunction.

A 2021 systematic review and meta-analysis focused specifically on patients with type 2 diabetes is particularly informative. A total of 22 trials with 867 participants were included. Soy product consumption led to a significant reduction in serum concentrations of triglycerides (WMD: −24.73 mg/dL), total cholesterol (WMD: −9.84 mg/dL), LDL cholesterol (WMD: −6.94 mg/dL), and C-reactive protein (WMD: −1.27 mg/L). In contrast, soy products had no effect on HDL cholesterol, fasting blood sugar, fasting insulin, glycated hemoglobin, HOMA-IR, systolic blood pressure, diastolic blood pressure, or BMI. This meta-analysis suggests that soy product consumption may improve cardiovascular parameters in patients with T2D, particularly in individuals with poor baseline risk profiles; however, larger studies with longer durations and improved methodological quality are needed before firm conclusions can be reached.

A separate systematic review and meta-analysis in postmenopausal women (n = 2,305) found that changes in the lipid profile showed statistically significant decreases of total cholesterol by −4.64 mg/dL and increased HDL-cholesterol by 1.15 mg/dL, though the changes in LDL-cholesterol and triacylglycerols did not reach statistical significance.

Evidence strength: The experimental evidence in support of soy's hypocholesterolemic effects is not as overwhelming as generally perceived, and critical analysis of the investigations to date indicates the data are not quantitatively impressive, raising substantial questions about the clinical importance of the hypocholesterolemic effects observed. Overall, the lipid-lowering evidence from RCTs and meta-analyses is moderate, with consistent but numerically modest effects, and most pronounced in hypercholesterolemic individuals. The FDA health claim (1999) was specifically for 25 g/day of soy protein as part of a diet low in saturated fat and cholesterol.

5.2 Menopausal Symptoms (Hot Flashes / Vasomotor Symptoms)

Hot flashes are the most common menopausal symptom in North America and Europe, with up to approximately 70% of women affected. Symptoms can begin during the menopausal transition up to 2 years before the cessation of menses. The average duration of hot flashes is 6 months to 5 years, although 20% of women continue with symptoms into their 70s and 80s.

Soy isoflavone supplements or soy protein may help to reduce the frequency and severity of menopausal hot flashes, but the effect may be small, and research results have not been completely consistent. Studies that tested isoflavones from soy or red clover for their ability to relieve menopause symptoms have had inconsistent results.

Soy isoflavone supplementation is beneficial in both perimenopausal and postmenopausal women, more so in perimenopausal women. Soy isoflavones are the most effective on the somatic and psychological symptoms, and therefore their use is more beneficial during perimenopause. Soy isoflavones are the least effective on the urogenital symptoms.

A 2022 systematic review and meta-analysis is among the most recent comprehensive analyses. The findings of clinical trials suggest that soy extract reduces the frequency of hot flashes in postmenopausal women; however, other studies reported that there was no significant difference between the soy and placebo groups.

One notable clinical trial, the SPARE (Soy Phytoestrogens As Replacement Estrogen) study, randomized women ages 45 to 60, without osteoporosis and within five years from menopause, to receive soy isoflavones 200 mg daily or placebo for 2 years. Participants had yearly measurements of spine and hip bone density, urinary phytoestrogens, serum lipids, thyroid stimulating hormone, and estradiol. Menopausal symptoms, mood changes, depression, and quality of life were assessed annually. The SPARE study recruited 283 women, 66.1% of whom were Hispanic white.

The FDA has approved a health claim for isoflavone-rich soy protein to reduce cholesterol with 25 g of soy protein consumption daily. However, it is important to note that it appears to require that soy isoflavones are consumed intact in soy protein.

Evidence strength: Moderate for modest reductions in hot flash frequency, particularly in the perimenopause period. Results across trials are inconsistent, possibly partly due to inter-individual variation in equol-producing capacity.

5.3 Bone Health and Osteoporosis

Epidemiological data suggest that populations with high intakes of soy (i.e., Asians) have a lower incidence of osteoporotic fractures. Asian women typically consume about 20 g of soy daily, which provides approximately 40 mg of isoflavones. However, lower rates of fractures in these populations may not be fully attributed to soy consumption, as there are a number of other confounding factors that can influence skeletal health.

Many scientists have examined the relationship between types of protein and urinary calcium excretion, and found that although animal protein was associated with increased urinary calcium excretion, soy protein was not. There is sufficient evidence suggesting soy isoflavones may have potential benefits for bone.

One prospective study with a 25 g/day soy protein and 60 mg/day isoflavone intervention found that one year supplementation of 25 g protein per se positively modulated markers of bone formation, but this amount of protein was unable to prevent lumbar and whole body bone loss in postmenopausal women.

Soy protein with naturally occurring phytoestrogens, mainly isoflavones, protects against bone loss, and synthetic soy ipriflavone in some studies has been shown to favorably affect bone, but a cause-and-effect relationship has not been established between the consumption of ipriflavone and maintenance of bone mineral density in postmenopausal women. Therefore it is too early to recommend it as a supplement for this group of women.

Evidence strength: Preliminary to moderate. Preclinical and some human data are promising for bone formation markers and fracture risk reduction, particularly in pre/perimenopausal women. Long-term RCTs have not consistently demonstrated preserved bone mineral density. Evidence is weaker than that for lipid effects.

5.4 Cancer — Breast Cancer

Equol is considered chemoprotective in specific endocrine-related pathologies, such as breast cancer, prostate cancer, cardiovascular diseases, and menopausal symptoms. However, the relationship between soy and breast cancer is complex and remains an area of active research with contested findings.

Several studies have demonstrated that regular consumption of phytoestrogens in the diet of Asian women has led to a reduction in menopausal symptoms, breast cancer, endometrial cancer, and an increase in bone mass compared to other regions. These are largely epidemiological observations subject to significant confounding.

Equol has been shown to significantly reduce hot flushes in menopausal women, prevent a reduction in bone mineral density in middle-aged women, and possibly help reduce the incidence of prostate cancer and prevent the occurrence of breast cancer. These findings are predominantly from observational and preclinical data.

Taking soy phytoestrogen supplements during tamoxifen or aromatase inhibitor treatment may promote tumor growth. Daidzein, equol, or genistein supplements might also promote breast cancer growth and proliferation under some other conditions, including in women with benign breast disease and in breast cancer survivors. This concern is based on laboratory and pharmacological evidence.

Evidence strength: Observational data in Asian populations are suggestive of a modest protective association for whole soy food intake. In contrast, evidence is insufficient and potentially concerning for high-dose isoflavone supplementation in women with hormone-sensitive cancers or on endocrine therapies. Human RCT data on cancer incidence are lacking.

5.5 Cancer — Prostate Cancer

A systematic review and meta-analysis including 22 articles comprising 1,409,213 participants found that soy products reduce the risk of prostate cancer (OR = 0.94, 95% CI = 0.91–0.97, p < 0.001), especially in cases of localized or low-grade prostate cancer (OR = 0.94, 95% CI = 0.90–0.97, p < 0.001), but exhibited no effect on non-localized or high-grade prostate cancer.

One study suggested that soybean isoflavones can prevent the onset of prostate cancer either through mechanisms directly involving equol or daidzein's transformation to equol. Subsequently, it was thought that being an equol producer provides a protective effect against prostate cancer.

Evidence strength: The overall pooled epidemiological evidence is suggestive of a modest protective association, particularly for localized prostate cancer, but is predominantly observational. Mechanistic data are largely preclinical or in vitro.

5.6 Cognitive Function

Isoflavone supplementation exerted a favorable effect on cognitive function, particularly verbal memory, in postmenopausal women. Short-term high-dose soybean intake altered the levels of total plasma testosterone and improved spatial cognition in women. Consumption of dietary phytoestrogens resulting in high plasma isoflavone levels may significantly affect sexually dimorphic brain regions, anxiety, learning, and memory.

Evidence strength: Preliminary. Most findings are from small or short-duration trials; the evidence is insufficient to draw firm clinical conclusions on cognitive benefits.

5.7 Skin Health

Preliminary findings from multiple studies indicate that dietary intake of soy-derived isoflavones exerts beneficial effects on the skin, including defense against oxidant damage, stimulation of collagen synthesis, and increased hydration. A 6-month prospective, randomized double-blind controlled trial in 44 postmenopausal women compared soy protein isolate with isoflavones (SPII) against casein protein. This 6-month prospective, randomized double-blind controlled study was conducted on 44 postmenopausal women with Fitzpatrick skin types I, II, and III who were randomized to receive either casein protein or SPII. A high-resolution facial photography system was used to measure wrinkle severity and pigmentation at 0, 8, 16, and 24 weeks.

Evidence strength: Preliminary. Limited human RCT data with small sample sizes; larger and longer trials are required.

5.8 Blood Pressure

There is no beneficial effect of soy isoflavone supplementation on lowering systolic blood pressure and BMI in the general population based on current pooled data. However, soy products decreased systolic blood pressure in patients with baseline hypertension (>135 mmHg) in a subgroup analysis of type 2 diabetic patients, suggesting that effects may be confined to those with elevated baseline values.

Evidence strength: Weak in the general population; some evidence for effect specifically in hypertensive individuals.

6. Body Systems and Health Areas of Association

• Endocrine/hormonal system: Phytoestrogenic activity via ER modulation; relevance to menopausal symptom management and sex hormone metabolism.
• Cardiovascular system: Lipid modulation (LDL-C reduction, mild HDL-C increase, triglyceride reduction), endothelial function, arterial compliance.
• Skeletal system: ER-β–mediated osteoblast stimulation; potential reduction in bone resorption markers; epidemiological association with fracture risk reduction.
• Oncological: Epidemiological associations with reduced breast and prostate cancer risk at dietary intake levels; uncertain or potentially adverse effects at supplemental doses in hormone-sensitive cancer.
• Metabolic/glycemic: Modest effects on glycemic markers in high-risk individuals with diabetes.
• Thyroid: Potential goitrogenic and levothyroxine absorption interactions (see Safety).
• Integumentary (skin): Preliminary evidence for photoaging protection and collagen support.
• Central nervous system: Preliminary evidence for cognitive and memory effects, particularly in postmenopausal women.
• Gastrointestinal: Source of prebiotic fiber; fermented soy products contribute to gut microbiome diversity.

7. Dosage Forms and Reported Study Dosages

The following dosages have been reported in cited clinical studies and systematic reviews. These are not recommendations, but are reported as observed in the sources:

• Soy protein (dietary): The FDA has approved a health claim for isoflavone-rich soy protein to reduce cholesterol with 25 g of soy protein consumption daily. One study examined soy-containing foods providing 25 g protein and 60 mg isoflavones per day for one year.
• Soy isoflavone supplements (menopausal symptoms): The SPARE trial randomized women to receive soy isoflavones 200 mg daily or placebo for 2 years. One randomized clinical trial administered 50 mg isoflavone once daily to the treatment group.
• Population dietary intake reference: Studies of isoflavone intake in Western countries indicate an average daily isoflavone intake of approximately <3 mg, and Asian populations consuming a high-soy diet or people consuming soy supplements have a daily intake of approximately 40 mg/day.
• Asian women dietary reference: Asian women typically consume about 20 g of soy daily, which provides approximately 40 mg isoflavones.

8. Safety Considerations and Drug Interactions

General Safety Profile

Phytoestrogens appear to be safe for short-term use, but their long-term safety has not been established. Because phytoestrogen supplements may have effects like those of the hormone estrogen, they may not be safe for women who should not take estrogen. The long-term safety of phytoestrogens such as soy, red clover, and flaxseed has not been established.

Possible side effects from soy include stomach pain and digestive problems such as diarrhea, prolonged menstrual cycle, and contact dermatitis.

Thyroid Function and Levothyroxine

There is some evidence to suggest that soy consumption is associated with thyroid disorders such as hypothyroidism, goitre, and autoimmune thyroid disease. The potential public health implications of the possible anti-thyroid effects of soy are important since as many as 10% of postmenopausal women, for whom soy foods have particular appeal, may be hypothyroid, and a large proportion of these may be undiagnosed.

In the 1930s, McCarrison described the goitrogenic effect of soybeans in rats. Subsequently, studies performed both in vitro and in animal models suggested that this was due to the inhibition of the activity of thyroid peroxidase by isoflavones, which reduce triiodothyronine and thyroxine synthesis. Importantly, however, the European Food Safety Authority (EFSA) concluded that soy isoflavones were without effect on thyroid function in healthy individuals with adequate iodine status.

It has been established that soy products can interfere with thyroid hormone absorption, resulting in continued hypothyroidism in individuals receiving recommended levothyroxine replacement. It has also been reported that achievement of euthyroidism in hypothyroid patients using soy products requires increased doses of levothyroxine.

Limited evidence was found for levothyroxine interactions with coffee, soy products, fiber, calcium or iron supplements, and enteral nutrition, but interestingly they all resulted in decreased levothyroxine absorption.

Much of the evidence for the goitrogenic effects of isoflavones is derived from experimental in vitro and in vivo studies. Goitrogenic effects were also noted in infants fed non-iodine-fortified, soy-based formula, a problem that was easily solved with iodine fortification. Recent studies suggest that genistein shows a good profile of safety on the thyroid, although definitive conclusions have not been reached.

Interactions with Endocrine Cancer Therapies

The American Association of Clinical Endocrinologists cautions that the effects of phytoestrogens are inconsistent and that women with a personal or strong family history of blood clots, cardiovascular disease, or breast, uterine, or ovarian cancer should not use soy-based treatments.

Taking soy phytoestrogen supplements during tamoxifen or aromatase inhibitor treatment may promote tumor growth. Daidzein, equol, or genistein supplements might also promote breast cancer growth and proliferation under some other conditions, including in women with benign breast disease and in breast cancer survivors.

Notably, one pharmacokinetic study in Asian American breast cancer patients found that there was no evidence that soy intake adversely affected levels of tamoxifen or its metabolites. The clinical significance of soy–tamoxifen interactions thus remains unresolved, with conflicting findings between pharmacokinetic and pharmacodynamic perspectives.

Soy Allergy

Soy is recognized as one of the major food allergens. The presence of soy-derived excipients in medicinal products highlights the need for clearer labeling of allergens and underscores the importance of vigilance for patients with soy allergies. Further research is required to address gaps in allergen disclosure by pharmaceutical manufacturers.

Hormone-Sensitive Conditions

Because isoflavones act as selective estrogen receptor modulators, women with a personal or strong family history of blood clots, cardiovascular disease, or breast, uterine, or ovarian cancer should not use soy-based treatments without appropriate clinical guidance.

Infant Formula

Goitrogenic effects were noted in infants fed non-iodine-fortified, soy-based formula, a problem that was easily solved with iodine fortification. This specific safety consideration has been addressed by modern formula manufacturing standards.

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