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10 Benefits and Side Effects of Soy Isoflavones (8 Contraindications to Be Noted)[Updated Feb/2023]

Soy isoflavones, often referred to as phytoestrogens, are a common health food for women’s menopause.

Here’s an explanation of why soy isoflavones are important and a discussion of the benefits of supplementation and contraindications to use.

What are soy isoflavones?

Soy isoflavones are a flavonoid in soybeans, the two most common of which are daidzein and genistein.

Soy isoflavones are structurally similar to estrogen (especially 17-β estradiol) and can cause weak estrogenic effects or anti-estrogenic effects depending on endogenous estrogen and estrogen receptors.

In addition, like other flavonoids, it has various functions such as antioxidant, inhibition of cancer cell proliferation, anti-inflammatory, and antibacterial.

Interestingly, the incidence of breast and prostate cancer in Asians is lower than in Western countries, which may be related to the significant difference in intake of isoflavones-containing foods in the Asian diet (15-47 mg/day) and the Western diet (0.15-1.7 mg/day).

Food sources of soy isoflavones

    Content per hundred grams (mg/100g)

  • Soy bean: 26–381mg
  • Soy flour: 83–466 mg
  • Tofu: 8–67 mg
  • Miso: 25–89 mg
  • Tempeh: 86.5 mg

What are the benefits of soy isoflavones?

  1. Soy isoflavones reduce arteriosclerosis

Arterial stiffness is associated with altered properties on the inside of blood vessels, which result in a decrease in the dilation of the artery wall and therefore reduced the ability of the artery to buffer blood from a pulsatile cardiac ejection.

With the accumulation of data from various epidemiological studies, it has been found that the sclerosis of the larger central artery system, such as the aortic tree, has an important impact on cardiovascular disease, and is positively correlated with systolic hypertension, coronary artery disease, stroke, heart failure and atrial fibrillation.

A systematic literature review and meta-analysis (8 randomized controlled trials) found that soy isoflavones reduced arterial stiffness compared with placebo (measured by the following values, such as: pulse wave velocity, systemic arterial compliance, augmentation index, Cardio-ankle vascular index. Note 1

*Conclusion: Soy isoflavone supplementation may help reduce arteriosclerosis, but limited by the short duration and small sample size, more studies are needed to support this

  1. Soy isoflavones are beneficial to cognitive function

As we age, cognitive function is essential for functional independence, including whether a person can live independently, manage finances, use medication correctly, and drive safely.

All cognitive degeneration is thought to begin unknowingly and develop gradually, with Mild Cognitive Impairment possibly representing this early stage

The key criteria for distinguishing mild cognitive impairment from dementia are the maintenance of independence of functional capacity and the lack of significant impairment in social or occupational functioning.

A systematic literature review and meta-analysis (16 randomized controlled trials, 1386 participants) suggested that soy isoflavones (daily doses ranging from 60 mg to 160 mg, median intervention duration of 17 weeks) improved cognitive function in adults, particularly in the area of memory. Note 1

*Conclusion: Oral administration of soy isoflavones may have a positive effect on cognitive improvement, but limited by the small sample size of some studies, more studies are needed to support this

  1. Soy isoflavones improve menopausal hot flashes symptoms

Hot flash is the most common vasomotor symptom in menopausal women, mainly intense fever, sweating, and flushing of the head, neck, chest, and upper back, and may be accompanied by rapid heartbeat, anxiety, nausea, and headache.

Hot flashes usually last 1 to 5 minutes, some up to an hour, and the frequency of attacks generally decreases 5 to 7 years after menopause, but some women can last more than 20 years, and the median duration of symptoms is about 4 years.

A meta-analysis (including 17 double-blind randomized controlled trials) showed that ingestion of soy isoflavone capsules (median dose 54 mg for 6 weeks to 12 months) reduced the frequency of hot flashes by 20.6% and the degree of hot flashes by 26.2%. Note 1

In addition, if the genistein content of soy isoflavone supplements exceeds 18.8 mg, it is twice as effective as the lower levels of hot flashes.

The mechanism behind it is related to the chemical and biological similarity of soy isoflavones to estrogen in mammals and selective estrogen receptor regulation.

*Conclusion: Soy isoflavone supplementation (whether naturally derived or chemically synthesized) is significantly more effective than placebo at reducing the frequency and severity of hot flashes.

  1. Soy isoflavones improve cholesterol

Although the normal aging process leads to a decrease in lean body mass and a decrease in metabolic rate, menopausal women tend to gain weight at an accelerated rate in the first few years of endogenous estrogen decline.

Another physiological change is a shift in lipids, including an increase in triglycerides, total cholesterol, and LDL cholesterol, and is often associated with increased abdominal fat mass.

A meta-analysis (12 randomized controlled trials of menopausal women with normal cholesterol) noted that oral soy isoflavones (mean daily dose of 70 mg, aglycone form for 1-3 months) were not significantly helpful in improving total cholesterol and low-density lipoprotein cholesterol (LDL-C) compared with placebo. Note 1

*Conclusion: To date, there is no evidence that oral soy isoflavone supplementation alone improves cholesterol levels

  1. Soy isoflavones are beneficial to osteoporosis

Osteoporosis is a multifactorial systemic bone disease characterized by low bone density and degeneration of the microstructure of bone tissue, resulting in bone fragility.

The most common sites of bone fragility fractures are the wrists, spine, buttocks, ribs and humerus, with the incidence increasing from 4% in women aged 50–59 years to 52% in women aged 80.

Many factors, including age, gender, ethnicity (especially Caucasians), genetics, reproductive status, low calcium intake, and exercise, can affect bone quality.

A systematic literature review and meta-analysis (including 28 randomized controlled trials) found that intake of soy isoflavones (56 mg aglycone equivalents daily for 10 weeks to 12 months) significantly reduced the osteolysis indicator: deoxypyridinoline (DPD) in menopausal women. Note 2

In addition, compared with placebo, the osteogenesis indexes of bone alkaline phosphatase and osteocalcin (osteocalcin) supplemented with soy isoflavones (daily doses of 84 and 73 mg for 12 months) increased by 8.0% and 10.3%, respectively, which were not statistically significant.

Another meta-analysis (12 randomized controlled trials, 1240 menopausal women) showed that intake of soy isoflavones (average daily intake of 82 mg for 6-12 months) significantly increased spinal bone mineral density by 22.25 mg/cm2 or 2.38% (but not in femoral neck, hip joint, and trochanter). Note 3

*Conclusion: For menopausal women, soy isoflavone supplementation may be of positive help in improving bone health and preventing osteoporosis, but due to heterogeneity, more large trials are needed to support it

  1. Soy isoflavones are beneficial for blood sugar regulation

Menopause is often associated with the accelerated development of glucose intolerance, as low levels of estrogen adversely affect the distribution and accumulation of body fat, appetite, energy expenditure, insulin secretion, and glucose homeostasis, which are key factors in the development of type 2 diabetes.

A systematic literature review and meta-analysis (17 randomized controlled studies of 1529 menopausal women) showed that taking soy isoflavones helped improve fasting blood glucose, insulin resistance, and insulin sensitivity index, especially with genistein formulations alone. Note 4

The mechanism behind it is related to the fact that soy isoflavones have alpha-glucosidase inhibitor, tyrosine kinase inhibitor, regulation of PPAR pathway and activation of alpha 1 adrenoceptors.

*Conclusion: Soy isoflavone supplementation may have a positive effect on glycaemic control in menopausal women, but more large trials are needed to support this due to the heterogeneity of the included studies

  1. Soy isoflavones reduce the incidence of breast cancer

Breast cancer is the most common cancer in women in the world (after cervical cancer). By 2030, the global breast cancer burden will increase to more than 2 million new cases per year.

Uncontrollable risk factors associated with breast cancer include: sex, age, genetics, family history, personal history of breast cancer, dense breast tissue, menstrual cycle, previous exposure to breast radiation, and estrogen use.

Lifestyle-related risk factors include no history of pregnancy or late childbearing, use of contraceptives, non-breastfeeding, alcoholism, overweight, obesity, physical inactivity, and abortion.

A meta-analysis (including 35 epidemiological studies) suggests that soy isoflavone intake is associated with a lower risk of breast cancer in pre- and postmenopausal women in Asia. Note 5

However, for premenopausal and late menopausal women in Western countries, there is no evidence that soy isoflavone intake is associated with breast cancer.

*Conclusion: Soy isoflavone intake is associated with lower breast cancer events (especially in Asian women), but randomized controlled trials are still needed to prove causation due to heterogeneity between studies and potential publication bias.

  1. Soy isoflavones are beneficial for blood pressure regulation

High blood pressure is a global public health problem that can lead to heart disease, stroke, kidney failure, blindness, ruptured blood vessels and cognitive impairment, especially over 50 years of age, where systolic blood pressure above 140 mmHg is a more important risk factor for cardiovascular disease.

Using 115/75 mmHg (systolic/diastolic blood pressure) as a baseline, the risk of cardiovascular disease doubles for every 20/10 mmHg increase in blood pressure.

A systematic literature review and meta-analysis (14 randomized controlled trials, 789 participants) showed that soy isoflavone extract supplementation with normotensive and prehypertension significantly reduced systolic blood pressure (-1.92 mmHg) compared with placebo. Note 11

Subgroup analysis showed that the associated improvement effect was most pronounced in subjects who used for more than 3 months, Western ethnicity, low dose, and low risk bias.

The mechanism behind it may be related to the fact that isoflavones can increase nitric oxide levels, which in turn improves vasodilation.

*Conclusion: Soy isoflavone supplementation may have the effect of regulating blood pressure (systolic blood pressure), but further confirmation is needed to determine whether it can be used to improve hypertension

  1. Soy isoflavones reduce the risk of prostate cancer

Prostate cancer is the second most common cancer in men after lung cancer, with an estimated 1.1 million new cases and more than 307,000 deaths worldwide each year.

In North America, the incidence of prostate cancer is 10 times higher than in Asia, although these regional differences may be influenced by ethnicity (genetic factors) and early diagnosis, and evidence suggests that environmental and dietary factors may also play an important role in prostate cancer development.

A systematic literature review and meta-analysis (including 8 randomized controlled trials) found that intake of soy products or soy isoflavone supplements reduced the risk of prostate cancer (by up to 51%) in subjects at potential risk of prostate cancer. Note 12

However, there were no significant changes in prostate-specific antigen (PSA) levels, sex hormone binding globulin, testosterone free, fractional testosterone, estradiol, and dihydrotestosterone.

The mechanism behind it may be related to the regulation of cell proliferation, cell cycle, apoptosis, tumor angiogenesis, tumor cell invasion and metastasis of soy isoflavones.

*Conclusion: Ingestion of soy isoflavones or soybean-related products can help reduce the risk of prostate cancer, but considering the limitations of sample size and study time, more long-term large-scale clinical trials are still needed for further verification

  1. Soy isoflavones help with lose weight (for menopausal women)

In addition to causing various discomforts, menopausal hormonal fluctuations also cause changes in body fat accumulation sites, greatly increasing the risk of overweight and obesity, and increasing the incidence of insulin resistance, type 2 diabetes and cardiovascular disease.

Compared with premenopausal women, it has been reported that the abdominal and subcutaneous fat of postmenopausal women will increase by 49% and 22%, respectively. Note 13

A meta-analysis of 30 studies involving approximately 2800 non-Asian postmenopausal women found that soy isoflavones supplementation had positive effects on weight loss, blood glucose, and insulin control compared with placebo. Note 14

The relevant weight loss effect was most significant in subjects with a body mass index of less than 30, a dose of less than 100 mg, and a period of use for no more than 6 months.

*Conclusion: Soy isorhuang supplementation in non-Asian postmenopausal women can produce weight loss effects, but more research is needed to confirm whether it has the same effect in Asian people

Are there any side effects of soy isoflavones?

Soy isoflavones, as part of the soy diet, have been consumed by humans for many years and have a high safety profile, but possible side effects or adverse reactions include gastrointestinal discomfort, nausea.

Severe allergies to soy-related products, although rare. However, if any possible symptoms occur after consumption, including: skin rash, itching/swelling (especially face, tongue, throat), severe dizziness, difficulty breathing, seek medical assistance immediately.

Safety precautions

1. A study in the elderly found that 100 mg of soy isoflavones per day for 6 months was well tolerated. Note 6

2. In vitro and animal studies have noted that intake of legumes may increase the risk of clinical hypothyroidism in subjects with hypothyroidism or iodine intake. Note 7

3. Do not use by pregnant and lactating women (due to unknown safety)

4. Although soy isoflavones have been found to reduce the incidence and recurrence rate of breast cancer (Note 8), women with a history of breast cancer, especially estrogen receptor-positive (ER+) breast cancer patients, are advised not to increase phytoestrogen intake, including soy isoflavones

5. Although some studies believe that long-term high doses of soy isoflavones may stimulate endometrial hyperplasia, and endometrial hyperplasia may be related to endometrial cancer, this thesis is still unproven. Note 9

6. Do not use if you have been allergic to soybean products, which may cause allergic reactions

7. Since isoflavone metabolism is associated with intestinal bacteria, antibiotic treatment may reduce the bioavailability of isoflavones

8. Do not use it with anti-estrogen drugs selective estrogen receptor modulators, such as tamoxifen (animal studies have found that high doses of soy isoflavones may interfere with drug effects) Note 10

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