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Is Soy Bad for You? Here’s What the Science Says

Published on October 03, 2023

By Lisa Stockwell

Lisa Stockwell

Lisa Stockwell has worked as a copywriter, writer, author, and editor for 35 years, specializing in the field of healthcare since 2009. She recognized the need for reliable health information while supporting friends through unique health challenges and refocused her career to bring clarity and compassion to healthcare communications. Lisa is a graduate of the University of California, Berkeley and a lifelong Northern Californian.

Will soy lead to infertility? Does it reduce a man’s testosterone level? Can it cause breast cancer? 

Over the years, soy has gained a reputation for being controversial, with numerous claims about its potential health hazards. It turns out, however, that the reports of soy’s health risks have been greatly exaggerated. In fact, you might benefit from eating more soy, not less.

Soy is a versatile legume that has been a dietary staple in various cultures for centuries. It’s most commonly enjoyed in the United States in the form of tofu, soy milk, and edamame (soy beans). It's known for its high protein content, making it a popular choice for vegetarians and vegans, and has been shown to have some other health benefits.

So why has soy gotten a bad rap?

In the Internet age, information spreads quickly, but not all is accurate or reliable. When making informed decisions about our health and diet, it's crucial to base your choices on the latest research. In this article, we will explore recent findings on soy to provide you with a clear understanding of its potential benefits and any potential risks.

Soy and Fertility

One of the persistent myths surrounding soy is its alleged link to infertility, a belief based on the fact the hormone estrogen affects the endocrine system, which produces and regulates hormones. Soy contains compounds called isoflavones that are similar in chemical structure to estrogen (or more specifically, estradiol). These isoflavones, which include genistein and daidzein, are known as "phytoestrogens" or plant-based estrogens. Their potential to mimic estrogen and possibly alter hormonal balance began creating concern over three decades ago.

According to some studies, phytoestrogens can bind to estrogen receptors throughout the body and exert weak hormonal effects. However, research also shows isoflavones have only about 1/1000th the potency of the body's native estrogens. Furthermore, soy isoflavones do not accumulate in the body long-term and are metabolized and excreted relatively quickly.

Phytoestrogens may have some beneficial estrogenic effects. But they may also, potentially, block the receptors from the real estrogen in your blood and reduce the amount of estrogen in your body. A study done in the 1990s showed that soy consumption suppressed the midcycle surge of luteinizing hormone (LH) and follicle-stimulating hormone (FSH) and lengthened a woman’s menstrual cycle, creating fear that soy could limit a woman’s ability to get pregnant. However, that study was small and involved only six premenopausal women.

A cross-sectional study showed a potential reduction in sperm count in men who’d eaten soy products.REF#2882 However, this was a small observational study and did not prove that soy consumption was responsible for that outcome.

Newer Research is More Optimistic

While studies have shown phytoestrogens to have some interaction with estrogen receptors, as noted above, they’re not as potent or long-lasting as human bioidentical estrogen. More recent clinical research on the effects of soy on fertility has shown no cause for concern.

A 2010 clinical trial studying the effects of soy protein on men determined that the consumption of soy protein had no adverse effect on sperm quality. REF#2883

A large 2020 study of women trying to conceive showed no measurable relationship between phytoestrogen consumption and the per-cycle probability of conceiving (also known as fecundability), and in some cases, found improved fecundability in the women who increased their isoflavone intake.REF#2884

In 2022, a comprehensive review of all available data on the effects of soy on women’s fertility concluded that soy “appears to have a negligible effect on hormonal network, menstrual cycle length, and fertility outcomes of healthy women.”REF#2885

Even more promising, in 2016, The “Journal of Clinical Endocrinology & Metabolism” published research that indicates that soy may have a beneficial effect on women who have poor in vitro fertilization (IVF) treatment outcomes due to high bisphenol A (BPA) levels. 

According to the U.S. Center for Disease Control and Prevention, BPA is found in 96 percent of the population. It is a chemical that can mimic estrogen and is found in low levels in polycarbonate plastics and epoxy resins used in food and beverage containers. The IVF outcomes of those who ate soy-rich diets (one serving of soy-based foods every two to three days) were not impacted by BPA.REF#2886

The latest research indicates that women trying to conceive do not have to avoid eating soy. However, as with any food plan you adopt, moderation is key. If you are trying to conceive, consult your healthcare provider before altering your diet. 

Soy and Menopause

There have been multiple claims that eating soy can make menopausal symptoms like hot flashes worse due to the estrogenic effects of isoflavones. However, current research debunks this myth and indicates soy may help relieve menopause symptoms. 

Multiple studies have found that consuming soy foods and isoflavone supplements can lead to modest but noticeable reductions in the frequency and severity of hot flashes, especially when foods have high levels of the predominant soybean isoflavone, genistein.REF#2887

Soy intake also may support bone health and reduce the risk of obesity in post-menopausal women. A meta-analysis of multiple trials concluded that soy foods may both delay the physical disabilities of menopausal women who have osteosarcopenia (the loss of bone and muscle) and obesity and also improve bone strength and muscle mass to prevent osteosarcopenia in postmenopausal women.REF#2888 

Soy and Breast Cancer

Estrogen plays a complex role in breast health. It involves stimulating the development and maintenance of breast tissue. However, excessive exposure to estrogen, particularly over a prolonged period, has been linked to an increased risk of breast cancer. This association led to concerns that soy, with its phytoestrogens, might also raise the risk of breast cancer.

Extensive clinical research has found that consuming soy does not increase your risk for breast cancer. Instead, studies have found that soy may be associated with a reduced risk of breast cancer in populations that eat soy-based diets from childhood on.REF#2889 More research is needed to determine if these benefits apply to women who begin consuming soy later in life.

It’s vital to emphasize moderation in soy consumption. Excessive intake of soy products or supplements may not yield these benefits and could lead to imbalances in hormonal regulation. If you’re considering adding more soy to your diet, consult a healthcare professional if you have concerns about breast health or a family history of breast cancer. 

Soy and Cardiovascular Health

Some early animal studies caused alarm by showing possible negative effects of isolated soy proteins and extracts on heart health. This sparked worries that eating soy could raise cholesterol and promote atherosclerosis.

However, an extensive body of research on soy foods in human clinical trials paints a different picture. The most recent studies and meta-analyses find soy intake benefits cardiovascular health and helps lower LDL (bad) cholesterol. One reason may be that soy is low in saturated fat and a good source of omega-6 and omega-3 fatty acids, which benefit cardiovascular health.

A review of 35 randomized controlled trials concluded that daily intake of soy protein leads to modest but significant reductions in LDL cholesterol, as well as increases in HDL (good) cholesterol, improving the LDL to HDL ratio.REF#2890 It found that the reductions in LDL cholesterol were greater in those who already have high cholesterol. The review also noted that eating soy foods provided more benefits than processed soy extracts.

Hypertension is another risk factor for cardiovascular disease. A 2020 meta-analysis of 17 randomized, double-blind placebo-controlled trials showed that soy significantly improved blood pressure in adults compared to controls. 

The FDA has approved a health claim stating that intake of 25 grams of soy protein daily may reduce heart disease risk by lowering cholesterol.REF#2891 

Sufficient human clinical data now supports soy foods as part of a heart-healthy diet that can contribute to reducing risk factors for cardiovascular disease. The beneficial effects seem to come from both the soy protein replacing less healthy animal proteins and the action of isoflavones reducing arterial stiffness and inflammation.REF#2892

The Nutritional Benefits of Soy

Soy products like tofu, tempeh, soy nuts, soy milk, and edamame are high-quality, plant-based proteins with excellent nutritional value. Soybeans are considered a complete protein source, providing all the essential amino acids needed in the diet. Soy also contains healthy unsaturated fats, fiber, vitamins, and minerals, such as calcium, magnesium, and potassium. It's also a good source of B vitamins, such as folate and B6, which are important for energy metabolism and overall vitality. Soy foods are among the few plant sources with sufficient levels of the amino acids methionine and cysteine. 

The form of food and the cooking method affect the amount of protein you can get from soy. One cup of raw soybeans contains 68 grams of protein, while one cup of boiled soybeans provides 29 grams. A cup of soy nuts (roasted soybeans) contains 32.75 grams of protein. One cup of tofu has 20 grams and a cup of tempeh has 31 grams of protein. Soy milk has only 8 grams. Raw sprouted soybeans have 9.2 grams per cup.

Unless you have a soy allergy, you should be able to enjoy soy foods in moderation (about 25 grams a day) without any risks to your health. The known side effects are diarrhea or constipation. Also, soy may affect your thyroid levels if you are deficient in iodine. If you are pregnant or breastfeeding, you should not consume soy in excess of the amount you get from food (i.e., in extract form). 

No matter what type of soy-based food you choose, incorporating soy into your diet helps provide well-rounded nutrition, including heart-healthy fats, fiber, vitamins, minerals, and top-quality complete vegetable protein.


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