Soy: Good or bad?

Soy is arguably one of the most controversial nutrition topics.

On one hand, it’s rich in nutrients, and diets containing it appear to be linked to health benefits, such as lower blood sugar levels, improved heart health, fewer menopause symptoms, and perhaps even a lower risk of certain cancers.

Yet, on the other hand, some people are concerned about the healthfulness of soy-rich diets. For instance, some fear that eating too much soy may increase the risk of breast cancer, hinder thyroid function, or have feminizing effects in men, to name a few.

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This article reviews the latest scientific evidence to determine whether eating soy is more likely to have positive or negative effects on your health.

Soy contains various nutrients

Soybeans are naturally rich in protein and contain all of the essential amino acids your body needs. They’re also rich in plant fats, fiber, and several important vitamins, minerals, and beneficial plant compounds.

• Calories: 172
• Protein: 18 grams
• Fat: 9 grams
• Carbs: 8 grams
• Sugar: 3 grams
• Fiber: 6 grams
• Calcium: 8% of your daily need
• Iron: 29% of your daily need
• Magnesium: 20% of your daily need
• Phosphorus: 20% of your daily need
• Potassium: 11% of your daily need
• Zinc: 10% of your daily need
• Copper: 45% of your daily need
• Manganese: 36% of your daily need
• Selenium: 13% of your daily need
• Thiamine: 13% of your daily need
• Riboflavin: 22% of your daily need
• Vitamin B6: 14% of your daily need
• Folate: 14% of your daily need

In addition to their vitamin and mineral content, soybeans are a natural source of polyphenols, a type of antioxidant that may help protect your body against cell damage and conditions like heart disease.

Soybeans are especially rich in isoflavones, a subclass of polyphenols referred to as phytoestrogens due to their ability to attach to and activate estrogen receptors in your body.

Soy isoflavones are believed to be one of the main reasons behind the many purported health benefits of soy-based foods. Boiled soybeans contain 90–134 mg of isoflavones per 3.5 ounces (100 grams), depending on the variety.

Due to their similarity in structure, soy isoflavones are often believed to mimic the hormone estrogen. However, research suggests that soy isoflavones differ from estrogen in many ways, with each having unique effects on the human body.

Suggested read: What is tofu, and is it good for you?

Summary: Soy and foods derived from it are typically rich in protein, fiber, vitamins, minerals, and antioxidants. They also contain soy isoflavones, which are believed to offer a variety of health benefits.

Soy may offer several health benefits

Soy-rich diets have been linked to a few potential health benefits.

Soy may help lower cholesterol levels

Several studies suggest that diets rich in soy foods may help lower LDL (bad) cholesterol and raise HDL (good) cholesterol.

For instance, one recent review suggests that a median intake of 25 grams of soy protein per day may help reduce total and LDL (bad) cholesterol levels by around 3%.

However, the authors believe that, in practice, reductions may be larger when people eat soy protein instead of animal protein. However, more research is needed to confirm this.

Another review suggests that soy-rich diets may help reduce total and LDL (bad) cholesterol levels by 2–3%. They may also raise HDL (good) cholesterol by 3% and reduce triglyceride levels by around 4%.

Currently, people with existing risk factors for heart disease, such as high cholesterol, obesity, or type 2 diabetes, appear to be among those who benefit most from soy-rich diets.

In addition, minimally processed soy foods, such as soybeans, tofu, tempeh, and edamame, appear to improve cholesterol levels more than processed soy products and supplements.

Soy may help protect heart health

Diets rich in legumes, including soy, may help lower your risk of heart disease.

It appears that soy isoflavones may help reduce inflammation in blood vessels and improve their elasticity — two factors believed to protect the health of your heart.

A recent review further links soy-rich diets to a 20% and 16% lower risk of stroke and heart disease, respectively.

Additional research suggests that diets rich in soy foods may reduce your risk of dying from heart disease by up to 15%.

Soy may lower blood pressure

Soybeans and foods made from them are generally rich in arginine, an amino acid believed to help regulate blood pressure levels.

Soybeans are also rich in isoflavones, another compound believed to offer blood-pressure-lowering benefits.

In one study, eating 1/2 cup (43 grams) of soy nuts daily was found to reduce diastolic blood pressure (the bottom number of a blood pressure reading) by around 8% in some, but not all women.

Other studies link daily intakes of 65–153 mg of soy isoflavones to blood pressure reductions of 3–6 mm Hg in people with high blood pressure.

However, it’s unclear whether these small blood-pressure-lowering benefits apply to people with normal and elevated blood pressure levels.

Some studies suggest both may benefit, while others suggest only people with high blood pressure would experience this effect.

More research is needed on this topic, but for the time being, the blood-pressure-lowering effects of soy, if any, appear to be very small.

Soy may lower blood sugar

One review including 17 randomized control studies — the gold standard in research — suggests that soy isoflavones may help slightly reduce blood sugar and insulin levels in menopausal women.

Soy isoflavones may also help lower insulin resistance, a condition in which cells no longer respond to insulin normally. Over time, insulin resistance can result in high blood sugar levels and lead to type 2 diabetes.

In addition, there’s some evidence that soy protein supplements may help slightly lower blood sugar and insulin levels in people with type 2 diabetes or metabolic syndrome.

Suggested read: 8 science-based health benefits of edamame

Metabolic syndrome refers to a cluster of conditions, including high blood sugar, cholesterol levels, blood pressure, and abdominal fat, which together, tend to increase a person’s risk of type 2 diabetes, heart disease, and stroke.

However, these results aren’t unanimous, and several studies have failed to find a strong link between soy foods and blood sugar control in healthy people and those with type 2 diabetes.

Therefore, more studies are needed before strong conclusions can be made.

Soy may improve fertility

Some research suggests that women eating soy-rich diets may benefit from improved fertility.

In one study, women with high intakes of soy isoflavones were 1.3–1.8 times more likely to give birth following fertility treatments than those with lower soy isoflavone intakes. However, men may not experience the same fertility-boosting benefits.

In another study, soy foods were found to offer some protection against the effects of bisphenol A (BPA), a compound found in some plastics believed to reduce fertility.

However, these findings in support of benefits for fertility aren’t universal.

For instance, one review suggests that ingesting 100 mg of soy isoflavones per day may reduce ovarian function and reproductive hormone levels — two important fertility factors.

Moreover, another review suggests that women consuming more than 40 mg of soy isoflavone per day may be 13% more likely to experience fertility issues than those with intakes below 10 mg per day.

However, most studies to date report that diets containing 10–25 mg — and perhaps even up to 50 mg of soy isoflavones per day — as part of a varied diet do not seem to have any harmful effects on ovulation or fertility.

These amounts of soy isoflavones are equivalent to around 1–4 servings of soy foods per day.

Soy may reduce menopause symptoms

Soy is rich in isoflavones, a class of compounds also referred to as phytoestrogens, or plant estrogens, due to their ability to bind to estrogen receptors in the body.

Suggested read: Soy protein: Nutrients, benefits, drawbacks, and more

During menopause, a woman’s estrogen levels naturally decrease, resulting in unpleasant symptoms, such as fatigue, vaginal dryness, and hot flashes.

By binding to estrogen receptors in the body, soy isoflavones are believed to help somewhat reduce the severity of these symptoms.

For instance, research suggests that soy isoflavones may help reduce the frequency and severity of hot flashes.

Soy isoflavones also appear to help relieve the fatigue, joint pain, depression, irritability, anxiety, and vaginal dryness experienced during menopause and/or the years leading up to it.

However, not all studies report the same benefits. Therefore, more research is needed before drawing solid conclusions.

Soy may improve bone health

The low estrogen levels experienced during menopause may cause calcium to leach from the bones.

The resulting bone loss may cause postmenopausal women to develop weak and brittle bones, a condition known as osteoporosis.

Some evidence suggests that intakes of 40–110 mg of soy isoflavones per day may reduce bone loss and improve markers of bone health in menopausal women. However, more research is needed to confirm these findings.

To put this into perspective, this would be the equivalent of eating around 5–15.5 ounces (140–440 grams) of tofu or 1/3–1 cup (35–100 grams) of cooked soybeans each day.

Soy may reduce the risk of breast cancer

Diets rich in soy have also been linked to a lower risk of certain cancers.

For instance, one recent review of 12 studies suggests that women with high soy intakes before receiving a cancer diagnosis may be at a 16% lower risk of dying from the condition, compared with those with the lowest intakes.

High soy intake pre-and post-diagnosis may also reduce the risk of breast cancer recurrence in postmenopausal women by up to 28%. However, this study suggests that premenopausal women may not experience the same benefit.

On the other hand, another study suggests that both pre-and postmenopausal women eating soy-rich diets may benefit from a 27% lower risk of cancer.

Suggested read: 11 foods high in phytoestrogens

However, the protective benefits of soy were only observed in Asian women, while Western women appeared to experience little benefit.

Based on these studies, a proportion of women eating soy-rich diets may benefit from a lower risk of breast cancer. Still, more studies are needed to determine which women may benefit the most.

Soy may reduce the risk of other types of cancer

Soy-rich diets may also help lower the risk of other types of cancer.

For instance, studies suggest that high intakes of soy isoflavones may reduce the risk of endometrial cancer by around 19%.

In addition, some studies have linked soy-rich diets to a 7% lower risk of digestive tract cancers and an 8–12% lower risk of colon and colorectal cancers, especially in women.

On the other hand, men eating soy-rich diets may benefit from a lower risk of prostate cancer.

Finally, one recent review of 23 studies linked diets rich in soy foods to a 12% lower risk of dying from cancer, particularly cancers of the stomach, large intestine, and lungs.

Summary: Soy-rich diets may improve heart health and lower blood pressure, blood sugar, and cholesterol levels. They may also improve fertility, reduce symptoms of menopause, and protect against certain cancers. However, more research is needed.

Why are some people concerned about soy?

Soybeans and foods derived from them have been part of the human diet for centuries. Nevertheless, some people worry about including soy in their diet due to the following areas of concern:

• Estrogen-mimicking effects. Soy isoflavones are often thought to mimic the female reproductive hormone estrogen. Although they’re similar in structure to this hormone, soy isoflavones have weaker and slightly different effects than estrogen.
• Cancer risk. Some people believe that soy isoflavones may raise the risk of breast or endometrial cancer. Yet, most studies find no negative effect. In some cases, they may even offer some protection against certain cancers.
• Thyroid function. Test-tube and animal studies suggest that some compounds found in soy may reduce thyroid gland function. Yet, human studies find little to no negative effects, especially in humans with healthy thyroid function.
• Feminizing effects in men. Some worry that soy isoflavones may reduce the production of the male hormone testosterone. However, human studies find a weak link between the two.
• Danger to babies. Some fear that soy formula may negatively affect the brain, sexual, thyroid, or immune development. Yet, studies typically fail to observe any long-term negative effects of soy formula in healthy, full-term babies.
• GMOs. Soybeans are often genetically modified (GMO). GMO soy may contain fewer nutrients and more herbicide residues than conventional or organic soy. More research about the long-term health effects of GMO soy is needed.
• Antinutrients. Soybeans contain compounds that may lower the body’s ability to absorb the vitamins and minerals they contain. Soaking, sprouting, fermenting, and cooking are ways to reduce these antinutrient levels in soy.
• Digestive issues. Animal studies suggest that the antinutrients in soy may reduce the gut’s barrier function, possibly resulting in inflammation and digestive issues. However, more human studies are needed to confirm this.

Keep in mind that while these concerns are common, few of them are supported by sound science. Moreover, when negative effects have been observed, they often followed the consumption of very large amounts of soy.

Suggested read: Soybeans: Nutrition facts, health effects & downsides

For instance, men who reported experiencing feminizing effects from soy consumed amounts up to 9 times larger than the average intake of men with soy-rich diets. Although possible, it would be difficult for most people to eat that much soy each day.

Summary: The concerns above are commonly cited when it comes to soy. Generally, few are supported by strong science, and more research is needed to confirm the remaining.

Not all soy-based foods are the same

It’s worth mentioning that not all soy foods are equally nutritious or beneficial.

Generally, the less processed soy food is the more vitamins, minerals, and beneficial compounds it may contain. On the other hand, the more processed soy food is, the more salt, sugar, fat, and unnecessary additives and fillers it likely contains.

That’s why minimally processed soy foods, such as soybeans, tofu, tempeh, edamame, and unsweetened soy milk and yogurts, are considered superior to soy-based protein powders, mock meats, energy bars, or sweetened soy milk and yogurts.

Minimally processed soy foods may also offer benefits beyond those associated with their nutrient content. For instance, they appear more effective at reducing blood sugar or cholesterol levels than processed soy-based foods or supplements.

In addition, fermented soy foods, such as soy sauce, tempeh, miso, and natto, are often considered more beneficial than non-fermented soy products. That’s because fermentation helps reduce some of the antinutrients naturally found in soy foods.

This can help improve your body’s ability to absorb the nutrients found in soy. Cooking, sprouting, and soaking are additional preparation techniques that can help reduce the antinutrient content of soy foods and enhance their digestibility.

Summary: Minimally processed soy foods, such as soybeans, tofu, tempeh, edamame, and unsweetened soy milk and yogurts, are considered superior to highly processed ones. Fermented soy foods may offer additional benefits.

The bottom line

Soybeans are rich in nutrients and beneficial plant compounds. Diets rich in minimally processed soy foods may offer various health benefits, including improved heart health, fewer menopause symptoms, and a lower risk of certain cancers.

Suggested read: 9 healthy beans and legumes you should try

However, some worry about soy’s potential downsides, including its GMO content, possible estrogen-like effects, and long-term influence on growth, digestion, sexual maturation, thyroid health, and breast cancer risk.

Currently, few of these concerns are backed by strong science. However, more research is needed. Those wishing to include soy in their diet would benefit from picking minimally processed foods over highly processed ones.