Is Eating Soy Healthy or Unhealthy?
Written by Alina Petre, MS, RD (NL) on August 27, 2020 — Medically reviewed by Adrienne Seitz, MS, RD, LDN
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.
This article reviews the latest scientific evidence to determine whether eating soy is more likely to have positive or negative effects on your health.
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.
Various soybean-derived products exist. Here’s a comparison of the nutrient content of several popular options, per 3.5-ounce (100-gram) portion (1Trusted Source, 2Trusted Source, 3Trusted Source, 4Trusted Source, 5Trusted Source, 6Trusted Source):
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 (7Trusted Source, 8Trusted Source, 9Trusted Source, 10Trusted Source).
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 (7Trusted Source).
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 (7Trusted Source, 11Trusted Source).
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 (12Trusted Source).
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.
May offer several health benefits
Soy-rich diets have been linked to a few potential health benefits.
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 (13Trusted Source, 14Trusted Source, 15Trusted Source).
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% (14Trusted Source).
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 (14Trusted Source).
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% (13Trusted Source).
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 (13Trusted Source).
May help protect heart health
Diets rich in legumes, including soy, may help lower your risk of heart disease (16Trusted Source).
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 (17Trusted Source).
A recent review further links soy-rich diets to a 20% and 16% lower risk of stroke and heart disease, respectively (18Trusted Source).
Additional research suggests that diets rich in soy foods may reduce your risk of dying from heart disease by up to 15% (19Trusted Source).
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 (20Trusted Source).
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 (21Trusted Source).
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 (22Trusted Source).
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 (22Trusted Source, 23Trusted Source).
Clearly, 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.
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 (24Trusted Source).
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 (24Trusted Source).
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 (15Trusted Source).
Metabolic syndrome refers to a cluster of conditions, including high blood sugar, cholesterol levels, blood pressure, and abdominal fat, that 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 (25Trusted Source, 26Trusted Source, 27Trusted Source).
Therefore, more studies are needed before strong conclusions can be made.
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 (28Trusted Source, 29Trusted Source).
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 (30Trusted Source).
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 (31Trusted Source).
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 (32Trusted Source).
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 (31Trusted Source).
These amounts of soy isoflavones are equivalent to around 1–4 servings of soy foods per day.
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.
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 (33Trusted Source, 34Trusted Source).
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 (34Trusted Source, 35Trusted Source).
However, not all studies report the same benefits. Therefore, more research is needed before drawing solid conclusions (36Trusted Source).
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 (37Trusted Source, 38Trusted Source).
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 (7Trusted Source, 12Trusted Source).
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 prior to receiving a cancer diagnosis may be at a 16% lower risk of dying from the condition, compared with those with the lowest intakes (39Trusted Source).
High soy intakes 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 (39Trusted Source).
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.
However, the protective benefits of soy were only observed in Asian women, while Western women appeared to experience little benefit (40Trusted Source).
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.
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% (41Trusted Source, 42Trusted Source).
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 (43Trusted Source, 44Trusted Source, 45Trusted Source).
On the other hand, men eating soy-rich diets may benefit from a lower risk of prostate cancer (46Trusted Source).
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 (19Trusted Source).
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 (12Trusted Source).
- 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 (12Trusted Source, 39Trusted Source, 40Trusted Source, 47Trusted Source, 48Trusted Source).
- 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 (49Trusted Source, 50Trusted Source, 51Trusted Source).
- 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 (12Trusted Source, 52Trusted Source).
- Danger to babies. Some fear that soy formula may negatively affect 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 (53Trusted Source, 54Trusted Source, 55Trusted Source, 56Trusted Source).
- 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 (57Trusted Source, 58Trusted Source).
- 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 (59Trusted Source, 60Trusted Source, 61Trusted Source, 62Trusted Source).
- 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 (63Trusted Source, 64Trusted Source, 65Trusted Source).
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.
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 (12Trusted Source).
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 a soy food is, the more vitamins, minerals, and beneficial compounds it may contain. On the other hand, the more processed a 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 milks and yogurts, are considered superior to soy-based protein powders, mock meats, energy bars, or sweetened soy milks 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 (13Trusted Source, 20Trusted Source).
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 (60Trusted Source).
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 (60Trusted Source, 61Trusted Source, 62Trusted Source, 63Trusted Source).
Minimally processed soy foods, such as soybeans, tofu, tempeh, edamame, and unsweetened soy milks 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.
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.
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