Top things to know:
Evidence suggests soy has cardiovascular and other health benefits.
There’s not a lot of data about how soy impacts hormones in humans.
More research is needed about soy to determine the risks and benefits.
You’ve probably heard the rumors about soy. Some of the rumors say it causes breast cancer. Some people are skeptical enough to remove soy from their diets because it contains plant estrogens. Even so, 35% of Americans report consuming soy food or beverages at least once a week according to the United Soybean Board's 2019 Consumer Attitudes about Nutrition study (1).
It’s hard, these days, to avoid soy in the typical American diet—it can be in everything from processed meats, margarine, chocolate and cereal, to soy milk (2). About 98 percent of U.S. soy consumption is by livestock (3), some of which might be used for meat products. Should you fear eating soy? Here’s what the latest research says.
Is eating soy bad for you?
It depends who you ask. The science on soy is still evolving. Studies have suggested that diets rich in soy can reduce cholesterol and improve kidney function (4). Based on this data, the United States Department of Health and Human Services has allowed products containing soy to claim that soy protein might reduce the risk for heart disease (5). Yet, other studies conflict with this data and in 2017, the Center for Food Safety and Applied Nutrition proposed that the positive health claims connected to eating soy should be revoked (6). As of January 2019, the claim had not been revoked (7) and new summary of the research supports the idea that consuming soy reduces cholesterol by approximately 3–4% in adults (8).
As you’ll read below, soy isn't just implicated in heart health. It has also been getting press (both positive and negative) for its possible effect on cancer risk, hormones and human development.
So, why do some people say soy could be harmful? Over the past several decades, animal studies have shown soy to have some negative effects on different animals (9). Researchers also believe soy allergies are present in some animals, and some soybean proteins are known to cause allergic reactions in humans (10).
But animal studies don’t always translate to humans. Some studies give animals different doses or types of soy compounds than seen in humans (11, 12). It’s also worth noting that humans and animals process foods differently (12).
Can eating soy cause cancer?
Compounds in soy called lignans and isoflavones mimic the sex hormone estrogen produced by the human body (13). In theory, these estrogen-like compounds in soy could stimulate hormone-dependent cancers like breast cancer (13).
However, researchers believe that lignans and isoflavones actually mimic sex hormones in ways that may make them cancer-fighting (14).
These compounds may even help to prevent the growth of cancer cells (15).
The American Cancer Society tells consumers that the benefits of eating soy foods outweigh the potential risks (16); eating these foods may even lower the risk of breast cancer somewhat (17). The Dieticians of Canada also suggest that people who have had estrogen-sensitive breast cancers do not need to avoid eating soy (18). These recommendations are about soy foods (like tofu) and not soy supplements. More research is needed to determine the benefits and risks of soy supplements and exactly how soy foods in general affect cancer risk.
Can eating soy impact your hormones?
Research suggests that eating soy products might decrease FSH and LH in people who are premenopausal, which may impact fertility. And it might increase estrogen in people who are menopausal (19), leading to a reduction in menopausal symptoms. The effects of soy might be based on the quantity of soy eaten as well as the type of soy consumed and when it is consumed (11). It’s not yet understood how these changes could impact people who menstruate (19).
There’s little evidence that soy impacts sperm quality or testosterone (11).
Animal studies suggest there may be slight hormonal and reproductive changes in male mice when they are exposed to soy consistently throughout their entire lifetimes (11). More research about the effects of soy on humans could help understand the effect of soy on hormones on all sexes.
Is soy bad for babies?
The American Academy of Pediatrics states that soy-protein infant formulas are safe for term infants (20). But there are few long-term studies of adults who were fed soy formula as babies, so there may be risks and benefits that are not yet fully understood (11).
One study of women who were fed soy formula as infants found that this group had somewhat longer and more painful periods than people who were fed cow's milk formula (21).
Another study of infants who were fed soy formula found slight differences in the sizes of their uteruses at 36 weeks compared to those fed cow’s milk formula, suggesting estrogen exposure (22). While these studies may indicate some soy-specific effects, research doesn’t raise concerns about the development of term infants fed soy formula (FYI, soy formula is not recommended or designed for preterm infants) (23).
Should you eat soy?
Soy is increasingly difficult to avoid in the American diet, particularly if you eat any processed food (24). It’s used in infant formula, flour, vegetarian cheese, tofu and many meat replacements (2). It is frequently used as a filler in meat products (2). And soy use is only on the rise: worldwide, consumption has increased 5-6% per year over the last 15 years (25).
The science about soy isn’t absolute. In fact, it’s often contradictory. It’s clear that more research is needed to determine all the positive and negative effects related to eating soy. At the end of the day, eating too much of anything, even if it is nutritionally good for you, can have negative effects.