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

Few foods have been as polarizing as soy. In the past few decades, some have praised soy as a healthy, nutritious food that can lower cholesterol and help with weight loss. At the same time, others have raised concerns about the possible estrogen-like effects of soy, which could lead to fertility problems or even increase your chance of cancer. So, is soy good or bad for you?

A comprehensive review of the evidence sheds light on this question. Researchers reviewed 417 reports based on human data and concluded that soy has no negative effects on thyroid health, estrogen levels, or fertility. And in some areas, scientists found soy to be beneficial. 

What are soy foods?

Soy foods are made from soybeans, which are a type of legume native to East Asia. They’re named this way because they grow in pods containing seeds or beans. Most soy products are innovations from cuisines across Asia. 

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Common nonfermented soy products include:

• Fresh soybeans (edamame)

• Fresh tofu

• Soy milk

• Soybean sprouts

• Soybean oil, a very common ingredient in processed foods

• Soy lecithin, a common emulsifier used to mix oil and water (used in everything from chocolate to baked goods to dietary supplements) 

Common fermented soy products include:

• Soy sauce

• Miso

• Soy pastes

• Tempeh

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• Miso and your health: Miso is a fermented soybean that you can use in many different recipes. Learn about its health benefits and get ideas on working it into your diet.

Why do some people think soy is bad for you?

Some people think that soy is bad for you because soybeans contain a high concentration of isoflavones. These are compounds that have a similar structure to estrogen. They are considered “phytoestrogens,” or plant-derived estrogens. Some people worry that because of this, eating soy will cause hormonal changes in the body. 

Concerns about soy have been mostly theoretical. For example, people worry that if phytoestrogen can act like estrogen, it might have feminizing effects on men or disrupt fertility.  Other concerns — like increased risk of breast cancer, thyroid problems, and fertility — have been based on animal studies. But animals and humans metabolize isoflavones very differently.

In 2016, the European Food Safety Authority reviewed over 25 years of data in humans and concluded that soy is safe and sometimes beneficial. There’s no evidence that soy is bad for you, and there’s emerging evidence that it may help prevent or treat certain chronic conditions. 

Processed vs. unprocessed soy products

Processed foods in general have a reputation for being bad for your health. This is for good reason — processed foods usually have more fat, salt, and simple sugars than unprocessed foods. Processed foods also provide less nutrients. Take a potato chip versus a baked potato. They’re both made from potatoes, but one is much more healthful than the other. Soy is no different.

Soy products that are unprocessed or minimally processed provide all the nutritional benefits that soy has to offer — without the unwanted additives. Examples of these products are edamame, tofu, soy milk, and tempeh. Highly processed soy products, on the other hand, often get stripped of their vitamins, minerals, and fiber, while fats and salt are added for flavor. Examples of processed soy products are soy proteins (used in things like protein bars and protein powders) and soy-based meat substitutes.

Do soy products increase estrogen in men or women?

No. Soy contains isoflavones, chemicals that look like estrogen on a molecular level. Because of the similarity between isoflavones and estrogen, people have worried that eating lots of soy could be similar to having higher estrogen levels in your body. Research has shown that this isn’t the case. 

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Even though isoflavones can interact with estrogen receptors in the body, this interaction tends to be weak and doesn’t seem to have harmful effects. Given the possibility of mimicking estrogen, there have also been worries that too much soy could lead to infertility in women. Studies have found that a diet high in soy can prolong the menstrual cycle by a day, but it doesn’t affect fertility. 

People have also worried about soy’s effect on men. Since isoflavones look like estrogen, it has been thought that men who eat lots of soy could become infertile or develop female characteristics, such as breasts. While some early studies showed that a high intake of soy was associated with lower sperm counts, more recent research has shown that this is not the case. 

The most current evidence suggests that there’s no association between soy intake and sperm counts or fertility problems. Despite the controversies, the truth is that soy hasn’t been found to have any effects on testosterone or estrogen levels in men.

Is soy bad for your thyroid?

No. In the past, some raised concerns after studies with rodents showed that giving them high amounts of isolated isoflavones led to a goiter and altered their thyroid function. But this was probably related to the high amount of isoflavones administered, and more importantly, the different ways in which rodents and humans metabolize soy products. 

More recent reviews of over 14 different studies showed that soyfoods didn’t affect thyroid function. 

Health benefits of soy products

Of the different kinds of legumes, soybeans are the only ones considered a “complete protein.” That means they contain all nine essential amino acids that the body cannot make. Soybeans are an excellent source of protein and unsaturated fat, and vitamins and minerals such as:

• Iron

• Calcium

• Potassium

• B vitamins

Let’s take a closer look at some of the possible health benefits of soy.

Weight loss

Eating soy food may help you lose weight when part of a nutritious diet. People often use soy as a replacement for animal protein, which usually has more calories. As part of a well-balanced diet, soy can be a good source of protein, fiber, and healthy vitamins and minerals. 

A small study showed that people who used soy-based protein as part of a high-protein, weight-loss diet and those who ate animal protein lost the same amount of weight. In other words, soy may be just as effective as animal protein in helping with a healthy and balanced weight loss. 

Another study found that people with polycystic ovarian syndrome (PCOS) who maintained a high-protein diet lost more weight when half their protein was from soy compared with those who had a high-protein diet just from animal protein. 

As part of a low-calorie diet, people doing a meal-replacement diet seemed to lose more weight with soy meal replacements than milk-based ones. 

May protect against some cancers

Studies have shown that eating soy can actually help protect against some forms of cancer, like breast cancer and prostate cancer. Multiple studies have shown that children and adolescents who consume more soy have lower breast cancer rates in adulthood. A large meta-analysis also showed that women who had higher soy intake had higher survival rates after being diagnosed with breast cancer. And a study looking at the link between soy and prostate cancer found that high soy consumption seemed to lower the risk of prostate cancer by as much as 50%. 

Soy also seems to play a role in stomach cancer, but the picture is a bit more complicated here. Multiple studies have found that fresh and unprocessed soy products can reduce the risk of stomach cancer, but highly processed or fermented soy products can actually increase your risk of it. Nobody knows why this might be the case, but some people think that the high salt content of some processed soy products can explain why they’re associated with higher rates of stomach cancer.

Can improve cholesterol level and heart health

In 2000, the American Heart Association recommended soy as a way to help lower cholesterol. But 8 years later, they retracted that recommendation due to inconsistent and conflicting studies. 

Part of the problem may be that some people with high soy intake eat higher amounts of processed soy — and processed foods are worse for heart health. More recent studies seem to suggest that soy intake can lower low-density lipoprotein (LDL), or “bad,” cholesterol by a small but notable amount. 

A review of research in 2019 looked at 46 different studies and found that eating about 25 g per day of soy lowered LDL by about 3% to 4% after 6 weeks. Soy also seems to have a beneficial effect on heart health overall. Eating one serving per week of tofu has been linked to an 18% lower risk of heart disease compared to those who never ate tofu. 

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Written by:

Karen Hovav, MD, FAAP

Karen Hovav, MD, FAAP, has more than 15 years of experience as an attending pediatrician in a variety of clinical settings. She has worked in a large academic center in an urban city, a small community hospital, a private practice, and an urgent care clinic.

Edited by:

Alex Eastman, PhD, RN

Alex Eastman, PhD, RN, is a California-based registered nurse and staff medical editor at GoodRx, where he focuses on clinical updates and Latino health.

Reviewed by:

Farzon A. Nahvi, MD

Dr. Nahvi is an emergency medicine physician and author of “Code Gray: Death, Life, and Uncertainty in the ER.” He works clinically at Concord Hospital in Concord, New Hampshire, and is a clinical assistant professor of emergency medicine at the Geisel School of Medicine at Dartmouth. Prior to this, he worked as an ER physician and clinical assistant professor of emergency medicine at the Mount Sinai Health System, NYU Langone Health, NYC Health + Hospitals/Bellevue, and the Manhattan VA. He is a graduate of Cornell University and NYU School of Medicine.

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