You may have heard things about soy, soy milk, and soy products that have made you think—are they safe? Many people unnecessarily worry about the natural phytoestrogens in soy. What exactly are phytoestrogens, and how do they affect the human body?

As a dietitian, I often recommend soy, tofu, and other healthy proteins in many recipes and meal plans. But people sometimes push back because they’ve heard negative things about soy. Despite rigorous research into soy’s pros and cons over the past few decades, many myths and misunderstandings persist.

Is Soy Beneficial? Does It Actually Benefit the Human Body?

Soy foods have long been recognized as an excellent source of protein, and in some parts of the world, soy has been part of our diet for thousands of years. In fact, a growing body of research supports numerous health benefits of soy. Nevertheless, some confusion remains.

In the Western world, soy is not a staple food. But traditional soy foods—such as soy milk, tofu, miso, and tempeh—have been the foundation of East Asian diets for centuries, valued not only for their versatility but also for the healthy nutrients they provide.

Soy’s nutritional profile is impressive.

While all legumes provide protein, soy ranks top in terms of protein quality. As a complete plant protein, soy contains all the essential amino acids needed to build the various specific protein structures required by the human body.

Soy is also low in saturated fat and naturally cholesterol-free—since only animals and animal products contain cholesterol. Combined with the fact that most fats in soy are polyunsaturated, soy is a heart-healthy food.

Adding soy foods to your diet can increase intake of vitamins like folate and vitamin K, as well as minerals such as calcium, magnesium, and iron. Additionally, if you eat whole soy, you’ll boost your fiber intake—a nutrient you can’t get from animal protein.

Soy Protein Aids Muscle Growth

When it comes to muscle-building protein, people usually think of whey protein. In reality, soy protein is also effective at promoting muscle growth, but biases against consuming soy and its natural isoflavones—phytoestrogens (or “plant estrogens”)—have led some consumers to avoid it. They believe these plant compounds may lower testosterone levels and hinder muscle development.

This myth is false. A meta-analysis of studies on soy protein and isoflavones concluded that neither soy foods nor isoflavone supplements have a significant impact on testosterone concentrations in men.

Soy is an important source of protein and is also rich in arginine, an amino acid the body uses to produce nitric oxide. Nitric oxide increases blood flow to muscles, helping deliver nutrients and oxygen during exercise.

Soy Is a Sustainable Alternative to Animal Protein

You can help the planet by incorporating more plant-based proteins like soy into your diet. Compared to the environmental impact of raising farm animals, soy production uses less water and land and generates fewer greenhouse gases.

Despite these benefits, some people remain reluctant to embrace soy foods, often citing rumors or outdated research. Let’s address these common questions and claims using science.

Misconceptions and Concerns About Soy: Are Phytoestrogens Harmful?

Phytoestrogens are different from the natural estrogen produced by the human body. Understanding this difference is key.

This misunderstanding seems to stem from the fact that soy (and many other foods, by the way) contains natural plant compounds called isoflavones, which are classified as “phytoestrogens” (“phyto” means “plant”).

Thus, when people hear that soy foods contain “phytoestrogens,” they may refuse to eat them for fear that soy foods will expose their bodies to “too much estrogen.” This is incorrect.

Let me explain the difference as clearly as possible:

• Isoflavones are called phytoestrogens or “dietary estrogens” because their chemical structure is similar—but not identical—to the natural estrogen produced by the human endocrine system.
• Phytoestrogens are part of plants’ natural defense systems, primarily functioning to protect plants from fungal damage, which is why you’ll find them in many foods.
• While soy is perhaps the richest source of phytoestrogens, they are also present in various legumes, nuts, whole grains, and seeds like flax and sesame. Certain fruits and vegetables, including apples, carrots, and yams, also contain small amounts.
• The key point: Phytoestrogens in soy are not the same as the estrogen produced by the human body, nor do they act the same way in the body.

How Do Phytoestrogens Work in the Body?

Explaining how they work may help you understand why these natural compounds in soy are so misunderstood:

• Certain tissues in the human body, such as the mammary glands, bones, and prostate, have structures within their cells called estrogen receptors (E.R.). These receptors are inactive in tissue cells until estrogen enters the cell. Once estrogen enters the cell and binds to the receptor, it can exert its effects within the cell.
• Here’s where things get a bit complex—but bear with me. In fact, there are two different types of estrogen receptors, and the body’s natural estrogen is not particularly picky about which one it binds to; it will readily bind to both types.
• Phytoestrogens (like isoflavones in soy), on the other hand, prefer to attach to only one type of receptor. This is a clear difference between phytoestrogens and the body’s own estrogen.
• Because phytoestrogens prefer to bind to one receptor over the other, this means two things. First, it means that on the receptors phytoestrogens don’t mind binding to, natural estrogen can attach and exert its effects within the cell. However, when phytoestrogens bind to their preferred receptor type, they “get in the way”—making it harder for the body’s natural estrogen to bind.

This is where phytoestrogens’ unique role comes in: In some tissues, they can act like estrogen, while in others, they can interfere with the body’s natural estrogen. Thus, these plant compounds may mimic estrogen’s effects in tissues where estrogen might be beneficial, while at the same time interfering with estrogen’s effects in other tissues where reduced estrogen exposure may be considered protective.

Most importantly: The effects of isoflavones on the human body have been extensively studied. A recent review article stated: “There is no evidence to suggest that consuming soy or soy isoflavones as part of a regular diet is harmful to human health… On the contrary, research over the past few decades has indicated that these compounds have protective effects.”

Does Soy Increase Breast Cancer Risk?

Soy isoflavones do not increase breast cancer risk. This confusion about phytoestrogens has led some women to avoid soy foods, mistakenly believing these phytoestrogens will increase their estrogen exposure and raise breast cancer risk. But evidence suggests the opposite may be true.

Can Soy Actually Reduce Breast Cancer Risk?

When it comes to breast cancer, concerns that soy increases natural estrogen levels are unfounded. In fact, breast cancer rates are lower in countries where soy is commonly consumed.

In epidemiological studies in Asia, eating more soy at an early age has been associated with a 25% to 60% reduction in breast cancer risk. Similarly, the North American Menopause Society states that soy isoflavones do not increase the risk of breast cancer or endometrial cancer.

Does Soy Have Feminizing Effects on Men?

Colloquially known as “man boobs,” this claim has led some men to avoid soy products entirely. While it certainly makes headlines, only one study documented “feminizing” effects in a 60-year-old man who drank three quarts of soy milk daily.

It’s estimated the subject consumed 360 milligrams of isoflavones per day—nine times the usual intake of elderly Japanese men, whose soy intake is already higher than the global average. The subject also had an unbalanced and nutritionally deficient diet, as most of his calories came from soy foods.

We cannot draw conclusions from a single case. In numerous clinical trials, no feminizing effects were reported in men consuming up to 150 milligrams of soy isoflavones per day.

How to Incorporate More Soy Into Your Diet

Soy is actually very versatile, and there are many convenient and practical ways to enjoy it daily. Here are a few examples:

• Edamame are fresh green soybeans. You can usually find them in the frozen section of grocery stores, either in pods or shelled. Simply boil them in salted water for a snack, or add them to soups and salads.
• Tempeh is made from partially cooked soybeans that are fermented into a firm block. As a fermented food, tempeh is a source of “good bacteria” or probiotics. It has a meaty, firm texture that holds its shape, making it perfect for salads and stir-fries.
• Miso is a paste made from fermented soybeans, so it also contains probiotics. It can be used as a soup base or as an ingredient in sauces, dressings, and marinades. It comes in different varieties, ranging in color from light yellow to dark brown. Generally, light miso is less salty and has a milder flavor than dark miso.
• Soy milk is made by soaking dried soybeans in water until dehydrated, then grinding them with water. The resulting milk is sold as a beverage or made into yogurt. An 8-ounce (250-milliliter) serving of soy milk and soy yogurt each contains about 7 grams of protein. You can drink soy milk on its own, or use it as a substitute for regular milk in most recipes or protein shakes.
• Soy nuts are roasted whole soybeans. They make a great snack on their own, and can also be added to salads, grain bowls, and cereals. Compared to peanuts or peanut butter, soy nuts (and soy nut butter made by grinding soy nuts) have slightly higher protein content and lower fat content.
• Soy protein powder and meat alternatives are made from soy flour with most of the fat removed. Soy flour can be added to smoothies or stirred into oatmeal, and soy meat alternatives can replace meat or poultry in various recipes.
• Tofu is essentially a cheese made from soy milk. It has a texture ranging from extra firm to silken and has a very mild flavor. It pairs perfectly with everything from spicy sauces to naturally sweet fruits. Firmer tofu is ideal for grilling or stir-frying, while softer, silken tofu works well in smoothies or as a dessert topped with fruit and sugar.