Does soy milk make men more feminine and cause their breasts to grow?

Concerns about soy’s effect on men probably arose because soybeans and other legumes contain isoflavones. These are phytoestrogens, or estrogen compounds structurally similar to human estrogen. However, “they have different biological properties than human estrogen and don’t promote harmful estrogen effects on the body,” Hensrud says. “In fact, they may be protective.”

Studies generally do not support the notion that drinking soy milk will cause men to grow breasts, or experience any other feminizing effects. Studies also refute the notion that soy increases the risk of breast cancer in women.

“I haven’t seen any data that leads me to believe that drinking soy milk or eating soy products promotes feminization in men, meaning breast growth, or any similar adverse health effects, such as breast cancer, in women,” says David Jenkins, university professor in the departments of nutritional sciences and medicine, Temerty Faculty of Medicine, at the University of Toronto. “I recommend soy intake to my patients — both sexes. I don’t think it has any ill effects.” He added that no major health organization has warned of any dangers associated with soy consumption.

At least two early studies each described a single case of soy consumption linked to feminizing effects in men, among them enlarged breasts, erectile dysfunction and decreased libido. But experts pointed out that the two subjects each consumed unusually high amounts of soy — in one instance, three quarts of soy milk a day — which is many times the intake in the average diet.

“That is a very large amount of soy milk on a consistent basis — most people don’t drink that much water daily,” Hensrud says. “These are two unusual cases, and they are not representative of the overall evidence on soy phytoestrogens.”

“Breast growth — gynecomastia — does occur occasionally in men and adolescent males,” he says, referring to other causes of gynecomastia in young men. “Most important, there is strong evidence that soy intake provides numerous health benefits with little risk.”

Many studies have also found that soy milk and other soy products may lower the risk of breast cancer, possibly accounting for the low incidence of breast cancer among Japanese women, whose traditional diets are heavy in soy. Soy intake also may lower the risk of breast cancer recurrence in women, according to studies.

Women who have estrogen-positive breast cancer may worry about soy, but Hensrud says data does not point to a greater risk. Other research indicates that breast cancer rates rise among Japanese women who reduce their soy consumption and adopt a Western diet.

Moreover, soy milk can have cardiovascular benefits by reducing blood pressure, inflammation and low-density lipoprotein (the bad) cholesterol, according to studies. Additional research shows soy appears to reduce the risk of prostate cancer and might prevent memory loss associated with aging and cognitive impairment.

What else you should know

Soy milk is a processed food that may contain added sugar, flavorings and other ingredients. Also, those who have a soy allergy should avoid it.

In addition to isoflavones, soy foods are rich in B vitamins, fiber, potassium and magnesium. Also, unlike some plant proteins, soy protein contains all nine essential amino acids, making it a complete protein. Soy foods can be unfermented or fermented, the latter meaning it has been cultured with beneficial bacteria, yeast or mold.

“Fermented soy milk may have even more health benefits, similar to other fermented foods,” Hensrud says.