As with all foods, soy has beneficial effects for many people but can be harmful in certain situations for others. Some people who start eating soy products may experience stomach upset or digestive problems such diarrhea and constipation. Typically, this passes as a person transitions to a soy diet or reduces a high amount of soy in the diet.

There has been speculation about other health risks associated with soy protein. Most of these concern focus on specific components of soy such as isoflavones rather than on the whole food or the soy protein. Isoflavones are phytoestrogens, a weak plant-based form of the hormone estrogen that may have an effect on the body that is similar to a medication. This effect is most likely to occur in postmenopausal women. However, the data is mixed. While some research suggests that high levels of isoflavones may lead to cancer, other research suggests that such levels may actually prevent cancer.

Most experts agree that a moderate soy intake, such as the 25 grams of soy protein a day outlined in the U.S. Food and Drug Administration (FDA)-approved health claim, is generally considered safe. However, it is important to note that FDA has limited claims about the health benefits of soy to foods containing intact soy proteins. The long-term health effects of other isolated substances from the soy protein or from eating larger quantities of soy are still under investigation. Any concerns about adding soy to the diet can be discussed with a physician.

There has been some confusion about the fact that soy products have only been granted GRAS (generally recognized as safe) status by the FDA for limited industrial use as binding glue for cardboard, not as a product for human consumption. Although this is true, other common foods have also never been granted GRAS status. The general opinion of the medical community is that soy products are safe.

There is no convincing evidence that moderate soy protein consumption has an adverse effect on thyroid function in healthy humans consuming adequate amounts of iodine. Early research identified that soy, in addition to a number of plant-based foods, may contribute to the formation of a goiter, an enlargement or swelling of the thyroid gland. But there is no risk of goiter in healthy people consuming soy products if they are not deficient in iodine.

Patients who are watching their weight should be aware that soy foods do contain moderate amounts of fat. However, most of the fat is unsaturated and does not contribute to clogged arteries. In addition, low-fat soy products often are available.

In recent years scientists have begun to question whether the use of soy infant formula may positively or adversely affect a child’s long-term health. Soy-based formula is an alternative to cow’s milk formula that may be used in children who need to avoid cow’s milk or milk sugar (lactose) for medical reasons. Researchers have examined the levels of isoflavones found in the blood of infants on soy formula. It is not known if levels of isoflavones in the blood of babies who use soy formula cause any biological effects. Further studies are underway to determine if these effects are positive or negative.

Some children who use soy formula experience vomiting, diarrhea, growth failure and damage or bleeding of the intestinal wall. However, this is rare. It is important to note that for infants who are allergic to dairy formula and/or cannot be given breast milk, soy infant formula can be life-saving.