The Soy Story

In our last blog for The Developing Story, we discussed how obesity and body fat may contribute to girls’ puberty. We did not, however, dive much into diet or specific dietary factors. One issue we are often asked about is soy. Many parents are concerned that soy may hasten puberty because of its estrogenic effects. It is true that soy contains phytoestrogens, which may mimic estrogen in the body, and as such, is often a suspect in early puberty. However, the evidence that soy disrupts puberty, and if so how, is far from clear cut. In fact, a recent longitudinal study by our colleagues in the NIH-funded Breast Cancer and the Environment Research Program (BCERP) suggests that soy intake might even delay puberty in girls.

Girls' early menarche, or first menstruation, has been linked to increased risk for breast cancer later in life. There is epidemiologic evidence that women who consume soy as young girls have lower breast cancer risk. However, breast cancer risk seems to adjust to background (higher) rates for these women’s daughters if they are raised in areas with less soy intake. For instance, if the food culture changes in a region or if there was immigration to a country with different eating habits. Thus, the lower rates of breast cancer in certain Asian countries, for instance, are not likely explained entirely by genetics but may reflect diet, and puberty may play a role.

Research from the BCERP longitudinal study of 1,200 girls suggests that girls with more soy intake may experience breast development later than those who eat less soy. In contrast, recent cross-sectional work has shown no effect of soy on age at menarche, but study design issues make those cross-sectional data difficult to interpret. Animal studies suggest that exposure to some soy proteins in immature animals may lead to earlier puberty. Some of these data focus on isolated isoflavones, not whole soy. Isoflavones are complex, and when separated, may have differing biological effects than the whole soy plant. For example, genistein exposure may cause earlier pubertal development in rats, but the same has not been shown with whole soy exposure. Of particular concern is soy protein isolate, which is a heavily processed form of soy with differing balance of isoflavones that the soy bean; some experts are concerned that this alternation in balance may lead to differing effects of processed soy versus the whole bean. In addition, the timing of soy intake may be important. There are very few studies measuring soy consumption during the early childhood years that have then followed girls over time as they entered puberty.

In terms of diet and soy, we suggest a cautious approach. Given that soy is an excellent source of protein in a balanced diet, we think that whole soy foods are a good choice for developing children, both boys and girls. Note the emphasis on whole soy foods—the evidence suggests that this should include relatively unprocessed soy foods such as edemame, tofu, soy milk, and tempeh. We suggest staying away from heavily altered soy products such as soy protein isolate. And to address a question we field regularly, the jury is still out on infant soy formula.