Soybeans are unique among legumes because they are rich in high-quality plant protein, as well as in phytoestrogens (plant estrogens) such as isoflavones, that can mimic estrogen and have hormone-like effects in your body. Isoflavones are found in varying amounts in tofu and other soy products such as mature dried soybeans, young green soybeans (edamame), soymilk, and other dairy alternatives made from soybeans.

Substituting soy foods, such as tofu, for some of the meat normally consumed in the average American diet has been hailed as a heart-healthy habit that may reduce serum cholesterol levels and help the heart in other ways. Soy foods have also been associated with decreasing symptoms of menopause, such as hot flashes, protecting against some forms of cancer, and improving bone health.

But at the same time, soy foods have also been linked to an increased risk of various types of cancer and other health issues.

The Good News

Studies of Asian and Asian-American women have linked higher soy intake in adults to a lower incidence of breast cancer, and even more substantial decrease of risk in women who also consumed soy throughout childhood and their teen years. A Korean women's study also linked the regular use of soybeans and tofu in the diet to a lower incidence of stomach cancer, while a University of Hawaii multi-ethnic study found that a high intake of phytochemicals found in soy foods may reduce the risk of endometrial cancer.

An analysis of the Shanghai Women's Health Study, published in a March, 2013 issue of the Journal of Clinical Oncology, revealed that those with a higher intake of soy foods lived longer on average after a lung cancer diagnosis than those with a low intake of soy foods. An earlier study of Japanese patients found that those with high-soy content in their diets had better lung function and were less likely to develop COPD. In this study, the researchers pointed out that much more research is necessary to see if these findings apply to Western populations, who may have higher rates of smoking and hormone replacement use post-menopause, both of which can affect lung cancer development and prognosis. Western populations are also likely to have a lower lifetime intake of soy food products.

The Not-So-Bad News

Eating tofu started to become controversial, and its benefits challenged, when some studies indicated possible links between soy foods and thyroid dysfunction, impaired mineral absorption, increased risk of breast cancer recurrence in women with the estrogen receptor-positive (ER+) form of the disease, and decreased testosterone levels in men. Mark Messina, an adjunct associate professor at Loma Linda University in California, executive director of the Soy Nutrition Institute, and longtime soy researcher, points out that the results of many of these studies have been challenged by larger and well-designed studies that showed no association at all, including for women with ER+ breast cancer.

Like many whole grains and other plant foods, soybeans contain phytates, substances that can interfere with mineral absorption. But according to Messina, there is no evidence that a diet containing soy foods results in mineral imbalances. Furthermore, he points out, the calcium in fortified soy products is absorbed as well as the calcium in dairy products.

While the FDA allows a "heart healthy" label on low-fat, low-cholesterol foods that contain at least 6.25g of soy protein in a serving, that label is controversial because of the modest effects soy foods actually have on cholesterol levels. It is important to understand, however, that including tofu and other soy foods in the diet is just one part of a heart-healthy lifestyle.

It is also important to remember that some studies use isoflavone or soy protein supplements, while others collect and compare data about the routine consumption of soy foods. Rarely are the results of one single study considered conclusive, and studies that measure the results of supplements cannot be compared to studies that measure food use.

The Bottom Line

A plant-based diet that includes moderate amounts of soy foods (up to 25 g/day) as a source of protein and other nutrients is not only safe, but potentially beneficial for most people. As with any specific food or nutrient, however, too much can throw your diet out off balance and may potentially cause problems for some people. "Eating a couple of servings of soy foods each day provides a lot of good nutrition, with the added potential of reducing risk of several chronic diseases," advises the expert. If you have any concerns, speak with your doctor about including soy foods in your diet.

Mark Messina, PhD, MS, reviewed this article.


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Last reviewed 2012 July 2012; accessed 2013 November.