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Soy has been a dietary staple in Asia for many centuries. Some studies have found that it may offer some cardiovascular benefits, though the evidence at this point is more suggestive than conclusive.

As far as any downside, most of the health concerns about soy stem from its concentration of phytoestrogens, a group of natural compounds that resemble estrogen chemically. Some experts have questioned whether soy might lower testosterone levels in men and cause problems for women who have estrogen-sensitive breast cancers. Animal studies have found, for example, that large doses of phytoestrogens can fuel the growth of tumors.

But phytoestrogens mimic estrogen only very weakly. A number of clinical studies in men have cast doubt on the notion that eating soy influences testosterone levels to any noticeable extent. And most large studies of soy intake and breast cancer rates in women have not found that it causes any harm, said Dr. Anna H. Wu of the Keck School of Medicine at the University of Southern California. In fact, work by Dr. Wu and others has found that women who consume the equivalent of about one to two servings of soy daily have a reduced risk of receiving a diagnosis of breast cancer and of its recurrence.

Still, some women who have developed breast cancer remain particularly worried about eating soy. But the evidence “is overwhelming that it’s safe,” said Dr. Bette Caan of the Kaiser Permanente Northern California Division of Research, who has studied soy intake and breast cancer. “If people enjoy soy as a regular part of their diet,” she said, “there’s no reason to stop.”

Last year, in its nutrition guidelines for cancer survivors, the American Cancer Society noted that eating traditional soy foods — like tofu, miso, tempeh and soy milk — may help lower the risk of breast, prostate and other cancers. But the guidelines do not recommend soy supplements, which tend to be highly processed and not very rigorously tested.