Soy: Poison or Superfood?

 

 

 

Sometimes people can become so invested in a particular belief or position that no amount of new information can move the needle on their outlook. As Scrooge's nephew Fred said in A Christmas Carrol, "It seems the less uncle Scrooge knows, the more stubbornly he knows it."

This is not to suggest that there was never a good reason to regard soy and soy products with suspicion. Some of the earliest studies conducted on soy suggested that genistein, which is one of the primary isoflavones in soy, increased the risk of breast cancer, specifically estrogen receptor-positive tumors. These studies were preliminary, and done in mice. They used massive doses of pharmacological genistein (not soy itself) and contained so many methodological flaws that even some of the original study authors have retracted their conclusions.

In 2011, it was shown that mice and humans metabolize genistein very differently. Some types of mice are unable to metabolize isoflavones effectively, and were shown to have up to 150 times the amount of genistein concentrations in their blood than the highest concentration ever observed in human subjects. This study casts doubt on the usefulness of conducting soy-related studies on mice, and emphasizes the importance of using human studies to draw meaningful conclusions.

Subsequent human studies have been conducted, and have found women who consume more than two servings of soy per day have the lowest incidence of breast cancer. Among breast cancer survivors, higher isoflavone intake was associated with a lower risk of death. There may even be implications for soy in the treatment of breast cancer, as preliminary research suggests it may be able to destroy cancer cells. Much more research will be needed in that area, though.

Soy, and in particular genistein, seems to protect against a variety of other cancers, including prostate cancer. It seems to inhibit tumor growth with no effect on testosterone levels. Even uterine fibroids, benign but often causing pain and heavy menstrual bleeding, may be able to be treated with genistein. However, these studies were done in vitro, so further research is needed to determine if the findings are consistent in the human body.

Another concern about soy consumption is mostly from men. They have heard that the phytoestrogens in soy will have a so-called “feminizing effect” on them. There were a few individual case studies that suggested this could happen. The first example was of a man who was consuming three quarts (12 cups) of soy milk per day, and the second is of a man consuming multiple types of soy products daily, which amounted to around 360 mg/day of isoflavones — roughly nine times the intake of older Japanese men, who consume soy daily.

Well-designed studies and literature reviews have revealed soy intake at usual amounts to have no effect on physical appearance or male reproductive hormones, and support the long-held advice of health care practitioners: too much of anything, even healthy foods, can potentially have adverse effects.

Meanwhile, soy has been shown to have a large number of salutary heath benefits that include reduced cancer risk, reduction of serum cholesterol levels and overall heart health. For example, Lunasin, a peptide contained in soy protein, now has a well understood mechanism of action for producing these and other desirable outcomes.

The clear majority of the evidence points to soy not only being safe, but even beneficial for most people to consume. While people with certain medical conditions might want to talk with their doctors about including soy in their diets, most can do so with peace of mind. If you would like to read more about the latest scientific research on soy safety, here is a place to start. It includes suggestions for those who might still do well to avoid it.