To soy or not to soy that is the question I am asked at least once a week. Unfortunately, theres not a clear answer, even in the research. Part of the problem is lies in the underwriters of many studies. For example, many of the pro-soy studies are funded, in part, by John Deere and other companies that have a vested interest in soy production. On the flip side, some of the anti-soy studies are funded, in part, by the dairy lobby, which has an interest in less soy, more dairy. This makes the already murky waters a bit murkier.
For me, I tend to lean toward less soy less often. Heres why.
For women with estrogen dominance or those suffering from perimenopause symptoms where estrogen levels are already a bit high, adding more estrogen (albeit phytoestrogen) to the mix doesnt seem to make rational sense. Now, Ive heard that the soy blocks the estrogen receptors and blocks you more potent estrogens from locking in and all that; but the reality is, you are adding an additional estrogen to a system thats already overtaxed. That cannot be good.
For women looking for relief from their menopause symptoms, soy can be a Godsend for the exact same reason its questionable for estrogen dominant women. Meaning, you are providing additional estrogen where estrogen levels are low. Sounds good, right? Well, maybe, maybe not.
A study from the May 2002 issue of Cancer Research looked at the interactions between dietary genistein and tamoxifen (an estrogen antagonist used in the treatment of estrogen-dependent breast cancer). Researchers implanted estrogen-dependent breast cancer cells in mice who had had their ovaries and thymus removed. They found that genistein negated or overwhelmed the inhibitory effect of tamoxifen. Based on these findings, they urged postmenopausal women to exercise caution when consuming dietary genistein while taking tamoxifen.
Other studies that shown a correlation between dietary intake of isoflavones and thyroid disease for several species of animals. Additionally, some studies have found that animals fed soy isoflavones developed enlarged organs, particularly the pancreas and thyroid, as well as increased deposition of fat in the liver.
Additionally, Dr. Lark has written in the past that women who have inflammatory bowel disease, autoimmune thyroiditis, and/or a known allergy or sensitivity to soy should avoid soy. Finally, soy is one of the two most genetically modified (GMO) foods in the United States. That for me is a huge red flag.
My Suggestion
Given all the controversy, the GMO issue, the interaction with tamoxifen, and the possible thyroid concern, I personally err on the side of caution with soy. I limit myself to true soy foods only, meaning edamame (the beans themselves), tofu, and tempeh and always, always organic (eliminates the GMO concern). I also limit my soy consumption to no more than three times a week.