Nowadays walk down any isle in the supermarket and you will almost definitely come across a soy-containing product. It seems it is in everything! Soy has dramatically increased in popularity in the last decade due to the idea that eating too much meat or dairy was bad for you and therefore soy being a great protein alternative. Soy crops are also very cheaply made into soybean oil and soy protein and so now it can be found in many processed foods. Many people are eating so much more soy then they realise and some even supplying their infants with large quantities through the use of soy based infant formula. With such an increased consumption, it really begs the question of what effect soy is actually having on your body.
A number of studies have suggested that soy contains compounds that may disrupt endocrine function. Soy contains phytoestrogens, which can mimic the estrogen in the body causing an inappropriate response (2). The results of one study suggested that phytoestrogens may act as estrogen antagonists in a high estrogen environment, or may act as estrogen agonists in a low estrogen environment (3). This hormone disruption is a cause for concern as this could potentially disrupt the body's normal functioning.
An article in the American Journal of clinical nutrition has also looked at the effects of soy proteins supplementation on epithelial proliferation in normal breast tissue. This study suggested that the consumption of soy could increase epithelial cell proliferation and therefore could potentially increase the risk of breast cancer (4).
Focusing on eating a diet full of natural, whole and unprocessed foods will see your intake of soy reduced considerably anyway. Studies have suggest that fermented soy products (such as tamari, tempeh or miso) could be better for you (5) so I'm not sure if you need to start panicking about the 1 tablespoon of tamari sauce you have with your sashimi just yet. With anything, the dose determines the toxicity so fermented and in small amounts should not be a cause for concern.
1. Russo, G. L. (2009). Dietary n-6 and n-3 polyunsaturated fatty acids: from biochemistry to clinical implications in cardiovascular prevention. Biochem Pharmacol, 77(6), 937-946. doi: 10.1016/j.bcp.2008.10.020
2. Morito, K., Hirose, T., Kinjo, J., Hirakawa, T., Okawa, M., Nohara, T., . . . Masamune, Y. (2001). Interaction of phytoestrogens with estrogen receptors alpha and beta. Biol Pharm Bull, 24(4), 351-356
3. Hwang, C. S., Kwak, H. S., Lim, H. J., Lee, S. H., Kang, Y. S., Choe, T. B., . . . Han, K. O. (2006). Isoflavone metabolites and their in vitro dual functions: they can act as an estrogenic agonist or antagonist depending on the estrogen concentration. J Steroid Biochem Mol Biol, 101(4-5), 246-253. doi: 10.1016/j.jsbmb.2006.06.020
4. McMichael-Phillips, D. F., Harding, C., Morton, M., Roberts, S. A., Howell, A., Potten, C. S., & Bundred, N. J. (1998). Effects of soy-protein supplementation on epithelial proliferation in the histologically normal human breast. The American Journal of Clinical Nutrition, 68(6), 1431S-1435S.
5. Buckle, K. A. (1985). Phytic acid changes in soybeans fermented by traditional inoculum and six strains of Rhizopus oligosporus. Journal of Applied Bacteriology, 58(6), 539-543. doi: 10.1111/j.1365-2672.1985.tb01709.x