VeganHealth.org Update: Soy
A 2010 study found that 3 years of a daily dose of 54 mg of the isoflavone genistein, in postmenopausal women, did not harm the thyroid in any way compared to placebo (12). They measured 11 different thyroid health parameters. This is the amount of genistein contained in about (
14.7*) 4 cups of soymilk, and the amount of isoflavones you would find in about ( 8.7*) 2 to 3 cups of soymilk (13).
*Correction: I was contacted by a soy expert who said, “54 mg genistein is the amount found in about 100 mg total isoflavones from soyfoods. One serving of a traditional soyfood, such as a cup of soymilk made using the whole soybean (think Silk) provides about 25 mg isoflavones. So 4 cups will provide 54 mg genistein.” Upon closer inspection of the USDA list of isoflavones in food, I noticed more entries for soymilk and a range of genistein from 1.5 to 6.7 mg per 100 g for genistein and a range of 2.6 to 10.7 mg per 100 g for total isoflavones.
In a 2008 study on soy intake and sperm characteristics, researchers found that sperm counts among men with higher soy intakes did not differ from those who ate no soy. Men who ate more soy had lower sperm concentrations, but this was due to a higher semen volume; this finding was more pronounced in overweight and obese men than among lean men (14).
12. Bitto A, Polito F, Atteritano M, Altavilla D, Mazzaferro S, Marini H, Adamo EB, D’Anna R, Granese R, Corrado F, Russo S, Minutoli L, Squadrito F. Genistein aglycone does not affect thyroid function: results from a three-year, randomized, double-blind, placebo-controlled trial. J Clin Endocrinol Metab. 2010 Jun;95(6):3067-72. Link
13. USDA Database for the Isoflavone Content of Selected Foods Release 2.0. (2008) Link
14. Chavarro JE, Toth TL, Sadio SM, Hauser R. Soy food and isoflavone intake in relation to semen quality parameters among men from an infertility clinic. Hum Reprod. 2008 Nov;23(11):2584-90. Link