Whey protein plus weight training promotes muscle protein synthesis in older adults and prevents sarcopenia— loss of muscle tissue that occurs with age. Rebecca Thompson and colleagues from the University of South Australia in Adelaide, in a study on aging men, found that supplementing diets with 27 grams of soy protein immediately after weight training caused lower strength gains, compared to a group receiving dairy protein. Estrogen-like compounds called phytoestrogens found in soya, tofu and legumes bind to estrogen receptors, decrease sperm counts and have feminizing effects. A product found in soy-based foods called genistein acts like a weak estrogen.
Soy protein might have reduced strength gains in older men because of its effects on testosterone. Messina and colleagues (Cancer Epidemiol Biomarkers Prev, 2007) found that soy protein supplements (approximately 55 grams per day for a month) decreased blood testosterone levels by 19 percent and increased estrogen receptor activity. Testosterone levels returned to normal within two weeks of discontinuing the soy supplement. (Clinical Nutrition, 35: 27-33, 2016)