The relationship between soy intake and cognitive function has been rigorously investigated for 20 years. The evidence overall is conflicting with some clinical studies showing improvement in one or more measures of cognition whereas a couple of epidemiologic studies suggest just the opposite. A just published review represents the most comprehensive look at the data to date. The authors concluded that “… the evidence to date is not sufficient to make any recommendations about the association between dietary intake of soy isoflavones and cognition in older adults.” It is unlikely given the amount of data already published, that at least in the near future this conclusion is likely to change.
Soni M, Rahardjo TB, Soekardi R, et al. Phytoestrogens and cognitive function: a review. Maturitas 2014.