Dementia risk may be reduced by consuming dietary soy, at least in those who have the right kind of gut microbes to produce a compound called equol from its soy-derived precursors. Equol is a metabolite of the soy isoflavone daidzein that is produced only by specific gut bacteria.
A recent study assessed serum levels of equol in 91 healthy elderly Japanese participants (who are known for consuming higher levels of traditional soy foods) for four years and then took MRI and PET scans 6 to 9 years later. They then compared non-producers, low producers, and high producers of equol to the white matter lesion percentage they found on the scans. Their findings showed a 50% decrease in white matter lesions in high producers of equol compared with low producers.
About 40-70% of Japanese have the right gut bacteria that can convert soy isoflavones into equol, while only 20-30% of Americans do. This may be due to variances in intestinal bacteria, types of soy isoflavones consumed and genetics. It’s possible that changes in dietary and lifestyle habits in Japanese migrants to Western countries may cause their microbiomes to reduce their equol-producing capability, as suggested by research showing that those migrants lose the protective effects of soy in their diets after they relocate.
Isoflavones are not widely consumed in the American diet and when they are they are of the modern, processed soy variety. In addition, without the specific microbes to convert daidzein, equol cannot be produced. Traditional, fermented soy preparations, such as “stinky” tofu, often naturally come with the those desired probiotic species, thus inoculating the microbiota at the same time as feeding it what it needs to make equol. Other foods containing daidzein are non-fermented soybeans, kudzu root, currants, raisins, and pistachio nuts.