Soy Products (Photo Credit: The Spruce Eats)
Higher intake of fermented soy products, such as miso and natto, is associated with a lower risk of mortality, claims a study conducted in Japan. The findings, published in the British Medical Journal, should be interpreted with caution as they may have been affected by unmeasured factors, the researchers said.
In Asian countries, especially Japan, several types of soy products are widely consumed, such as natto -- soybeans fermented with Bacillus subtilis bacteria, miso or soybeans fermented with Aspergillus oryzae bacteria, and tofu or soybean curd.
However, it is still unclear whether different soy products, especially fermented soy products, are associated with specific health effects, according to the researchers, including those from Osaka University Graduate School of Medicine in Japan.
They analysed the association between several types of soy products and death from any cause, and from cancer, total cardiovascular disease, respiratory disease, and injury. The team based its findings on 42,750 men, and 50,165 women aged 45-74 years, who were taking part in a study based in 11 of Japan's public health centre areas.
Participants filled in detailed questionnaires about their dietary habits, lifestyle, and health status. Deaths were identified from residential registries and death certificates over a follow-up period of nearly 15 years.
The researchers found that a higher intake of fermented soy (natto and miso) was associated with a significantly lower (10 per cent) risk of all-cause mortality. However, total soy product intake was not associated with all-cause mortality, they said.
The study also found that men and women who ate natto had a lower risk of cardiovascular mortality than those who did not eat natto. However, there was no association between soy intake and cancer related mortality, the researchers noted.
These results persisted even after further adjusting for intake of vegetables, which was higher among those consuming larger portions of natto. The researchers noted that fermented soy products are richer in fibre, potassium and bioactive components than their non-fermented counterparts, which may help to explain their associations.
However, this is an observational study, so it cannot establish cause, and the researchers cannot rule out the possibility that some of the observed risk may be due to other unmeasured factors.
"In this large prospective study conducted in Japan with a high rate of soy consumption, no significant association was found between intake of total soy products and all cause mortality," the researchers said. "In contrast, a higher intake of fermented soy products (natto and miso) was associated with a lower risk of mortality," they said.