However, writing in the BMJ, the researchers stress that the findings should be interpreted with caution as they may have been affected by confounding factors.
The team from Japan detailed how it sought to investigate the association between several types of soy products and all cause mortality, cancer, total cardiovascular disease, respiratory disease, and injury.
A total of 42,750 men and 50,165 women aged 45-74 took part in the study, filling in 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%) risk of all cause mortality, but total soy product intake was not associated with all cause mortality.
Men and women who ate natto also had a lower risk of cardiovascular mortality than those who did not eat natto, but there was no association between soy intake and cancer related mortality.
These results persisted even after further adjusting for intake of vegetables, which was higher among those consuming larger portions of natto.
The paper states: “This large prospective study investigated the association between intake of several kinds of soy products and mortality. The findings showed that the consumption of total soy products was not significantly associated with a decrease in all cause mortality, whereas intake of fermented soy products was significantly inversely associated with all cause mortality in both sexes.
“The risk of mortality in the highest fifth of fermented soy intake, including natto and miso, was 10% lower than that in the lowest fifth. Intake of non-fermented soy products was not significantly associated with all cause mortality.”
However, this is an observational study, so the reserachers stressed thy couldn’t establish cause, nor rule out the possibility that some of the observed risk may be due to other unmeasured factors.
“Our study does have several limitations,” they wrote.
“The most important one was that we could not eliminate residual unmeasured confounding completely. Although we adjusted for known and measured factors and excluded known diseases diagnosed before baseline, such as cancer, stroke, or myocardial infarction, we could not adjust for socioeconomic status variables other than the area deprivation index or diseases other than diabetes and hypertension.”
They added in a linked editorial piece that Increasing evidence has suggested that fermented soy products are associated with health benefits.
“Whether people eat those products depends on their food culture, but some countries already include soy and fermented soy products in their dietary guidelines,” they stated.
“Further studies are still required, however, to refine our understanding of the health effects of fermented soy, and perhaps to inform the development of healthier and more palatable products. These efforts should be collaborative, including not only researchers but also policy makers and the food industry.”
The study was supported by National Cancer Center research and development fund (since 2011) and a grant-in-aid for cancer research from the Japanese Ministry of Health, Labour, and Welfare (1989 to 2010). The funders had no role in the study design, data collection and analysis, decision to publish, or preparation of the manuscript.
“Association of soy and fermented soy product intake with total and cause specific mortality: prospective cohort study”
Authors: Ryoko Katagiri, et al