A new study reports finding a link between higher intake of fermented soy products such as miso and nato and a lower risk of all-cause mortality. However, at this point, the findings aren’t conclusive; further research is needed to establish a cause-and-effect relationship between the two.

Fermented soy products are quite widely consumed in Asian countries, particularly Japan. These include natto (soybeans fermented with Bacillus subtilis), miso (soybeans fermented with Aspergillus oryzae), and tofu (soybean curd). A new study aimed at studying the potential health benefits associated with these products reports that they may help reduce mortality from any cause, although the link is not yet properly established.

Spilling the beans

“In this study a higher intake of fermented soy was associated with a lower risk of mortality,” the authors write. “A significant association between intake of total soy products and all cause mortality was not, however, observed.”

“The findings should be interpreted with caution because the significant association of fermented soy products might be attenuated by unadjusted residual confounding.”

The team set out to investigate the association between several types of fermented soy products and death from any cause (“all-cause mortality”), from cancer, total cardiovascular disease (heart disease and cerebrovascular disease), respiratory disease, and injury.

The findings were drawn from a pool of 42,750 men and 50,165 women aged 45-74 who were taking part in the Public Health Centre-based Prospective Study, which includes 11 public health center areas in Japan. As part of the study, participants were asked to fill in detailed questionnaires regarding their dietary habits, lifestyle, and personal health. Residential registries and death certificates were used to track the evolution of these participants over a 15-year-period after filling in the questionnaires. Roughly 13,300 deaths were identified during this time.

All in all, the team reports, a higher intake of fermented soy was associated with a 10% lower risk of all-cause mortality. Participants who ate natto also had a “significantly” lower risk of cardiovascular mortality than those who did not eat natto, the team adds. Total (unfermented) soy and soy product intake, however, had no observable link to all-cause mortality, they add, or to cancer-related mortality.

The association stood firm even after the team adjusted for vegetable intake — higher rates of which are associated with better overall health and reduced mortality. Participants who consumed higher portions of natto were, on average, also chowing down on more vegetables than their peers, the team explains. Fermented soy products are higher in fiber, potassium, and other beneficial compounds than non-fermented soy products, the team explains. They believe this might underpin the association observed in this study.

However, they also strongly stress that this is an observational study. The team simply found that people who eat more of these products are less likely to die from certain causes, but that doesn’t mean the products themselves lead to decreased mortality. The authors controlled for several factors, including overall diet, body-mass index (BMI), smoking status, alcohol intake, and engagement in sports among many others. Still, unaccounted for factors (these are the ‘confounders’ they mention) could be causing the observed link.

Further research is needed “to refine our understanding of the health effects of fermented soy,” they add, noting that there is some evidence linking fermented soy products with various health benefits — so it’s a promising line of study.

“These efforts should be collaborative, including not only researchers but also policymakers and the food industry,” they conclude.

The paper “Association of soy and fermented soy product intake with total and cause specific mortality: prospective cohort study” has been published in the journal BMJ.