A new large-scale study conducted in Japan suggests that fermented soy products, in comparison to those with unfermented soy, may minimize risk of mortality. The research is observational, however, and it has limitations.
Soy products have been popular in Asia since ancient times, and have become increasingly popular in Western regions over the last few decades.
Researchers are keen to understand, with growing interest in nutritional science, whether any of the different forms of soy could bring health benefits.
Recent studies, for example, have concluded that consuming fermented soy products is associated with lower blood pressure and a lower risk of cardiovascular mortality.
To date, few studies have investigated whether consuming fermented soy impacts overall mortality, and conflicting results have been produced by those who have looked into this subject.
For example, one study concluded that soy intake “may have moderate but beneficial effects on total mortality,” while another found that “intake of soy products was not statistically significantly associated with all-cause mortality.”
A group of Japanese researchers recently reopened the discussion. They published their findings in BMJ journal.
Soy and mortality
The authors explain that they aimed to “investigate the association between intake of several types of soy products and all-cause and cause-specific mortality.”
Data from 11 public health centers in Japan were available to the scientists. The data came from 42,750 men and 50,165 women aged 45–74 years.
Each participant completed lifestyle, health, and nutrition questionnaires. For nearly 15 years the scientists followed the group of participants and also collected information on deaths that occurred during the period of study.
The researchers paid particular attention to fermented soy products, including natto— soybeans fermented with bacteria Bacillus subtilis— and miso— a soybean product fermented with Aspergillus oryzae fungi. Researchers also analyzed the consumption of unfermented soy, such as tofu (soybean curd), and abura-age (fried tofu), from the participants.
The authors conclude, overall, that higher intakes of miso and natto— fermented soy products — reduce the risk of mortality. Participants with the highest intakes of fermented soy, in particular, had a 10 per cent lower risk relative to those with the lowest intakes of these products.
The risk of cardiovascular mortality also has been significantly reduced. The authors write:
“In this large, prospective study carried out in Japan, with a high soy consumption rate, there was no significant association between total soy products intake and all-cause mortality. In contrast, a higher intake of fermented soy products (natto and miso) was associated with a lower mortality risk.”
According to the analysis, soy products, whether fermented or unfermented, did not affect the mortality risk associated with cancer.
The scientists noted that people who eat more natto often consumed more vegetables, which could help explain why these people were at a lower risk of mortality. Nevertheless, the beneficial effect of natto on mortality risk was still statistically significant when they adjusted for vegetable intake.
Strengths, limitations, and queries
Current research has considerable strengths, including the large sample size and extended follow-up period.
There are however shortcomings. For example, this was an observational study which means the relationship could result from factors not measured by the researchers.
As the authors note, “While substantial mortality decreases have been reported, our results should be viewed with caution.”
There is also room for error because the research focused on self-reported food intake. In addition, the participants provided only dietary information at one point in time, and diets can change significantly over the years.
In short, the study adds to the evidence that fermented soy may have health benefits but far from being conclusive. Scientists are sure to continue researching thanks to the soy’s popularity.
The study was published in conjunction with an editorial written by Kayo Kurotani, Ph.D., and Dr Hidemi Takimoto, both from Tokyo’s National Institutes of Biomedical Innovation, Health and Nutrition.
The editorial writers are wondering whether the reported drop in mortality risk with increased intake of fermented soy may actually be an underestimation.
We clarify that items like miso are often served in dishes with a high salt content. High intake of salt is a risk factor for conditions which increase the risk of mortality.
Because the researchers behind the study did not control their salt intake analysis, the editorial authors wonder if “the association between higher miso intake and lower mortality could be confused and possibly underestimated.”
In other words, people who eat lots of fermented soy are likely to have a high salt intake that increases their mortality risk. The editorial writers doubt whether fermented soy would protect against the adverse effects of salt on the diet. This is a matter which will need further study, of course.