A new large-scale study carried out in Japan concludes that fermented soy products, as opposed to those with unfermented soy, might reduce mortality risk. However, the study is observational, and there are limitations.
Soy products have been popular in Asia since ancient times and, over recent decades, they have become increasingly popular in Western regions.
With growing interest in nutritional science, researchers are keen to understand whether any of the various forms of soy might impart health benefits.
To date, few studies have investigated whether consuming fermented soy impacts overall mortality, and those that have looked into this topic have generated conflicting results.
For instance, one study concluded that soy intake “may have moderate but beneficial effects on total mortality,” while another found that “Soy products intake was not statistically significantly associated with all-cause mortality.”
Recently, a group of researchers from Japan reopened the debate. They have published their findings in the journal BMJ.
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.”
The scientists had access to data from 11 public health centers in Japan. The information came from 42,750 men and 50,165 women, aged 45–74.
Each participant completed questionnaires about lifestyle, health, and diet. The scientists followed the group of participants for almost 15 years and also collected information about deaths that occurred during the study period.
The researchers paid particular attention to fermented soy products, including natto — soybeans fermented with Bacillus subtilis bacteria — and miso — a product of soybeans fermented with Aspergillus oryzae fungi. They also looked at participants’ intake of unfermented soy, such as tofu (soybean curd), and abura-age (fried tofu).
Overall, the authors conclude that higher intakes of miso and natto — fermented soy products — reduce mortality risk. Specifically, participants with the highest intakes of fermented soy had a 10% lower risk, compared with those who had the lowest intakes of these products.
There was also a significant reduction in cardiovascular mortality risk. The authors write:
“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. In contrast, a higher intake of fermented soy products (natto and miso) was associated with a lower risk of mortality.”
According to the analysis, soy products, whether fermented or unfermented, did not influence the risk of cancer-related mortality.
The scientists noted that individuals who ate more natto also ate more vegetables, which could help explain why these people had a lower mortality risk. However, when they adjusted for vegetable intake, the beneficial effect of natto on mortality risk was still statistically significant.
The current research has significant strengths, not least the large sample size and extended follow-up period.
However, there are shortfalls. For instance, this was an observational study, which means that the relationship could result from factors that the researchers did not measure.
As the authors explain, “Although a significant reduction in mortality was observed, our findings should be interpreted with caution.”
Also, because the study relied on self-reported food intake, there is room for error. Moreover, the participants only provided dietary information at one point in time, and diets can change substantially over the years.
In short, the study adds to the evidence that fermented soy may have health benefits, but it is far from definitive. Thanks to the popularity of soy, scientists are sure to keep investigating.
The study has been published alongside an editorial, which was written by Kayo Kurotani, Ph.D., and Dr. Hidemi Takimoto, both from the National Institutes of Biomedical Innovation, Health and Nutrition, in Tokyo.
The authors of the editorial ask whether the reported drop in mortality risk with increased intake of fermented soy might, in fact, be an underestimation.
They explain that products such as miso are often served in dishes with high levels of salt. High salt intake is a risk factor for conditions that increase mortality risk.
Because the researchers behind the study did not control their analysis for salt intake, the authors of the editorial wonder whether “The association between higher miso intake and lower mortality might be confounded, and possibly underestimated.”
In other words, individuals who eat lots of fermented soy are likely to have a high salt intake that increases their mortality risk. The authors of the editorial ask whether fermented soy might be protecting against the negative impact of salt in the diet. This is a question that will, of course, need further investigation.