Trendy fermented soy products like miso and natto may reduce the risk of an early death, research suggests.
Few foods have sparked as many health debates as soy.
While some studies suggest the Japanese staple could ward off osteoporosis and tame hot flushes, others link it to breast cancer, thyroid issues and even dementia, Harvard reported.
To better understand its health effects, scientists from the National Cancer Centre in Tokyo looked at more than 92,000 adults, aged 45-to-74.
The participants completed a questionnaire that asked about their diet, lifestyle and overall wellbeing.
Fatalities were recorded via residential registries and death certificates.
Results, published in The BMJ, suggest the women who ate the most fermented soy were 11% less likely to die over the next 14 years than those who consumed the least.
Among men, the odds went down by 10%.
Total soy intake, including non-fermented products like tofu, were not linked to a reduced risk of early death.
“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 scientists wrote.
“In contrast, a higher intake of fermented soy products (natto and miso) was associated with a lower risk of mortality.”
Fermented products are higher in fibre, potassium and “bioactive components” than their non-fermented counterparts, the scientists claim.
Soy fibre has been shown to lower cholesterol and boost weight loss, while fibre from beans is linked to a reduced risk of heart disease.
The result also demonstrate both the men and women who ate natto, soybeans fermented with Bacillus subtilis, were less likely to die from heart disease.
No link was found between fermented soy consumption and cancer deaths.
The results remained true after adjusting for vegetable intake, which was higher among natto consumers.
The scientists stress, however, other factors that influence death risk may be at play.
Writing in a linked editorial, scientists from the National Institute of Health and Nutrition in Tokyo note the questionnaires were completed between 1995 and 1998.
This is before Japan introduced salt-reduced foods, including miso, off the back of the UK’s successful programme.
Miso is traditionally high in salt, with 12.4g per 100g of the seasoning.
Salt has been linked to an early death and was not adjusted for in the study.
The team from the National Institute of Health and Nutrition reference a study that found adults who eat large portions of miso are at no greater risk of high blood pressure than those who consume the least.
This suggests “the fermented soy in miso might protect consumers from adverse effects of a high salt content”, they wrote.
The National Cancer Centre scientists hope future studies will “refine our understanding of the health effects of fermented soy, and perhaps inform the development of healthier and more palatable products”.
For example, tempeh - soybean fermented with Rhizopus - which has unclear effects, the National Institute of Health and Nutrition team added.