A higher intake of fermented soy products, such as tofu, miso and natto, is associated with lower risk of an early death, say researchers.

In Asian countries, especially Japan, several types of soy products are widely consumed, such as natto (soybeans fermented with Bacillus subtilis), miso (soybeans fermented with Aspergillus oryzae), and tofu (soybean curd).

"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," said study researchers from National Cancer Centre in Japan.

"In contrast, a higher intake of fermented soy products (natto and miso) was associated with a lower risk of mortality," they added.

For the findings, published in the journal The BMJ, researchers set out to investigate the association between several types of soy products and death from any cause ("all-cause mortality") and from cancer, total cardiovascular disease (heart disease and cerebrovascular disease), respiratory disease, and injury.

They base their findings on 42,750 men and 50,165 women aged 45-74 years who were taking part in a study based in 11 of Japan's public health centre areas.

According to the study, participants filled 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 said.

The study found that a higher intake of fermented soy (natto and miso) was associated with a significantly lower (10 per cent) 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 research said.

According to the researchers, fermented soy products are richer in fibre, potassium and bioactive components than their non-fermented counterparts, which may help to explain their associations

However, this is an observational study so can't establish cause, and the researchers cannot rule out the possibility that some of the observed risk may be due to other unmeasured factors.