In brief
A cohort study published in The BMJ (2020) described that in the Japanese population a higher intake of fermented soy (including natto and miso) was associated with a lower risk of overall mortality. Importantly, no similar relationship was observed for non-fermented soy. The study describes a correlation, not a cause and effect relationship. This article is educational in nature and does not make any claims about treating or preventing disease.
The key conclusions from the study
- The study covered more than 90,000 people in Japan, observed for an average of about 15 years.
- In the group with the highest intake of fermented soy, the risk of death was about 10% lower.
- For non-fermented soy (tofu) no significant relationship was recorded.
- The authors stress that this is an observational study, which indicates a correlation and does not prove causation.
What sets fermented soy apart
The fermentation of soy, carried out among others by Bacillus subtilis or Aspergillus oryzae, changes the profile of the product: it improves protein digestibility and increases the content of bioactive compounds such as nattokinase, polyamines and isoflavones. This describes the composition and the process, not a declaration of a health effect. None of these compounds (apart from vitamin K) has an authorised health claim in the EU.
How the study was conducted
The cohort study covered more than 90,000 Japanese people aged 45 to 74. The observation period averaged about 15 years. The data came from the resident register and death certificates, which ensured high reliability of the analysis.
| Type of soy | Examples | Observation in the study |
|---|---|---|
| Fermented | natto, miso | Lower overall mortality in the group with the highest intake |
| Non-fermented | tofu | No significant relationship |
| Soy in general | various forms | No significant association with overall mortality |
Numerical results
In people with the highest intake of fermented soy:
- men: HR = 0.90 (0.83 to 0.97)
- women: HR = 0.89 (0.80 to 0.98)
The result was statistically significant (Ptrend < 0.05). The authors note that the observation concerns the Japanese population and its dietary habits, so it should not be generalised directly to other populations nor interpreted as proof of the action of a specific component.
How to interpret such results
Observational studies show the co-occurrence of phenomena, but they do not settle the cause. The lower mortality in populations that consume a lot of fermented soy is probably influenced by the whole lifestyle, not a single product. That is why results of this type are worth treating as a premise for further research, not as confirmation of the health properties of natto or miso.
How to include fermented soy in the diet
Fermented soy can be introduced into the menu in various ways. Natto is best eaten raw, with rice or vegetables. Miso is a base for soups and sauces. Tempeh works well in main dishes. A few portions a week are enough for fermented soy to become part of a varied diet.
Is tofu also valuable
Tofu remains a good source of plant protein and calcium. It is not a fermented product, so it does not contain the enzymes and bacterial cultures present in natto or miso. It can complement the diet, but it has a different profile from fermented products.
Conclusion
The 2020 study describes a correlation between the intake of fermented soy and lower overall mortality in the Japanese population. It does not prove a causal relationship and does not justify attributing specific health properties to natto or miso. It remains a valuable starting point for people interested in traditional soy products.
Scientific source
Katagiri R. et al. (2020). Association of soy and fermented soy product intake with total and cause specific mortality: prospective cohort study. The BMJ, 29 January 2020.
https://www.ncbi.nlm.nih.gov/pmc/articles/PMC7190045/
FAQ
Does fermented soy extend life?
The study describes a correlation between its intake and lower overall mortality in the Japanese population, but it does not confirm a causal relationship.
Can natto be replaced with tofu?
Tofu is not fermented and has a different component profile from natto. They are different products.
How often is it worth eating natto or miso?
A few times a week, as part of a varied diet. This is a matter of individual preference.
Does miso contain a lot of salt?
Yes, miso contains salt. People limiting sodium should take this into account.
Do fermented products contain live bacterial cultures?
Yes, natto and miso contain bacterial cultures from the fermentation process.
This article was prepared by the Natto.pl team. It is educational in nature and does not replace medical consultation. Natto is a food, not a medicine.
