Does natto (fermented soybeans) increase the risk of cancer, depending on how it is consumed? No, that's not true: Contrary to a claim circulating on social media, high intakes of fermented soybean foods have been associated with a lower risk of death. Also, no evidence suggests that eating natto in specific ways causes cancer.
The claim appeared in a video (archived here) on TikTok on March 13, 2024. It opened (translated from Japanese to English by Lead Stories staff):
Danger! 240% increased risk of cancer. A dangerous way to eat natto, recommended ways to eat natto that dramatically reduce the risk of cancer.
This is what the post looked like on TikTok at the time of writing:
(Source: TikTok screenshot taken on Thu Mar 28 00:56:43 2024 UTC)
A Japan Public Health Center prospective study on the relationship between intake of soybean foods and fermented soybean foods and mortality risk (archived here) shows that a higher intake of fermented soy foods lowers the overall mortality risk. Although total soybean food intake was not clearly associated with mortality, higher intakes of fermented soybean foods were associated with a lower risk of overall mortality (all-cause mortality) in both men and women.
According to the Japan Natto Association (archived here) the components of natto -- protein, isoflavones, vitamin E, selenium and saponin -- are effective in preventing cancer.
This traditional Japanese food is, in general, considered safe, even though it can cause bloating and could create problems in people allergic to soy (archived here).
A Google Scholar search (archived here) conducted on March 29, 2024, with the following keywords: "natto, 240%, increase, risk, cancer" did not return any result that could corroborate the claim. It highlighted, though, several studies on the potential health benefits of consuming natto.
A review of the nutritional health perspective of natto, for example, published on Biochemistry Research International online on October 21, 2022, (archived here) stated that "natto intake has been linked to health advantages such as a decreased incidence of certain illnesses and a lower risk of death."
