Both fermented (natto, miso) and unfermented (tofu, soymilk) soyfoods have been widely consumed throughout Asia for centuries. It is often stated that fermentation improves the digestibility of soy protein. However, the digestibility of soy protein from unfermented products is already excellent—typically exceeding 90%.1
Fermentation does reduce the phytate content of soy, which is quite high, but it is not clear that this reduction leads to a meaningful increase in mineral absorption in humans.2,3,4 There may also be a decrease in trypsin inhibitor content,5 but the relevance of this change is unclear since in commercially available soyfoods, the amount of trypsin inhibitor is already so low as to be clinically irrelevant.6,7
Finally, fermentation does lead to the creation of antioxidants and peptides, but the effects of these newly created compounds in humans have not been established.8,9
There is little basis for claiming that fermented soyfoods are superior to unfermented ones, and there is certainly no evidence that fermented foods are better able to provide the proposed health benefits of soyfoods than unfermented ones. Both types of soyfoods are good choices for individuals wanting to incorporate more soy into their diet.
Dr. Messina is the co-owner of Nutrition Matters, Inc., a nutrition consulting company, an adjunct associate professor at Loma Linda University, and the Executive Director of the Soy Nutrition Institute. Full bio >>
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2. Zhao Y, Martin BR, Weaver CM. Calcium bioavailability of calcium carbonate fortified soymilk is equivalent to cow’s milk in young women. J Nutr. 2005;135:2379-82.
3. Murray-Kolb LE, Welch R, Theil EC, Beard JL. Women with low iron stores absorb iron from soybeans. Am J Clin Nutr. 2003;77:180-4.
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8. Fan J, Zhang Y, Chang X, Saito M, Li Z. Changes in the radical scavenging activity of bacterial-type douchi, a traditional fermented soybean product, during the primary fermentation process. Biosci Biotechnol Biochem. 2009;73:2749-53.
9. Kim NY, Song EJ, Kwon DY, Kim HP, Heo MY. Antioxidant and antigenotoxic activities of Korean fermented soybean. Food and Chemical Toxicology. 2008;46:1184-9.