Natto and Viscous Vegetables in a Japanese-Style Breakfast Improved Insulin Sensitivity, Lipid Metabolism and Oxidative Stress in Overweight Subjects with Impaired Glucose Tolerance

This randomized crossover trial enrolled 11 overweight subjects with impaired glucose tolerance and crossed them between a Japanese-style breakfast including natto and viscous vegetables versus a Western-style breakfast, measuring insulin sensitivity (by HOMA-IR), lipid profiles, and oxidative stress markers after each arm.

The Japanese-style breakfast arm produced improvements in insulin sensitivity, modest LDL-C reduction, and reduced urinary 8-isoprostane (a marker of oxidative stress) compared with the Western breakfast comparator. The natto component was credited with contributing the fermented soy-specific bioactives, though it was not isolated from other dietary elements (viscous vegetables also contributed soluble fiber with metabolic effects).

The most fundamental limitation is sample size: n=11 is insufficient to establish any clinical conclusions. Additionally, the intervention bundles natto with other dietary changes (viscous vegetables, lower saturated fat from Western foods), making it impossible to attribute the observed effects specifically to natto. The outcomes are biomarker endpoints, not cardiovascular events.

There is no large randomized trial evidence establishing that natto supplementation prevents cardiovascular events, reduces atherosclerosis progression, or provides outcomes benefits beyond what can be achieved with established therapies.

We rate the evidence limited. An extremely small crossover study (n=11) suggesting metabolic benefits from a Japanese-style breakfast containing natto — insufficiently powered and methodologically confounded to support any clinical recommendations about natto specifically.