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OBJECTIVES In our previous study, we showed that among Japanese diets from different time periods, the 1975 Japanese diet has the greatest health benefits and is the most effective to prevent obesity. In addition, exercise is also effective to reduce obesity. Therefore, we conducted a human clinical trial combining the 1975 Japanese diet and exercise and, as a result, found a reduction in body weight, visceral fat, and serum lipids. However, the mechanism of this phenomenon was not determined. Therefore, in this study, we examined this mechanism in mice using a diet that was similar to that used in the human trial. METHODS The modern and 1975 Japanese diets were cooked, lyophilized, powdered, and fed freely to 5 wk old male C57 BL/6 J mice for 8 wk. In addition, the mice exercised on a treadmill. RESULTS Total white adipose tissue weight decreased significantly due to the interaction between the 1975 Japanese diet and exercise. A histologic examination revealed that the hypertrophy of adipocytes was suppressed. To clarify this mechanism, the mRNA levels for lipid metabolism-related genes in epididymal adipose tissue were measured, and the mRNA level of hormone sensitive lipase (Hsl), which is related to lipolysis, was found to be significantly increased after intake of the 1975 Japanese diet combined with exercise. In the gut microbiota analysis, the Bacteroidetes to Firmicutes ratio, which is decreased in obese people, was increased by the 1975 Japanese diet and exercise. At the genus level, there was an increase in butyrate-producing bacteria as a result of the 1975 Japanese diet intake and exercise. CONCLUSIONS A combination of the 1975 Japanese diet and exercise increased lipolysis in white adipose tissue and increased butyrate-producing bacteria in gut microbiota, and thereby suppressed fat accumulation.