Objectives: Although the school lunch program is expected to reduce obesity and underweight among children in Japan, there had been no individual-level study examining the impact of school lunch on body size (overweight or underweight). The present study examined the association between school lunch and body size in Japanese junior high school students. Methods: This cross-sectional study was conducted based on data from the Japanese National Health and Nutrition Survey in 2014 and 2018. The present analysis included 323 individuals (12–15 years old). The exposure factor was school lunch usage. The primary outcome measure was body size (normal weight, overweight/obesity (including both overweight and obesity), and underweight). Results: Of 323 individuals, the proportion of school lunch users was 65.6%. School lunch was not statistically significantly associated with normal weight; the multivariate-adjusted odds ratio (95% confidence interval) of normal weight in school lunch users was 1.07 (0.66–1.75) in comparison with non-users. No significant associations were found for overweight/obesity or underweight outcomes. Conclusions: The present findings did not support the expectation for the Japanese school lunch program.

Background: Previous studies have shown that Japanese dietary patterns are associated with high nutrient density. However, these studies were limited to the Japanese population. We examined this association in the US population. Methods: A cross-sectional analysis was conducted using data from the National Health and Nutrition Examination Survey (NHANES) 2017–2018. We included 3138 people aged 20–79 years. Food and nutrient intake data were based on the 24 h recall method. Three Japanese diet indices were used: (1) Japanese Diet Index (JDI, based on 9 food items), (2) modified JDI (mJDI, based on 12 food items), and (3) weighted JDI (wJDI, selected and weighted from mJDI food items). The nutrient density (ND) score was calculated based on the Nutrient-Rich Food Index 9.3. Spearman’s rank correlation coefficients were calculated. Results: The correlation coefficients with the ND score were 0.24 (p < 0.001) for the JDI and 0.38 (p < 0.001) for the mJDI. The correlation coefficient between the wJDI and ND score was 0.48 (p < 0.001). The three Japanese diet indices were correlated with the ND score in all racial groups (p < 0.001). Conclusions: Even among the US population, higher degrees of Japanese diet defined by the JDI or mJDI were associated with higher nutrient density.