Introduction The aim of the present study was to investigate the relationship of overweight to chewing thoroughly and eating until full and to examine the combined effect of chewing thoroughly and eating until full on overweight among schoolchildren in Japan.

Methods Subjects included all fourth-grade schoolchildren (9 or 10 years of age) in Ina-town, Saitama prefecture, Japan, during 1999–2009. Information about subject's sex, age, and lifestyle, including chewing thoroughly and eating until full, was collected using a self-administered questionnaire. Measurements of height and weight were made for each child. Overweight in children was defined according to the criteria of the International Obesity Task Force. To calculate the OR and 95% CI for overweight, a logistic regression model was used.

Results Data from 4027 children were analysed. Chewing thoroughly revealed significantly decreased OR for overweight when compared to not chewing thoroughly (OR: 0.39, 95% CI 0.33 to 0.48). Eating until full showed significantly increased OR for overweight compared with not eating until full (1.24, 1.02 to 1.51). Among children who reported chewing thoroughly, OR of eating until full was not statistically significant (0.97, 0.76 to 1.24). On the other hand, eating until full illustrated significantly increased OR among the not chewing thoroughly group (1.67, 1.21 to 2.30).

Conclusion Chewing thoroughly and eating until full were associated with overweight. Furthermore, a combined effect of not chewing thoroughly and eating until full on overweight was noted. This study suggests that chewing thoroughly is useful for the prevention of childhood overweight.