The relationship between seasonal variation of body weight and the degree of obesity was examined in 133 Japanese primary school children living in Tokyo. Body weight was measured monthly for six years (except August) and stature three times a year (in April, September, and January). Time-series analysis of body weight was made by the Program of Census Method IIX-11 in which the original time-series 0 is assumed to consist of three components, i.e., the trend-and-cycle factor C, the seasonal factor S, and the irregular factor I; O=C×S×I. In general, it is said that peaks for weight appear in fall or winter and trough in summer. In this study, 79 out of 111 children whose percentages of standard weight were under 120% at the sixth grade showed weight gain in fall, winter, or spring. 17 children indicated no significant seasonal variation. However, 19 out of 22 children with percentage of standard weight >120% at the sixth grade showed weight gain in summer. Furthermore, in most of obese children at the sixth grade, their percentages of standard weight increased gradually during school life for six years and the peaks of seasonal factor observed in summer were getting more predominant. This suggests that weight gain in summer may be one of risk factors for obesity.
