The mean respondent age was 16.6 years. About 51.5%, 26.4%, and 22.1% of the sample were white, black, and Hispanic, respectively. Forty-percent of the girls had mothers who were overweight or obese; 46.9% had mothers who attended college, 32.8% of their mothers graduated from high-school and 20.2% of their mothers were high-school dropouts. The mean BMI of the girls was 23.4, with 18% being overweight and 12.2% being obese. About half of the girls were born before the admission cut-off-dates. Overall, 10% of girls with early birthdates and 3.4% with late birthdates entered school later than expected. Late starters had higher BMIs and a higher prevalence of overweight and obesity; the results were consistent across age groups ().
Body weight status of adolescent girls by age group, birth dates, and admission status
Model A in shows that one additional year of schooling was correlated with a reduced BMI, meaning adolescent girls who had more schooling were less likely to be obese (OR = 0.862, p < 0.001).
Results of linear models for the association between body weight status and early school status among adolescents aged 12–19 years in the National Longitudinal Survey of Youth 1997 (NLSY97)*
Model B in demonstrates the effect of school entry on body weight status. For BMI and BMI z-score, all except one coefficient of an early birth date were significantly negative, indicating that one additional school year was associated with reduced BMIs. Among girls with birthdays within one-month of the cutoff-dates, the coefficient of late starting was significantly positive (β = 0.311; p = 0.02), indicating that late starting may be correlated with weight gain in adolescence. The interactive term was significantly negative in the sample within one month of the cut-off dates, indicating that late starters with early birthdays were slimmer than those with later birthdays, possibly due to the additional one year of schooling. As expected, these terms became insignificant within 3 and 6 months of the cutoff-dates.
For overweight and obesity, all ORs of early birth date were below 1, indicating that one additional school year for girls decreased the risk of overweight and obesity in adolescence.