Maintaining a Healthy BMI: Data From a 16-Year Study of Young Australian Women.

INTRODUCTION: The aims of this prospective cohort study were to examine 16-year trajectories of weight and BMI in young adult women who had a healthy BMI in 1996 and determinants of remaining in the healthy BMI category.

METHODS: A total of 4,881 women with healthy BMI at baseline and either healthy, overweight, or obese BMI at 16-year follow-up reported their weight, height, health, and health behaviors in six surveys of the Australian Longitudinal Study on Women's Health between 1996 (aged 18-23 years) and 2012 (aged 34-39 years). Determinants of BMI maintenance were estimated using binary logistic regression and generalized estimating equations in 2015.

RESULTS: Almost 60% remained in the healthy BMI category from 1996 to 2012, (mean weight gain, 0.19 kg/year), 29% transitioned to overweight BMI (0.83 kg/year), and 11.6% transitioned to obese (1.73 kg/year). The mean rates of annual weight gain in each group were consistent over time. Only three factors (low alcohol, moderate/high physical activity, having a university degree) were positively associated with maintaining a healthy BMI. Additional behavioral factors (smoking, high sitting time, energy intake, dieting, takeaway food, and use of oral contraceptives), as well as blue collar occupation, separation/divorce/widowhood, and major illness were negatively associated with BMI maintenance.

CONCLUSIONS: To prevent the transition from healthy to overweight/obese BMI, weight gain must be limited to <0.5 kg/year. Women with healthy BMI, but with higher rates of weight gain in their early 20s, could be identified by health professionals for assistance with prevention of becoming overweight/obese.