BackgroundThere are few longitudinal studies investigating the association between changes in body weight and the development of prediabetes or diabetes, and the extent to which the association is attenuated by changes in health behaviors and demographic characteristics has not been well-characterized. The purpose of the present study is to examine the association between changes in body mass index (BMI) and the development of prediabetes defined by measures of impaired fasting glucose (IFG) and impaired glucose tolerance (IGT) over 8 years in an existing longitudinal cohort study of older adults aged 60 years and older. Methods Participants enrolled in the Rancho Bernard Study, a longitudinal cohort study, who took an oral glucose tolerance test at the 4th measurement visit (between 1984-1987) and at the 7th measurement visit (between 1992-1996) and had normal glucose tolerance (NGT) at the 4th measurement visit were included in the study (n = 532). IFG and IGT were determined using WHO criteria. Weight and height were measured by a trained investigator, with participants wearing light clothing and no shoes, and BMI was calculated as weight (kilograms) divided by square of the height (meters). Health behaviors (i.e., smoking status, alcohol consumption, and exercise engagement) and demographic characteristics were self-reported to a trained investigator at both timepoints. We evaluated the association between changes in BMI and the development of IFG and IGT using Poisson regression. Results At the 4th measurement visit, participants had a mean age of 64.7 years, 58.8% were female, and mean BMI was 24.5 kg/m2. Over 8 years, 173 of 532 NGT participants (32.5%) developed impaired glucose metabolism. The burden of prediabetes increased significantly by approximately 12% (95% confidence interval (CI) 1.04 to 1.21) for each unit increase of BMI. The corresponding prevalence ratio increased after adjustment for changes in demographics (1.15, 95% CI 1.07 to 1.25) and health behaviors (1.17, 95% CI 1.07 to 1.26). Conclusion Among a population of older adults, increases in body weight were strongly associated with the development of prediabetes, replicating the findings of prior research. Furthermore, the association was not attenuated by changes in health behaviors and demographics, suggesting that progression from NGT to prediabetes may be largely driven by pathophysiological complications attributable to increased adiposity.