Body Mass Index and Diabetes Incidence Across the Adult Lifespan: The Baltimore Longitudinal Study of Aging.

Context: Body composition and glucose metabolism change with aging. Whether different levels of body-mass-index (BMI) are needed to define diabetes risk across the adult lifespan is unknown.

Objective: This work aimed to investigate whether BMI similarly reflects relative fat mass (FM) and diabetes risk across age groups.

Methods: Participants without diabetes from the Baltimore Longitudinal Study of Aging (973 men, 1073 women), stratified by age (<50, 50-59, 60-69, ≥70 years) and categorized by either World Health Organization (WHO)-defined BMI categories (for normal weight, overweight or obesity) or BMI quartiles. The primary exposure was BMI. The primary outcome was diabetes incidence. The relationship of BMI to dual-energy x-ray absorptiometry-derived FM was also investigated in older vs younger participants.

Results: The median (range) follow-up time was 7.1 years (range, 0-29.0 years). Within WHO-defined BMI categories, different age groups demonstrated significantly different FM percentage, FM/lean mass, and waist circumference ( < .05). WHO-defined BMI categories for overweight and obesity were generally related to higher diabetes risk compared to normal weight in all ages except 50 to 59 years. When BMI was categorized by quartiles, diabetes incidence increased dramatically beginning in quartile 2 (23-25 kg/m) in older groups. BMI cutoffs with equivalent diabetes incidence rate as BMI 25 kg/m and 30.0 kg/m in individuals younger than 50 years were 22.7 kg/m and 25.2 kg/m for ages 50 to 59 years; 22.8 kg/m and 25.0 kg/m for ages 60 to 69 years; and 23.2 kg/m and 25.8 kg/m for ages 70 years and older, respectively.

Conclusion: WHO-defined BMI categories do not reflect similar diabetes risk across the lifespan. Diabetes incidence is greater at lower levels of BMI in older adults and may lead to underestimation of diabetes risk with aging, particularly among those traditionally classified as normal-weight individuals.