Changes in the Relationships Between Body Mass Index and Health Outcomes Across Middle Age and Older Adulthood

To examine patterns of the incidence of diabetes, hypertension, and mortality by single units of body mass index (BMI) and 5-year age groups using longitudinal data from middle-aged and older women.

Middle-aged (born between 1946 and 1951; N = 13,715) and older (born between 1921 and 1926; N = 12,432) participants in the Australian Longitudinal Study on Women's Health completed surveys in 1996 and at approximately 3-year intervals thereafter until 2011. Proportions of women with diabetes, hypertension, and mortality over 3-year intervals were estimated for each unit of BMI and 5-year age group (45 to < 50, 50 to < 55, 55 to < 60, 70 to < 75, 75 to < 80, and 80 to < 85 years) using generalized additive modeling with adjustment for time-varying covariates.

Three-year incidence of diabetes (1.2%-3.6%), hypertension (5.2%-17.8%), and death (0.4%-9.5%) increased with age. For both diabetes and hypertension, the associations with BMI were curvilinear in middle-aged women and became almost linear in older women. With increasing age, the slope became steeper, and the increase started at lower BMI values. For hypertension, there was a marked increase in intercept from 75 years onward. In contrast, mortality risks were highest for low BMI (≤ 20) in all age groups. A clear U-shaped curve was observed only in the oldest age group.

The shapes of the relationships between BMI and incidence of diabetes, hypertension, and mortality change with age, suggesting that weight management interventions should be tailored for different age groups.