Associations of aging and birth cohort with body mass index in a biethnic cohort

Objective: To examine associations of aging and birth cohort with body mass index (BMI) in a biethnic cohort. Research Methods and Procedures: This was a longitudinal closed cohort study of 14,500 white and African-American men and women, 45 to 64 years of age, followed for 9 years. Aging was defined as the length of the interval in years between baseline and following visits. Birth cohort was defined by the year in which participants were born. Mixed model analyses were used to examine associations of aging, birth cohort, and BMI in four ethnicity-gender groups. Results: We found that aging was associated with an increase in BMI in white and African-American men and women. The associations between aging and BMI were stronger in the younger birth cohorts. Except for white women, younger birth cohort was associated with a higher BMI. After adjusting for aging, birth cohort was associated with an increase in BMI of 0.1 kg/m² [95% confidence interval (95% CI): -0.1, 0.3] among white women. The corresponding values for African-American women, white men, and African-American men are 0.5 kg/m² (95% CI: 0.1, 0.9), 0.6 kg/m ² (95% CI: 0.4, 0.8), and 0.6 kg/m² (95% CI: 0.2, 1.0), respectively. Discussion: Our analyses show that, in all except white women, people in this age range who were born later have a higher BMI at the same attained age. In all groups, people who are born later gained more weight as they aged. In general, subjects ages 45 to 64 years gained weight as they aged 9 years.