Effects of birth cohort and age on body composition in a sample of community-based elderly

Abstract

Background: The effect of the recent obesity epidemic on body composition remains unknown. Furthermore, age-related changes in body composition are still unclear. Objective: The objective was to simultaneously examine the effects of birth cohort and age on body composition. Design: A total of 1786 well-functioning, community-based whites and blacks (52% women and 35% blacks) aged 70-79 y from the Health, Aging, and Body Composition Study underwent dualenergy X-ray absorptiometry annually from 1997 to 2003. Results: At baseline, mean ± SD percentage body fat, fat mass, and lean mass (bone-free) were 28 ± 5%, 24 ± 7 kg, and 56 ± 7 kg, respectively, for men and 39 ± 6%, 28 ± 9 kg, and 40 ± 6 kg for women. Mixed models were used to assess the effects of cohort and age-related changes on body composition. Later cohorts in men had a greater percentage body fat (0.32% per birth year, P < 0.0001) than did earlier cohorts. This cohort effect was due to a greater increase in fat mass than in lean mass (0.45 kg and 0.17 kg/birth year, respectively). With increasing age, percentage body fat in men initially increased and then leveled off. This age-related change was due to an accelerated decrease in lean mass and an initial increase and a later decrease in fat mass. Similar but less extreme effects of cohort and age were observed in women. Conclusions: The combination of effects of both birth cohort and age leads to bigger body size and less lean mass in the elderly. © 2007 American Society for Nutrition.