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Effects of birth cohort and age on body composition in a sample of community-based elderly ^1,2,3

¹ From the Sticht Center on Aging, Wake Forest University School of Medicine, Winston-Salem, NC (JD, SBK, and BJN); the Departments of Epidemiology and Medicine, University of Pittsburgh Graduate School of Public Health, Pittsburgh, PA (ABN); the School of Human Movement Studies, University of Queensland, Brisbane, Australia (DRT); the Institute of Health Sciences, Faculty of Earth and Life Sciences, Vrije University, Amsterdam, Netherlands (MV); the Institute for Research on Extramural Medicine, Vrije University Medical Center, Amsterdam, Netherlands (MV); the Department of Biostatistics, University of North Carolina, Chapel Hill, NC (JSL); the Department of Epidemiology and Biostatistics, University of California, San Francisco, CA (MN and SMR); the Department of Preventive Medicine, University of Tennessee Health Science Center, Memphis, TN (FAT); the Department of Aging and Geriatric Research, College of Medicine, Institute on Aging, University of Florida, Gainesville, FL (MP); the Geriatric Research, Education and Clinical Center, Malcom Randall Veterans' Affairs Medical Center, North Florida/South Georgia Veterans Health System, Gainesville, FL (MP); and the Laboratory of Epidemiology, Demography, and Biometry, National Institute on Aging, Bethesda, MD (TBR)

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 dual-energy 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.

Key Words: Birth cohort • age • body composition • elderly • fat • lean

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