Evidence for the incompatibility of age-neutral overweight and age-neutral physical activity standards from runners

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Abstract
In contrast with earlier versions, the 1995 Dietary Guidelines for Americans uses the same definition of overweight for both younger and older adults. These guidelines state that prevention of weight gain at any age is achievable by balancing food intake with physical activity. The purposes of our study were to assess 1) whether vigorous exercise prevents weight gain with age, and 2) whether weight maintenance and an age-neutral adult overweight standard are consistent with a constant activity level over time. These hypotheses were tested in a national cross-sectional survey of 4769 and 2150 male runners aged 18-49 y and > 49 y, respectively. Before age 50 y, the rates at which body mass index (BMI; in kg/m2) and waist circumference increased in association with age were the same in shorter-distance (< 16 km/wk) and longer-distance (> 80 km/wk) runners. Regardless of weekly running distance, before age 50 y, BMI increased at (+/-SE) 0.045 +/- 0.006 kg.m-2.y-1 and waist circumference increased at 0.186 +/- 0.014 cm/y. The percentage of runners who were moderately overweight (BMI > or = 25) was 21.5% before age 30 y and 30.1% in those between 45 and 49 y old. Men who ran greater distances were nevertheless leaner because weekly running distance was inversely related to BMI (slope +/- SE: -0.033 +/- 0.002 kg/m2 per km/wk) and waist circumference (-0.083 +/- 0.004 cm.km-1.wk-1). To be consistent, guidelines should either 1) recommend substantial increases in physical activity over time when promoting an age-neutral adult overweight standard, or 2) accept an age-adjusted overweight standard when recommending a constant level of physical activity over time.