by Paul T. Williams

Current physical activity recommendations assume that different activities can be exchanged to produce the same weight-control benefits so long as total energy expended remains the same (exchangeability premise). To this end, they recommend calculating energy expenditure as the product of the time spent performing each activity and the activity's metabolic equivalents (MET), which may be summed to achieve target levels. The validity of the exchangeability premise was assessed using data from the National Runners' Health Study.

Physical activity dose was compared to body mass index (BMI) and body circumferences in 33,374 runners who reported usual distance run and pace, and usual times spent running and other exercises per week. MET hours per day (METhr/d) from running was computed from: a) time and intensity, and b) reported distance run (1.02 MET•hours per km).

When computed from time and intensity, the declines (slope±SE) per METhr/d were significantly greater (P<10^-15) for running than non-running exercise for BMI (slopes±SE, male: -0.12±0.00 vs. 0.00±0.00; female: -0.12±0.00 vs. -0.01±0.01 kg/m² per METhr/d) and waist circumference (male: -0.28±0.01 vs. -0.07±0.01; female: -0. 31±0.01 vs. -0.05±0.01 cm per METhr/d). Reported METhr/d of running was 38% to 43% greater when calculated from time and intensity than distance. Moreover, the declines per METhr/d run were significantly greater when estimated from reported distance for BMI (males: -0.29±0.01; females: -0.27±0.01 kg/m² per METhr/d) and waist circumference (males: -0.67±0.02; females: -0.69±0.02 cm per METhr/d) than when computed from time and intensity (cited above).

The exchangeability premise was not supported for running vs. non-running exercise. Moreover, distance-based running prescriptions may provide better weight control than time-based prescriptions for running or other activities. Additional longitudinal studies and randomized clinical trials are required to verify these results prospectively.