The Impact of Excluding Non-Leisure Energy Expenditure on the Relation between Physical Activity and Mortality in Women

The purpose of this study was to examine the relation between physical activity and mortality in a 7-year follow-up of a sample of women more than 30 years of age (N = 6,620) from the Canada Fitness Survey cohort, which was initiated in 1981. Age-adjusted relative risks relating quartiles of average daily energy expenditure (kilocalories per kilogram of body weight per day) to mortality were estimated using logistic regression. Compared with the least active, the risk of all-cause mortality was 0.73 for those in the highest quartile (P for trend = 0.03). The associations were stronger for cardiovascular disease mortality (odds ratio = 0.51; P for trend = 0.01) and fatal myocardial infarction (odds ratio = 0.61; P for trend = 0.04) for those in the highest quartile. These relations were due mainly to the contribution of non-leisure (household chores) energy expenditure, which represented, on average, 82% of women's total activity. The accompanying study on the same cohort by Villeneuve et al reported estimates based on a subset of leisure-time physical activity only, which underestimates the activity of many women (Villeneuve PJ, Morrison HI, Craig CL, Schaubel DE. Physical activity, physical fitness, and risk of dying. Epidemiology 1998;9;632-635). The resulting bias illustrates the importance of including non-leisure energy expenditure in the assessment of total activity. These data support the hypothesis that physical activity is inversely associated with risk of death in women.