U-shaped relationship between exercise intensity and cardiovascular health

The Million Women Study has many strengths. By its size and length of follow up, it has been able to overcome the notorious methodologic challenges of how to accurately measure not only the frequency of physical activity but also its type, intensity, and duration as well as variations in activity level over time. And, to its credit, the Million Women Study is one of the few large cohort studies to measure housework and to include it as a form of moderate physical activity.

Its findings may be subject to confounding, nonetheless. Such as data showing that the prevalence of current smokers was 25% in the group that exercised every day and in the group that reported no strenuous exercise – considerably higher than the prevalence estimates for women who did strenuous exercise one to six times per week. Even though the results were adjusted for smoking (and other risk factors), the authors acknowledged that residual confounding may have persisted and, thus, may have explained some of the association between physical activity and vascular risk in the most active women.

Further perplexing is the fact that even its sedentary participants were far from inactive. Current guidelines for the level of exercise needed to maintain cardiovascular health call for at least 30 minutes of moderate-intensity aerobic activity at least 5 days per week in bouts of 10 minutes of more moderate or 25 minutes of vigorous aerobic activity at least 3 days per week, or a combination thereof. In total, this amount of activity would equate to approximately 8-12 MET-hrs per week. Thus, it is puzzling that the women in the Million Women Study, even women who reported doing no physical activity at study baseline, still accrued over 15 excess MET-hrs per week (after excluding housework), predominantly through walking and gardening.

Dr. Rachel Huxley, D.Phil., is from the University of Queensland in Herston, Australia. She did not disclose whether she had any financial conflicts of interest. Her remarks were distilled from an editorial (Circulation 2015 Feb. 16 [doi: 10.1161/CIRCULATIONAHA.115.014721] accompanying the research report.