To the Editor In their Original Investigation published in a recent issue of JAMA Internal Medicine, O’Donovan et al1 claim that “weekend warrior” and other leisure time physical activity patterns characterized by 1 or 2 sessions per week may be sufficient to reduce all-cause, cardiovascular disease, and cancer mortality risks. We notice that the authors may have overlooked a major confounder in this study: the geographical variations within the sample. Since the analyzed data covered various geographical areas in England and Scotland, we expect the geographical variations within the sample would be an important factor affecting mortality rate, independent of the physical activity level of the participants. Such a claim can be justified by the fact that the mortality rate for respiratory cancers were higher in northern Italy than southern Italy2 due to different levels of exposure to air pollution. The survival rate for cancers was also different across countries, due to different availability of effective treatments.3 In this connection, geographical areas should be an important confounder that needs to be adjusted in the analyses.
Lam MHS, Lee PH, Lee KY. Is Weekend-Only Physical Activity Enough to Compensate for a Sedentary Lifestyle? JAMA Intern Med. 2017;177(8):1224. doi:10.1001/jamainternmed.2017.2380
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