Low physical activity and mortality in women: Baseline lifestyle and health as alternative explanations

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Abstract
Aims: The aim of this study was to investigate the association between physical activity and mortality in post-menopausal women. In addition, the authors wanted to investigate to what extent this association could be attributed to confounding from other lifestyle factors, and to reverse causation due to a number of common health problems that may inhibit the ability to exercise. Methods: A total of 27,734 women aged 51—83 years from the Swedish Mammography Cohort were investigated. In 1997 they responded to a detailed questionnaire including questions on physical activity, diet, alcohol intake, smoking, and medical problems. During follow-up in 1999—2004, 1,232 deaths were identified by linkage to the National Population Register. Results: Women with low physical activity (≤35 MET*h/day) had a 3.22 times increased mortality (95% confidence interval (CI)=2.35—4.43) compared with the most active women (>50 MET*h/day). No increased risk was seen in women with moderate compared with high physical activity. Sedentary women tended to have a less healthy lifestyle and more health problems at baseline, e.g. almost 30% of them reported high blood pressure compared with less than 20% of active women. Baseline medical problems and lifestyle factors such as smoking, diet, and education accounted for 30% of the excess risk seen in sedentary women (24% and 6% respectively). Conclusions: This study indicates that even fairly small amounts of activity will reduce mortality in older women. However, sedentary women seemed to be a selected group with more medical problems and a less healthy lifestyle. The findings indicate that the association between physical inactivity and mortality will be overestimated if this is not taken into account.