Comment Re:Correlation? Causation? (Score 1) 341
Several factors could explain the positive association between time spent sitting and higher all-cause death rates. First, time spent sitting might be more easily measured than physical activity and/or may reflect a different aspect of inactivity than other indices usually used in epidemiologic studies. However, this potential misclassification of exposure is unlikely to fully explain our findings, because time spent sitting was significantly associated with mortality even among men and women with the highest levels of physical activity.
Second, time spent sitting might be associated with other unhealthy behaviors that are either not captured or incompletely captured through questionnaires. Total energy expenditure is reduced among individuals who are sedentary. However, consistent with previous studies, the present study found no correlation between physical activity and time spent sitting (r = –0.03). Time spent sitting is also associated with greater food consumption and subsequent weight gain, especially when watching television (16, 34, 35). Time spent sitting was previously shown to be associated with increased weight gain in this cohort (18). While residual confounding by obesity could contribute to the association between sitting time and mortality, this association was attenuated but not eliminated by controlling for or stratifying on body mass index.
Third, prolonged time spent sitting, independent of physical activity, has important metabolic consequences that may influence specific biomarkers (such as triglycerides, high density lipoprotein cholesterol, fasting plasma glucose, resting blood pressure, and leptin) of obesity and cardiovascular and other chronic diseases (8–11). Animal studies have also shown that sedentary time substantially suppresses enzymes centrally involved in lipid metabolism within skeletal muscle, and low levels of daily life activity are sufficient to improve enzyme activity (36–38). Furthermore, substantial evidence in both adults and children from observational studies and randomized clinical trials shows that reducing time spent sitting lowers the risk of obesity and type II diabetes (19, 39–42).