For more than a decade, researchers have been turning out studies to show that more time spent sitting correlates with a greater risk of mortality. Today's research materials are a recent example of the type, the analysis carried out in a sizable study population. The most obvious cause to suggest is that people who spend more time sedentary also spend less time exercising, and it is the amount of exercise that actually matters. There are studies controlling for exercise level that show that sitting time correlates with mortality independently of exercise level, and there are studies that claim the opposite, that it is all about the degree of exercise rather than the time spent sitting. This is business as usual in the field of epidemiology; one can't take any one study at face value. On any question of this nature, a survey of a dozen or more studies is a good idea.
Why would greater time spent sitting raise the risk of mortality even in people who obtain the recommended level of exercise? One possible line of evidence to consider is the use of accelerometers in daily life to show that even light activity in older people, such as casual walking, gardening, and the like, is associated with reduced mortality. It is possible that more is always better, even given a normal exercise schedule. Further, work on establishing a dose-response curve for exercise has indicated that the presently recommended level of exercise is too low to be optimal. Which again might suggest that adding more, even light activity, could improve matters. Another way of looking at it is to consider whether altered metabolic states, both beneficial and harmful, may produce larger effects the longer that they are maintained. Thus lengthy immobility may produce a greater cost to health and shorter periods of immobility broken by up by light activity. Supporting any of these hypotheses with data is, of course, the challenge.
This population-based cohort study included participants aged 35 to 70 years recruited from January 1, 2003, and followed up until August 31, 2021, in 21 high-income, middle-income, and low-income countries with a median follow-up of 11.1 years. Daily sitting time was measured using the International Physical Activity Questionnaire. The measured outcome was a composite of all-cause mortality and major cardiovascular disease (CVD), defined as cardiovascular death, myocardial infarction, stroke, or heart failure.
Of 105,677 participants, 61,925 (58.6%) were women, and the mean age was 50.4 years. During a median follow-up of 11.1 years, 6,233 deaths and 5,696 major cardiovascular events (2,349 myocardial infarctions, 2,966 strokes, 671 heart failure, and 1,792 cardiovascular deaths) were documented. Compared with the reference group (less then 4 hours per day of sitting), higher sitting time (more than 8 hours per day) was associated with an increased risk of the composite outcome (hazard ratio [HR] 1.19), all-cause mortality (HR 1.20), and major CVD (HR 1.21).
When stratified by country income levels, the association of sitting time with the composite outcome was stronger in low-income and lower-middle-income countries (≥8 hours per day: HR 1.29) compared with high-income and upper-middle-income countries (HR 1.08). Compared with those who reported sitting time less than 4 hours per day and high physical activity level, participants who sat for 8 or more hours per day experienced a 17% to 50% higher associated risk of the composite outcome across physical activity levels; and the risk was attenuated along with increased physical activity levels.