Michael O’Riordan

July 29, 2014

LONDON, UK — Individuals who are physically active and don’t spend a lot of time sitting down are less likely to be obese than those who report high levels of leisure-time sitting and low levels of physical activity, and the combination of the two behaviors is important, according to a new study[1].

“The present findings, based on longitudinal comparisons of separate and combined associations of physical activity and leisure-time sitting, add to the literature by suggesting that the combination of high physical activity and low leisure-time sitting is a stronger protective factor against becoming obese than either behavior on its own,” report Dr Joshua Bell (University College London, UK) and colleagues.

Published July 28, 2014 in Diabetologia, the study included 3670 participants from the Whitehall II cohort. As the investigators explain in the paper, physical activity and sedentariness are two separate but related lifestyle factors linked with obesity and metabolic health, as well as with type 2 diabetes, cardiovascular disease, and all-cause mortality. Individuals who are physically inactive often spend more time being sedentary, but few studies have examined the combined effects of these two behaviors on obesity.

Among those who reported high levels of physical activity—defined as more than 4.25 hours per week—the rate of incident obesity was 4.8% compared with 8.0% among those who reported low levels of activity after five years. At 10 years, the rate of obesity was 6.9% and 11.9% in the high and low physically active groups. Compared with low levels of physical activity, high levels of exercise were associated with a 36% reduction in the risk of obesity at five years and 37% in risk at 10 years.

In contrast, leisure-time sitting was not associated with a risk of obesity at five or 10 years.

When the two behaviors were studied together, however, the combination of high physical activity and low leisure-time sitting—defined as spending between zero and 11.5 hours sitting down—was associated with a significant 74% lower risk of obesity at five years compared with those who exercised the least and sat the most. At 10 years, the most active/least sedentary had a significant 49% lower risk of obesity compared with the least active/most sedentary.

“The mechanisms underlying this interaction are unclear,” write Bell et al. “In principle, lower levels of leisure-time sitting may strengthen protective effects of higher physical activity, either through independent physiological mechanisms or as a marker for greater engagement in low-intensity activity, such as standing.”

The researchers did not observe a combined effect of physical activity and leisure-time sitting on the development of metabolic risk factor clustering, which was surprising, “given known dose-response associations of moderate to vigorous physical activity” on the metabolic risk factors.

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