Several studies indicate that people who spend a lot of time on sedentary activities, especially watching TV, are more likley to be overweight. This ralation between sedentary behaviour and weight status appears to be rather independent of the amount of physical activity people engage in. Indeed, people who do a lot of physical activities, may also engange in a lot of sedentary behaviour. Sport activities may make you tired and sit down! A study by Saskia te Velde and others published in BMC Public Health suggest that sedentary behaviours and sport activities cluster among teenage boys (see http://www.biomedcentral.com/content/pdf/1471-2458-7-16.pdf).
But what is a more important risk factor for becoming overweight, lack of physical activity or being sedentary? A recent study published in the International Journal of Behavioral Nutrition and Physical Activity (http://www.ijbnpa.org/) shows that 'screen time' (TV, video, computer) at age 15 did predict obesity at age 21, while medium and high intensity physical activity did not. This effect was strongest among girls (http://www.ijbnpa.org/content/pdf/1479-5868-4-26.pdf). This is one of very few longitudinal studies on this topic, and the results suggest that getting adolescents away from their screens is important for obesity prevention.
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