There is a surprising amount of controversy about the ability of physical activity to prevent the development of obesity. Sure, obese individuals tend to perform less physical activity than their lean counterparts, but that doesn't prove causation. And almost every week it seems that there is a news story reporting that the obesity epidemic is caused by diet. Period. If you believe these articles, physical activity plays a minor role, if any role at all. Some have even (erroneously) suggested that physical activity increases the risk of weight gain (for a thorough debunking of a recent TIME article on this subject, click here).
One of the problems of trying to untangle the role of physical activity in the development of obesity is that most studies use indirect measures of physical activity, like self-report questionnaires. Not surprisingly, there is a lot of error when people are reporting a socially-desirable behaviour like physical activity, as they tend to err on the positive side. And questionnaires also often give several fixed options, for example "Are you normally active for 15, 30, 45, or 60 minutes per day?". If you are active for 20 minutes per day, would you pick 15 or 30? Either way, it introduces a lot of error, which makes it very difficult to determine the specific role that your current physical activity levels play in the development of obesity down the road.
All of this brings me to a very interesting paper that has just been published in the British Medical Journal, which is available for free on the BMJ website. Author Chris Riddoch and colleagues asked 7159 12-year-old children to wear accelerometers for a full week. Accelerometers measure movement, and allow for the direct measurement of both the volume (minutes, hours, etc) and intensity (light, moderate, vigorous) of physical activity. This is the gold-standard for measuring physical activity, and a tremendous improvement over self-report questionnaires. They then had all of these participants come back to the lab at age 14, where they had their body fat mass directly measured using Dual Energy X-Ray Absorptiometry (also a gold-standard).
What did they find? The more physically active kids were at age 12, the lower their fat mass at age 14. Interestingly, these results were independent of fat mass at age 12. In other words, no matter how much fat mass a child had at age 12, if they were more physically active, they had less fat at age 14. In fact, for every 15-minute increase in daily moderate-to-vigorous physical activity at age 12, there was a 10% reduction in fat mass at age 14. Further, the authors also estimate that 12 year old children "who meet current health related recommendations of 60 minutes of moderate-vigorous physical activity a day would be expected to have around 4.3 kg less fat mass at age 14 than children who do no moderate-vigorous physical activity" [emphasis added]. That's almost 10 lbs of body fat in just 2 years!!! As the authors point out, this type of strong relationship is not likely to play a trivial role in the development of obesity or chronic disease.
So, what is the take-home message? This well-designed study, with a HUGE sample size, clearly suggests that kids who are more active today have less fat mass tomorrow. That's certainly not a huge surprise, but it's an important finding, and one that directly counters the argument that physical activity is useless for obesity prevention. It also suggests that previous studies might not have seen any relationship between physical activity and obesity because their measures just weren't very sensitive. Traditional measures like body weight and self-reported physical activity are far less precise than those used in this study, and it makes sense that when we reduce the amount of error involved, we might get a clearer picture of what is truly going on (Not surprisingly, when the authors substituted body weight instead of directly measured fat mass in the present analysis, the relationships between physical activity and obesity diminished dramatically).
Even if you still don't buy that physical activity could reduce your risk (or your child's risk) of gaining excess fat mass down the road, keep in mind that irrespective of body weight, increased physical activity is associated with dramatically lower risk of just about every chronic disease there is. Just a few more reasons to work a bit of physical activity into your day!
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Riddoch, C., Leary, S., Ness, A., Blair, S., Deere, K., Mattocks, C., Griffiths, A., Davey Smith, G., & Tilling, K. (2009). Prospective associations between objective measures of physical activity and fat mass in 12-14 year old children: the Avon Longitudinal Study of Parents and Children (ALSPAC) BMJ, 339 (nov26 2) DOI: 10.1136/bmj.b4544
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