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!
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
I was wondering what your policy on republication is? I am interested in republishing this article on psnews.com.au - we are an online site for Australian Public Servants. We have over 50,000 subscribers who receive our publication each week.
Would it be ok to republish? We would attribute the article to yourself and Peter and provide a link to scienceblogs.
Please let me know by sending me an email: email@example.com as soon as practicable as our next edition goes online in a few days.
What were the gender breakdowns? I'm just thinking that 12-14 is the age when a lot of girls start to develop "womanly fat"--i.e., hips, breasts. And, also, it's the age when many girls are likely to start decreasing their physical activity (for various reasons, including peer pressure, societal expectations, and bodily changes that can be awkward and/or embarrassing). At the same time, it's an age when boys start to be able to develop more muscle mass (b/c they have more testosterone), and are also more likely to be physically active (again, for peer pressure and societal reasons).
Basically, I'm wondering how much of the extra fat mass for the less-physically-active kids can be attributed to those kids being female. If they're disproportionately female, then that extra fat mass might not necessarily indicate anything about their lifestyle--simply that they've started to develop a womanly shape.
I'm always surprised how little physical activity the average kid engages in nowadays. When I was in elementary school (early 1960's) we had two recess periods, one of which lasted 40 minutes. On the school field we often played serious team sports. We expended a lot of energy; this encouraged social interaction and made learning easier. After school, we often played street baseball for two hours (there was less traffic in those days). Obesity was virtually non-existent.
Good question, Kate. In the study they did their actual analyses for each gender separately (they were pretty similar, so I reported them together in the post). Here is the info for each gender, taken from the study abstract:
An extra 15 minutes of moderate-vigorous physical activity per day at age 12 was associated with lower fat mass at age 14 in boys (by 11.9% (95% confidence interval 9.5% to 14.3%)) and girls (by 9.8% (6.7% to 12.8%)).
I heard about a study on the news this morning that said that adults with a healthy BMI need to exercise at least an hour a day to maintain their weight long-term. Because this was on a basic news show, I don't fully trust their interpretation of the data, but nor could I follow links to investigate it. Have you heard of this study, and can you explain what it really means?
I'm not aware of it off the top of my head, but I'll keep my eyes open - it sounds like a nice topic for a future post. If you come across any news stories about it online, please pass it along.
I think bananacat is talking about the recent JAMA article (JAMA. 2010;303(12):1173-1179.) The abstract is at: http://jama.ama-assn.org/cgi/content/short/303/12/1173
Here's the text from a press release at Physicians First Watch:
An Hour of Daily Exercise Helps Ward Off Weight Gain in Middle-Aged Women
Patients may ask about a widely reported JAMA study suggesting that 1 hour of moderate exercise daily is needed to prevent weight gain in middle-aged women.
Roughly 34,000 women (mean age at enrollment, 54) in the Women's Health Study answered questionnaires about their weight and exercise habits at baseline and periodically over 13 years' follow-up.
After adjusting for reported diet at baseline, researchers found that over any 3-year period, women who exercised <150 minutes weekly or 150 to <420 minutes weekly were, respectively, 11% and 7% more likely to gain 2.3 kg (5 lb.), compared with women who exercised â¥420 minutes a week.
The authors point out that the association was observed only among women with a body-mass index less than 25, noting that "among heavier women, there was no relation, emphasizing the importance of controlling caloric intake for weight maintenance in this group."
Found a new JAMA article:
Lee I-M, DjoussÃ© L, Sesso HD, Wang L, Buring JE. Physical Activity and Weight Gain Prevention JAMA. 2010;303(12):1173-1179.
Subscription required, so this link may not work.
A prospective cohort study involving 34,079 healthy US women (mean age, 54.2 years) from 1992-2007.
Conclusions: "Among women consuming a usual diet, physical activity was associated with less weight gain only among women whose BMI was lower than 25. Women successful in maintaining normal weight and gaining fewer than 2.3 kg over 13 years averaged approximately 60 minutes a day of moderate-intensity activity throughout the study."
Terrific, thanks Todd and Nora!
I'm suddenly much more happy that my parents forced me to play on a sports team during most of the year as a child even though I didn't really want to... And it makes me curious just to what extent I owe that for the shape I'm in today.