Why giving children more opportunity to exercise is unlikely to put a dent in rates of obesity

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I can across this report today. It’s based on a study which finds that British children are generally most active between 5.00 and 8.00 pm. Between late March to late October, here in the UK we run on British Summer Time (Greenwich Mean Time minus 1 hour). From late October to late March we revert GMT. The suggestion is if we ran on British Summer Time all year around, it would be lighter at the end of the day, and kids would be more active, and they’d be slimmer too.

However, as is often the case, things are not as straightforward as they seem. I know it makes sense that being more active will help with weight control by burning more calories, but study after study has shown that aerobic exercise has little or no bearing on weight in the long term. I’ve written about this a few times.

The natural reaction some have when faced with the evidence is to deny it and believe it must somehow be flawed. One observation many will cite to ‘prove’ the research is wrong is to point to, say, elite marathon runners as an example of what happens as a result of regular distance running – you end up looking like a marathon runner (slight in stature and very lean).

Here’s the problem though – just because leanness and regular distance running go hand-in-hand, does not mean running causes leanness. Could it be the other way round? Could it be, in other words, that people who are naturally lean are more likely to end up as marathon runners?

I anecdotally know many who have lost little or no weight clocking up the massive mileages while in training for a marathon.

If we’re at least a bit open to the idea that leanness may ‘cause’ marathon running or greater levels of activity, then we should also be open to the fact that being obese might cause sedentary behaviour (rather than the other way round). So, when we witness an obese child slouched in front of the TV or X-box, it’s easy to conclude that such sedentary endeavours are contributing to their weight issue (and that all would be well if they spent more time running around). But, again, could it be that the weight problem caused the sedentary behaviour?

Far-fetched though this may sound to some, there is some evidence for it in the form of a study [1] which was published on-line last year and I reported on here. In this research, the relationship between physical activity and body fatness in children were assessed over a 3-year period. Basically, the most sedentary children were most likely, also, to be carrying excess body fat. Following children over time allowed the researchers to ascertain that lower levels of activity did not lead to increased body fatness. In fact, it was the other way round: children appeared to accumulate fat first, and then this was followed by them becoming more sedentary.
The authors note that this finding “may explain why attempts to tackle childhood obesity by promoting PA (physical activity) have been largely unsuccessful.”

I’m a fan of exercise and rate it highly as a lifestyle factor for enhanced health and wellbeing. However, the idea that extending daylight hours will help tackle childhood obesity is not rooted in science. A better strategy, I believe, would be for kids to consume less foods with added sugar as well as relatively crappy starchy carbohydrates like bread, rice, pasta and breakfast cereals.


1. Metcalf BS, et al. Fatness leads to inactivity, but inactivity does not lead to fatness: a longitudinal study in children (EarlyBird 45). Arch Dis Chil 2011;96(10):942-7

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Walking versus running

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