Physical activity is incredibly important to our health. Exercise improves almost all aspects of physical and mental health, and people who are physically active are also usually thinner.
But that doesn’t necessarily mean that exercise will make you shed extra fat.
The makers of junk and fast food are sponsors of exercise, from backyard games and pee wee leagues to the World Cup and the Olympic Games, and suggest that in this little imbalance we have between caloric intake and caloric expenditure we shouldn’t lay the blame on eating too much but rather on exercising too little.
And it would have been nice if we could go for a brisk walk and forget about inconvenient eat-less guidance. I, too, would like to believe that. But is it true?
Exercise as a weight-loss tool
A new study in the British Medical Journal (BMJ) looked at 30 randomized controlled trials in which kids were assigned an intervention aimed at promoting exercise. What’s special about this analysis is that the review includes only studies in which physical activity was objectively measured using accelerometry devices (accelerometers measure body movement, and are an unbiased way to sum up activity – they’re much better than asking people how active they’ve been over the day).
The results were disappointing. Kids on exercise regimens added on average only a tiny and negligible amount of active time – about 4 minutes -- to their daily tally compared to controls. These extra 4 minutes would amount to about 15 extra calories burnt. Definitely not enough to justify the sports drink or a fast food meal many kids are rewarded with after they exercise.
How come adding several extra hours of physical activity at school ends up adding close to nothing? Well, one of the explanations the authors offer is that the exercise interventions simply replaced equally intense exercise that would have taken place after school – instead of running in the playground kids were exercising at school. Our natural instinct might be to regulate activity around a set point, and when we expend more than our usual energy we follow that activity burst with inactive periods of rest.
The authors conclude:
“Physical activity interventions have little effect on the overall activity levels of children, which may explain, at least in part, why such interventions have had a limited effect on body mass index or body fat. The outcome of this meta-analysis questions the contribution of physical activity to the prevention of childhood obesity.”
Are we less active than our ancestors?
Kids need plenty of physical activity. Current recommendations are that kids get 60 minutes a day of any activity that makes their heart race and gets them breathing hard – they need to sweat. Only a third of US kids get that much.
On the other hand, while obesity rates have doubled and tripled over the past few decades, physical activity levels have been quite stable over that time period, and studies have shown that kids today move no less than when the obesity rates were much lower. TV was invented a long time ago.
A neat recent study in PLoS ONE further debunks the notion that our sedentary lifestyle is the culprit in the obesity crisis. The study quantified daily energy expenditure (by measuring carbon dioxide production) among the Hazda people of Tanzania, one of the few remaining populations of traditional hunter-gatherers. The researchers found that despite covering many miles a day in search of food, the number of calories that the Hadza burned was pretty much the same as that of typical adults in Europe and the US.
How could that be? The researchers think that the Hadzas’ bodies have adjusted to the higher activity levels required for hunting and gathering by saving energy on other activities. Our bodies spend energy even when we seem not to move much, and their guess is that these ‘background’ activities can be done more efficiently when energy is spent elsewhere.
That doesn't mean that ecercise is a waste of time. Exercise will make you healthier, and is a key habit change that can open the door to healthier eating and other good habits, some of which can also help with weight loss and weight maintenance.
But there’s very little support for the idea that exercising more without eating less can change your weight.
It’s the food; it really is! We’re getting fat because we eat too much and too much of the wrong things. Our food is tempting and sweet and plentiful.
Leading a sedentary lifestyle is another major health issue, one that should be addressed because physical activity is a wonder power for wellbeing.
But addressing physical activity alone will do close to nothing to obesity rates.