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Why Exercise Doesn't Actually Help You Lose Weight

With news stories coming out this week about mandatory BMI screenings and Disney's new emphasis on nutrition in their programming, obesity is yet again making headlines.

And that means the blatantly unworkable solutions to weight gain — eating less and exercising more — have also made an appearance in your favorite news source. Losing weight simply by trying to eat less is problematic for many reasons, and we've covered it here extensively. But isn't exercise a foolproof way to slim down? Maybe not.

Though exercise is considered a necessity for anybody looking to lose weight, a surprising amount of research shows that exercise may do little to help overweight people shed excess weight, and may even make them eat more.

In a recent lecture about exercise, science writer Gary Taubes put the issue to the audience this way: if you were going to a big dinner party and the host told you to come hungry, how would ensure you showed up ready to eat? The audience responded, as most people would, by suggesting that they'd fast for most of the day and engage in some form of exercise, a long walk perhaps.

The point, says Taubes, is that exercise makes us hungry, which leads us to replace the calories burned by exercising and encourages weight gain. It's a clever hypothetical and many readers would probably treat it as no more. But the data backs it up.

At least four clinical trials have demonstrated that exercise tends to suppress resting metabolic rate. In all four studies overweight participants who engaged in 300-600 calories worth of daily exercise experienced a significant drop in resting metabolism. According to Drs. Jeff Volek and Stephen Phinney, “Although genetically lean people as a group may respond differently, when overweight humans do more than one hour of endurance exercise daily, resting metabolism on average declines between 5% and 15%.” 

Commenting on the pair's findings, Dr. John Briffa points out that this down-regulating of the metabolism is probably the effect Taubes is describing. Much like what happens when caloric intake is severely restricted, "The idea that the body would down-regulate the metabolism in response to exercise makes ... intuitive sense," says Briffa. "It’s not too difficult to imagine that the body would have a similar response to increased calorie expenditure ...."

Critics of this argument would likely cite any number of studies which have reached the opposite conclusion. But as the American College of Sports Medicine explains, the best that can be said about the relationship between weight loss and exercise is that "it is reasonable to assume that persons with relatively high daily energy expenditures would be less likely to gain weight over time, compared with those who have low energy expenditures. So far, data to support this hypothesis are not particularly compelling.”

But if this is the case, why do we assume that exercise is key to weight loss? Volek and Phinney argue that it's a matter of mixing up cause and effect.

Thin people exercise a lot compared to overweight people, and assume that they're slim because of all the exercise. Meanwhile, overweight people tend exercise much less, and we all assume that explains their bigger waistlines.

The truth, very likely due to genetics, is that the body composition of both groups explains the exercise habits, not the other way around. 

Diet also plays a role. According to researchers at the University of California, San Francisco, people who consume a lot of easily digestible carbohydrates (most Americans) are going to be less willing to exercise because of the metabolic effects of such a diet. Energy that ends up trapped in fat cells (just one of the awesome side effects of a high-sugar diet) isn't available to fuel the rest of the body, and one of the results is lethargy. 

None of this means that exercise is unhealthy. To the contrary, exercise is beneficial for many reasons. Research suggests that it's good for your memory, reduces risk of death from cardiovascular disease and some cancers, and may help people manage their depression. So we should exercise because it yields all sorts of health benefits. But, we shouldn't count weight loss among those benefits, because it probably isn't one of them. 

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