Exercise alone isn’t key to losing weight

A new study starts to make sense of weight loss contradictions

Everyone seems to have advice for losing weight. Family, friends, TV shows, magazines, and those web ads revealing “one weird trick“. Worryingly, the advice is rarely consistent. You’ve probably heard someone say you can eat all you want as long as you burn it off, so get out there and work it! It’s just as likely you’ve heard advice along the lines of, “just put down the fork.” It’s hard to know who to believe when exercise clearly works for some people, yet others report little weight loss with exercise alone. So what’s going on? A new study in Current Biology might be starting to provide answers.

Dodgy health forums aren’t the only place we find weight loss contradictions; scientists have been pondering some puzzling observations for quite a while. People who exercise burn more calories than sedentary people. If you started as a couch potato and ended up being a fitness freak, you’re burning more calories than you used to. It was presumed that the more you exercised, the more calories you burned. However, large studies on highly active humans (and other animals) reveal that they burn the same amount of calories as less active groups. For example, farmers and hunter-gatherers are a lot more physically active that city dwellers like myself but burn a similar amount of calories. On the one hand, more exercise means more calories burned. On the other, why are people with highly active lifestyles burning the same number of calories as less active people?

Herman Pontzer, City University of New York, led the study to make some sense of what’s going on. The group looked at 332 individuals living in 5 different countries and studied their energy expenditure over the course of a week. The results revealed that moderately active people were burning more calories than sedentary individuals, but going beyond moderately active didn’t burn extra calories. Clearly there’s something else going on. Somehow we’re able to adapt to high activity levels and stop burning calories. What this means for weight loss is that there must be a sweet spot where exercising any further won’t burn extra calories. I’m sure many people will find these results reflect their own experience as it’s common for weight to be lost when beginning a healthy lifestyle but then eventually reaching a plateau.

The revelation that exercise alone isn’t going to burn off extra calories is only relevant beyond a certain level of exercise. Going from a sedentary to active lifestyle will help you lose weight so there’s no need to stop. Indeed, exercise helps our bodies in many ways beyond losing any unwanted weight. For example, an active lifestyle is good for mental health. The study doesn’t suggest we should stop working out, it suggests that we need to think more about our diet. Consuming more calories means you need to do more exercise to burn them. So in theory you could eat as much as you want as long as you exercised enough to burn it off. If we only burn calories up until moderate activity as the study suggests, then we can’t simply eat all we want. We’re limited to how many calories we can actually burn, which means we should be limiting our calorie-intake if we’re trying to lose weight. Exercise alone isn’t the answer after all; our diets play a big part. Similarly it doesn’t mean all we need to do is “put down the fork”, as moderate activity does indeed burn calories.

So the plateau is a real thing. What next? The researchers are now working to figure out how our bodies adapt to the high activity levels. Why is it that we can increase our energy demands but stop burning calories? It’s an intriguing question and the answer might be worth knowing for anyone wishing to lose weight. Understanding how it works might help us figure out what the sweet spot is, which I would guess is going to be different for all of us. If you want to lose some weight in the meantime, don’t fall into either of the two extremes of “exercise is all it takes” or “diet is all it takes”. Get out there and burn some calories, but consider how much you’re eating too.


Main image © iStock/Christopher Futcher