Why exercise doesn’t help much when you’re trying to lose weight

Exercise is hugely beneficial for our health in many ways, but it’s not so effective when it comes to weight loss – and now we have the best evidence yet to explain why.

People who start exercising more burn more calories. Still, they don’t lose nearly as much weight as would be expected based on extra calories burned. Now an analysis of 14 human trials has revealed that our bodies compensate by burning less energy for other things.

Moreover, this compensatory effect is greater when people eat less while exercising more—and can completely cancel out the extra energy expended in exercise. In other words, while eating less will lead to weight loss, exercising while dieting may not lead to any additional weight loss.

“The real killer is that when you combine exercise with diet, your body says, ‘Okay, well, then I’m going to overcompensate,'” he says. Herman Pontzer at Duke University in North Carolina. “It’s still good for you, just not for weight loss.”

When Pontzer studied Hadza hunter-gatherers in Tanzaniahe was amazed to find that despite their physically active lives, they did not use more energy overall than people who sit at a desk all day. This discovery led Pontzer to propose in 2015 that our bodies evolved to limit the amount of energy we burn and compensate for greater physical activity by conserving energy in other ways.

There are studies that support the idea, but not everyone in the field is convinced. Now Pontzer and Eric Trexleralso at Duke University, identified studies conducted for other reasons whose findings could be analyzed for evidence of compensation. These studies should not be biased in relation to whether compensation occurs, Pontzer says. “They had no dog in the fight when the data was collected.

Specifically, the pair looked at 14 studies involving a total of about 450 people (the numbers are small because tracking total energy expenditure requires the use of specialized, expensive methods). Pontzer and Trexler found that people’s total energy expenditure increased on average by only a third of what would be expected based on the increase in exercise.

For example, says Pontzer, suppose people did enough exercise to burn an extra 200 kilocalories a day. Their total energy expenditure increased by only about 60 kilocalories in these studies.

But there was a lot of variation in that. In people who continued to eat normally, total energy expenditure increased on average by half of what would be expected. But for those who ate less while exercising more, total energy expenditure often did not increase. “They burn 200 kilocalories a day through exercise, but it doesn’t show at all,” says Pontzer.

It also depended on the type of exercise. Compensation only occurred with aerobic exercise such as running. With weightlifting or resistance training, energy expenditure increased more than expected. For example, people who expended an extra 200 kilocalories lifting weights increased their total energy expenditure by 250 kilocalories per day.

It’s hard to measure how much energy people use to lift weights, Pontzer says, so these findings should be treated with caution. However, he speculates that weightlifters could be burning extra energy to repair and build muscle.

Pontzer used to think that the type of exercise didn’t matter. “It’s really a surprise to me,” he says. “I think it’s exciting and it points to something we didn’t know before.” However, the people who did weightlifting in these studies gained muscle and lost almost no fat, Pontzer says. “So it’s still not a good way to lose weight.”

So why doesn’t our overall energy expenditure increase as much as we would expect when we do more aerobic exercise? Analysis suggests that our bodies compensate by reducing the amount of energy devoted to all the background tasks it is performing. Resting metabolic rate, especially during sleep, may decrease in response to more aerobic exercise.

“We change what our different organ systems do.” [after exercising],” says Pontzer. “And if we can figure out exactly what’s changing, we’ll understand a lot more about how exercise affects our bodies. [and] why some people seem to benefit more from exercise than others.’

While Pontzer sees the findings as very clear evidence of compensation, others are still not convinced. Dylan Thompson at the University of Bath in the United Kingdom points to a meta-analysis conclusion that aerobic respiration does not change resting metabolic rate.

There are also some keys limitations of study analyzed, he says Javier Gonzalezalso at the University of Bath. For example, the extra exercise that people have been asked to do may have replaced other forms of exercisesuch as gardening. That could explain why people’s energy consumption hasn’t increased as much as expected, Gonzalez says.

But Pontzer says that can be ruled out in some studies. Compensation has also been observed in animal research, supporting the results in humans. However, Thompson and Gonzalez believe more rigorous studies are still needed. “We really need to carefully design randomized controlled trials in humans,” says Thompson.

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