You should exercise to lose weight, right?

What people overestimate.


So what if you had extra helpings at Thanksgiving? So what if you shared a large bucket of popcorn at the movies? You can always work it off, right?

“People totally overestimate how many calories they’ll burn when they exercise,” says Susan Roberts, director of the Energy Metabolism Laboratory at Tufts University in Boston.

“We asked people, ‘What do you need to do to lose weight?’ About three-quarters of them said, ‘My trouble is that I stopped going to the gym’ or ‘I’ve been lazy. I need to go to the gym.’ They were beating themselves up because they weren’t doing enough exercise.”

In fact, most studies find that people lose more weight when they’re told to cut calories rather than exercise more, though a mix of diet plus exercise is best in many studies.


More exercise sometimes doesn’t work

“Exercise for weight loss is often unhelpful,” says Roberts. “Several years ago, a study looked at the individual response to an exercise program. It was discouraging because they found a huge variability.” Some people lost a lot of weight, but some gained weight.

UK researchers had 35 overweight or obese men and women exercise on a stationary bike, treadmill, or stepping or rowing machine long and hard enough to burn 500 calories per session, five times a week.

After  12 weeks, the average weight loss was 8 pounds, but some people lost 32 pounds, while others gained 4 pounds. Those who lost the least weight also reported more hunger and ate more on a given day near the end of the study than did those who lost the most weight.

“For some people it was clearly working, but for others it was counter-productive,” says Roberts.


Other factors are influential

It’s not just individual differences. Other factors, like temperature, might also matter. In a recent study, researchers assigned  16 overweight people to walk for 45 minutes on a treadmill when the room temperature was either 46º or 68º Fahrenheit. They ate more calories (1,300) at an all-you-can-eat buffet after walking in the cold temperature than after walking in the typical indoor temperature (1,170 calories).

“You need to keep doing exercise for your health, for preventing disabilities and problems in old age,” says Roberts. “But for most people, exercise is not the solution for losing weight.”


What about trying to speed up your metabolism?

“This boosting of metabolism is pretty overblown,” says Susan Roberts. “Magazines and supplements love to talk about ways to boost your metabolism. But you can eat a brownie with 700 calories in about  10 seconds, and there is nothing that you can remotely do to change your metabolism by anywhere near that much, except to give up your job and spend all your day in the gym.”

Companies that sell supposed metabolism-boosting foods or supplements  need virtually no evidence to make their claims.

Take green tea. Some small studies report that it leads to a bump in metabolic rate, but long-term studies find that it makes little or no difference in weight.

The largest study: With funding from Coca-Cola, researchers randomly assigned 572 people to a weight-loss program with or without three daily cans of diet cola, each fortified with green tea extract (83 mg of EGCG) and caffeine (100 mg). After three months, the diet-cola drinkers had lost no more weight than the others.


Does anything help you burn more calories per minute?

“Exercise is going to help a bit because muscle is more metabolically active than fat,” says Roberts. “So if you have five more pounds of muscle and five pounds less fat, it will make a bit of difference, but not a huge difference.

“Weight control is dominated by how many calories you eat. That’s the honest truth.”


The bottom line: Exercise when you can, but don’t count on it alone to lose—or keep off—extra pounds. And don’t expect to lose much weight from “metabolism boosters.”


Sources: Obesity 22: 325, 2014; PLoS ONE 9:e109849, 2014.; Int. J. Obes. 32: 177, 2008; Med. Sci. Sports Exerc. 47: 49, 2015; Int. J. Sport Nutr. Exerc. Metab. 22: 139, 2012;
Brit. J. Nutr. 111: 372, 2014.