Big Debate: Does exercise lead to weight loss?

The answer may change what you think about exercise.

Post Summary

• This post will help clear some recent confusion over whether exercise helps us burn more calories across a day.

• We once thought exercise would lead us to burn more calories in a day.

• But a new theory, called the "constrained energy hypothesis," suggests exercise doesn't help you burn any more daily calories.

• The theory has gained popularity. But is it legit?

• More research shows that exercise does indeed help you burn more calories, but you won't burn as many as you think.

• And if you want to lose weight through exercise, you'll have to do a lot of it.

• You'll learn just how many calories exercise really burns and how much you'll have to exercise to see a difference.

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The post

We've long thought that the calories you burn from exercise add to the total calories you burn in a day. For example, burning 500 calories on a run would lead you to burn 500 more calories that day.

Hence, if you just exercise enough, you can burn more calories than you eat and, in turn, lose weight.

But the relationship between exercise and the number of calories you burn a day—and, by proxy, whether you'll lose weight—is much more complicated.

A new-ish theory called the "constrained energy hypothesis" argues that we don't actually burn more calories a day when we exercise. Works like this:

• Yes, if you exercise, you'll burn some calories.

• But then, quite deviously, your body works some internal magic and you'll burn fewer calories elsewhere.

• For example, your metabolism and spontaneous movement will drop, balancing the books.

The overall effect: You'll end up at a net zero and won't lose weight.

Duke researcher Herman Pontzer popularized the idea. He wrote a book about this idea called Burn: New Research Blows the Lid Off How We Really Burn Calories, Lose Weight, and Stay Healthy.

On page 103, he sums up the takeaway:

“The bottom line is that your daily (physical) activity level has almost no bearing on the number of calories that you burn each day.”

Pontzer argues calorie control is mostly determined by what you eat rather than what you burn from exercise.

Perhaps this is why all those old bodybuilders always said, "Abs are made in the kitchen." Or the runners say "You can't outrun a bad diet."

But is that true? And if it is, how should that determine how we exercise if our goal is weight loss?

Let's dive in.

Why do some researchers think exercise doesn't increase total calorie burn?

• Section summary: Pontzer's work with highly active hunter-gatherers shows that they burn about the same number of calories we do in a day.

Pontzer's theory came from his work with the Hadza hunter-gatherer tribe in Tanzania.

The Hadza and most hunter-gatherers are about 14 times more active than people living in the modern, built environment. So you'd expect them to burn far, far more total calories across a day than we do.

Pontzer used the doubly-labeled water method to measure exactly how many extra calories they burn. It's the most accurate way to track how many calories a person burns in a day.

But his results were a shocker. Pontzer wrote in Scientific American:

When the analyses came back from (the lab), the Hadza looked like everyone else. Hadza men ate and burned about 2,600 calories a day, Hadza women about 1,900 calories a day—the same as adults in the U.S. or Europe. We looked at the data every way imaginable, accounting for effects of body size, fat percentage, age and sex. No difference.

Other studies comparing the energy burn of highly-active people—like traditional farmers—show that they, too, burn about the same amount of energy across a day as we do.

How could that be? Pontzer's theory is that your body compensates and saves calories elsewhere. When you ramp up your exercise:

• You fidget and pace less, and are more likely to sit, avoid the stairs, and more. This all happens unconsciously.

• More importantly, your body reduces many internal activities that burn calories. For example, your immune system dials back inflammation, you may produce fewer hormones, etc. That saves energy.

This is a brilliant evolutionary trick. It theoretically helped our ancestors survive times of famine where we had to walk and run long distances to find food. In other words, our bodies won't let us exercise into starvation.

The graphs below can help us visualize this.

• These two graphs show your body's non-exercise/metabolic calorie burn in grey and your exercise calorie burn in red.

• Graph A shows the "Additive" model. This is our old idea that you'll keep burning more calories if you exercise more.

• Pontzer's idea, called the "Constrained" model, is shown in graph B.

• Notice how in the constrained model your daily non-exercise/metabolic calorie burn drops as your exercise burn goes up, capping the number of calories you'll burn across a day.

Pontzer wrote in a review of the topic:

“The human body adapts dynamically to maintain (daily calorie burn) within a narrow physiological range. Rather than increasing with physical activity in a dose-dependent manner (Michael's note: as seen in Graph A), experimental and ecological evidence suggests the hypothesis that (daily calorie burn) is a relatively constrained product of our evolved physiology (Michael's note: as seen in Graph B)”

Do you really not burn more calories if you exercise more?

• Section summary: Most research suggests a middle ground between the "additive" and "compensatory" models. Exercise does lead you to burn more calories, but not as many as you'd think.

Studies do indeed show a "compensatory" effect. This means that if you were to burn 100 calories from exercise, your body wouldn't actually burn 100 more total calories during the day.

But there's still some extra burn.

One study found that it nets to about 72 percent of what you'd expect, but the figure varies. That comes from a study that stated:

We used the largest dataset compiled on adult (daily calorie burn) and (daily calorie burn from metabolism and not from exercise) of people living normal lives to find that energy compensation by a typical human averages 28% due to reduced (daily non-exercise calorie burn); this suggests that only 72% of the extra calories we burn from additional activity translates into extra calories burned that day. Moreover, the degree of energy compensation varied considerably between people of different body compositions.

Some people in that group, primarily people with more body fat, compensated by about 46 percent, meaning that only 54 percent of their burn translated to additional calories burned that day.

This means that, in the real world, if your exercise burned 1,000 calories, your actual increase in total daily calorie burn would be 720 to 540 calories.

Other randomized controlled trials found that exercisers got only 50 to 66 percent of the calories they burned from exercise.

This graph shows the data from those two trials. The black bars show what you'd expect the total energy burn to be, while the red bars show what it actually was.

I.e., more evidence shows that the truth is somewhere in the middle of the additive and constrained models.

Here are a few other figures that push back against the constrained energy hypothesis.

• One study discovered that sedentary people who started training for a half-marathon that was 44 weeks away boosted their daily total calorie burn by 33 percent. The average total daily burn of men rose by 669 calories for men and 430 for women.

• Another study took a group of sedentary, overweight or obese people and put them on an exercise plan of five 45-minute sessions a week for more than a year. The men increased their total daily burn by 371 calories and the women 209.

• Yet another found that people who exercised five days a week for 10 months burned 289 extra calories.

Important: In all those studies, people weren't asked to change their diet.

Does exercise help you lose weight and burn fat, though?

• Section summary: Well-designed studies show that exercise often does lead to weight and fat loss.

Calories are abstract. When ordinary people talk about calorie burn, we're usually talking about weight loss.

"So the real question that comes up for people is, is exercise a fat loss tool? Is it useful for energy expenditure (total calorie burn)?"

That's according to Layne Norton, a friend who I called to discuss this question. Layne is a Ph.D. biochemist known for being a top mind in the world of nutrition and exercise. He also founded the Carbon diet app (I have no affiliation but use it).

He's well known on the internet as something of a sheriff around the science and analyzing nutrition and fitness claims. You may have heard him on popular podcasts like Andrew Huberman, Peter Attia, or Joe Rogan.

"There's kind of an extreme viewpoint which says that if you exercise, you're just going to compensate some other way, and you're not actually going to (burn more calories across a day) for it," Layne explained.

To be fair and transparent, Pontzer's book uses some strong language. But neither Layne nor I had heard him publicly say anything like "exercise won't lead you to burn more calories."

For example, on Dr. Rangan Chatterjee's podcast, Pontzer said, "Even if you're more physically active and are getting more exercise and physical activity every day, that doesn't necessarily mean that you're burning more calories every day than someone who is more sedentary than you."

I've bolded "doesn't necessarily mean" because that is very different than "exercise doesn't burn more calories." (My thought: The discrepancy between the language in Pontzer's book and what he says in public might be because book editors like bold statements—bold statements are better for book marketing).

Still, the implication is that exercise won't make you burn more calories and lead to weight loss.

"If you look at free-living research studies on exercise and weight loss, you get some that kind of say yes and kind of say no," said Layne.

• For example, one study had people exercise for up to 210 minutes a week. They didn't lose a significant amount of weight—only a pound or two.

But, Layne said, "If you look at tightly-controlled randomized controlled trials, it is a unanimous yes."

He continued. "When scientists actually monitor to make sure people are eating a certain amount of food and doing a certain amount of activity, the people doing the activity absolutely lose more body fat. It's not negotiable."

But for exercise alone to spur weight and fat loss, you have to exercise a lot.

Let's examine how the three calorie burn studies mentioned above changed the participants scale weight and body composition.

• The first study found that the participants' scale weights didn't change much. But their bodies did. The men lost 8 pounds of fat and added 4 pounds of muscle. The women lost 5 pounds of fat and gained 3 pounds of muscle.

• In the second study, the men lost 11 pounds, 10 of which were from fat. The women didn't lose weight or fat, but they did avoid the 6 pounds of weight gain the control group experienced.

• In the third study, the participants lost roughly 11 total pounds, most of it fat.

Should you exercise to lose weight?

• Section summary: Exercise alone can increase calorie burn and fat loss. But it takes a lot of exercise and also changing what you eat will make your life much easier.

Exercise and its relationship to weight and fat loss is so murky because we've set the bar rather low. It takes a lot of exercise to reduce your weight.

When people exercise far more than the government recommends, we see total daily calorie burn go up. We see fat and weight loss.

Recall that the Hadza are highly physically active. They get about two hours and 15 minutes of moderate physical activity a day (mostly from brisk walking and carrying). That's likely the level of exercise our bodies are adapted to.

We're nowhere near that—the government guidelines, for example, are 1/6th of that. And only 20 percent of people meet the guidelines.

Not to mention, it's easier than ever to replace everything you've burned with the delicious food we've surrounded ourselves by.

Layne put it this way:

I feel like the message (around the constrained theory) has kind of been misconstrued. I think the message should be, 'if you think going out and walking a couple miles is going to offset the plate of doughnuts, you just ate? No. But is doing exercise beneficial on the whole for body composition, health, weight loss? I think the answer is unequivocally yes.'

On Wednesday, we'll dive into how exactly how you should use this information to meet your goal.

Have fun, don't die, exercise.

-Michael

P.S., What's been your experience with exercise and calorie burn? Do you think it's helped you change your weight? Weigh in in the comments.

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