About Human Metabolism

Exercising to lose weight? Science says it rarely works.

A metabolism researcher dispels myths about how we burn calories and how this changes as we age.

November 27, 2024 7 min

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(Tierney L. Cross for The Washington Post)

By Gretchen Reynolds When Herman Pontzer began studying the metabolisms of the Hadza, a tribe of modern hunter-gatherers in Tanzania, for a 2012 study, he assumed they’d be incinerating calories like a furnace. They were in almost constant motion — walking, jogging, tugging and lifting all day long.

Get concise answers to your questions. Try Ask The Post AI. But when he and his colleagues compared the Hadza’s typical daily energy expenditure, controlled for body size, with that of your average couch potato office worker back in the United States, the totals were nearly identical.

“That was unexpected,” said Pontzer, now an evolutionary anthropologist at Duke University and author of the 2021 book “Burn,” about the science of metabolism.

The findings were so surprising that he wound up developing a new theory for how we use calories — called the “constrained total energy expenditure model.” It suggests that our bodies and brains can rejigger how many calories we burn — recalibrating within a narrow range — by slowing or shutting down some biological operations if we’re especially active, keeping our total calorie burn at the same level.

“We’ve done studies where we look at people who are really active,” including runners racing across America, Pontzer said, “and people who are really sedentary, and actually they’re burning the same number of calories,” which helps explain why people rarely lose much weight with exercise, even as the exercise makes them healthier.

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Follow That discovery is only one of many from Pontzer and his colleagues, who have been busily upending entrenched ideas about our metabolisms in recent years.

Now, with calories and weight control becoming top-of-mind topics as we enter the holiday season, I talked with Pontzer about what the latest science says about our metabolisms and whether they’re affected by exercise, differ between men and women, and can be “boosted.” We also talked about why he thinks Thanksgiving is the most-human holiday we have. (The following interview was edited for length and clarity.)

Let’s start with some rapid-fire yes-or-no’s: Men’s metabolisms are faster than women’s.

No.

Our metabolisms slow down in middle age.

No.

Exercising more means we’ll burn more calories.

No, not overall.

A more-basic question, then, and one I don’t think I could answer: What is our metabolism?

Your metabolism is all the work that all 37 trillion of your cells do every day. You can measure that work in the energy required. That’s why we measure your metabolism in calories, which are a measurement of energy.

Most of us probably think our metabolisms are fastest when we’re going through puberty and start slowing down drastically when we reach midlife, and also that men’s are faster than women’s. But none of that is true?

So, how to begin? The main determinant of how many calories you burn every day is how many cells you have at work. Bigger people burn more calories. The type of cells matters, too. Fat cells burn a little energy, but not much. So, when we look at women versus men, men will burn more calories than women, but it’s because men tend to be a bit bigger and carry a bit less fat. If I compare a man and woman who have the same body size and same fat percentage, I would expect exactly the same energy expenditure per day.

And teenagers are burning the most calories?

Yes, the highest total calories burned in your lifetime probably happens in late adolescence. But pound for pound, nobody burns calories like a 3- or 4-year-old child. Their metabolisms are white-hot because of just how much work their cells are doing with all that growth and development. But they’re tiny, so the total calories is still quite small.

Does someone’s metabolism slow down a lot during middle age? It sure feels like it.

I’m in my late 40s and I definitely feel different in the way I respond to food when I eat than when I was in my 20s. But, shockingly, no. We’ve done this measurement now on thousands of adults from their 20s through their 50s and beyond. But in that block, the 20s to 50s, we don’t see any changes in the metabolic rate, after you control for size. There does seem to be some slowing after age 60.

But many of us gain weight during middle age. If we can’t blame our slowing metabolisms, what’s going on?

If it’s not the energy-burned side of the equation, it must be the energy we’re eating. Why might that change? One reason is, we get more stressed. It’s easy to develop unhealthy eating habits. Plus, if we look at how people actually gain weight, it won’t surprise anybody that we typically gain a few pounds around Thanksgiving and into the Christmas season, then lose it with New Year’s resolutions. As we get older, we seem to get a little better at putting it on and not quite as diligent about taking it off.

So where does exercise fit in? I’m a runner and I’d like to think the more I run, the more calories I burn and the more weight I lose. But no?

I thought so, too. But it turns out, it’s not so simple. If you exercise today, you’ll burn more energy today. But if you really change your lifestyle and start exercising regularly and that becomes your new normal, your body adjusts, and you wind up not burning more calories overall. Basically, if you spend more energy on exercise, your body finds ways to spend less on other stuff. But here’s the good news. The adjustments the body makes, which include reductions to inflammation and stress reactivity, may be a big part of why exercise is so good for us.

Can we boost our metabolisms with the right pills or foods?

No, there’s no way to boost your metabolism by what you eat. You can go high-carb, low-carb, low-fat, high-protein. None of those seem to have any measurable effects on calories burned per day.

What about thinking hard? Won’t that burn extra calories?

Wouldn’t that be nice, if all it took was enlightened conversations around the Thanksgiving table to burn away dessert, or if discussing chess strategy helped us lose weight? But no. It’s true your brain is incredibly hungry. Basically, it runs a 5K every day, burning through about 300 kilocalories, the same as a 5K. But that’s true whether you’re deep in thought or totally zoned out.

You mentioned Thanksgiving. Will eating one enormous meal, as many of us do at Thanksgiving, raise our metabolisms and help us burn those extra calories?

You do burn about 10 percent of the calories you consume during digestion, thanks to everything from chewing the food to digesting it and dealing with it in your cells. But that’s pretty minor. Realistically, if you want to avoid gaining weight at Thanksgiving, you’ll need to eat less. But let’s stop for a minute here, and I’ll put on my anthropologist’s hat, and we can talk about what’s really amazing about Thanksgiving, which is that we all sit around and share food. It’s the strangest thing we do.

Strange how?

No other ape shares like we do. It’s anomalous in the natural world. But we do it every day. And whenever there’s a big moment to celebrate, we do it by sharing food. So Thanksgiving is, in my mind, the ultimate human-evolution holiday. You’ve got hunted meat. You’ve got gathered plant foods. You’ve got language happening. You’ve got tools. You share this bounty with your extended family and friends, your kin group. What other celebration puts human evolution on display like that? So, of course, we should be aware of the health aspects. But let’s not lose sight of the fact that, in humans, food is a social bond, and especially on Thanksgiving. Maybe give yourself one day to focus on that aspect and not worry too much about the calories. It’s okay, honestly, to just enjoy the pie.