When it comes to calories, sometimes the numbers simply don’t add up. Check the tally on your smartwatch or Peloton, keep track for a few months, and then check the bathroom scale. No matter how many calories you’ve torched, the resulting weight loss will be less than you hoped.
That’s the riddle at the heart of a recent review published by Lewis Halsey, who heads the University of Roehampton’s Behaviour and Energetics Lab, in the journal Physiological and Biochemical Zoology. It’s clear from the review’s title, “The Mystery of Energy Compensation,” that final answers aren’t yet forthcoming – but as Halsey explains, a flurry of studies using new techniques to measure calorie burn are overturning some long-cherished tenets of conventional wisdom about the effects of exercise.
The key technology in the debate is “doubly-labelled water,” which is water that contains rare – and easy trackable – isotopes of both hydrogen and oxygen. In the 1950s, a researcher named Nathan Lifson figured out how to accurately measure the total daily calorie burn of mice by having them drink some of this special water and then collecting urine over subsequent days to see how long it took to metabolize. The problem: it would have cost more than $300,000 to run the same experiment on a human.
It’s only in the past few decades that the cost has dropped enough for scientists to use the technique widely, with a current price of about $750 per person. The results have been unexpected, most notably when a team led Herman Pontzer of Hunter College in New York (he’s now at Duke University) tried it with the Hadza, a group of hunter-gatherers who still live a traditional lifestyle in northern Tanzania.
Hunting with bow-and-arrow and foraging with digging sticks, the typical Hadza adult gets more exercise in a day than the average couch-bound Canadian gets in a week. But as Pontzer recounts in Burn, an entertaining book about the new science of metabolism that he published earlier this year, the results defied his and everyone else’s expectations. Once you take into account factors such as age and body size, Hadza adults burn pretty much exactly the same number of calories each day as Canadians – and as everyone else from populations around the world tested with doubly-labelled water.
We know, from the fundamental laws of thermodynamics, that physical activity must burn calories. So the fact that high levels of physical activity don’t lead to greater overall calorie burn means that we must have other ways of saving calories: the “energy compensation” of Halsey’s title.
The first question is how we do it. One theory is that we fidget and move around less after exercise. Another is that we spend less energy on under-the-hood bodily functions like immune response and tissue repair. A third is that our cells become more efficient at converting food into usable energy, squeezing more power out of each calorie we eat. At this point, Halsey says, there’s simply not enough evidence to choose between them.
The second question is what it means for our ongoing societal struggle to lose weight. Another double-labelled water study, published in July, suggests that energy compensation kicks into high gear when you’re losing weight, but is less of a factor when your weight is stable or increasing. That’s consistent with the widespread view that exercise is more useful for preventing weight gain than promoting weight loss.
Even when energy compensation is working against you, it doesn’t erase all your efforts. In one study that’s often cited as evidence of the effect, people training for a half-marathon didn’t burn as many calories as their training predicted – but they still burned about 30 per cent more calories than before they started training, which may be enough to move the needle of the scale.
The broader message, though, is that good health isn’t just about hitting a certain weight. In Burn, Pontzer suggests that some of the health benefits of exercise may be a direct result of energy compensation. If your body foils your weight-loss goals by ramping down immune function, that might reduce chronic low-grade inflammation in response to things like food and allergens, while preserving the ability to respond to real threats. The same is true of the body’s stress response.
“We need to get over the idea that if we start doing, for example, 300 calories of exercise per day, then we have burned that many more calories per day,” he says. “That appears not to be the case, in the long run.”
Alex Hutchinson is the author of Endure: Mind, Body, and the Curiously Elastic Limits of Human Performance. Follow him on Twitter @sweatscience.