Ever wonder why it seems so hard to exercise off those winter kilos, even as the weather warms and you become increasingly active?
The problem, says Lewis Halsey, an environmental physiologist at the University of Roehampton, London, isn’t that you’re doing something wrong. Rather, you’re up against a mysterious aspect of human physiology known as “energy compensation”.
“Put simply,” Halsey says, “if we start a period of enhanced activity [such as] taking out a gym membership or training for a marathon, our body partially compensates for the extra calories we burn on that activity by cutting energy spent on other things.”
This, he says, “is presumably an adaptation to stop us losing too much weight during tough times. But it isn’t very helpful in the modern day, when food it plentiful and many are more worried about their waistline getting too big, rather than too small.”
Much of this has been suspected for a long time. In a paper in Physiological and Biochemical Zoology, Halsey notes that studies of the world’s few surviving hunter-gatherer cultures have found that these people burn no more calories per day than the average Westerner, even though other research, such as that by Daniel Lieberman, a professor of evolutionary biology at Harvard University, shows that they lead far from sedentary lives.
In his 2020 book Exercised: Why Something We Never Evolved to Do Is Healthy and Rewarding, Lieberman describes how he once equipped Hadza hunter-gatherers in Tanzania with activity-tracking devices of the type often used by recreational runners, to see just how active they were. He found that they walked, jogged, or sometimes sprinted an average of 11.5 kilometres per day – a whopping total of 80km per week, about equivalent to the training program of a serious recreational marathoner.
By all rights, these people should be burning up hundreds of calories a day more than the average Westerner. But, Halsey says, they aren’t. Other studies, he says, have been done in exercise physiology labs, where researchers had volunteers change their activity levels, then monitored the effect.
These, too, suggest that the body attempts to reduce its base metabolism to compensate for increased exercise. But is this just a short-term effect—a temporary form of energy compensation that will eventually disappear? That wasn’t clear.
To find out, Halsey led a second study, published in Current Biology, in collaboration with a worldwide team of more than 60 other scientists. It confirmed that the more active we are, the more our bodies lower their base metabolic rates in an effort to compensate, not just in short-term laboratory studies, but in the real world.
This study took advantage of the increasing accessibility of data from large numbers of other studies (hence Halsey’s astounding number of co-authors). What he wanted were “doubly labelled water” studies of energy expenditure – a testing method that can be used to assess the daily energy expenditures of not just laboratory volunteers but “free living” individuals, i.e. you and me.
“Doubly labelled water” sounds like a technical mouthful. But while the biochemistry is complex, the idea isn’t. It involves giving people a drink of water whose H2O contains rare (but harmless) isotopes of both oxygen and hydrogen.
The oxygen is oxygen-18, while the hydrogen is “heavy” hydrogen, also known as deuterium. Chemically, they might look and behave similarly to normal oxygen (oxygen-16) and normal hydrogen, but they are easily distinguishable with the right analytic instruments.
Over the course a day, your body uses this water for various purposes, with some of the oxygen coming back out in your exhalations as CO2, and some of the hydrogen appearing in your urine, in water. By tracking these, scientists can figure out a lot about your total metabolic rate.
If they also know your exercise level, they can then figure out how much of your energy expenditure comes from exercise, and how much from base metabolism.
It’s not a cheap test, but from parsing the literature, Halsey found a “nice, big, juicy data set” of 1,754 people of all ages, nationalities and ethnicities for whom it had been done.
He found that for the average person, there’s a 28% base energy compensation effect for increased exercise: meaning that the body adjusts its base metabolic rate by enough to wipe out 28% of your hard work.
And he notes: “We’re not talking about elite athletes. We’re not talking about marathoners. We’re not talking about people being dragged into labs to pedal on bikes for two hours. Just people mowing lawns, etc.”
Why this is happening isn’t fully clear. One possibility might simply be that regular exercise makes muscles more efficient. This is part of how Olympic coaches try to make athletes faster and more competitive.
Another possibility is that if we are fat and sassy (or at least well-fed) we may tend to fidget more (or less, if we are trying to cut calories).
But attractive as that idea sounds, it doesn’t seem to be the case. “Our bodies are designed to survive,” Halsey says. When faced with what they perceive as starvation, “they will sacrifice the least important things” – including the immune system, wound healing, or even reproductive systems (a major problem in women who get too thin). “The body is shutting down, in a desperate attempt to limit how far negative you go in your energy deficit,” Halsey says.
Which is useful for the survival of anorexics and famine victims. “If you take us back a few thousand years and go through a lean patch where you can’t get much food, it’s great if your body is able to ratchet down, because otherwise you’re going to waste away [and die],” Halsey says. “It’s a competition between groups of cells. Who’s singing loudest for their supper?”
But it’s a disaster for overfed Westerners seeking to stave off obesity, diabetes, heart disease, and stroke.
Worse, the more you need to deal with these problems, the more your body is likely to resist. For the stoutest people – those with a body mass index (a common measure of height vs weight) in the top 10 percentile, Halsey says, the energy compensation effect reaches a staggering 46%. In other words, the more you need lose weight, the more strongly your body tries to sabotage you.
“The fault, Dear Brutus, is not in our stars, but in ourselves,” Shakespeare writes in Julius Caesar. In this case, however, Halsey thinks the fault is most likely in our mitochondria, the sub-cellular powerhouses that burn sugar, fat, and oxygen to run our muscles.
Exercise, he says, may make these more efficient: a good thing for elite athletes trying to win medals, but a bad thing for those of us trying to control our weight by burning off as much fat as possible.
Other scientists think Halsey might be on the right track in looking at mitochondria. Matthew Robinson, an exercise physiologist at Oregon State University, calls Halsey’s findings “very interesting”, adding: “mitochondrial efficiently is a hot topic in exercise physiology”.
What to do about these findings, however, isn’t clear.
The good news is that they don’t mean that exercise is useless. Yes, they show that for each kilometre you walk, run, bicycle, or swim, you’re only getting 540 to 720 metres of fat-burning benefit.
That’s a far cry from zero.
And there’s plenty more research showing that being active is good for pretty much whatever ails you (or might ail you). But if you wonder why you aren’t losing as much weight as you hoped, “energy compensation” is probably the answer.
Don’t blame yourself. Blame your mitochondria.