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who drive to a job to sit all day, with the men using up about 2600 calories and the women 1900. “I couldn’t believe it,” says Pontzer. The findings caused a stir. They called into question the widely accepted idea that sedentary lifestyles in many societies are responsible for the obesity epidemic. Instead, Pontzer and his team began to wonder whether our daily energy expenditure could have evolved to be fixed at these levels, regardless of whether we sit at a desk all day or search the plains looking for our next meal. To back up the idea, what’s needed is to study other ways of living too, including populations with Western lifestyles. That’s where Lara Dugas of Loyola University Chicago comes into the story. Her team kitted out nearly 2000 people from the US, Ghana, Jamaica, South Africa and the Seychelles with activity monitors for eight days to gauge their basic pattern of physical activity. She then tracked their weight over several years. The upshot? Activity levels didn’t predict weight two years later. In fact, those who met the US guideline of 150 minutes of moderate-intensity exercise per week, according to the monitor data, tended to have put on more weight than those that did less. A paradox indeed. In 2016, Pontzer and Dugas joined forces. They looked in more detail at over 300 of the people in Dugas’s original study. It turned out that those who were moderately active used up about 200 more calories per day than sedentary people, but after that, calorie burning plateaued. Those who exercised every day didn’t burn any more than those who worked out a few times a week. “Only at the very, very low end did we see anything like a trend of lower activity being paired with lower energy expenditure,” says Pontzer. This view tallies with calculations of how much people exercise when viewed over longer time spans, says Glenn Gaesser at Arizona State University. “If you add up the amount of calories individuals would expend doing 150 minutes of exercise a week, times 52 weeks of the year, you come up into the literally tens of thousands of calories that are expended.” And yet exercisers only weigh around 2 kilograms less on average, he says. As the evidence piles up, says Pontzer, the idea that activity dictates how many calories you burn looks “pretty naive”. It seems time to put the calories in, calories out equation to rest. But how can it be that people do more exercise without seeming to expend extra energy? The assumption has been that they eat more to make up for it, whether because they are hungrier or feel like they have earned it. “You can consume a doughnut in less than a minute,” says Gaesser. “But that minute of consuming the doughnut might take an hour or more of walking to match in terms of calories.” It also doesn’t help that people grossly overestimate their energy use during exercise. In one study, people were assigned a treadmill workout and then told to estimate how many calories they burned and eat an equivalent amount from a buffet. They guessed they used up 800 calories and ate about 550. In reality, they had burned just 200. That might help explain why Dugas found that those meeting US exercise guidelines tended to have put on more weight. But it wouldn’t explain why the Hadza’s prolific activity doesn’t add up to much more energy consumption over the course of a day than a sedentary lifestyle. So another suggestion for this exercise paradox is that our bodies compensate for a hard workout by moving less the rest of the day. Some clues have come from mice. When given running wheels to prompt exercise, they were found to move around less than usual in between bouts of activity. The number of calories saved from moving less the rest of the day almost exactly negated the calories burned from running. It seems people make similar sorts of adjustments when they embark on a new exercise regime, even if they don’t realize it.