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There's a Limit to How Many Calories You Can Burn in a Day

Why your diet matters: The most physically active people have the same energy expenditure as those who are moderately active.

If you eat with wild abandon and put all your faith in your gym regimen’s ability to take care of your physique alone, boy are you in for a surprise. Most likely: unexpected weight gain. New research published in Cell Press claims exercise by itself may not be enough to help you drop the pounds—and keep them off. 

In the week-long study, researchers measured the daily energy expenditure and activity levels of more than 300 men and women using a "doubly-labeled water" technique. Participants drank a certain quantity of water with a known concentration of hydrogen and oxygen. As participants expend energy, carbon dioxide and water are produced; the difference between the elimination rates are then used to calculate total energy expenditure, giving the researchers a precise measure of calories expended per day. The participants' exercise levels weren't manipulated, nor were their weight changes measured.

Surprisingly, physical activity had a weak (but measurable) effect on the participants’ daily energy/calorie expenditure. And further examination showed this pattern of exercise burning calories only held true with participants on the lower portion of the physical activity spectrum.

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Essentially, people with moderate activity levels had higher daily energy expenditures—about 200 calories more—than the most sedentary subjects. But, people who exceeded moderate activity levels saw no effect of their extra work in terms of burning more calories.

"The most physically active people expended the same amount of calories each day as people who were only moderately active," says study author Herman Pontzer. 

Our bodies adapt to higher activity levels, he explains, so you don't necessarily burn extra calories even if you exercise more. We know; it’s devastating news. Go overboard and your body makes adjustments and takes action to adapt; do too little and you’re unhealthy. There's a "sweet spot" for physical activity. (Similar research has found a supposed sweet spot for gaining muscle and losing fat, too.)

"That's the first thing I mention to anyone asking about the implications of this work for exercise,” Pontzer says. “There is tons of evidence that exercise is important for keeping our bodies and minds healthy, and this work does nothing to change that message. What our work adds is that we also need to focus on diet, particularly when it comes to managing our weight and preventing or reversing unhealthy weight gain."

So is there an actual ceiling or specific number on how many calories we can burn in a day? Pontzer says, yes, but each person has their own unique cap judging by the variability he and his team saw in daily energy expenditures.

Why? Evolution. "All animals, not just humans, would be expected to have evolved mechanisms to keep energy expenditures in check, so our daily energy expenditures don't outstrip the available food resources," Pontzer explains. "In fact, we find evidence for this metabolic 'ceiling' and plateau with energy expenditure vs activity, in many mammal species."

But, hey, this isn't a call to stop exercising; just balance a healthy diet with your current exercise regimen and you'll have no problems getting, being, and staying fit. 

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