Losing weight and staying in shape has to do with both diet and exercise, and it’s really important not to underestimate the importance of the diet. You simply can’t just eat whatever you want and then go work it off in the gym, and some new research is pointing to some interesting reasoning about why.

This study published in Cell Press is making claims that there is actually a cap off point where you aren’t going to burn more calories no matter what you’re doing. They suggest that extremely active people don’t burn more calories than moderately active people, which means that what you eat can be the deciding factor between how much weight you end up gaining or not.

The study took place over a week’s time and measured the energy expenditure and activity levels of the participants who were a group of over 300 men and women. They used a technique called “doubly-labeled water” which allows researchers to track how many calories they burn. Basically they drink water that has a known amount of oxygen and hydrogen, and then as they expend energy they produce water and carbon dioxide. The elimination rates provide numbers that show the calorie burn through how much energy they have expended.

Strangely, their activity level had a fairly low effect on their calorie burn. The body tends to adapt to higher levels of physical activity so there’s not a huge calorie burn difference between moderate to high activity levels as there is low to moderate.

According to the study author Herbert Pontzer:

“The most physically active people expended the same amount of calories each day as people who were only moderately active.”

He went on to say:

“That’s the first thing I mention to anyone asking about the implications of this work for exercise. 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 yes, there is sort of a cap on how many calories you can burn in a day, but it varies from person to person. As for why this occurs, it’s just the body’s way of following the rules of nature.

“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. In fact, we find evidence for this metabolic ‘ceiling’ and plateau with energy expenditure vs activity, in many mammal species.”

So the point is certainly not to skip the exercise (obviously), but to pay close attention to what you’re putting into your body because it makes a big difference.