Losing weight is an undertaking most of us have to do at some point (or several points) in our lives. For a lot of years the central piece of wisdom was to start working out, in order to burn more calories. And while there's certainly a lot of value to exercise, researchers are finding more and more that it's not the central method for losing weight. 

Instead -- though the foodies among us may not like it -- eating less is really the key to weight loss. 

A new study looked at how much energy people burned, relative to how active they were throughout the week. Some of the participants were not very active, of course, while some were moderately, and some very, active.

It turned out that people's energy expenditure tended to plateau above a certain point: That is, moderately active people burned more calories than inactive people. But those who were very active didn't expend much, if any, more energy than the moderately active ones. Which suggests that there's an upper limit to the number of calories we can burn just by increasing our activity levels. 

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

And the results didn't totally surprise him. Pontzer had previously studied hunter-gatherers in Tanzania, who spend much of their days walking; but when he measured their energy output, they didn't burn any more calories than people in the Western world, who tend to do a lot of sitting. 

Other researchers in recent years have also singled out calorie restriction as being the most effective way to lose weight. Exercise is important for a variety of different reasons, both physical and mental. But by itself, it just doesn't seem to help people lose much weight over the long-term. This is why people's exercise routines sometimes seem to become less effective over time. 

"Our study joins others in pointing to diet as the better tool for managing weight," Pontzer tells me. "Exercise is still important!, for heart health, mental health, healthy aging, and so on. But our study shows that our bodies adapt to higher physical activity workloads to keep daily energy expenditure in check. As a result, it's very hard to tip the 'calories in = calories out' equation in your favor by adding more exercise."

So for weight loss, reducing caloric intake seems to be the more important part of the equation. But again, exercise is valuable for a multitude of other reasons, including reducing the risk of disease -- from heart disease to cancer to depression to dementia -- not to mention living a longer life. 

"This work does nothing to change that message," says Pontzer. "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."