Anyone who's spent hours slaving away on the treadmill will know that more exercise doesn't always equal more weight lost, and now new research could explain why. Scientists have found that, after a certain point, our bodies seem to adapt to higher activity levels, and actually stop burning extra calories. Hence the dreaded plateau that many of us have faced after starting a promising new workout routine.

If confirmed, it means we might need to rethink the ol' energy in vs. energy out formula, which we're frequently told is the key to weight loss, as the study makes a strong case for diet being just as important - if not more so - when it comes to shifting stubborn weight.

But before you cancel your gym membership - don't. Seriously. First of all, the research showed that it's only when we exercise beyond a moderate activity level that our bodies seem to stop burning extra calories. And secondly, the benefits of exercise go far, far beyond weight loss.

"Exercise is really important for your health," said lead researcher Herman Pontzer, from the City University of New York. "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," he explained.

The study also helps provide some insight into a long-standing contradiction that exists in weight loss research. While some studies show that increasing exercise levels causes people to burn more calories, large, comparative studies in humans and animals have shown that populations that lead very active lifestyles - like hunter-gatherers in Africa - seem to burn around the same number of calories as those that are far more sedentary.

To explore the link further, Pontzer and his team monitored the daily energy expenditure and activity levels of more than 332 adults living in five countries across Africa and North America over the course of a week.

Their results showed that physical activity did have a weak influence on the amount of calories burnt... but only up to a certain point.

What that means is that people with moderate activity levels did indeed burn around 200 calories more than the most sedentary people. But anyone who exercised beyond moderate activity levels didn't see any extra benefits in terms of calories burnt.

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

The good news in all this is that Pontzer believes there may be a 'sweet spot' for physical activity - if we don't do enough, we become unhealthy, but if we do too much our body may end up compensating for it.

The findings have been published in Current Biology, and the team will now investigate exactly how the body can adapt to all that extra exercise without burning additional calories.

In the meantime, it's probably a good idea to take a closer look at your diet if you want to lose weight this year... just don't forget that exercise could still save your life.

"It is an interesting study and there is a possibility that if we are very, very active there may be some adaptation," dietician Frankie Phillips, who wasn't involved in the study, told The Guardian. "But for most people even moderate activity isn’t what they are achieving at the moment and that’s crucial. Let’s not put people off before they have even got to a stage where they are moderately active."