We all know that in order to lose weight, we need to do more exercise to burn more calories, and eat controlled portions of the right foods. But it could be that too much of a good thing – in this case physical activity – might not be as advantageous as many had thought. Research suggests that as exercise increases, our bodies adapt and counteract the effects, meaning that they end up burning as many calories as those who are more sedentary.

When obese people increase their physical activity, it is often matched with significant weight loss, but this often declines after a few months. Even when some then increase the amount of exercise they’re doing, the pounds no longer seem to shift. What the researchers in this study claim is that there is a big jump in energy expenditure, or calories burned, from sedentary people to moderate exercisers, but no such shift in energy expenditure when people then participated in even more intense exercise.

Interestingly, this might also help to explain another curiosity noticed by biologists studying how hunter-gatherers' bodies expend energy, as they lead incredibly active lives walking long distances and doing hard physical work. “Despite these high activity levels, we [find] that they [have] similar daily energy expenditures to people living more sedentary, modernized lifestyles in the United States and Europe,” explains Herman Pontzer, coauthor of the study published in Current Biology, in a statement. “That was a real surprise, and it got me thinking about the link between activity and energy expenditure.”

To examine this, Pontzer and his team measured the daily energy expenditure of over 300 men and women from five countries, for seven days. The data showed how there was a weak increase in energy expenditure when people first start on an activity regime, and that there is a “sweet spot” during moderate exercise where people are burning on average around 200 calories more than they would have been without this level of exercise. But they found that above this level of activity, they had nothing to show for it in terms of an increase in energy expenditure, as the body adapts to the extra work.

“Exercise is really important for your health,” continues Pontzer. “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.”

The researchers say that their study just goes to show that we need to stop instantly assuming that more physical activity means more calories burned, as the relationship is not that simple.