Exercise alone is not enough for humans to lose weight, say researchers from City University of New York. They suggest that the body adapts to changes in energy consumption, therefore using more energy on a regular basis does not necessarily result in the burning of more calories.
The report, published in Current Biology, says that our bodies can adapt to a more demanding lifestyle by reducing the amount of energy we need to keep going. That is to say that people starting exercise programmes would gradually see a decrease in the amount of weight they lose over time – and sometimes they may even start putting it back on again despite keeping up with routines.
The scientists use the example of the Hadza, a group of traditional hunter-gatherers in Tanzania. They say their lifestyle is much more arduous than those of most people in the US and UK, but at the end of the day, their energy expenditure is almost exactly the same.
Herman Pontzer, researcher on the study, said: "The Hadza are incredibly active, walking long distances each day and doing a lot of hard physical work as part of their everyday life. Despite these high activity levels, we found that they had similar daily energy expenditures to people living more sedentary, modernised lifestyles in the United States and Europe. That was a real surprise, and it got me thinking about the link between activity and energy expenditure."
In the study, researchers looked at the amount of energy used daily in 332 men and women, and the amount of activity completed to use that energy. Over the course of seven days, it became apparent that people carrying out a huge amount of activity generally used more energy that those who did very little exercise. However, upon closer inspection, the researchers found that people undergoing high levels of activity ultimately saw no benefit compared with those carrying out 'moderate levels' of activity.
Essentially, that means that the difference in calories burned between someone who runs a marathon every day and someone who goes for a short jog is very little. The body, it appears, adapts to higher activity levels to keep energy expenditure within a narrow range.
"The most physically active people expended the same amount of calories each day as people who were only moderately active," Pontzer explained. The researchers believe the findings should be taken into account by public health messages - especially in light of the obesity epidemic.
"There is tonnes of evidence that exercise is important for keeping our bodies and minds healthy, and this work does nothing to change that message," he added. "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."