Exercise by itself isn't always enough to take off the weight. Now, evidence reported in the journal Current Biology helps to explain why that is: our bodies adapt to higher activity levels, so that people don't necessarily burn extra calories even if they exercise more.
The results suggest it's time to rethink the effect of physical activity on daily energy expenditure, the researchers say. They are also a reminder of the importance of diet and exercise in supporting weight loss goals.
People who start exercise programs to lose weight often see a decline in weight loss (or even a reversal) after a few months. Large comparative studies have also shown that people with very active lifestyles have similar daily energy expenditure to people in more sedentary populations.
To explore this question further in the new study researchers measured the daily energy expenditure and activity levels of more than 300 men and women over the course of a week.
In the data they collected, they saw a weak but measurable effect of physical activity on daily energy expenditure. But, further analysis showed that this pattern only held among subjects on the lower half of the physical activity spectrum. People with moderate activity levels had somewhat higher daily energy expenditures--about 200 calories higher--than the most sedentary people. But people who fell above moderate activity levels saw no effect of their extra work in terms of energy expenditure.
"The most physically active people expended the same amount of calories each day as people who were only moderately active," author says.
The researchers say it's time to stop assuming that more physical activity always means more calories. There might be a "sweet spot" for physical activity--too little and we're unhealthy, but too much and the body makes big adjustments in order to adapt.
Researchers now plan to study how the body responds to changes in activity level. They'll start by looking for other changes--for example, in immune function or the reproductive system--that might explain how the body adapts to greater physical demands without consuming extra calories.