Here's why you exercise so much and still can't lose weight

Story highlights

  • People who exercise a lot may hit a plateau above which they do not burn more calories
  • Experts think we may have evolved to limit our calorie burn to keep from starving
  • You may be able to break the ceiling by changing up your exercise regimen

(CNN)If you are intensifying your running regimen in hopes of losing weight, you might be running around in circles: There is a limit to how many calories we can burn through exercise, a new study suggests.

The grim message comes from a small study of a group of 332 adults living in the United States, Jamaica and Africa, some of them more sedentary and some more active. A team of researchers measured their activity level for seven days using an accelerometer, similar to the kind in the Fitbit and other wearable devices, and also measured the number of calories the participants burned over the week.
The researchers found that the participants who moved more also burned more calories, but only up to a point. The most active people hit a plateau and did not burn any more calories than their less-active peers.
    Although the researchers did not look at the specific activities that participants were doing, the level on the accelerometer at which calorie burning peters out would be achieved "if you're walking a couple miles a day, like to work and back, taking the stairs instead of the elevator and trying to exercise a couple times a week," said Herman Pontzer, associate professor of anthropology at Hunter College, and lead author of the study, which was published on Thursday in the journal Current Biology.
    The study is in step with a growing body of research suggesting that burning a bunch of calories is a less realistic weight loss strategy than we might have thought, or hoped. "We can't push the calories out [value] around too much," Pontzer said. "Our bodies work very hard to keep it the same."
    It might be time to shift that standard public health message: To lose weight, simply exercise more.
    "We would say that 'If you want to lose weight, you probably ought to focus on changing your diet and watching how much you eat.' Exercise can help and it's really important [for health in general], but they are two different tools," Pontzer said.

    Making the most of exercise

    The challenge of trying to lose weight just by exercising more is no secret to some clinicians. "This study actually explains a phenomenon that I see quite commonly," said Dr. Holly F. Lofton, director of the Medical Weight Management Program at NYU Langone Medical Center.