When your smart watch tells you 500 calories were burned in exercise, is it telling the truth?

This morning my fitness watch showed that I burned 720 calories between a one-hour aerobic dance class and a 45-minute walk with my dog. Yeah, me! But, a new study, says not so fast!  

My last post reported on a new study that said middle-age weight gain is not tied to a slower metabolism (click here for the post). and another new study says that we compensate for energy expended in activity. In other words, for most of us 28% of the extra calories we spend on additional activity does not translate into extra calories burned that day. Only 72% of the extra calories spent in physical activity is the number of extra calories burned that day. (Click here for the study in Current Biology).

The authors of this large study of 1754 adults aren’t sure why this is so, but they speculate that “compensation would have been adaptive for our ancestors because it minimized food energy demands and hence reduced the time needed for foraging, the advantages of which may include reducing exposure to predation.”  The only foraging I do for food is drive to the grocery store and I’ve never seen a predator trying to steal my grocery cart at the local Ingles!

The study also noted there was no difference in compensation between men and women, young or old, but there was a difference based on body composition. Those individuals who were at the 90th percentile for BMI and had highest body fat levels burned only 51% of their activity calories. The authors suggest that “It appears that individuals with greater fat levels are predisposed to increased adiposity either because they are stronger energy compensators or because they become stronger compensators as they get fatter. If the former, then two people can be equally active, yet one puts on fat mass while the other stays lean. If the latter, then such a positive feedback loop may imply that using exercise as a strategy to escape high adiposity becomes less effective.” It’s important to remember that we exercise for more than just burning calories. Just look at all the benefits of physical activity listed in this slide:

Rosenbloom & Murray, ACSM Health & Fitness Summit presentation

This study also made me realize that some of my advice to athletes was simplistic. I developed a series of visuals for collegiate athletes called “Eat This, Do That” (a riff on the popular “Eat this, not that) to show them how much exercise would be needed to burn off the calories in some of their favorite foods. I tried to be as scientific as I could; looking up the energy expenditure values for an average female and male athlete for a variety of activities. For example, a serving of chicken wings contains 1590 calories, so a female athlete needs to play basketball for 6 hours and male athlete for 4 hours to burn those calories. This new study shows I was off on my calculations!

This new study shows us biology is complicated! We should stop the simplistic advice to eat less and move more as a cure for those with obesity. As Ted Kyle says in his ConscienHealth commentary, “this is just one more reason that you can’t outrun a bad diet. Exercise is great for physical health. But the popular concept that a person can simply work out to “burn off” excess fat is a lie that won’t die. If the goal is to reach and maintain a healthier weight, advice from a nutrition professional (an RDN) can be quite helpful.” (Click here for his commentary on the study on energy expenditure.)

So, that 720 calories my watch told me I burned in exercise translates into about 520 calories. So, I think I will enjoy the feeling that being physically active gives me and not be lulled into the false idea that I can have an extra dessert today!

Chris Rosenbloom is a registered dietitian nutritionist and nutrition professor emerita at Georgia State University. She is co-author of Food & Fitness After 50. Follow her blog devoted to optimal aging by clicking here.