The fat-burning heart-rate zone is a myth: How exercise and weight loss really work
Yes, we know. If you look at the wall charts or cardio equipment in a gym, or listen to many personal trainers, you’ll be indoctrinated about the “fat-burning zone.” The standard advice for getting in this zone is to workout at about 60 percent of your maximum heart rate. That level of exertion is relatively low intensity; most people can talk in complete sentences while exercising at it. Working in this zone, it’s said, will burn more fat and result in greater long-term weight loss, compared with doing the same exercise at higher intensities.
There’s substance to part of this claim. Your body primarily fuels itself by burning a mix of stored fat and carbohydrates. The less active you are at a given moment, the greater the percentage of that fuel mix comes from fat. As your intensity of activity increases, the percentage of carbohydrates in that fuel mix also increases . At rest, fat constitutes as much as 85 percent of calories burned. That figure shifts to about 70 percent at an easy walking pace. If you transition to a moderate-effort run, the mix becomes about 50 percent fat and 50 percent carbohydrates, and it moves increasingly toward carbohydrates the faster you go.
