Max Heart Rate & Fat Burn

Your body uses mostly carbohydrate and fat for energy, preferring to conserve finite carbohydrate stores and burn practically inexhaustible fat whenever possible. However, fat metabolism is slow, so as your energy demands increase, a greater proportion of energy comes from carbohydrate. At your maximum heart rate -- measured in how many times your heart beats in a minute -- nearly all of the energy you use comes from carbohydrate.

Significance

Sports professionals often describe exercise intensity as a percentage of your maximum heart rate, or MHR. At heart rates up to 70 percent of MHR, 85 percent energy used to sustain the activity comes from fat. Above 70 percent of your maximum heart rate, carbohydrate picks up the slack for what fat metabolism can’t keep up with. Beyond 80 percent of your maximum heart rate, nearly all of your energy comes from carbohydrate.

Misconceptions

Many people mistakenly believe that working out in the “fat burning zone” below 70 percent of MHR is the best way to lose weight. In fact, you are burning almost as much fat per minute at higher intensities. However, because you are burning more total calories, a smaller proportion of those calories come from fat. Since your muscles store carbohydrate like a gas tank stores gasoline, intense workouts not only burn fat, but also require more calories afterward to replenish carbohydrate stores. Since those calories won’t be converted to fat, high-intensity exercise lets you eat more without gaining weight. Losing weight is a matter of consuming fewer calories than you burn, so burning more calories through an intense workout is a faster route to weight loss.

Time Frame

The more intense your exercise, the less time you need to spend working out to burn the same number of calories. Also, according to the website Exercise Prescription, after half an hour of exercise your body gradually uses more fat and less carbohydrate to preserve its dwindling carbohydrate stores.

Potential

Your target heart rate depends on your MHR. As a rule of thumb, MHR 220 minus your age for men and 226 minus your age for women. However, as Sally Edwards of Cycling Fusion explains, maximum heart rate varies greatly by individual; up to 12 beats per minute according to the American Council on Exercise. Therefore, the best way to determine your real MHR is to do a maximal stress test, exercising nearly to the point of collapse. Most people are not fit enough to perform a maximal exercise test, so you can estimate your maximum heart rate by performing a sub-maximal field test such as the Rockport walking test or a timed step test.

Warning

Since heart rate testing requires a high level of initial fitness, get a doctor’s approval before doing MHR testing. Also, if you have a heart condition or are at risk for cardiovascular disease, then your doctor may recommend that you stay below a certain heart rate in your workouts.