You may be familiar with the idea that exercising within a certain range of intensity puts your body into the “fat burning zone.”

Sometimes this level of exercise intensity is referred to as “Zone 2.” (These two concepts aren’t identical but they overlap and for our purposes we’ll discuss them together.)

How can you put the fat burning principle to work for you?

Here’s the theory of the fat burning zone

The body uses both carbohydrates and fats as fuel when you exercise. They’re used in different combinations based on the type of exercise you’re doing and its intensity. Tapping your carbohydrate stores is the quickest way to give your muscles an energy boost. But when steady-state activity is called for, your body will access the much larger stores of energy found in fat tissue.

So if you exercise with a moderate level of intensity, in the “fat burning zone,” as much as 65% of the energy you’re using could be coming from fat. With higher intensity exercise, only about 35% of the total energy you’re using might be from fat.

Measuring exercise intensity

A simple way to gauge exercise intensity is to measure heart rate. And, in particular, heart rate as a percentage of your (hypothetical) age-related maximal heart rate.

Everyone’s maximal heart rate will be different. But the general trend is that your maximal achievable rate will diminish with age. The most commonly used formula to derive your hypothetical maximal heart rate (the “Fox” formula) is to start with 220 and subtract your age. But a more accurate formula is the “Tanaka” formula: start with 208 and subtract 70% of your age.

For example

I’m 70, so by the Tanaka formula, my maximal heart rate would be 208 – (70 x 0.7) = about 159 beats per minute. So my “fat burning zone” would be to exercise with a level of intensity to keep my heart rate within 60-70% of its maximum, or about 94 to 111 beats per minute.

There are a wide range of moderate-intensity activities that will get your heart rate into the 60-70% of maximum range: walking, biking, hiking, even doing housework.

An even simpler way

If you don’t want to go to the trouble of measuring your heart rate, you can estimate that you’re in the fat burning zone if you’re still able to carry on a conversation though with a bit of a strain.

Despite these benefits

Despite these benefits, here’s why I suggest that you not focus your exercise strategy around workouts in Zone 2:

1. Yes, it’s true that with more intense exercise, a smaller fraction of your caloric needs will be drawn from fat. But when you work more intensely, you’re using more calories overall, so you’re drawing down a larger total of calories from your fat stores.
2. I’m lazy. If you keep your exercise within the fat burning zone, it just takes more time to achieve your fitness goals. Moderate exercise only stresses the body moderately, so it only moderately stimulates beneficial fitness adaptations. I’d rather exercise for a shorter period of time than stretch it out.
3. Exercising within the fat burning zone isn’t the most efficient way to improve your body composition and overall metabolism. Instead, focus on building muscle. To do that, you have to stress your muscles closer to their limit with weight training or other higher intensity exercise.

I do recommend moderate exercise (within the fat burning zone) as part of an “off” day. If you’ve been in the gym lifting weights, or running high intensity intervals, you need a day off once in a while. But rather than being entirely sedentary on your “off” day, it’s better to take a moderate walk or enjoy a bike ride in the neighborhood, staying within your fat burning heart rate zone.

Dr. Lavine has been an innovator in the use of movement and touch to promote health since 1981. He practices in New York City and Princeton, NJ.