If you’ve ever used any piece of cardio training equipment you’ve seen it: the “heart rate zone chart.” This handy reference tool provides you with some research-based general guidelines for where to keep your heart rate to achieve specific fitness goals.
The target zone for most people, and the focus of most exercise programs, is the “weight loss zone.”
Although there is merit to the “weight loss zone” and related programs, people often take it a step further and claim that exercising above this zone will make your body burn muscle for fuel. However, this has also led to many bodybuilders choosing to avoid cardio altogether during their bulking cycles so that they don’t risk losing any muscle.
But what’s the science at work here? Is the fat loss zone real? Is it possible to work out so intensely that you actually burn muscle?
The Fat Loss Zone
It is true that the fat loss zone exists but it is often misrepresented. Your body will not burn a higher amount of fat in this zone but it will burn a higher percentage. It’s not a matter of whether your body is using carbs or fat for fuel, because you are always using both — in different proportions — depending on the needs at hand.
According to Active.com, exercising at lower intensities forces your body to use about 50 percent of both carbs and fat. At higher intensities this mixture switches to about 35 percent fat, 65 percent carbs, but your total caloric expenditure is much higher so it will likely balance out.
For example, if you run at eight miles per hour for an hour, you burn about 860 calories and 300 of those are from fat. A lower-intensity exercise, like a jog at five miles per hour, will burn 600 total calories with 300 from fat.
Muscle For Fuel
Is there any point at which your body will start to use muscle for fuel? Yes, but you’re not likely to reach it during an average workout. Muscle is precious, used for literally everything you do on a daily basis and your body isn’t eager to destroy it. Using muscle for fuel is called a catabolic state and occurs only during periods of starvation. Interestingly, crash diets can create catabolisis by restricting the caloric intake so much that the body has no option but to turn on its own muscle for fuel.
A study published in the American Journal of Clinical Nutrition found no correlation between exercise intensity and whether your body burns muscle or fat for fuel. The researchers concluded that, when it comes to conserving muscle, exercise intensity can be left up to you.
Can You Work Out Too Hard?
Although it’s not likely that your workout will destroy your muscles, is it possible for you to work out too hard? Of course it is. Working out too intensely puts you at an increased risk for muscle overuse and other injuries, which can put you out of commission for a while and throw off your exercise routine.
Listen to your body when deciding on the appropriate exercise intensity for you and your fitness level.