Fat Burning Vs. Carb Burning During Exercise Over Time

Knowing when the body begins to burn fat can help the dieter know how to lose weight in the most optimal way.
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For all forms of physical exercise, fat and carbohydrates are the main fuel sources for your body. While your body does burn both of these fuel sources simultaneously, the way you exercise determines how much of each your body burns. Knowing this information can help endurance athletes conserve energy and help dieters lose fat.


Fat is the more abundant of the fuel sources maintained in your body. Your body has a better capacity for storing fat than it does carbohydrates. It is natural for your body to store excess calories as fat instead of other forms of energy, as fat is more useful as an energy source. This fat is not always stored in muscles, but can also be in tissue around your body’s organs. Healthy fat sources include foods such as salmon, nuts, seeds and plant oils.


Carbohydrates differ from fats in a couple meaningful ways: They are stored in your muscles and liver and your body has fewer reserves for carbs than it does for fat. Your body uses carbs by breaking them down into glucose, or sugar, an energy source. Carbs come from food sources that contain sugar or grains. Healthy examples include whole grains, beans, fruits and vegetables.

Carb and Fat Burning

Carbs are the go-to source for energy when you begin to exercise. While you exert yourself, you are telling your body to recruit the glycogen, or carbohydrates stores, in your muscles as fuel. Your body burns its storage of carbs quickly during an exercise. Usually, but depending on the intensity of the exercise, your body will rely mainly on carbohydrates as fuel for the first 30 minutes of a workout. When your body runs out of its glycogen reserves or begins to get low on glycogen reserves, it switches its fuel source to fat. Fat is a reliable fuel source that your body can use well after you’ve depleted your glycogen reserves and makes it possible for athletes to perform for periods extending over the course of several hours.

Importance of Intensity

The intensity of your exercise helps determine how your body recruits its energy source of choice. Lower intensity exercises done over a long period tend to use more fat sources as energy, while higher intensity exercises tend to use more carbohydrate sources. Depending on the intensity, the proportion of fat your body is burning as an energy source can change from around 30 percent to 60 percent.

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