Fat Burning vs. Carbohydrate Burning
Both fat and carbohydrates are macronutrients and play a pivotal role in supplying cellular energy during exercise. The body stores a certain amount of fat and carbohydrates in various locations throughout the body, although these nutrients need to be continually replenished through diet. The degree to which fat or carbohydrates are used to fuel exercise depends on multiple factors (see reference 1).
Carbohydrate Burning Basics
Muscles require a chemical called adenosine triphosphate to contract. Muscle cells store a certain amount of this chemical, although ATP needs to be replenished through the process of anaerobic glycolysis, or the breakdown and conversion of carbohydrates -- sugars -- into usable energy. For exercises of higher intensity and shorter duration, carbohydrates supply the majority of the cell's energy.
Fat Burning Basics
Fats are stored in adipose tissues throughout the body. When exercise begins, lipolysis, the breakdown of fats into fatty acids and glycerols, occurs. Fatty acids are then mobilized, oxidized within the cell and used for energy. Because fat oxidation occurs aerobically, fat metabolism is maximized during exercise of less intensity and longer duration.
Carbohydrates and fat have an interrelated role in supporting exercise. Carbohydrates serve as a metabolic primer and are essential to completely burn stored fats. Insufficient carbohydrate intake may result in the incomplete combustion of fat and the production of ketones, which have a number of negative health effects. Adequate carbohydrate intake also prevents the depletion of protein stores for energy and the breakdown of muscle tissue. However, excess carbohydrate intake may increase triglyceride levels within the bloodstream, and these are converted to fats and stored as adipose tissue. This can increase body fat and suppress the oxidation of fats as a fuel source.
Elite athletes, especially those in aerobically challenging sports, have an increased capacity to oxidize fats during exercise. These athletes can burn fat stores when carbohydrates are depleted, allowing them to run for longer times without fatiguing. There is some evidence of a "fat-burning zone," or exercise intensity at which fat metabolism is maximized. According to Peak Performance, highly trained athletes burn the most fat at about 70 percent of their maximal heart rate, while untrained exercisers burn the most fat at about 50 percent.
- Essentials of Strength Training and Conditioning (3rd. Ed.); Thomas R. Baechle and Roger W. Earle
- Peak Performance: Fat Burning; Using Body Fat Instead of Carbhydrates as Fuel
- ExRx.net: Carbohydrates
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