For example, on page 39 of The Paleo Diet for Athletes, the authors state, “For high-intesity exercise sessions, the body will turn to protein for a fuel source as glycogen stores run low.” They are saying that carbohydrates need to be consumed during high-intesity exercise otherwise the body will turn to it’s own muscle and use it for energy.
Now, high-intesity exercise is called anaerobic exercise and it uses only the following non-oxidative energy systems for fuel sources:
- Adenosine triphosphate (ATP) – which is chemical energy stored in skeletal muscle;
- Creatine phosphate (CrP) – which is a high-energy compound that can be broken down quickly to provide energy for ATP re-synthesis; and
- Glycogen – which is broken down through the lactacid system glycolysis, in which glucose units, derived primarily from intramuscular glycogen reserves, are broken down to lactate.
Or they may have meant to say, “…for aerobic exercise the body will turn to protein for a fuel source as glycogen stores run low…”. This is closer to the truth, however still misleading. According to Louise Burke’s Clinical Sports Nutrition, 4th Ed. “During prolonged exercise, the oxidative metabolism of CHO and lipid provides the vast majority of ATP for muscle contraction. Although amino acid oxidation occurs to a limited extent during exercise, CHO and lipid are the most important oxidative substrates.”
In other words, the body prefers to use carbohydrates (CHO) for fuel first and foremost (which it metabolizes into glucose) and may turn to fatty acids (lipid) in case glycogen stores run low. The body oxidizes protein as an energy source but only in small amounts, particularly when the other two fuel sources are struggling to provide (ie. as in the case of starvation). Proteins are required primarily for structural and regulatory functions, not energy.