Energy Content of Carbohydrates, Fats, and Protein
by Stanley Phillips
The energy contained in equal weights of carbohydrate, fat, and protein is not the same. Energy content is measured in Calories (note the capital C). Carbohydrates and protein both contain 4.1 Calories per gram (120 Calories per ounce) while the energy “density” of fat is more than double at 9 Calories per gram. The disadvantage of fat as a fuel for exercise is that it is metabolized through pathways that differ from carbohydrates and can only support an exercise level equivalent to 50% VO2 max. It is an ideal fuel for endurance events, but unacceptable for high level aerobic (or sprint) type activities.
Carbohydrate metabolism is much more efficient than fat metabolism assuming adequate oxygen is available (ie aerobic metabolism). But once VO2max has been reached, and anaerobic metabolism takes over, the efficiency of carbohydrate metabolism drops off dramatically. Carbohydrate will produce 19 times as many units of ATP per gram when metabolized in the presence of adequate cell oxygen supplies (aerobic) as opposed to its metabolism in an oxygen deficient (anaerobic) environment.
In the well fed and rested state, the human body contains approximately 1500 carbohydrate Calories (stored as glycogen) in the liver and muscle tissue, and over 100,000 Calories of energy stored as fat. The carbohydrate Calories are adequate energy for several hours of brisk cycling (80 to 100 % VO2max), and if one slows the pace to 50 – 60 % VO2max where fat Calories can be utilized, there are enough energy stores to support cycling at this reduced speed for days.
How can these facts help you in designing a program to maximize your performance?? If one does not supplement glucose stores in the body (snacking while riding), you will run out of carbohydrate stored in your muscle and liver cells after 2 hours of aerobic activity, and the bonk occurs. This term describes the fatigue resulting from muscle glycogen depletion. Without adequate carbohydrate to fuel continues high level muscle activity, it is impossible to maintain a high level of energy output and one has to slow to speeds of 50% VO2max where fat metabolism can provide the needed Calories. The bonk can be delayed by using oral glucose to supplement muscle glycogen stores. On a long ride, a rider that snacks will have more glucose available to fuel that final sprint.
Two other strategies are to 1) minimize extremely energy inefficient anaerobic sprints earlier in the ride (remember they are very inefficient in terms of ATP production) and 2) whenever possible, ride closer to 50% VO2max to take advantage of supplemental Calories available from fat metabolism. In addition to eating while riding, these two strategies will help to save a few more grams of muscle glycogen for that final sprint to the line.
