ACSM'S Health & Fitness Journal:
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Thompson, Dixie L. Ph.D., FACSM
Dixie L. Thompson, Ph.D., FACSM, is the director of the Center for Physical Activity and Health and department head for the Department of Exercise, Sport, and Leisure Studies at the University of Tennessee, Knoxville.
It has been observed since the early 1900s that exercise intensity influences the relative reliance on fats and carbohydrates for energy. In the mid-1990s, the term crossover concept was coined, and a new debate about its meaning emerged.
WHAT IS THE CROSSOVER CONCEPT?
Fats and carbohydrates are the primary sources of energy during exercise. The energy stored in fats and carbohydrates is released as these substrates are broken down. This energy is captured in the form of adenosine triphosphate (ATP). ATP is then used by cells for many purposes, including muscle contraction. Although each gram of fat stores relatively more energy than a gram of carbohydrate (9 kcal vs. 4 kcal, respectively), the rate at which ATP can be formed is higher for carbohydrates than for fats.
At rest and during light to moderate activity, fat breakdown can supply the majority of the body's need for ATP. To sustain higher exercise intensities, the body needs to resupply ATP more rapidly. Thus, for vigorous aerobic exercise, carbohydrates become the primary supplier of ATP. The Figure illustrates this concept.
FIGURE. Influence of...Image Tools
The precise point at which the "crossover" from primary reliance on fats to carbohydrates occurs varies and is particularly influenced by training. An important adaptation of regular aerobic exercise is skeletal muscle's increased ability to break down fat. After training, a person can more readily use the energy stored in fat. However, regardless of one's state of training, fat cannot supply ATP at the rate needed to sustain high-intensity exercise.
IMPLICATIONS FOR COMPETITION
Athletes need fats and carbohydrates to supply their energy needs during training and competition. Individuals who are aerobically trained and can resupply ATP more rapidly from fats may have an advantage in avoiding glycogen depletion. Although this increase in the ability to use fats as a source for ATP increases with training, the intensity at which most competitive events occurs will require that athletes have an exceptional ability to supply ATP from carbohydrates. As you might expect, trained athletes have enhanced ability to break down carbohydrates without oxygen (glycolysis) and with oxygen (oxidation).
IMPLICATIONS FOR WEIGHT LOSS
The recognition that lower-intensity exercise is more reliant on the breakdown of fats is sometimes used to encourage people to engage in low-, rather than high, intensity exercise for weight loss. Evidence suggests that total caloric expenditure, rather than the percentage reliance on fats, is the most important factor influencing weight change. Certainly, low-intensity exercise can be used to create a caloric deficit, but the time devoted to exercise must be expanded to allow for the same expenditure of calories that can be achieved with moderate or vigorous activity. The good news is that individuals wanting weight loss can choose the intensity and type of activity that best fit their fitness, time, and enjoyment requirements.
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