SSE #59: Fat Metabolism During Exercise: New Concepts
Edward F. Coyle, Ph.D.
SPORTS SCIENCE EXCHANGE
FAT METABOLISM DURING EXERCISE
SSE#59, Volume 8 (1995), Number 6
Edward F. Coyle, Ph.D.
Professor, Department of Kinesiology and Health Education
The University of Texas atAustin
Member, GSSI Sports Medicine Review Board
1. People store large amounts of body fat in the form of triglycerides within fat (adipose) tissue as well as within muscle fibers (intramuscular triglycerides).When compared to carbohydrate stored as muscle glycogen, these fat stores are mobilized and oxidized at relatively slow rates during exercise.
2. As exerciseprogresses from low to moderate intensity, e.g., 25-65% VO2max, the rate of fatty acid mobilization from adipose tissue into blood plasma declines, whereas the rate of total fat oxidation increases due to a relatively large use of intramuscular triglycerides. Intramuscular triglycerides also account for the characteristic increase in fat oxidation as a result of habitual endurance-training programs.3. Dietary carbohydrate intake has a large influence on fat mobilization and oxidation during exercise; when dietary carbohydrate produces sufficient carbohydrate reserves in the body, carbohydrate becomes the preferred fuel during exercise. This is especially important during intense exercise because only carbohydrate(not fat) can be mobilized and oxidized rapidly enough to meet the energyrequirements for intense muscular contractions.
The two main sources of energy during muscular exercise are fat (triglyceride) and carbohydrate (glycogen and glucose) stored within the body, and there has been much research and practical experience over the past 30 y demonstrating the importance of muscle and liver glycogen for reducing fatigue and improving athletic performance. Forexample, it is well known that diets containing predominantly carbohydrate are necessary to maintain glycogen stores at high levels during bouts of intense exercise and that such diets are apparently optimal for promoting training-induced improvements in performance (Simonsen et al., 1991). The primary reason that glycogen reserves are essential is that athletes can only slowly convert their body fatstores into energy during exercise. Therefore, when muscle glycogen and blood glucose concentrations are low, the intensity of exercise must be reduced to a level that can be supported by the body's limited ability to convert body fat into energy. With endurance training, athletes can markedly increase the rate at which body fat can be oxidized, thus allowing them to exercise longer beforebecoming exhausted due to glycogen depletion. Of course, exercise training also increases an individual's ability to exercise more intensely, so trained athletes must continue to derive most of their energy from carbohydrate during intense training and competition because their increased ability to oxidize fat cannot meet their increased energy demands.
What limits the rate at which people can converttheir body fat into energy during exercise? Recent research using new techniques has begun to shed light on this question, and the emerging picture will be discussed in this article. Although we do not yet have a complete understanding of fat metabolism during exercise, there is now enough information available to cast serious doubts on many of the recent advertising claims for special diets andnutritional supplements that stress more fat, and less carbohydrate.
BODY FAT STORES
Fat is stored in the body in the form of triglyceride, which is comprised of three fatty acids attached to a molecule of glycerol. The fatty acids consist of chains of carbon atoms with hydrogen atoms attached. There is more stored energy (9 kcal) in a gram of fat than in an equal weight of...