Dietary fat is frequently undervalued as a contributor to health and performance of athletes. Fat is an extremely important fuel for endurance exercise, along with carbohydrate, and some fat intake is required for optimal health. Dietary fat provides the essential fatty acids (EFA) that cannot be synthesized in the body.
The fat stores of the body are very large in comparison with carbohydrate stores. In some forms of exercise (e.g., prolonged cycling or running), carbohydrate depletion is possibly a cause of fatigue and depletion and can occur within 1 to 2 hours of strenuous exercise (see chapter 6). The total amount of energy stored as glycogen in the muscles and liver has been estimated to be 8,000 kJ (2,000 kcal). Fat stores can contain more than 50 times the amount of energy contained in carbohydrate stores. A person with a body mass of 80 kg and 15% body fat has 12 kg of fat (see table 7.1). Most of this fat is stored in subcutaneous adipose tissue, but some fat can also be found in muscle as intramuscular triacylglycerol (IMTG). In theory, fat stores could provide sufficient energy for a runner to run at least 1,300 km.
Table 7.1 Availability of Substrates in the Human Body
|Energy kJ (kcal)
|Plasma fatty acid
Ideally, athletes would like to tap into their fat stores as much as possible and save the carbohydrate for later in a competition. Researchers, coaches, and athletes have therefore tried to devise nutritional strategies to enhance fat metabolism, spare carbohydrate stores, and thereby improve endurance performance. Understanding the effects of various nutritional strategies requires an understanding of fat metabolism and the factors that regulate fat oxidation during exercise. This chapter therefore describes fat metabolism in detail and discusses various ways in which researchers and athletes have tried to enhance fat metabolism by nutritional manipulation. Finally, the effects of both low-fat and high-fat diets on metabolism, exercise performance, and health are discussed.
Fat Metabolism During Exercise
FAs that are oxidized in the mitochondria of skeletal muscle during exercise are derived from various sources. The main two sources are adipose tissue and muscle triacylglycerols. A third fuel, plasma triacylglycerol may also be utilized, but the importance of this fuel is subject to debate. Figure 7.1 gives an overview of the fat substrates and their journey to the muscle. Triacylglycerols in adipose tissue are split into FAs and glycerol. The glycerol is released into the circulation, along with some of the FAs. A small percentage of FAs is not released into the circulation but is used to form new triacylglycerols within the adipose tissue, a process called reesterification. The other FAs are transported to the other tissues and taken up by skeletal muscle during exercise. Glycerol is transported to the liver, where it serves as a gluconeogenic substrate to form new glucose.
Besides the FAs in plasma, two other sources of FAs for oxidation in skeletal muscle are available. Circulating triacylglycerols (for example in a very low-density lipoprotein [VLDL]) can temporarily bind to lipoprotein lipase (LPL), which splits off FAs that can then be taken up by the muscle. A source of fat exists inside the muscle in the form of intramuscular triacylglycerol. These triacylglycerols are split by a hormone-sensitive lipase (HSL), and FAs are transported into the mitochondria for oxidation in the same way that FAs from plasma and plasma triacylglycerol are utilized.