Athletes need adequate energy, especially during high-intensity and/or long duration training. Together, the three macronutrients—carbohydrates, proteins and fats—are essential to providing athletes with the energy they need.
In focusing on the levels of protein and carbohydrates in an athletic diet, it is easy to lose sight of the importance of healthy fats. Your body requires two classes of fatty acids to function properly: omega-3 and omega-6 fatty acids. Moderate to intense training requires both carbohydrates and fat for fuel. Hormones and other molecules produced from fat are important for maintaining the balance of the biochemical reactions that drive life.
Fats provide the body with energy to drive activity. Where carbohydrates account for the majority of energy during short-duration or low-intensity exercise, fats make up the majority of energy during longer or more intense workouts. Fats should never fall below 15 percent of your daily caloric intake, according to the Colorado State University Extension Service. For endurance athletes, up to 75 percent of energy demand may be met by fat in your body.
Hormones are chemicals that control the balance of biochemical reactions in your body, driving growth, development, recovery and overall health. Many hormones are produced from molecules derived from essential fatty acids, such as omega-3 and omega-6 fats. Steroid hormones that control how your body responds to high energy demands and maintain mineral balance, and sex hormones that drive muscle growth are derived from fat. A lack of fat in your diet will prevent these hormones from being in balance, impairing athletic performance and recovery.
Exercise-induced damage to your muscles triggers increases in strength and endurance. This damage also leads to inflammation in the muscles. When muscles are inflamed, they are sore, also losing strength and range of motion. Omega-3 fatty acids especially are needed to regulate the level of inflammation in your body. A diet low in omega-3 fats, while high in the more common omega-6 fats, can bias your body towards inflammation, impairing exercise recovery, according to the Linus Pauling Institute.
Integrating fat in your diet does not mean succumbing more often to bacon cheeseburgers and jelly donuts. Omega-6 fats are very common in the modern diet; they can be found in most vegetable and nuts oils as well as meats and dairy. Omega-3 fats are much more rare, although some are found in most foods containing omega-6 fats. Foods rich in omega-3 fats include walnuts, cold-water fish such as salmon, flaxseed, soybeans, soybean oil, tofu and canola oil. Many experts recommend eating no more than four to five times the amount of omega-6 fats as omega-3 fats, according to the Linus Pauling Institute. Athletes may wish to take an omega-3 supplement derived from fish or krill oil to ensure dietary balance.
Fat as Exercise Fuel
The longer-term effects of dietary fat on an athlete are not the only consideration; it is important to understand more acute issues as well. Regarding dietary fat as fuel during exercise, two major phenomena are the “metabolic crossover effect” (picture above (4.2)) and the “duration effect,” or “fat shift." The former involves a crossover from fat oxidation at rest at lower intensities toward carbohydrate usage at high intensities. That is, an inverse relationship exists between direct fat “burning” (measured by respiratory exchange ratio) and exercise intensity (measured via heart rate or VO2max) (increased exercise = decreased fat). Biochemical control and the immediacy of need for energy are reasons for this crossover. Even highly trained aerobic endurance athletes, with their enhanced capacity to oxidize fat, eventually “cross over” to carbohydrate use, albeit at higher intensities than less aerobically fit persons.
Asker Jeukendrup, PhD and Michael Gleeson, PhD
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. 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.
Overview of fat metabolism and the main organs involved. TG = triacylglycerol; FA = fatty acid;
IMTG = intramuscular triacylglycerol; LPL = lipoprotein lipase; VLDL = very low-density lipoprotein.
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.