Metabolism - Getting the Energy We Need
To make the most of an exercise program, it is important to understand how exercise affects the metabolic process as well as how it can be enhanced through diet and nutrition.
After food is consumed, it is broken down into components used for energy. Organic molecules, including amino acids, lipids, and simple sugars, are broken down by a process called catabolism. Simple sugars, mainly glucose, are the body’s primary source of energy, followed by fats. Only when these two energy sources are depleted is protein, or muscle mass, used for energy. In general, metabolizing protein for energy is not desirable. More energy is needed to metabolize protein than carbohydrates or lipids. Also, protein catabolism (breakdown) produces ammonia as a byproduct, which is harmful to cells. Continued catabolism of protein will damage cells and body systems as well as reduce the effectiveness of any exercise program.
Ultimately, catabolism ends in the production of adenosine triphosphate (ATP) (i.e., the body’s main energy molecule) in the mitochondria. ATP is necessary for virtually every energy-requiring process in the body. Furthermore, ATP is essential for anabolism, or the synthesis of new organic molecules used to perform repairs, support growth, and produce secretions. When living cells use ATP to create new molecules, a high-energy phosphate bond is broken to release the energy, thereby creating adenosine diphosphate (ADP).
Muscle Activity During Exercise
The goal of a nutritionally sound exercise program is to support healthy muscle function by providing enough energy for the exercise and recovery period. To design a healthy exercise program, it is valuable to know how energy is consumed by working muscles.
The first energy source to be used by a muscle is ATP, which is stored in the muscles in very limited quantities—enough for only one contraction. When exercise begins, more ATP must immediately be synthesized from creatine phosphate, which is also stored in muscle tissue. Like ATP, creatine phosphate stores are consumed quickly.
ATP and creatine phosphate are supported and replenished through the metabolism of glucose. Almost as soon as the muscle goes to work, glucose is released from glycogen reserves in the muscles in a process known as glycogenolysis. When adequate oxygen is available, glucose is burned through oxidative (aerobic) metabolism, with a high yield of ATP. When adequate oxygen is not available (as in sudden bursts of activity), anaerobic metabolism occurs. A byproduct of anaerobic metabolism is lactic acid. When lactic acid builds up, it creates the “burn” that is familiar to weight lifters and others who get a lot of anaerobic exercise.
As glycogen stores are depleted, the body turns to fat and then protein for energy. After a workout, during recovery, oxygen demand is high while muscles restore ATP, creatine phosphate, and glycogen.
Muscle performance and energy metabolism are determined by physical conditioning and the type of muscle fibers being used. Anaerobic activity is characterized by brief, intense workouts (e.g., 50-meter dashes or weight lifting). Strength training, which usually relies on short bursts of activity with relatively heavy weights (Aniansson 1981), builds muscle mass.
Aerobic endurance training (e.g., jogging and distance swimming) involves sustained, low-level muscle activity. Increased aerobic function is used to produce weight loss (providing fewer calories are consumed than expended) as well as improved respiration and cardiovascular function. Since aerobic activity does not result in increased muscle mass, a combination of aerobic and anaerobic activities (interval training), along with reduced caloric intake and other factors (e.g., nutritional status and body type) will result in both weight loss and increased muscle mass (Martini 1995).