The Science of Energy Systems and How They Relate to Performance Indicators and Dietary Modifications.
The energy in food does not transfer directly to cells for biologic work. “Macronutrient energy” releases and funnels through the energy-rich compound adenosine triphosphate (ATP).
The immediate energy system of the body is the ATP-PCr System. This is utilized for short duration, high intensity performances such as a 100m sprint, 25m swim or lifting a heavy weight.
The body stores only 80-100g of ATP. This provides enough intramuscular stored energy for several seconds of explosive, all-out exercise. Because the body only stores a small quantity of ATP, it must be re-synthesized continuously at its rate of use. This re-synthesis is achieved by way of cellular respiration.
There are two forms of cellular respiration: Anaerobic and Aerobic.
Anaerobic metabolism provides short term energy. This energy system is utilized during a 400m sprint, a 100m swim, or in multiple sprint sports like soccer or hockey. Anaerobic metabolism occurs in the cytoplasm of the cell, requires no oxygen, produces 2 ATP, and results in lactate as an end product.
So what are the implications for training.
Exercise intensity at the point of lactate build-up (blood lactate threshold) is a powerful predictor of aerobic exercise performance. The blood lactate threshold, the point at which lactate begins to accumulate in the blood, occurs at a higher percentage of an endurance athlete’s aerobic power as a result of training adaptation (compared to an untrained person or undertrained athlete). Cellular adaptations that increase the rate of lactate removal include increased capillary density and increased mitochondria size and number. Blood lactate levels are therefore an objective measure of training gains and subsequent performance capacity.
Aerobic metabolism occurs within the mitochondria of the cell. This process requires oxygen and produces 30+ molecules of ATP. The end product of aerobic metabolism is carbon dioxide and water.
So how does this translate to exercise.
Liver and muscle glycogen (the storage form of glucose) provide 40-50% of the energy required for moderate exercise. As exercise continues, energy is produced by fat breakdown as glycogen stores become depleted. Free fatty acid uptake by active muscles increases with exercise duration. In the first 90 minutes, less than 40% of energy comes from fat catabolism (breakdown of stored fat). By the third hour, 50-60% of energy comes from fat catabolism. This is the basis for ketogenic diets in high endurance athletes. However, it is interesting to note that carbohydrate depletion decreases work capacity. After 2 hours, exercise capacity progressively decreases to 50% of the starting exercise intensity. Reduced power output directly results from the slow rate of aerobic energy release from fat oxidation.