Understanding how energy is supplied for activity helps make sense of exercise and diet regimens.

Energy — we all need it, we all use it, and we need a constant supply for physical activity. There are many different definitions of what energy is, but the one most accepted in the exercise science world is, “the capacity to perform work.”1 The body is a power plant that provides energy for all physical activity, converting chemical energy to mechanical energy and heat. This chemical energy is derived from food (carbohydrates, fats and proteins), stored in the body and ultimately converted into adenosine triphosphate (ATP).

ATP is the energy molecule that the body uses for work. It is a chemical compound stored primarily in the muscle cells that, in its chemical breakdown, provides energy for cells. Surprisingly, only a small amount of ATP is stored in the cells at any time. Four metabolic pathways provide ATP for the body as food is broken down — two are anaerobic (oxygen not used) and two are aerobic (oxygen needed). Which pathway is used to provide energy depends on the intensity and duration of the activity being performed.

Anaerobic energy systems

The two anaerobic energy systems are the ATP-PC system and the glycolysis system. Neither of these systems need oxygen to complete their metabolism. Both systems are used for high-intensity, short-duration work. They are used exclusively for activities such as sprinting, throwing, jumping and striking — high-intensity, forceful activities that last less than 30 seconds for the ATP-PC system, and less than three minutes for the glycolysis system.

The ATP-PC system provides a small amount of energy very quickly. This metabolic system has only two chemical reactions — one where ATP is broken down and energy is produced, and one where PC (phosphocreatine) is broken down to continue to restore ATP. This system is a true “coupled system” — the energy produced from the breakdown of one reaction is used by the other reaction for work. Although the body has a small amount of ATP present in the muscle cells, there is about three times more PC in the muscle cells. This amount of stored energy allows for high-intensity work for approximately 30 seconds. After that, the body needs to get ATP from another energy system.

Anaerobic glycolysis provides a slightly larger amount of energy than the ATP-PC system. It is the energy system of choice for high-intensity activities that last from 30 seconds to about three minutes. This system has 11 chemical reactions that incompletely metabolize glucose to provide ATP and a byproduct of metabolism that causes acute muscle soreness, lactic acid. In all-out activity, lactic acid in the muscle cells can stop the muscles from functioning — commonly called “lactic acid pain.” Lactic acid is produced by this energy system because oxygen is not available in the short time this system is producing ATP. But, even though lactic acid is produced by this system, about twice the amount of energy is produced compared to the ATP-PC system.

Aerobic energy systems

To perform activities that last longer than three minutes, energy comes from two other systems — the aerobic system and beta-oxidation, or fat metabolism. Again, intensity and duration play a role. Higher-intensity activities lasting between three and 20 minutes primarily use the aerobic system to receive the necessary ATP. These activities include walking, running, jogging, aerobic dance, cycling, swimming, etc. Activities of average to low intensity that last longer than 20 minutes receive ATP primarily from beta-oxidation. This may include walking, cycling, marathon running, triathlons and even being a couch potato.

The aerobic energy system is a completion of glucose metabolism. There are three parts to this system: aerobic glycolysis, the Krebs cycle and the electron transport system. Aerobic glycolysis has the same metabolic pathway as anaerobic glycolysis, but it uses oxygen, and the metabolic byproducts continue to the Krebs cycle for continued metabolism. No lactic acid is accumulated. The Krebs cycle and the electron transport system complete the metabolism of glucose, producing approximately 10 times the ATP as the anaerobic energy system. This energy system completes about 40 chemical reactions before ATP is generated.

Beta oxidation is the energy system that breaks down fat molecules for their entry into additional systems. Fats are found in the form of triglycerides — a structure that contains a backbone of glycerol and three fatty acids. The fatty acids are broken into parts by the beta oxidation pathway, and these parts then enter the Krebs cycle and electron transport system for final metabolism. Although this system may provide a high ATP turnover, there is a drawback. It takes 15 percent more oxygen to completely oxidize fats than it does for carbohydrates. If plenty of oxygen is available, as in lower-intensity, long-duration activities, fats represent a highly concentrated reserve of stored energy that can be used for physical activity performance.

Energy continuum

So, what energy system is used when? Basically, they are all used at the same time, with one system providing the primary source of energy, depending on activity intensity and duration. As noted previously, the ATP-PC system is the primary energy source for ATP in activities of very short duration — 30 seconds or less. The anaerobic glycolysis system provides energy for activities lasting 30 seconds to three minutes. For activities lasting more than 3 minutes, the aerobic system is the primary energy source, and in lower-intensity activity lasting longer than 20 minutes, beta oxidation provides the greatest amount of ATP. Figure 1 depicts the relative contribution of ATP for physical activity depending on time and intensity.

Energy expenditure

The amount of energy expended by the body daily is dependent on three factors: resting metabolic rate, the thermic effect of eating, and the thermic effect of physical activity. Resting metabolic rate is the energy expended by physiological functions, including ventilatory and cardiovascular activity, protein, glycogen and triglyceride synthesis, and cellular electrical activity. It typically accounts for 60 to 75 percent of daily energy expenditure. The thermic effect of eating is the increase in energy expenditure from digestion, absorption, transport and storage of nutrients, resulting in 10 to 15 percent of daily energy expenditure. The thermic effect of physical activity accounts for all activities done above resting levels, averaging 10 to 30 percent of daily energy expenditure.3 This average will vary depending on frequency, duration and intensity of physical activity, as well as an individual’s body weight and aerobic capacity.2

As active individuals set body-weight goals, trainers need to look at the “energy in = energy out” formula. If the food consumed is equal to daily energy expenditure, no change in body weight will occur. If the food consumed is greater than the daily energy expenditure, body weight can increase. Finally, if food consumed is less than daily energy expenditure, body weight can decrease. The amount of physical activity will aid in the formula — it may be that the same food intake can be maintained if additional physical activity is performed. The energy systems used for physical activity need to be examined to determine the proper diet needed for recovery.

For example, physical activity that uses the anaerobic glycolysis system or aerobic system for its energy will burn glucose (carbohydrates). The proper amount of carbohydrates in the daily diet contributes to recovery, allowing the active individual to perform more physical activity on the succeeding days. Improper recovery of carbohydrates in the diet decreases energy supply, which decreases the ability to perform. If the physical activity uses the beta oxidation system for its energy, fat will need to be replaced.

Carbohydrates (starches) are the source of glucose for the body. Active individuals should eat 55 to 60 percent carbohydrates, with 45 percent complex carbohydrates and 10 to 15 percent mono and disaccharides (compared to a general-population average of 24 percent from complex carbohydrates and 26 percent from mono and disaccharides).1 More active individuals may need up to 60 to 70 percent carbohydrates. Fats and oils provide the energy source for the beta-oxidation system, and presently average 34 to 38 percent of the American daily diet. This amount should be no higher than 30 percent, with 10 percent from saturated fat, 10 percent from monounsaturated fat and 10 percent from polyunsaturated fat. Protein completes the food sources, accounting for approximately 10 to 15 percent of daily calories. The normal recommended daily allowance for protein is eight-tenths of a gram per kilogram of body weight (0.8 g·kg-1·d-1). This amount of protein is easily found in the daily diets of adults.1 Individuals in heavy resistance training programs may need additional protein in their diets, the suggested amounts ranging from 0.96 g·kg-1·d-1 to 1.8 g·kg-1·d-1. Many individuals may already be getting these additional amounts in their typical diets.1

Conclusion and application

Everything a person physically does requires energy. This energy is produced from the metabolism of food. Without the proper amounts of nutrients, not only percentage amounts but also calorie amounts, the body’s ability to perform physical activity is decreased. A well-balanced diet will allow energy systems in the body to work efficiently, and produce the necessary amounts of ATP.

The changing media environment for Power 5.


1. Foss, M.L., & S.J. Keteyian. Fox’s Physiological Basis for Exercise and Sport, 6th ed. The McGraw-Hill Companies Inc.: Boston, Mass., 1998.

2. Melby, C.L., & J.O. Hill. Exercise, macronutrient balance, and body weight regulation. Sport Science Exchange, 12(1): 1-6, 1999.

3. Thompson, J.L., & M. Manore. Energy Balance in Berning, J.R., & S.N. Steen, Nutrition for Sport and Exercise, 2nd ed. Aspen Publishers Inc.: Gaithersburg, Md., 1998.

By Donna J. Terbizan & Brett A. Dolezal

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