Energy is released when chemical bonds – the bonds that hold elements together to form molecules – are broken. Substrates are composed primarily of carbon, hydrogen, oxygen, and in some cases nitrogen. The molecular bonds that hold these elements together are relatively weak and therefore provide little energy when broken. Consequently, food is not used directly for cellular operations. Rather, the energy in food molecular bonds is chemically released within our cells and then stored in the form of the high – energy compound adenosine triphosphate (ATP).

At rest, the energy that the body needs is derived almost equally from the breakdown of carbohydrates and fats. Proteins serve important functions as enzymes and as structural building blocks but usually provide little energy for metabolism. During intense, short – duration muscular effort, more carbohydrates are used, with less reliance on fat to generate ATP. Longer, less intense exercise utilizes carbohydrates and fats for sustained energy production.


The amount of carbohydrates utilized during exercise is related to both the carbohydrate availability and the muscles well – developed system for carbohydrate metabolism. All carbohydrates are ultimately converted to the simple six – carbon sugar, glucose, a monosaccharide (one – unit sugar) that is transported through the blood to all body tissues. Under resting conditions, ingested carbohydrate is stored in muscles and in the liver in the form of more complex polysaccharide (multiple linked sugar molecules), glycogen. Glycogen is stored in the cytoplasm of muscle cells until those cells use it to form ATP. The glycogen stored in the liver is converted back to glucose as needed and then transported by the blood to active tissues, where it is metabolized.

Liver and muscle glycogen stores are limited and can be depleted during prolonged, intense exercise, especially if the diet contains an insufficient amount of carbohydrates. Thus, we rely heavily on dietary sources of starches and sugars to continually replenish our carbohydrate reserves. Without adequate carbohydrate intake, muscles can be deprived of their primary energy source. Furthermore, carbohydrates are the only energy source utilized by brain tissue, therefore severe carbohydrate depletion results in negative cognitive effects.


Fats provide a large portion of the energy utilized during prolonged, less intense exercise. Body stores of potential energy in the form of fat are substantially larger than the reserves of carbohydrate, in terms of both weight and potential energy. For the average middle – aged adult with more body fat (adipose tissue), the fat stores would be approximately twice as large, whereas the carbohydrate stores would be about the same. Fat is less rapidly available for cellular metabolism because it must first be reduced from its complex form, triglyceride, to its basic components, glycerol and three fatty acids (FFAs). Only FFAs are used to form ATP.

Substantially more energy is derived from breaking down a gram of fat (9.4 kcal/g) than from the same amount of carbohydrate (4.1 kcal/g). Nonetheless, the rate of energy release from fats too slow to meet all of the energy demands of intense muscular activity. Other types of fats found in the body serve non – energy – producing functions. Phospholipids are a key structural component of all cell membranes and form protective sheaths around some large nerves. Steroids are found in cell membranes and also function as hormones or as building blocks of hormones such as estrogen and testosterone.


Protein also can be used as a minor energy source under some circumstances, but it must first be converted into glucose. In the case of severe energy depletion or starvation, protein may even be used to generate FFAs for cellular energy. The process by which protein or fat is converted into glucose is called gluconeogenesis. The process of converting protein into FFAs is termed lipogenesis. Protein can supply up to 5% or 10% of the energy needed to sustain prolonged exercise. Only the most basic unit of protein – the amino acids – can be used for energy. A gram of protein yields about 4.1 kcal.


Guyton and Hall – Medical physiology 12 ed.

Berne and Levy – Physiology 6ed.

Costanzo – Physiology 5ed.

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