Metabolism and energy

All energy originates from the sun as light energy. Chemical reactions in plants(photosynthesis) convert light into stored chemical energy. In turn, we obtain energy by eating plants or animals that feed on plants. Nutrients from ingested foods are provided and stored as carbohydrates, fats, and proteins. These three basic fuels, or energy substrates, can be broken down in our cells to release the stored energy. Each cell contains chemical pathways that convert these substrates to energy that can then be used by that cell and other cells of the body, a process called bioenergetics. All of the chemical reactions in the body are collectively called metabolism.

Because all energy eventually degrades to heat, the amount of energy released in a biological reaction can be calculated from the amount of heat produced. Energy in biological systems is measured in calories. By definition, 1 calorie(cal) equals the amount of heat energy needed to raise 1g of water 1°C, from 14,5°C to 15,5°C. In humans, energy is expressed in kilocalories(kcal), where 1 kcal is the equivalent of 1,000 cal. Sometimes the term Calorie(with a capital C) is used synonymously with kilocalorie, but kilocalorie is more technically and scientifically correct. Thus, when one reads that someone eats or expends 3,000 cal per day, it really means that person is ingesting or expending 3,000 kcal per day.

Some free energy in the cells is used for growth and repair throughout the body. Such processes build muscle mass during training and repair muscle damage after exercise or injury. Energy also is needed for active transport of many substances, such as sodium, potassium, and calcium ions, across cell membranes. Active transport is critical to the survival of cells and the maintenance of homeostasis. Myofibrils also use some of the energy released in our bodies to cause sliding of the actin and myosin filaments, resulting in muscle action and force generation.

Energy sources

Energy is released when chemical bonds – the bonds that hold elements together to form molecules – are broken. Foods are composed primarily of carbon, hydrogen, oxygen, and(in the case of protein) 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 that aid chemical reactions and as structural building blocks, but usually provide little energy for metabolism. During intense, short-duration muscular effort, more carbohydrate is used, with less reliance on fat to generate ATP. Longer, less intense exercise utilizes carbohydrate and fat sustained energy production.

Carbohydrate

The amount of carbohydrate 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 glucose, a monosaccharide(one-unit, or simple, sugar) that is transported through the blood to all body tissues. Under resting conditions, ingested carbohydrate is stored in muscles and liver in the form of a more complex sugar molecule, 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 unless the diet contains a reasonable amount of carbohydrate. 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.

Fat

Fat provides a large portion of the energy 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. Table below provides and indication of the total body stores of these two energy sources in a lean person(12% body fat). 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. But fat is less readily available for cellular metabolism because it must first be reduced from its complex form, triglyceride, to its basic components, glycerol and free fatty acids(FFAs). Only FFAs are used to form ATP.

Body stores of fuels and energy
	g	kcal
Carbohydrates
Liver glycogen	110	451
Muscle	500	2,050
Glucose in body fluids	15	62
Fat
Subcutaneous and visceral	7,800	73,220
Intramuscular	161	1,513
Total	7,961	74,833

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 fat is 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 also found in cell membranes and function as hormones and building blocks of hormones such as estrogen and testosterone.

Protein

Protein can also be used as a minor energy source, 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 fatty acids is termed lipogenesis. Protein can supply up to 5% or 10% of the energy needed to sustain prolonged exercise. Only the most basic units of protein – the amino acids – can be used for energy. A gram of protein yields about 4.1 kcal.

Rate of energy release

To be useful, free energy must be released from chemical compounds at a controlled rate. This rate is partially determined by the choice of the primary fuel source. Large amounts of one particular fuel can cause cells to rely more on that source than on alternatives. This influence of energy availability is termed the mass action effect.

Specific protein molecules called enzymes control the rate of free-energy release. Many of these enzymes facilitate the breakdown(catabolism) of chemical compounds. The way these enzymes speed catabolism has been characterized as a “lock-and-key” mechanism. However, many enzymes also become altered in structure after binding to the chemical compound. Thus, the structure and function of enzymes may be more complex, but the concept of the lock and key provides a useful model of the interactions between energy compounds(e.g. glucose) and enzymes important to energy transfer within the cell. Although the enzyme names are quite complex, most end with the suffix –ase. For example, an important enzyme that acts to a break down ATP and release stored energy is adenosine triphosphatase(ATPase).

“Physiology of sport and exercise”, fourth edition; Jack H. Wilmore, David L. Costill, W. Larry Kenney