Metabolism refers to all of the chemical reactions that take place in a living body to keep that body alive. Metabolic reactions are of two general kinds: catabolic and anabolic reactions. Catabolic reactions are chemical reactions by which nutrients are broken down to produce the energy needed to keep a body alive and active. Anabolic reactions are chemical reactions that result in the formation of complex chemical compounds used to build body parts and for other purposes.
The human body makes use of three types of nutrients in the metabolic reactions it performs: carbohydrates, proteins, and lipids. Carbohydrates are complex chemical compounds that consist of carbon, hydrogen, and oxygen, the latter two in the proportion of about two to one (as in H2O), thus accounting for the expression “hydrate.” Starches and sugars are the most common kind of carbohydrates included in the human diet. Starches are large, more complex compounds, accounting for their designation as complex carbohydrates. Sugars are simpler carbohydrates, hence the name simple carbohydrates. The body requires other types of nutrients, such as vitamins and minerals, although these compounds are used primarily as catalysts for catabolic and anabolic reactions, and are not used up in those reactions.
When a person eats carbohydrates, they pass through the digestive system into the stomach, where they are broken down into simple sugars, primarily the sugar known as glucose (C6H12O6). Glucose, also known as “blood sugar,” then passes through the stomach walls into the bloodstream, where it is transported to cells throughout the body. The primary means by which the human body produces energy is the breakdown of glucose within cells, a process known as glycolysis. When glucose breaks down, it produces carbon dioxide and water, as indicated by the following equation:
This equation is somewhat deceptive since the breakdown of glucose does not occur in a single step. Instead, the process involves a complex series of more than 30 individual reactions. In the first set of those reactions, the six-carbon glucose molecule is broken down into a three-carbon molecule called acetyl coenzyme A (CoA). Acetyl CoA then feeds into a series of reactions known as the Krebs cycle or the tricarboxylic acid cycle, a series of reactions that produce an energyrich compound known as adenosine triphosphate (ATP). ATP is the key chemical compound that stores and provides energy for many chemical reactions needed to keep a human body alive and active. Energy is provided by the breakdown of ATP molecules to adenosine diphosphate (ADP). ATP is a very unstable molecule that spontaneously breaks down into ADP and a burst of energy very rapidly, the human body must produce ATP continuously in order to keep up with its energy needs.
The human body can obtain energy from nutrients other than carbohydrates. The next most important source of energy in the body is the family of compounds known as lipids, primarily fats and oils. When a person ingests fats and oils, they pass through the digestive system into the small intestine, where they are broken down into simpler compounds known as fatty acids and glycerol. These compounds then pass through cell walls into the bloodstream, where they are carried to cells for metabolism. As with carbohydrates, fatty acids go through a complex series of reactions that result in the formation of acetyl CoA and ATP. Lipids are actually a much richer source of energy than are carbohydrates, providing more than twice as much energy per gram than do carbohydrates. The body relies on its lipid sources of energy for long-term sources of energy compared to its carbohydrate sources, which are used for short-term energy needs. Proteins can be metabolized to produce energy also. Protein oxidation normally only contributes a small amount to whole body substrate oxidation (less that 5%) but may increase up to 10% depending on duration of activity and energy balance. Protein oxidation will be greater as the duration of exercise increases, particularly when there is low carbohydrate availability. Amino acid oxidation can be influenced by training and nutritional status. The branch chain amino acids are catabolized as a fuel source by contracting skeletal muscle. They are oxidized to form acyl CoA that can be converted to acetyl CoA for entry into the tricarboxylic acid (TCA) cycle and respiratory chain.
The course of metabolic reactions has significance for exercise and sport activities. The breakdown of ATP provides an immediate burst of energy over a very short period of time, usually no more than about three seconds. At that point, a secondary metabolic reaction takes over in which a substance known as creatine phosphate (CP) begins to produce additional ATP. That reaction lasts a very short period of time, no more than about seven seconds. When supplies of both ATP and CP become insufficient, another source of energy is required. Because the total time during which ATP and CP are available is so short, they provide the energy needed for very short exercises, such as a 100-m dash. A runner who continues beyond 100 m relies on the body to move to the next source of energy.
Anaerobic glycolysis is the next source of energy. During this process glucose is converted to carbon dioxide, water, and energy. This process is not very efficient, but it is available quickly, usually within a matter of seconds. Anaerobic metabolism continues until a point is reached at which lactic acid is being produced more rapidly by muscle cells than it can be removed by the blood. This point is known as the lactate threshold (LT), or lactate inflection point (LIP). Beyond the lactate threshold, lactic acid begins to accumulate in the blood, an event that not only causes discomfort for the athlete, but also interferes with efficient functioning of muscle tissue. Anaerobic metabolism is able to provide the energy required for activities that involve short bursts of energy, such as the brief shifts of about 60 seconds expected of ice hockey players. After a brief rest (for hockey players, about three or four minutes), their bodies have recovered sufficiently to allow aerobic metabolism to take over energy production once more.
The energy needed for longer, more sustained exercise comes primarily from aerobic metabolism. Aerobic metabolism is a very efficient method for producing energy, especially compared to anaerobic metabolism. However, it takes much longer to develop, primarily because the oxygen needed by cells for aerobic metabolism must be transported from the lungs to the cells, a process that can take a few minutes. Aerobic metabolism is the primary (but not exclusive) method of energy production used by athletes who must continue to function over long periods of times, such as long-distance runners.
Discussions of metabolism focus on two different aspects of the topic. First, the human body has to process nutrients at some given rate simply to stay alive. That is, cells, tissues, and organs need a constant supply of energy just to keep fundamental chemical reactions going in the body. The rate at which metabolism must occur for this basic level of functioning is known as the basic metabolic rate (BMR). Scientists measure a person's BMR by determining the rate at which oxygen is consumed or carbon dioxide is produced when a person is completely at rest. Any activity performed by a person beyond this most fundamental resting level requires an increased rate of metabolism. For example, a person playing tennis for an hour will need to increase her or his metabolic rate far beyond the BMR.
A team of researchers led by Edward Melanson at the University of Colorado at Denver in 2009, studied the effects on the BMR of a group of 65 volunteers who engaged in various forms of exercise. The group included moderately active people who engaged in both low intensity and high intensity cycling; endurance athletes, such as competitive runners and triathletes; sedentary individuals, both obese and of normal weight; and older men and younger men. The results obtained for all experimental groups were essentially the same: Beyond the immediate increase in BMR as a direct result of the exercise, no effects on BMR beyond a 24-hour period could be detected. Therefore, exercise is not an effective means for increasing an individual's BMR beyond its current level.
This information is important because many people try to lose weight by finding a way to use up one's food stores (the fat in one's body) faster than they are replenished. Experts who advise people about losing weight almost always suggest that they increase their level of exercise. A regular program of exercise makes a person healthier overall; but it is increasingly clear that just exercising by itself has limited effect on one's weight, because it does not affect one's BMR. Some weight may be lost during an exercise, but the weight will not continue to be lost because of an increased metabolic rater after the exercise has been completed. The key to weight loss is to reduce the intake of nutrients, so that excess nutrients are not stored in the body as fatty tissue.
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American College of Sports Medicine, 401 W Michigan St., Indianapolis, IN, 46202-3233, (317) 637-9200, Fax: (317) 634-7817, http://www.acsm.org .
David E. Newton, AB, MA EdD