A metabolic response is any reaction by the body to a specific influence or impact. Metabolism is a general term describing the organic process in any cellular structure. A metabolic response can occur with respect to individual cells, a gland, an organ, or a process such as the cardiovascular system. Metabolism is often understood in terms of the metabolic rate, which is the amount of energy expended by the body in a given period. Metabolic response, when stated without reference to a specific action, is a neutral term; metabolic responses occur and may be correspondingly assessed or measured in respect of a wide variety of circumstances.
Metabolism is a variable in the assessment of human performance. Metabolic function is subject to such individual factors as age, heredity, gender, level of physical fitness, and others. It is also well understood that while metabolisms are unique in every individual, there are certain generalizations regarding body types and structures that can be used in any consideration of metabolic function.
There are three general body types, also known as somatotypes, that have a specific metabolic function, namely ectomorphs, mesomorphs, and endomorphs. An ectomorph is a person with a generally slim, light build, with a faster metabolic function that makes it generally difficult to both gain weight and build muscle. A mesomorph is a more solid, muscular build, one that tends to be suited to explosive movements and contact sports. The mesomorphic build is usually associated with a moderate metabolism. An endomorph is a person with a shorter, more rounded and thicker frame, often with a higher percentage of body fat, who usually has a slower metabolic function. Most persons have a body type that is a blend of two or all three of these body types.
The body may exhibit a metabolic response to any type of external factor or change. Some common metabolic responses to different stimuli received by the body are improved physical performance, an ability to synthesize proteins, weight change, and response to injury.
Changes in diet or nutritional practices are used to achieve an improvement in a physical attribute or capability. Athletes often tinker with their diet by means of both foods and dietary supplements to assist in achieving improvements in their performance. Diet alone can significantly influence the physical performance of an athlete. As a single factor or agent for change, diet will not improve athletic performance; it will permit athletic improvement to occur through its support of more intensive physical training, while reducing the risks of both injury and illness. Diet will also facilitate changes in the manner in which the body processes particular proteins, as the effect of exercise on systems such as protein synthesis into muscle continues beyond the extent of the particular training period.
When a diet is altered from a traditional balanced ratio of approximately 60% to 65% carbohydrates, 12% to 15% protein, and less than 30% fat to one of greater fat composition, a prominent metabolic response is the generation of a greater number of low density lipoproteins (LDLs), a form of cholesterol that contributes to reduced cardiovascular function and long-term arterial disease.
The manipulation of diet may also achieve a metabolic response with respect to how the body uses its stores of carbohydrates and fats for the production of energy.
The most common desired metabolic response through diet is that of either weight gain or weight loss. A significant industry has grown around the sale of products that are described as metabolic response modifiers, all of which profess to modify the user's metabolism to stimulate weight loss. The efficacy of these products is not well established. If an athlete consumes a reduced number of calories through the diet, and the athlete does not alter his or her particular pattern of physical activity, the metabolic response will be weight loss. When an athlete seeks to build muscle and adds a protein supplement to the diet, in concert with a specific weight training routine, the athlete will expect to gain some desired muscle. An undesirable metabolic response may occur if the athlete consumes excessive amounts of protein in the training diet, causing the formation of acids that tend to interfere in normal bone function, making the athlete a long-term subject for reduced bone density and osteoporosis.
An injury or illness produces a pronounced metabolic response. A significant trauma to the body will trigger a "fight or flight" response, which will trigger the production of the hormone adrenaline. The perceived threat to the body will also initiate an increase in the body's energy demands, reflected in increased body heat and an increased rate of conversion of glucose from glycogen.
Changes in the physical intensity of athletic activity, either by training practices or competitive schedule, will generate a metabolic response. This response is particularly evident when assessing the nature of muscle composition in an athlete. When an athlete seeks to improve endurance ability, the training program will correspondingly focus on endurance exercise. The muscle groups involved in the generation of power in the exercise, each with a set pattern of distribution between fast-twitch and slow-twitch fibers, will respond by making a slight adaptation in which more fast-twitch fibers are utilized for the muscle.
Overtraining will produce the metabolic response of decreased immune system function, among other possible responses.