Metabolic Set Point

 Metabolic 

 by Christina Body and Fitness

The Metabolic Set Point

The metabolic set point is the average rate at which study individual's metabolism runs. During an intensive study of weight loss, researchers discovered that the body seeks to maintain a certain base rate of metabolism. This set point is controlled by genetics and environmental factors, but research has demonstrated that it can be changed through dietary means and physical activity.

The existence of a metabolic set point causes the body to also have a body-composition set point. People with a slow metabolism seem to store fat easily, while people with a fast metabolism seem to be able to eat whatever they want and never get fat. However, studies have demonstrated that going on a low-calorie diet causes the body to lower its metabolic set point in an effort to conserve energy. The body actually resets itself to burn fewer calories. Exercising tends to keep the metabolic rate up, and exercising aerobically tends to cause the body to burn more fat for energy.  

To Carb or Not to Carb

Food also influences metabolism. Diets that are low in fat and high in protein and carbohydrates can increase the BMR. The processing of the excess protein seems to require additional energy. Certain substances found in food also increase the metabolic rate. Prime examples are caffeine and ephedrine. Because of this, the herbs ephedra, which contains ephedrine, and guarana, which contains caffeine, are popular ingredients in supplements called metabolic boosters. Metabolic boosters, also known as thermogenic aids, are substances whose digestion causes the body to produce more than the normal amount of heat (energy).

A common question among athletes is which of the macronutrients-carbohydrates, protein, or lipids-is the primary producer of energy. The answer is not easy to determine because it varies from person to person. However, researchers have devised a method for calculating the specific "fuel mix" that is used by a particular person. Called the reparatory quotient (RQ), the method measures the ratio of fat, carbohydrates, and protein that is burned for energy.

The RQ compares the volume of carbon dioxide exhaled to the volume of oxygen inhaled. Because different amounts of oxygen are used to burn fat, carbohydrates, and protein, this ratio can indicate which macronutrient is the predominant energy source. According to research, persons consuming a normal diet derive about 40 to 45 percent of their energy from fatty acids, 40 to 45 percent from carbohydrates, and 10 to 15 percent from protein. When the diet is high in carbohydrates, more energy comes from carbohydrates. When the diet is low in carbohydrates and high in fat, more energy comes from fat.

Training intensity also affects which energy source is primary. During exercise, a training intensity below 60 percent of VO2 max (the maximum rate at which oxygen can be consumed) causes fatty acids and carbohydrates to be equal sources of energy. The more the training intensity is increased above 60 percent of V02 max, the more carbohydrates are used for energy. When the training intensity reaches 100 percent of VO2 max, a rate that can be sustained for only a few minutes, carbohydrates become the sole source of energy. Keep in mind that amino acids, particularly the BCAAs, are also used for energy during both exercise and rest. During exercise, they may even supply as much as 10 percent of energy.

In general, physical conditioning causes the body to obtain more energy from fatty acids. However, more energy is also obtained from protein in trained individuals. Carbohydrates are always used for energy. In research comparing trained individuals with untrained individuals, both groups used mostly carbohydrates for fuel during exercise, but the trained individuals used more fatty acids than the untrained individuals did. High-intensity exercise causes the use of more carbohydrates for energy, while low-to-moderate-intensity exercise causes the use of more fatty acids in addition to the carbohydrates. Fatty acids are the predominant energy source during rest .

 Exercise and Metabolism

Exercise influences metabolism by affecting the body's anatomy, physiology, and biochemical makeup. Different kinds of exercise promote different kinds of changes. Exercise type, duration, and intensity are all important factors that shape the body.

Low-intensity exercise, which is usually long in duration, promotes the development of slow-twitch muscle fibers and stimulates the use of the oxidative energy systems. Slow-twitch muscle fibers are called into action when endurance is needed, since they produce a steady, low-intensity, repetitive contraction. In general, aerobic exercise causes increases in the mitochondria density of muscle cells, especially of slow-twitch muscle fibers, and in the increases in the number of capillaries and in cardiac output. At the same time, aerobic exercise causes decreases in the percentage of fast-twitch muscle fibers, in the resting heart rate, in body fat, and in muscle size. High-intensity exercise, which is usually short in duration, promotes the development of fast-twitch muscle fibers and stimulates the use of the nonoxidative energy systems. Fast-twitch muscle fibers are called into action when strength and power are needed, since they contract quickly, providing short bursts of energy. Some of the major changes resulting from anaerobic exercise, such as heavy-weight lifting, are muscle fibers, in the ability to tolerate higher blood-lactate levels, and in enzymes involved in the nonoxidative phase of glycolysis. Other changes are increases in the levels of ATP, CP, creatine, and glycogen in resting muscles and of GH and testosterone after short bouts (forty-five to seventy-five minutes) of intensive weight training.

 Energy Metabolism

Energy metabolism consists of a series of chemical reactions that break down foodstuffs and thereby produce energy. The body traps about 20 percent of the energy that is produced and releases the remaining 80 percent as heat. This is why the body heats up during exercise.

Energy production in the human body revolves around the rebuilding of ATP molecules after they have been broken down for energy. ATP is the molecule that stores energy in a form that the body can use. This rebuilding of ATP molecules is accomplished in a number of ways, all of which correlate to the four main purposes for which energy is utilized during athletic performance - power, speed, strength, and endurance - and to the four basic types of physical activity - strength - power, sustained power, anaerobic power - endurance, and aerobic endurance.

When resting, the body derives most of its energy from the oxidative energy systems. But when physical activity begins, the nonoxidative energy systems kick in. When the exercise level becomes very intense, the nonoxidative immediate energy systems predominate. First, the ATP reserves in the muscle cells are drafted. However, these ATP reserves are depleted instantly - within a second. Therefore, if the exercise level remains very intense, resting CP stores are called upon, regenerating the ATP molecules, which can then continue to serve as the intense, the next nonoxidative energy system, glycolysis, jumps in. Nonoxidative glycolysis functions during near-maximum efforts lasting up to about one and half minutes. If helps maintain the intensity of the muscle contractions, even though the muscles' force output becomes diminished. When an ATP molecule releases its energy to power a muscle contraction, it is reduced to 1 adenosine- diposphate (ADP) molecule and 1 phosphate atom. In nonoxidative glycolysis, a glucose molecule is split in half to regenerate ADP back to ATP. Every glucose molecule releases enough energy to regenerate 2 ATP molecules. Nonoxidative glycolysis also results in the formation of lactic acid. Glycolysis takes place in the cytoplasm of a cell. Since the amount of free glucose in a cell is generally low, the glucose that is used is usually derived from the breakdown of glycogen. During intensive exercise, the oxidative energy systems also supply energy, but they contribute a much smaller share.

As the intensity of the exercise is reduced and the duration is increased - generally to more than three to four minutes - the oxidative systems include oxidative glycolysis and beta oxidation. In oxidative glycolysis, when the glucose molecule is split in half, it forms 2 molecules of pyruvate. These pyruvate molecules enter the Kerbs cycle, a process in which short chains of carbon atoms from glucose, fatty-acid, or protein molecules are broken down and the energy that is released is used to regenerate ATP molecules. The breakdown products of the Kerbs cycle are then shuffled into the electron transport system, a process in which electrons are passed between certain protein molecules, releasing energy that is used to regenerate additional ATP molecules. The complete catabolism of 1 molecule of glucose yields about 36 ATP molecules. In beta oxidation, fatty acids are metabolized. Each fatty-acid molecule is broken down into 2 carbon acetyl fragments. The acetyl molecules then combine with coenzyme A to form acetyl-coenzyme A (actryl-CoA). The acetyl-CoA molecule is shuffled to the krebs cycle and broken down, yielding energy to regenerate ATP. Oxidative energy metabolism is a slower process that nonoxidative energy metabolism, but it more completely breaks down the energy molecules - for example, glucose and fatty acids - and generates a lot more energy. The complete breakdown of 3 molecules of fatty acids containing 18 carbon atoms each yields about 441 ATP molecules.

 Glycogen Depletion and The Metabolism of Fatigue

Glycogen is essential for good performance in both anaerobic and aerobic activities. Muscles that are being strenuously exercised rely on glycogen to power their strength - generating muscle contractions. In aerobic - endurance activities, the primary fuel is fatty acids, but glycogen is also utilized. In fact, fat catabolism works better when carbohydrates are metabolized at the same time. Research on aerobic - endurance exercise and work performance has shown that when glycogen is depleted in the body, fatigue sets in. Therefore, adequate carbohydrate intake and glycogen replenishment are vital factors for peak energy output.

Glycogen depletion is just one factor that contributes to the onset of fatigue, however. Some other factors are ATP and CP depletion, accumulation of lactic acid, buildup of calcium ions in the muscles, oxygen depletion, dehydration, and decreased blood pH. The proper conditioning of the body to enable it is to appropriately utilize the preferred energy systems is also important. If the body is poorly conditioned, it cannot properly use the energy systems appropriate for the activity, which can lead to early onset of Fatigue. To avoid this, focus your physical - conditioning efforts on developing the primary energy systems needed for your diet supplies the ideal fuel mix to enhance your physical conditioning.