ENERGY AND ITS USE DURING EXERCISE By Dr. John Troup, U.S. Swimming This article appeared in Australian Swim Coach News, March 1987 THE ENERGY MOLECULE OF THE BODY All organisms require energy to do biological work. For the human, this work includes the transport of materials across cell membranes, the conduction of nerve impulse and the contraction of skeletal and heart muscle. The energy is obtained from the breakdown of a high energy chemical compound called adenosine triphosphate (abbreviated ATP) located in all the body’s cells. “Triphosphate” refers to the fact that this molecule has three (3) phosphates (P) attached to it. When it breaks down, one of the phosphates is split off, releasing energy which can be used by the cell… (1) ATP ADP + P + “useful energy” The problem is that very limited amounts of ATP are available in the muscle cell, probably only enough to sustain muscle contractions for about three seconds. ATP is the universal energy molecule; it must be present if work is to be done. The problem of the cell is to reform ATP from its component parts, ADP and P. Basically, this process is a reverse of the chemical reaction listed as equation (1) above. (2) ADP + P + “energy” ATP Notice that the process of ATP formation and breakdown is cyclic…

Notice also that it TAKES ENERGY to link the ADP and P to make ATP. The energy needed to reform ATP comes from that stored in the energy yielding foods from the diet – carbohydrate, fat and protein. These foods or nutrients are broken down in stepwise chemical reactions within the cell, at certain points giving off energy which can be used to remake the ATP. A simplified schematic of this process is shown below.

A BRIEF DIVERSION We use the term energy to describe our feelings, “I don’t have much energy today”, and to relate our metabolic needs during exercise or other activities to the actual ATP requirement of the task “I need quick energy before the race.” In all cases, the term energy is used rather loosely and can mean anything from the ATP requirement to the amount of food energy consumed. How can we best use these terms? THE CALORIE The term “Calorie” is used to describe both the energy associated with food intake and that associated with food breakdown and ATP production. What follows is a brief, schematic illustration of how Calorie is defined and used in conjunction with food consumption and metabolism in the body, especially during exercise. BASIC CONCEPT #1 All matter in the universe whether it be water, wood, a protein or muscle is made up of various combinations of atoms – in the case of water (H2O), two hydrogens and an oxygen link together; the other are much more complex, involving hundreds of thousands of linkages. Each of these attachments (bonds) requires some amount of energy (ATP use) to attach the component parts. Likewise, if the bond is broken, some energy can be given off. Thus, inherent in the STRUCTURE of the molecule (the bonds), there is some energy stored. Some substances have much more energy stored in them than others. For the foods we commonly eat, carbohydrates, fats and proteins are rich in stored energy. FUNDAMENTAL PRINCIPLE OF THE UNIVERSE #1 Matter or energy cannot be created or destroyed, but can be inter-converted from one form to another. Applied to energy systems studied in the laboratory or metabolism in the body, this means

that the energy found in the foods we eat can be stored or converted to other forms, but theoretically one must account for the total. The description of food energy is fairly simple.

Calorie = heat needed to raise the temperature of 1 Kg water, 1 degree of centigrade. 1g protein = approximately 4.1 calories 1g fat = approximately 9.0 calories 1g CHO = approximately 4.1 calories As shown above, a food item is placed inside a sealed chamber and pure oxygen is pumped in. The food-oxygen mixture is combusted (ignited by electric spark) giving off waste products (atoms like carbon, nitrogen, etc.) and heat. Heat is a form of energy. In other words, the energy originally available in the structure of the food is released by combustion and shows up as heat. The heat raises the temperature of the surrounding water bath. By definition we now have a way of describing the energy in the food … i.e. it is a Calorie. All of the original stored energy is released as heat. Note that if 1.0 grams of carbohydrate, protein or fat is completely combusted, it has a certain number of Calories (stored energy) associated with it. This is how the numbers in your calorie books or on food labels were derived. Notice that we can also measure the amount of oxygen used in the combustion chamber just described.

In this case, the energy available from the nutrients (either delivered by the blood or those stored in the cell) is released by combustive processes in the cell (metabolism) to give off waste products like carbon dioxide (CO2) and heat. The difference is that some of the original energy in the nutrient is recovered in the structure of ATP. The process of metabolism is relatively inefficient about 70% of the energy originally present in the carbohydrate, fat of protein is lost as heat. This is why your body temperature is about 98°F – you are constantly metabolising foods to provide ATP to run the body’s energy requiring processes. The heat lost keeps the internal environment warm. BASIC CONCEPT #2 As energy (ATP) requirements increase because of exercise, so too do all the processes associated with metabolism … i.e. more oxygen and nutrients are provided – more wastes and heat are produced. What Limits Exercise Capacity? Depending upon the specific circumstances of the exercise (like intensity, duration, etc.), the environment (is it hot or cold?), your present training status (in shape or not) and your genetic potential, anyone of several factors associated with metabolism (energy/ATP production) could limit performance. These could include inability to provide nutrients or oxygen or inability to get rid of wastes like lactic acid or heat produced. BASIC CONCEPT #3 The capacity to do physical work requires the very complex integration of a wide variety of body functions – everything from the respiratory systems taking air into the lungs, to the thermo-regulatory processes (sweating) which get rid of excess body heat. Neglect of any factor may have negative effects upon the total system.

AEROBIC & ANEROBIC METABOLISM The total amount of energy (ATP) needed for a given amount of work can come from two sources within the cell (1) aerobic and/or (2) anaerobic. These terms refer to the type of metabolism; aerobic means with oxygen, anaerobic without oxygen. A schematic illustration of these processes is illustrated.

Under resting conditions, carbohydrates, fats and protein are broken down aerobically providing the needed ATP. The use of oxygen and nutrients and the production of wastes are low because the demand for energy is also low. If you begin to exercise, you need more energy (ATP) so all the processes involved speed up. Respiration increases to bring in more air. The heart pumps harder and faster to deliver the nutrients and carry away wastes in the blood. The reactions within the cell producing ATP also increase. BASIC CONCEPT #4 All of the processes of the body involved in energy production generally increase in proportion to the demand. This can be illustrated graphically…

At some point the body processes can no longer increase with increased work intensity and they level off. This is the maximum and is determined by a number of factors including age, sex, training status (condition) and genetic endowment. When work intensity is easy, all the energy (ATP) needed can be provided by aerobic processes – meaning that the speed at which carbohydrate is broken down is matched to the speed at which the breakdown Intermediate (see diagram) can be fed into the aerobic side of metabolism. In this situation, the waste product LACTIC ACID does not begin to accumulate in the muscle cell nor appear in the blood. This is illustrated conceptually in the left side of the diagram below.

At some point the total energy (ATP) demand is so great that the reactions of carbohydrate metabolism producing ATP go faster than those which accept the Intermediate into the aerobic side. In this case, the intermediate is converted to the waste product LACTIC ACID. The accumulation of this product in the muscle and its distribution in the blood has effects upon your capacity to continue exercising. The importance and implications of this are discussed in another section. SUMMARY The energy you need to exercise comes from the breakdown of food nutrients. A wide variety of body systems are involved. These are listed in the table below. It is important to recognise that these processes must work together for optimal functioning. Neglect or misuse of one, may reduce the body’s capacity to tolerate intense training or prevent one from achieving best results in competition. Testing gives the Sports Scientist information on how these body systems are adapting to training. SYSTEM FUNCTION TEST(s) Respiratory Breathing in air (oxygen) exhale CO2 Pulmonary Function Deliver blood (nutrients, oxygen), carry away Heart rate cardiac output, blood Cardiovascular wastes, heat test, nutritional status Muscle composition strength, Muscular Use nutrients to produce contractile force power and fatigue