To understand how the body obtains the energy it needs to make it “go”, a useful analogy for how a car and the human body obtain the energy to make them “go.” In an automobile engine, the proper mixture of gasoline fuel with oxygen ignites to provide the energy necessary to drive the pistons. Various gears and linkages harness this energy to turn the wheels, and increasing or decreasing the energy supply either speeds up or slows down the engine. similarly, the human body must continuously be able to extract the energy from its fuel and harness this energy to perform its many complex functions.

How the body gets energy from food
During exercise, the ability to swim, bike, run long distances is determined largely by the body’s ability to extract energy from food nutrients and transfer it to the contractile elements of skeletal muscle.

This transfer occurs through thousands of complex chemical reactions that require a continual supply and utilization of oxygen. Such oxygen-requiring reactions are termed aerobic. In contrast, anaerobic chemical reactions generate energy rapidly for short periods (less than 90 seconds in duration) and do not require oxygen. This rapid method of energy transfer is crucial for maintaining a high standard of performance in activities requiring all-out bursts of exercise such as sprinting in track and swimming, or stop-and-go activities like soccer, basketball, and tennis. Most activities are not purely aerobic or anaerobic, but are classified on an anaerobic-to-aerobic continuum. Depending upon the intensity and duration of the effort, these activities will use short-term, immediate, and long-term sources of energy derived from the foods we ate.

This energy is not transferred directly to our cells for biologic work. Instead, the chemical energy trapped within the bonds of carbohydrates, fats and proteins (the nutrients foods are made of) is used to produce the energy-rich compound adenosine triphosphate (ATP) through complex, enzymaticaly-controlled reactions. Carbohydrate is the only nutrient whose stored energy can be used to generate ATP anaerobically. This is important in vigorous exercise that requires rapid energy release.

The sprinter vs the marathoner.
For the sprinter who is exercising at a maximal level of intensity and for the weight lifter, an immediate energy source is necessary for performance. This comes from high-energy phosphate compounds (derived from ATP) and carbohydrates (glycogen) already stored in your muscles. During the first few seconds of an anaerobic exercise, ATP and its derivatives creatine phosphate (CP), and adenosine diphosphate (ADP) work like tiny, supercharged battery packs to supply your muscles with immediate energy. After the first few seconds, the longer you can continue exercising, the more heavily your muscles will depend upon carbohydrate stores. When these stores are depleted, muscle fatigue and exhaustion sets in. When we eat in relationship to when we exercise is important.

The marathon runner, in contrast to the sprinter, is exercising at a submaximal intensity level. Exercise intensity is usually light-to-moderate and remains relatively stable throughout the exercise period. This “steady state” represents a balance between the energy required by the working muscles and the rate of ATP production through aerobic metabolism.

During endurance exercise, carbohydrate supplies about one-half to three-quarters of the body’s energy requirements. With prolonged endurance exercise, there is a shift to a greater use of fats as a primary fuel source. This results in a decreased rate of utilization of both carbohydrate (glucose) and muscle glycogen stores. Despite the fact that fats are predominately used by muscle during endurance exercise of moderate intensity, the rate at which fats break down to provide sufficient energy for prolonged periods of time cannot occur if your diet lacks adequate carbohydrate intake. It is therefore important to eat a well-balanced diet.