How the Body Uses Food for Energy

Understanding the body's natural energy systems helps to appreciate what you should eat and when, especially if you are an athlete.
Created: March 2011
Revised: October 2016
Latest Feedback: October 2016

Muscular Energy from ATP

All the energy needed by your body for growth, development, bodily processes, exercise and repair, comes from the foods and fluids you eat and drink - Carbohydrates, Proteins and Fats. As an athlete, you should know the science of metabolism to ensure you are making the best possible decisions with your nutrition choices for performance and recovery.

The movement of your muscles is powered by a compound called adenosine triphosphate (ATP). Your body produces ATP continuously because it can only store a small amount in your cells - in fact just enough to power a few seconds of high-intensity energy output.

To produce ATP, your body has three separate energy systems to call upon, all of which produce ATP. A variety of factors, including exercise intensity, duration of activity, and availability of fuel, will determine which energy system is used. An athlete can target each of these energy systems and condition the body to generate power more effectively through training.

Energy Systems in the Body

During exercise, an athlete with move through the following metabolic pathways to produce ATP.


This energy system is called anaerobic (doesn't rely on oxygen) and is used almost exclusively by the body in the first few seconds of exercise. Initially, this energy system uses ATP stores from muscles, and then uses creatine phosphate (CP) to produce additional ATP until CP is depleted. All this occurs in the first 10 seconds or so of exercise. To continue activity the body will call upon another energy system to produce ATP.


This is the energy system that you may be more familiar with as this is the one that utilises glucose (supplied by food and stored as glycogen in the muscles or liver). There is both fast glycolysis (which produces lactic acid) and slow glycolysis (produces pyruvic acid, which leads to the Krebs cycle in the Oxidative System).

Fast glycolysis is an anaerobic stage of metabolism and produces energy for short, high-intensity burts of activity. Our bodies have a lactate threshold at which point we endure fatigue and muscle pain. After approximately 45 seconds of continuous activity, the body will switch its energy demands to the Oxidative System.

Oxidative System

This energy system is complex and includes a series of chemical reactions that continue the oxidation of glucose to produce additional ATPs. It is therefore an aerobic state of metabolism. However, this system enables the body to metabolise fat (Lipolysis) as well as carbohydrate to produce ATP and is the ideal energy system for long duration activity.

However, if fat is metabolised in this stage, it will produce significantly more ATPs than if carbohydrate is the source however more oxygen is required to do so. This means that to use fat for fuelling your body's energy needs you will put a greater demand on your cardiovascular system. If you are unable to supply your cells with sufficient oxygen quickly enough, your body will revert back to using carbohydrate stores (from your liver and muscles) and if you deplete that (such as in endurance events) your body will switch back to utilising fat but because you cannot keep up the demand for oxygen to metabolise the fat, your exercise intensity will drop off.

So, this means that ingesting quick release carbohydrates during exercise is of benefit when intensity is high and prolonged to avoid a performance drop off. For high intensity performance, you should ensure that muscles are not depleted and a diet high in proteins will ensure minimum damage and maximum repair occurs.

In order for your body to burn fat for energy supply, lower intensity exercise demands are required, however as the body has adequate stores of fats (unlike carbohydrates), you can sustain activity for prolonged periods. If your body fat is low however, your body will not be able to rely on fat as a fuel and your demands for carbohydrate refueling during exercise will be critical to avoid fatigue.

Avoiding Fat Gain

When we consume carbohydrates they are stored as glycogen, ready for use in our muscles and liver. Generally, we can store around 2000 carbohydrate calories in our glycogen stores however once filled, any excess is stored as fat. The simple rule is energy in, energy out. If the equation doesn't balance, then excess energy becomes a fat gain. It is far easier for our bodies to utilise carbohydrates by exercising regularly, for once it has been converted to fat it is a much more complex process to burn fat back into energy.


More more information see our article Weight Management.

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