When you exercise, your body must start producing energy faster than it does when it’s at rest. The muscles start to contract more strenuously, the heart beats faster to pump the blood around the body more rapidly and the lungs work harder. All these processes require extra energy. Where does it come from and how can you make sure you have enough to last through a training session or race?

Before you can fully answer such questions, it is important to understand how the body produces energy and what happens to it. You need to understand what takes place in the body when you exercise, where extra energy comes from and how the fuel mixture used differs accordingly to the type of exercise.

What is energy and ATP?
Firstly we need to understand what energy is. Although we cannot see energy, we can see and feel its effects in terms of heat and physical work.

Energy is produced by the splitting of a chemical bond in a substance called adenosine triphosphate (ATP). This is often referred to as the body’s ‘energy currency’. It is produced in every cell of the body from the breakdown of carbohydrate, fat, protein and alcohol (four fuels that are transported and transformed by various biomechanical processes into the same end product).

ATP is a small molecule consisting of an adenosine ‘backbone’ with three phosphate groups attached. Energy is released when one of the phosphate groups splits off. When ATP looses one of its phosphate groups it becomes adenosine diphosphate, or ADP. Some energy is used to carry out work(such as muscle contractions), but most (around three quarters) is given off as heat. This is why you feel warmer when you exercise. Once this has happened, ADP is converted back into ATP. A continuous cycle takes place in which ATP forms ADP and then becomes ATP again.

The conversion of ATP and ADP.
The body only stores very small amounts of ATP at any one time. There is just enough to keep up basic energy requirements when you are at rest, sufficient to keep the body ticking over. When you start exercising, energy demand suddenly increases, and the supply of ATP is used up within a few seconds. As more ATP must be produced to continue exercising, more fuel must be broken down.

Where does energy come from?
There are four components in food and drink that are capable of producing energy; carbohydrate, protein, fat and alcohol. When you eat a meal or have a drink, these components are broken down in the digestive system into their various building blocks. Then they are absorbed into the bloodstream. Carbohydrates are broken down into small, single sugar units; glucose, fructose and galactose. Fats are broken down into fatty acids and proteins into amino acids. Alcohol is mostly absorbed into the blood.

The ultimate fate of all these components is energy production, although carbohydrates, proteins and fat also have other important functions. Carbohydrates and alcohol are used mainly for energy in the short term, while fats are used as a long term energy store. Proteins can be used to produce energy either in ‘emergencies’ (for instance, when carbohydrates are in short supply) or when they have reached the end of their useful life. Sooner or later all food and drink components are broken down to release energy.

In Part 2 next week we’ll look at energy sources and which fuels are most important for exercise.

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