During exercise, glucose in the blood is used for the first few minutes. After this, the body’s stores have to be used. Glucose is stored in the muscles and liver as glycogen. Glycogen stored in the muscles can be broken down to glucose very quickly.

For the next 20 minutes, half of the body’s energy comes from glycogen and the other half from fat stored under the skin. Fat is a much more efficient energy store because it contains less water and more energy.

As time goes on, the glycogen is used up and after three or four hours, 80% of the energy comes from fat. Long distance runners experience the switch to burning fat as they enter a period of exhaustion called ‘the wall’.

The average of 10.5kg of fat stored under the skin can provide enough energy for a walk of several thousand miles. There is only 450g of stored carbohydrate in the body. This is enough for a 20 mile run. Changing the diet can alter the amount of stored carbohydrate in the body. Marathon runners use a high carbohydrate diet to build up their glycogen stores. However, once the glycogen stores are full, the carbohydrate is converted into fat.

Transport of Oxygen and Carbon Dioxide
In the body, gases can move in and out of cells by diffusion. This process is slow. The structure of the lungs, red blood cells and capillaries all provide a large surface area to speed up movement.

In the blood, oxygen combines temporarily with the red pigment haemoglobin. This happens when there is plenty of oxygen available (in the lungs). The blood is then pumped around the body until it reaches an oxygen-poor environment, such as in exercising muscles. Where there is little oxygen this temporary combination breaks down releasing the oxygen.

The blood capillaries have very thin walls and are so narrow that friction slows down the movement of the blood. This makes it easier for diffusion to take place.

Carbon dioxide is more soluble than oxygen and can be carried dissolved in the blood plasma. Carbon dioxide dissolves into the blood where it is plentiful - in exercising muscle. In the lungs there is less carbon dioxide and the gas comes out of solution and diffuses into the alveoli. The walls of the alveoli are also very thin.

Changes in Respiration with Exercise
A fit and active adult can breathe in 12 litres of air per minute. This will allow 3 litres per minute of oxygen to pass into the blood of a normal person. A well-trained athlete can increase this to 6 litres per minute. To do this, the distribution of blood flow changes and the breathing rate increases.

These changes are controlled by the brain. During exercise, receptors detect changes in the oxygen and carbon dioxide levels in the blood. The brain sends nerve impulses to the diaphragm and intercostal muscles causing an increase in the depth and rate of breathing. At the same time, small arteries in the muscles widen, increasing the supply of blood.

These changes affect the composition of the air breathed out. Inhaled air always contains 21% oxygen and 0.04% carbon dioxide. At rest, exhaled air may contain 18% oxygen and 2% carbon dioxide. During intense periods of exercise, only 16% of the air is oxygen whilst carbon dioxide has risen to 4%.

Different Energy Needs
Daily energy requirements vary a great deal:

The factors responsible for these differences are:

• body mass
• growth rate - energy requirements per kg of body mass are very high for teenagers in growth spurts
• basal metabolic rate - the rate of energy consumption of body cells at rest
• activity
• gender - in addition to differences in body mass and activity, there are differences in average metabolic rate
• pregnancy and lactation - providing a growing foetus or baby with energy increases the mother's energy needs

Energy Balance
Energy balance is becoming an important issue for young people in more affluent societies. A combination of high-energy processed foods and low-energy pastimes is resulting in a significant rise in the proportion of young people who are overweight. This in turn leads to concern over future rates of heart disease and other illnesses linked with obesity.

Current trends in counselling and health education promote the value of regular exercise combined with education about energy balance. Young people should understand the importance of balancing energy input and output and of having positive attitudes towards regular exercise.