Every cell and tissue in the body contains protein. It is in the muscles, bones, hair, nails and skin, accounting for 20 percent of total body weight. In addition, different proteins work as enzymes, hormones, neurotransmitters, antibodies and specialised proteins such as haemoglobin and others, constantly repairing body tissues to keep it healthy.

Proteins are made of amino acids. There are 20 amino acids. Rather like the alphabet, the amino acid 'letters' can be arranged in millions of different ways to create 'words' and an entire protein 'language'. Depending on the sequence in which they are combined, the resulting protein carries out specific functions in the body.

The body gets amino acids from protein-containing foods. Digestive enzymes break proteins into their component parts. The body absorbs the amino acids and then rebuilds them into new sequences that are needed for body growth and maintenance and for the control of body processes.

There are two types of amino acids-essential and non-essential. The eight essential amino acids cannot be made in sufficient amounts in the body, and must therefore be supplied in the food. The 12 non-essential amino acids can be made from other amino acids in the diet. Foods containing animal protein, such as meat, milk and eggs, contain ample amounts of all the essential amino acids. Vegetable protein sources have one or more of the essential amino acids missing or have less than the adequate amounts. These foods, however, can be combined in a diet that supplies the required amounts. Most people need to have 10-15 per cent of their total calorie intake in protein; this is about 0.75 grams of protein per kilogram of body weight per day. Hence, a 70-kilogram man and a 55-kilogram woman need 50 to 60 grams and 40 to 50 grams per day, respectively. Two or three servings of animal protein foods or four servings of mixed vegetable-protein sources, such as whole grain cereals, vegetables, legumes, nuts and seeds can easily provide the needed protein.

The body cannot store amino acids so it is constantly breaking down and remaking proteins. This 'protein turnover' or recycling process must be constantly fuelled by proteins in the diet. There will often be excesses of some amino acids and of total protein. The liver converts these to glucose, used to provide energy.

Since muscles are made of protein, athletes sometimes consume extra protein to achieve greater strength and muscle size. Typical intakes are around 1.4 grams of protein per kilogram of bodyweight per day. But eating higher amounts of protein does not improve power or strength. The only way to make muscles bigger and stronger is to exercise them.

The body uses only the protein it needs and as many high-protein foods are also good sources of fat, a diet with a high amount of protein can also have too many calories, resulting in weight gain and layers of fat. It is essential to good health to eat a well-balanced diet, but there is no need to go overboard on protein

References

• Bean, A. (2000) The complete guide to sports nutrition. How to eat for maximum performance. 3rd edition. A. and C. Black Ltd, London
• Department of Health (1991) Dietary reference values for food energy and nutrients for the United Kingdom. Report on Health and Social Subjects 41. HMSO, London
• Williams, C. and Devlin, J. T., Editors (1992) Foods, nutrition and performance: an international scientific consensus. Chapman & Hall, London
• World Health Organisation (1985) Energy and protein requirements. Report of a Joint FAO/WHO UNU Meeting, Geneva. WHO Technical Report Series, 724

FOOD TODAY 09/2001