• fats
• carbohydrates
• proteins.

Fats are needed by the body for things like making new cell membranes, protective padding for internal organs, the storage of energy and to make certain steroid hormones.

Perhaps the most obvious job of fats is in the storage of energy. If you take in too much food, then the excess energy that it contains is stored as fat underneath the skin. This is healthy up to a certain point. You need to have some fat reserves but unfortunately many people in developed countries are now eating too many high calorie foods and so becoming dangerously overweight or obese.

Excess fat in the diet can also cause problems with the heart and circulation. Animal fats contain lots of cholesterol and saturated fats. They can form deposits called fatty plaques on the inner surface of blood vessels which reduce the blood flow and contribute to an increase in blood pressure. If the blockage is in one of the coronary arteries, that supply blood to the heart muscle, it can cause a heart attack (see The heart and circulation).

Plant-fats and oils are thought to be less harmful than animal fats. Plant oils, like olive oil, contain more unsaturated fats and less cholesterol. However, this does not mean that they can be eaten to excess. Too much of any one food group will always cause problems.

One of the most important parts of the diet is carbohydrate for energy. Body cells need to maintain respiration to generate energy for processes such as growth, repair and active transport. Carbohydrates in the diet provide this immediate source of energy.

Simple sugars, such as glucose taste sweet. They are small enough to pass through the intestine wall and into the bloodstream so do not require any further digestion. Many energy drinks contain glucose so that they give an immediate energy boost.

Starch is a complex carbohydrate found in potatoes, rice and pasta. It is made of many glucose molecules joined together and is too large to be absorbed into the blood in this form. Starch must first be digested into its constituent glucose molecules before it can be used.

Excess carbohydrates can cause problems such as obesity and possibly contribute to developing diabetes in later life. A high content of sugar in the diet often results in tooth decay, especially in young children who eat sweets and have carbonated drinks that have large amounts of sugar added to them.

Proteins are the building blocks of all cells. They are really versatile molecules and can make enzymes, be used for active transport, work as hormones and make cell structures. They are essential for the healthy growth and repair of body cells and tissues. Proteins cannot be stored in the body and about 80g per day is required every day. Any excess protein is broken down in the liver and excreted as urea, which is the yellow part of urine.

Proteins are made up of a long chain of amino acids. When digested, these amino acids can be reassembled in any sequence to produce the proteins that the body requires. This will vary according to an individuals needs. For example if a person has lost blood due to a cut they may need to direct more of the amino acids from their diet into the production of haemoglobin to replace the lost red blood cells. If a woman is pregnant, amino acids will be needed supplied to the developing embryo for its own growth. Good sources of protein in the diet are any meats, fish, eggs, pulses and beans.