Nutrients

A mixture of different types of food in the correct amounts is needed to maintain health. The main food groups are:

The main food groups

food group	found in	required by our bodies for
	potatoes, pasta, bread, bananas, sugar and rice	A source of energy for other life processes. Sometimes referred to as fibre, which is actually just one - very common - type of carbohydrate.
	cheese, butter, margarine and oils	Fats are needed to make cell membranes and to insulate our bodies. They also contain important fat-soluble vitaminsvitamins: organic substances which are essential in small amounts to regulate the metabolism and maintain the immune system.
	meat, fish, eggs and cheese	Growth and repair.
	wholemeal bread, fruit, vegetables and pulses	The fibre or roughage in our diet is not digested, but is important because it allows the muscles in our intestines to move food through our system by peristalsisperistalsis: wave-like muscular contractions in the smooth wall of the gut which move food through the alimentary canal.

Metabolic rate

A healthy diet contains all the different nutrients in the correct amounts, and provides the right amount of energy for each individual. An unbalanced diet can lead to a person becoming malnourished. They may be too thin or too fat as a result, and they may suffer from deficiency diseases.

Chemical reactions

Respiration is the chemical reaction that allows cells to release energy from food. The metabolic rate is the speed at which such chemical reactions take place in the body. It varies because of several factors, including:

• age
• gender - male or female
• the proportion of muscle to fat in the body
• the amount of exercise and other physical activity
• genetic traits

The metabolic rate increases as we exercise and stays high for a while afterwards.

The right amount of food

Not enough food

If you don't eat enough food, you will become too thin and may suffer from health problems. These include:

• irregular periods in women
• reduced resistance to infection
• deficiency diseases

Deficiency diseases include rickets - which affects proper growth of the skeleton and is caused by insufficient vitamin D - and kwashiorkor - which causes a swollen abdomen and is a result of insufficient protein.

Problems such as these are more likely to affect people in the developing world, where it can be more difficult to get enough food.

Too much food

In warm weather, or when you don't do much exercise, you do not need to eat as much food as when it is cold or when you have exerted yourself physically. If you eat too much food without taking enough exercise, you will become overweight. Very fat people are described as obese. Overweight people may suffer from health problems, including:

• diabetes - an illness in which the body is unable to control the amount of sugar in the blood
• arthritis - an illness in which the joints become worn, inflamed and painful
• high blood pressure
• heart disease

The heart

The heart is an organ that needs its own supply of blood to keep it working. If the blood supply is reduced, the heart muscle will not work properly and will become weaker. A heart attack happens when part of the heart does not get any blood because of a blocked artery.

Cholesterol

A molecular model of a cholesterol molecule

Cholesterol is a substance found in the blood. It is made in the liver and is needed for healthy cell membranes. However, too much cholesterol in the blood increases the risk of heart disease, and of diseased arteries.

Good and bad cholesterol

The bloodstream transports cholesterol around the body attached to proteinsproteins: organic compounds made up of amino acid molecules. One of the three main food groups, proteins are needed by the body for cell growth and repair.. The combination of cholesterol and protein is called lipoprotein, and there are two types.

1. Low-density lipoproteins - LDLs - carry cholesterol from the liver to the cells.
2. High-density lipoproteins - HDLs - carry excess cholesterol back to the liver.

LDLs are often called 'bad' cholesterol because they lead to fat building up on artery walls, which causes heart disease. HDLs are often called ‘good’ cholesterol because they help to stop fat building up in the arteries.

Improving the balance

A high proportion of HDLs to LDLs is good for a healthy heart. Monounsaturated and polyunsaturated oils - as found in vegetable oils - help to reduce cholesterol levels in the blood, and also increase the proportion of HDLs compared with LDLs. Check your understanding of such oils by looking at Vegetable oils.

There are also drugs that can improve high blood pressure and high cholesterol levels.

Salt

Table salt is sodium chloride. Too much salt in the diet can lead to high blood pressure, which in turn leads to an increased risk of heart disease and strokes.

Salt is found naturally in many kinds of food, but more is added by food manufacturers - and many people add even more when they are eating. Processed foods often have a high proportion of salt and fat. Salt added to food during processing accounts for about two-thirds of the average salt intake.

On average, men in the UK eat about 10g of salt a day and women about 8g a day. However, food scientists recommend no more than 6g of salt a day in the diet. Reducing the amount of salt in the diet will help to reduce the risk of heart disease, and to a greater extent the risk of suffering a stroke.

There is some evidence that the food industry has persuaded government bodies to set higher targets for the recommended amount of dietary salt in our food than doctors might wish.

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