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There are seven different categories of nutrients:

  • carbohydrates
  • proteins
  • fats
  • minerals
  • vitamins
  • fibre
  • water

Some foods are particularly rich in certain nutrients. The table shows why we need each nutrient, and some good sources of each.

Nutrient Use in the body Good sources
Carbohydrate To provide energy Cereals, bread, pasta, rice and potatoes
Protein For growth and repair Fish, meat, eggs, beans, pulses and dairy products
Fat To provide energy. Also to store energy in the body and insulate it against the cold. Butter, oil and nuts
Minerals Needed in small amounts to maintain health Salt, milk (for calcium) and liver (for iron)
Vitamins Needed in small amounts to maintain health Dairy foods, fruit, vegetables
Fibre To provide roughage to help to keep the food moving through the gut Vegetables, bran
Water Needed for cells and body fluids Fruit juice, milk, water

Some effects of a poor diet

If you have too little of a particular nutrient, we say that you have a deficiency in that nutrient. For example, fibre is needed to keep food moving through the intestines easily, and people who have a fibre deficiency in their diet may get constipation.

Mineral deficiencies

woman's neck swelling due to thyrotoxic goitre

Thyrotoxic goitre, causing neck swelling

People with iron deficiency may getanaemia and have too few red blood cells. People with iodine deficiency may get a swelling in the neck called a goitre (pronounced “goy-ter”).

Vitamin deficiencies

Although we only need small amounts of the different vitamins in our diet, we become ill if we don’t get enough. For example, vitamin A deficiency can cause blindness. Vitamin C deficiency causes scurvy, which makes the gums bleed, and vitamin D deficiency causes rickets, which makes the legs bow outwards in growing children.

Too thin, too fat

Food is a store of chemical energy. If you look on the side of food packets you will see the food’s energy content. This is usually measured in kilojoules, kJ. If we eat too little food, we will use up our store of fat and become too thin. If we eat too much food, especially foods rich in sugar and fat, we will increase our store of fat and become too fat.

It is important to balance the amount of food we eat with who we are and what we do. The amount of energy we need from our food depends on our age, our height and how much exercise we get.

For example, a one-year old baby needs 3850 kJ each day to continue to grow, whereas an adult Olympic swimmer in training needs 15,600 kJ each day. Someone who sits at a desk all day will need less food than their twin who climbs ladders all day to wash windows.

Digestive system

When you eat a piece of bread, you don’t wake up next day to discover it growing out of your arm! The food we eat has to be broken down into other substances that our bodies can use. This is called digestion. Without digestion, we could not absorb food into our bodies and use it.

Digestion happens in the digestive system, which begins at the mouth and ends at the anus.

After we swallow, our food passes through these organs in turn:

  • oesophagus or gullet
  • stomach
  • small intestine
  • large intestine.

Stages of digestion

  • Food is digested in the mouth, stomach and small intestine.
  • Digested food is absorbed into the bloodstream in the small intestine.
  • Excess water is absorbed back into the body in the large intestine.
  • Any undigested food passes out of the anus as feces  (pronounced “fee-seez”), or excrement, when we go to the toilet.

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