Micronutrients are required in only small amounts and fibre is not digested but they are both necessary for health.

Small amounts of minerals and vitamins are required in the diet each day. However, they are essential for healthy growth and the correct functioning of the body. For example, a lack of the mineral iron can cause anaemia because not enough haemoglobin can be made for replacement red blood cells. Even though the body only requires 0.015 grammes of iron per day, a lack of this mineral can lead to a serious problem. Folic acid is another important vitamin. Even though the body requires less than 0.0002g per day, if a pregnant woman does not have enough it can cause birth defects like cleft lip and neural tube defect (spina bifida). To counteract this, women who are planning a pregnancy are advised to have double this amount of folic acid in their diet.

Even though fibre, or roughage, is not digested it still plays an important part in the proper functioning of the digestive system. It adds bulk to the food which allows it to be pushed along smoothly by the process of peristalsis. Vegetables, whole-grain cereals and brown bread are good sources of fibre.

A lack of fibre in the diet causes problems like constipation. Food spends longer than normal in the large intestine and this causes the faeces to become dried and hard to pass out. It also allows more bacteria to grow and these are thought to produce toxins that are linked with an increase in the chance of developing bowel cancer.

Health authorities have studied the amounts of the major food groups that are required each day in a balanced diet. The Dietary Reference Value (DRV) describes a range of amounts for each food that is required.

This range reflects the natural variation and differences in activity levels of the population. For example, a pregnant woman would have different nutritional needs to a male office worker but confusingly many food labels still quote the Recommended Daily Amount (RDA). This is the amount that would be enough to meet the dietary needs of the majority of the population.