Carbohydrates and Fibre in the Diet

Carbohydrates are our most important source of energy and an essential part of a healthy, balanced diet. They are also necessary for the metabolism of proteins needed to build and repair body tissue, and for the functioning of the central nervous system. Complex carbohydrates provide the body with fibre – a form of carbohydrate that is indigestible, yet essential for a healthy and efficient digestive system.

Carbohydrates and fibre have important roles to play as part of a balanced diet. They consist of chains of sugar molecules formed from combinations of carbon, hydrogen and oxygen atoms. Carbohydrates and fibre are readily available from many plant sources, and are easily-incorporated into the diet.

The Role of Carbohydrates and Fibre

Carbohydrates and fibre have important roles to play in maintaining a healthy, efficient body – carbohydrates provide the body with most of its energy and fibre is essential for a healthy digestive system.


Classed as a macronutrient), carbohydrates are required in large quantities in the daily diet; most of our energy needs are met by them. When we eat carbohydrates, our body digests them and then releases their potential energy as glucose -the body’s most accessible energy source.

Different carbohydrates are digested at different rates – making blood-sugar and energy levels rise slowly or rapidly. In general, starches (complex carbohydrates), such as pasta, take some time to digest and so release their energy slowly, whereas sugars (simple carbohydrates), such as honey, are rapidly digested, providing a very quick boost of energy.

This explanation is a rather simplistic one, however. The combination of nutrients in a meal can affect the rate at which carbohydrates are broken down. Also, some complex carbohydrates, such as those in potatoes and bread, can produce a faster rise in blood-sugar levels than the simple carbohydrates found in apples and pears, for example. The glycaemic index is used to indicate the rate at which different foods are broken down into glucose.

The body converts carbohydrates into glucose and glycogen (the stored form of glucose). During activity, the muscles are fuelled by glucose in the blood and by glycogen stored in the liver and muscles. When the body has enough glucose, carbohydrates are converted into glycogen; when there is insufficient glucose, the body turns the glycogen back into glucose.

Carbohydrates are also needed by the central nervous system – the brain uses about 20-30 per cent of the glucose from digestion and carbohydrate metabolism. Carbohydrates are also needed to metabolize proteins that are used to build and repair body tissue.


Although the body cannot digest fibre, and it does not supply any nutrients, it still plays a vital role in maintaining health. Most people know that fibre is good for preventing constipation and its attendant ailments, such as haemorrhoids, but scientific studies show that dietary fibre can also help to lower blood cholesterol, control diabetes, and even prevent certain forms of cancer, although the reasons for these benefits are not yet fully understood. A diet that is high in fibre tends to be low in fat and calories, so a high-fibre diet can also make weight loss easier. There are two types of fibre and each has a different role.

Insoluble fibre acts like a sponge, absorbing large quantities of water, adding bulk to waste matter, and hurrying it through the digestive system. Stools are larger and softer, and can pass out of the body easily and quickly; this is why insoluble fibre is so good for regulating bowel action. Insoluble fibre can also bond with harmful substances, including potential cancer-causing substances (carcinogens), and expel them from the body, helping to prevent bowel disease.

Instead of hurrying waste through the digestive system, soluble fibre slows down the rate of digestion in the stomach, and, in doing so, provides the body with a steady supply of energy. That is why oats are good to eat for breakfast – they provide energy for the whole morning. Because soluble fibre slows the rate at which the body absorbs sugar into the blood, encouraging stable blood sugar levels, it is useful in controlling some forms of diabetes. When it reaches the intestines, soluble fibre is broken down by micro-organisms into short-chain fatty acids. These may help to lower blood cholesterol.