Carbohydrates – sugars, starches and fibre – are made of the elements carbon, hydrogen and oxygen. Their main role is to provide a source of energy for the body.
The basic building block of all carbohydrates is a single sugar molecule, such as glucose or fructose, known as a monosaccharide. A disaccharide is two monosaccharides joined together – sucrose, for example, contains glucose and fructose.
Sugars in fruit and vegetables are “intrinsic” as they are incorporated into the structure of foods, often
hidden within cell walls. In some foods, such as biscuits and sweetened cereals, the sugar is not bound into the structure of food and so these are called “extrinsic” sugars. Generally, foods with intrinsic sugars are healthier than those with extrinsic sugars; an apple is healthier to eat than a piece of cake. Foods containing intrinsic sugar tend to release energy more slowly into the bloodstream compared to foods rich in extrinsic sugar.
Also referred to as complex carbohydrates, dietary starches are made up of chains of sugar molecules. Starch-based foods include vegetables, bread, pasta, rice, potatoes, beans, pulses and breakfast cereals. Starches come in two main forms. Refined starches, as found in white bread, white rice and most commonly available types of pasta, have lost much of their fibre, vitamin and mineral content.
Unrefined starches are richer in fibre and nutrients than their refined counterparts and are therefore considered nutritionally superior. They also tend to give a slower, more sustained release of sugar into the bloodstream, which may be important for health in both the short and long term. Examples of unrefined starches include wholemeal bread, brown rice, wholewheat pasta and rolled oats.
Fibre is plant material that is indigestible and is sometimes referred to as non-starch polysaccharide (NSP). Fibre comes in two main forms. Soluble fibre dissolves in the gut to form a thick gel-like substance that slows down the release of some nutrients, particularly sugar, into the bloodstream. It appears to help control the levels of cholesterol in the blood, which may help to reduce the risk of coronary artery disease.
Insoluble fibre does not dissolve in the digestive tract and therefore adds bulk to the faeces. It is useful for
preventing constipation and there is evidence that a highfibre diet is associated with a reduced risk of cancer of the colon. A diet that is rich in insoluble fibre may also reduce the risk of other conditions, including haemorrhoids (piles) and diverticular disease (abnormal pockets on the lining of the colon that can become infected and cause bleeding or perforation of the gut wall).
Good sources of soluble fibre include fruit, vegetables, beans, oats, barley and rye. Good sources of insoluble fibre include wholegrain (unrefined) cereals, such as wholemeal bread, brown rice and wholewheat pasta, as well as beans and pulses, nuts, seeds, and fibrous vegetables, such as
carrots, celery and cabbage.