Carbohydrates are readily available in the diet, providing 17 kJ (4 kcal) per gram of energy (15.7 kJ (3.75 kcal) per gram monosaccharide equivalent).
Carbohydrate intake comprises the polysaccharide starch, the disaccharides (mainly sucrose) and monosaccharides (glucose and fructose). Carbohydrate is cheap compared with other foodstuffs; a great deal is therefore eaten, usually more than required. Dietary fibre, which is largely non-starch polysaccharide (NSP) (entirely NSP according to some authorities), is often removed in the processing of food. This leaves highly refined carbohydrates such as sucrose which contribute to the development of dental caries and obesity. Lignin is included in dietary fibre in some classification systems, but it is not a polysaccharide. It is only a minor component of the human diet. The principal classes of NSP are:
- cellulose
- hemicelluloses
- pectins
- gums.
None of these is digested by gut enzymes. However, NSP is partly broken down in the gastrointestinal tract, mainly by colonic bacteria, producing gas and volatile fatty acids, e.g. butyrate.
All plant food, when unprocessed, contains NSP, so that all unprocessed food eaten will increase the NSP content of the diet. Bran, the fibre from wheat, provides an easy way of adding additional fibre to the diet: it increases faecal bulk and is helpful in the treatment of Constipation.
The average daily intake of NSP in the diet is approximately 16 g. NSP deficiency is accepted as an entity by many authorities in the UK. It is suggested that the total NSP be increased to up to 30 g daily. This could be achieved by increased consumption of bread, potatoes, fruit and vegetables, with a reduction in sugar intake in order not to increase total calories. Each extra gram of fibre daily adds approximately 5 g to the daily stool weight. A high intake of fruits and vegetables probably reduces the risk of cancer. A high intake of dietary fibre reduces blood lipids.
Pectins and gums have been added to food to slow down monosaccharide absorption, particularly in type 2 diabetes.
|
