sugars

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sugars carbohydrates, along with fats and proteins, water, some minerals, and vitamins, constitute the diet. Thus what the body is made of, plus what is needed to keep it functioning, must be derived from these elements. The building blocks of carbohydrates are the sugars. The general formula for sugars is (CH2O)n — that is, carbon combined with water multiplied n times — indicating the derivation of the word ‘carbohydrate’. The value of ‘n’ can be from 3 to 7, in trioses, tetroses, pentoses, hexoses, and heptoses respectively. These are all monosaccharides.

The common sugars that form part of the diet are sucrose, lactose (milk sugar), and maltose. They are all disaccharides and are split into monosaccharides during digestion; sucrose to glucose and fructose, lactose to galactose and glucose, and maltose into two glucose molecules. All three monosaccharides — glucose, galactose and fructose — can be metabolized by the body and used as a source of energy, although glucose is by far the most important, and is normally maintained within a narrow range of concentration in the blood. Fructose is also a dietary component in fruit. Some people congenitally lack lactase, the enzyme which splits the lactose molecule in the intestine, and consequently suffer abdominal discomfort and diarrhoea after ingesting lactose. Lactose is present in milk and while few Europeans suffer lactase deficiency, some 97% of Thais, for example, are without this enzyme; in most instances this is likely to be the consequence rather than the cause of the rarity of milk-drinking and cheese-eating in this and Eastern communities, since their infants have no problem. Starch (a polyglucose) is broken down during digestion into maltose and then into glucose.

Sugars have multiple functions within the body and are used as a primary source of energy, through the synthesis of ATP, and for energy storage. When the body stores glucose it does so as glycogen (body starch). Glycogen stores in muscle provide readily available glucose for exercise; glycogen stores in the liver are crucially involved in the maintenance of blood sugar — the concentration of glucose in the blood. This in turn is vital as a nutrient supply to the brain, which normally utilizes only glucose for energy production.

Five-carbon sugars (pentoses) also have a crucial significance in the body. For example, ribose and deoxyribose are essential for the formation of the backbone of RNA and DNA. As well as forming part of the molecules determining inheritance and phenotype, sugars are linked to proteins and lipids, to form glycoproteins and glycolipids. Glycoproteins include antibodies and clotting factors. Other proteins and lipids expressed in the membranes of cells and linked to sugars are responsible for cell-to-cell recognition and adhesion, and are involved in the processing of messages brought by hormones and neurotransmitters and, in cells in the spleen and elsewhere, in recognizing old circulating red cells and removing them.

Alan W. Cuthbert


See also blood sugar; carbohydrates; insulin; metabolism.