Simple sugars
Monosaccharides are the simplest carbohydrate units and are classified according to whether they are aldehyde or ketone derivatives and the number of carbon atoms in the molecule. Hexoses, sugars containing six carbon atoms, are the nutritionally significant sugars found in foods, whereas others, particularly the pentoses, ribose, and deoxyribose, which each contain five carbon atoms, are produced in the metabolism of food stuffs. The single hexoses-glucose, fructose, and galactose-require no digestion and are readily absorbed from the intestine directly into the bloodstream.
Glucose, also called dextrose, is a moderately sweet sugar found in fruits and vegetables. Linked with another molecule ofa monosaccharide it is a component of all the disaccharides and is the basic structural unit of the polysaccharides starch and cellulose. It is prepared commercially as corn syrup by the acid hydrolysis of starch. Fermentation of glucose by the enzyme zymase in yeast results in the formation of carbon dioxide and ethyl alcohol. Glucose is the form of carbohydrate to which all other carbohydrates are eventually converted for transport in the blood and for utilization by the cells of the body. The following terms are used to describe the blood glucose level in the body: normoglycemia, which refers to a blood glucose level within normal range (65-115 mg/dl), hyperglycemia, which indicates a blood glucose level above normal range, and hypoglycemia, which means a bloud glucose level below normal range.
Fructose, also called levulose or fruit sugar, is found associated with glucose in many fruits and vegetables, and especially in honey. It is a highly soluble sugar and in water solution appears to be the sweetest of the simple sugars. It is fermented by yeast. It is combined with glucose to form sucrose and is the structural unit in inulin, a polysaccharide found in certain roots Qerusalem artichoke) and bulbs (onions and garlic). Inulin, however, has limited dietary significance because very little is digested in the gastrointestinal tract.
Now that a commercially economical method of separating glucose from fructose in digested sucrose has been developed, fructose is now available in liquid, powder, and tablet form. Because of its sweetness, it has been
uggested that fructose replace saccharin as a sweetener in diabetic and low-calorie diets. Although fructose does not require insulin to enter the cell, in insulin deficiency, fructose can lead to increased glucose production. As the long-term effects on metabolic function of substituting fructose for glucose in the diet are not knovm, caution is recommended. Moreover, the sweetness of fructose varies to such a degree in different products, depending on the temperature, acidity, flavor, and other characteristics, that the reduction in sugar and, hence, calories may be minimal. Its significance in calorie-restricted diets, therefore, depends on how much of the low-calorie product is consumed and its total calorie content. High-fructose corn syrup (HFCS), which is from 42% to 90% fructose, is also being used in many commercial products.
Galactose is seldom found free in nature but is derived chiefly by hydrolysis from the disaccharide lactose found in milk. It is less soluble in water and less sweet than glucose. Some galactosans, also called galactans, occur in food. Since little is known about the digestibility of these galactose-containing polysaccharides, they are frequently excluded in the dietary treatment of galactosemia. Galactose is also a constituent of the glycolipids and glycoproteins found in many tissues. In the body, glucose is changed to galactose so that the mammary glands can produce lactose.
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