Sugar alcohols
Sugar alcohols called sorbitol, mannitol, and xylitol have a sweetening effect similar to glucose. Sorbitol, which is made commercially from glucose by hydrogenation and also is found in many fruits and vegetables, is very slowly absorbed into the bloodstream and can apparently be metabolized without insulin. It has the same caloric value as glucose, the sugar from which it is derived. Manni tol, obtained commercially by hydrogena tion of mannose, occurs naturally in pineapples, olives, as paragus, and carrots, and may also be added as a drying agent to other foods. Since it is poorly absorbed, mannitol supplies about one-half the energy value of glucose.
Xylitol, a five-carbon sugar, is the reduced form of xylose. It is equal to sucrose in sweetness and caloric content. Although it is found naturally in such foods as raspberries, strawberries, spinach, and cauliflower, it is commercially produced from birch tree chips. Because metabolism ofxylitol is independent of insulin, its possible use in diabetic diets was suggested. Also, its lower cavity-causing potential led to its use in chewing gum. Recently, however, food and gum manufacturers have ceased using xylitol because of reports of its carcinogenic effects. The status of xylitol is currently under review by the Food and Drug Administration.
Inositol occurs in many foods, especially in the bran of cereal grains. Inositol, when combined with certain phosphate groups, forms phytic acid, which reduces the absorption of calcium and iron from the intestines. Dulcitol, obtained from galactose by hydrogenation, is sometimes added to foods.
Alcohol or ethanol is produced from the fermentation of glucose by the enzymes in yeast and may, for certain individuals consuming large quantities of alcoholic beverages, represent a significant part of the total energy intake. One g of alcohol yields 7 kilocalories (kcal). One jigger, approximately 1 oz (30 ml), of 100-proof alcohol (50% alcohol) contains about 15 g of alcohol, which contributes 105 kcal.
Ethanol requires no digestion and can be absorbed throughout the gastrointestinal tract. Its concentration in the body is in proportion to the water content of the tissue; for this reason, the blood can contain large amounts of alcohol while bone and adipose tissue have little. Alcohol is metabolized primarily in the liver where the enzyme alcohol dehydrogenase, essential for the conversion of ethanol to acetaldehyde, is found. Because alcohol also requires NAD+ (nicotinamide adenine dinucleotide) for this reaction, the rate of alcohol metabolism is increased by the simultaneous metabolism of carbohydrate (pyruvate). For the same reason, fasting or starvation decreases the rate of alcohol metabolism.
Another enzyme system for metabolizing ethanol oxidizes ethanol and NADPH so that the first step in oxidation consumes metabolic energy. It depends on two enzymes, cytochrome P-450 and a reductase, and is associated with the endoplasmic reticulum of cells. In chronic alcoholics who consume a pint or more of alcohol a day, this pathway may account for much of the metabolism of ethanol. Tolerance to alcohol may occur from an increase in the enzyme cytochrome P-450 as a result of the frequent consumption of large amounts of alcohol. Moreover, due to the metabolic inefficiency of this system, the alcoholic derives fewer calories per gram of alcohol. Because the same system is also responsible for the metabolism of barbiturates, it is not surprising that there is an interaction beween alcohol and drugs.
