Absorption and utilization of nutrients in alcoholism
Absorption and Utilization of Nutrients in Alcholism
It is well known that alcohol can cause primary malnutrition by displacing other foods from the diet. However, balanced meals alone will not ensure adequate nutrition in alcoholics, because alcohol impairs the absorption and utilization of nutrients. These effects, known as secondary malnutrition, are the subject of this article.
ALCOHOL AND ENERGY
Because the U.S. population derives, on average, 4 percent of dietary calories from ethanol, and very heavy drinkers derive well over 50 percent of their calories that way, it is important to consider the implications of ethanol as an energy source. Data from the physical chemistry laboratory predict that the combustion of ethanol could yield 7.1 kilocalories (kcal) of energy per gram. In most situations, however, this theoretical value probably is not achieved by the body. For example, subjects given additional calories as ethanol failed to gain weight (Lieber et al. 1965), an observation confirmed when 1,800 kcal of ethanol were added to the 2,600 kcal diets of hospitalized alcoholics (Mezey and Faillace 1971).
As additional calories, ethanol causes less weight gain than equivalent calories from carbohydrate or fat (Pirola and Lieber 1972). When carbohydrate was withdrawn from the diet and was replaced by ethanol for up to 50 percent of total calories, subjects actually lost weight (Pirola and Lieber 1972). This can be explained in part by the fact that ethanol increases oxygen consumption and metabolic rate in normal subjects, and more so in alcoholics (Tremolieres and Carre 1961; Stock and Stuart 1974; Stock et al. 1973). Some of this effect is due to increased heat production after eating, a phenomenon known as dietary thermogenesis. Another mechanism of energy wastage is the metabolism of some of the ethanol by pathways that do not result in the usual storage of energy in high-energy phosphate compounds (Pirola and Lieber 1976).
DIGESTION AND ABSORPTION
There are three main types of foodstuffs: carbohydrates, proteins, and lipids (fats). To reach the tissues of the body where they are needed, these substances must pass through the lining of the digestive system and enter the blood or lymph. However, the molecules that compose these foodstuffs are far too large to pass through cell membranes. To be absorbed, they must be broken down into smaller molecules in a process known as digestion. Some of the enzymes that accomplish this task are contained in the lining of the digestive tract; others are secreted by organs such as the pancreas. Through their actions, carbohydrates are converted into simple sugars, proteins into amino acids, and fats into fatty acids and other substances.
Most digestion and absorption takes place in the duodenum, the first section of the small intestine. The internal surface of the small intestine is corrugated and studded with microscopic, finger-like projections, or villi, that increase the absorptive surface. Special mechanisms built into the cell membranes help transport digested foodstuffs into the villi, where sugars and amino acids enter blood vessels and fatty molecules enter the lymph. This type of...