Alcohol is a commonly consumed psychoactive drug that also provides supplementary energy to the diet. Each gram of ethanol contains 7.1kcal and therefore regular drinkers can add considerable energy intake to their dietary consumption when consuming alcoholic beverages. Estimates for the amount of ethanol energy contributed to the total energy intake of individuals vary from around 5% for casual drinkers to over 10% for heavy drinkers and to around half of daily calories in alcoholics. However, despite this fact evidence suggests that drinkers do not have higher body weights than non-drinkers, and this is especially true for women. There is the possibility that ingestion of ethanol alters the intake of other macronutrients. For example, in women it is known that as ethanol consumption increases, a compensatory decrease in carbohydrates occurs, mainly as sugars.
Researchers1 have investigated the contribution of ethanol to energy intake in a metabolic ward using 14 alcoholic subject ages 41 to 59 years. The patients had abstained from alcohol for 6 weeks or longer and had no gastrointestinal problems that could alter absorption of nutrients. Subjects were given a diet to maintain bodyweight and systematically their carbohydrate levels were reduced and replaced with an equal amount of calories from ethanol, until 50% of carbohydrate calories were substituted. At the higher concentrations of ethanol, body weight declined. The mean loss of bodyweight at the end of day 16, following 7 days of the highest ethanol intake, was 0.9kg. This was significantly different to the weight change in the control period (+0.04kg). Therefore when 50% of the total energy intake of carbohydrates is substituted for ethanol there is a loss of bodyweight.
To further elucidate the role of ethanol in bodyweight and energy metabolism, two subjects were given the ethanol in addition to the standard diet. In these subjects the addition of ethanol caused an increase in total energy intake from 2500 to 4500kcal per day. After 30 days on this diet the weight gain in the subjects was only 0.006kg per day with a total gain of 0.190kg. This weight gain resulted from an initial increase, followed by a slowing and then cessation in weight gain as the study progressed. When the same protocol was repeated but with chocolate as the extra energy, the daily weight gain was 0.198kg with a total weight gain of 2.780kg. Analysis of the faeces showed that there was no extra energy present suggesting that this was not the route of loss of the extra calories.
Administration of alcohol has been shown to increase oxygen consumption, metabolic rate and thermogenesis in moderate drinkers. One possible explanation is that alcohol is metabolised in the endoplasmic reticulum of hepatocytes by the microsomal ethanol oxidising system (MEOS) using cytochrome P450-II-e1. Normal cytosolic metabolism of ethanol uses alcohol dehydrogenase to synthesise acetaldehyde with the production of 3ATP equivalents. However MEOS uses the high energy compound NADPH in the synthesis of acetaldehyde but no high energy compound if formed and the energy is released as heat. This results in the consumption of 3ATP equivalents and thus a total reduction of 6ATP over normal cytosolic ethanol metabolism. The Km of MEOS is also higher than that of cytosolic alcohol dehydrogenase suggesting that it would be more active at high blood ethanol concentrations. Chronic ethanol consumption may also induce MEOS thus increasing the contribution it makes to ethanol metabolism.
RdB
