This section is from the book "Alcohol, Its Production, Properties, Chemistry, And Industrial Applications", by Charles Simmonds. Also available from Amazon: Alcohol: Its Production, Properties, Chemistry, And Industrial Applications.
Except for the small proportion excreted unchanged, alcohol taken into the body is oxidised therein as completely as the carbohydrates are. Definite proof of this has been obtained by analysing the air inspired and that expired by subjects treated with experimental doses of alcohol, and thus determining the effect of the treatment upon the value of the "respiratory quotient." This quotient is the ratio: volume of carbon dioxide produced / volume of oxygen used up and for the oxidation of carbohydrates its value is unity.
1 Rosemann, Pfluger's Archiv, 1889, 78.
This will be seen from the following equation, denoting the oxidation of a sugar: -
Oxygen, 6 vols.
Carbon dioxide, 6 vols.
The volume of the carbon dioxide produced is equal to that of the external oxygen supplied and used up in the oxidation, so that, for carbohydrates, the above ratio - 1.
With alcohol, however, a larger proportion of oxygen is required: -
Oxygen, 3 vols.
Carbon dioxide, 2 vols.
Hence the respiratory quotient for alcohol is only 2/3. Its value is in fact lower, not only than that for carbohydrates, but for fats and other ordinary foodstuffs.
Thus if alcohol is oxidised in the body, the addition of alcohol to the diet should be followed by a fall in the value of the respiratory quotient. This, in fact, is found to occur (Atwater and Benedict).1 It has been shown further that the energy liberated by this oxidation of alcohol is not lost as mere superfluous heat. Up to the extent of one-fifth of its total needs, the body can utilise energy derived from the metabolism of alcohol.
Energy so utilised will conserve that supplied by carbohydrates and fats in the diet. Of this there is experimental proof. Thus a man was kept on a standard measured diet, rich in carbohydrates, and his respiratory quotient was determined. A ration of alcohol was then added to the diet. The respiratory quotient fell promptly, and to such an extent as to show that, while the alcohol was being oxidised, carbohydrate was being economised and added to the food reserve of the man's body.
Similarly as regards fats; if a fixed diet sufficient for maintenance is given, an addition of alcohol protects some fat from oxidation, and allows more fat to be added to the body's reserve supply.
Thus alcohol can to some extent replace foodstuffs such as sugars, starches, and fats in the nutrition of the body. Moreover, unlike starch and fat, it requires no preliminary digestion, but passes unchanged to the tissues which utilise it. Since, however, it contains no proteids, alcohol is not a food in the sense of being a flesh-forming material. Nevertheless, it can apparently serve in some degree to economise protein, by reducing to a minimum the amount which the body requires.
1 "Physiological Aspects of the Liquor Problem," New York, 1903.
Excess of protein in the diet, over the amount necessary to repair tissue-waste, is oxidised to furnish energy, and alcohol can supply the place of this excess, so that a smaller ration of protein suffices. Whilst the experiments made to determine this point are not all in agreement as to the results, the general conclusion is that stated above.
The method of experiment is to put the subject on a mixed ration, containing known amounts of protein, carbohydrate, and fat. The nitrogen excreted in the urine is determined; this indicates the amount of protein that is being oxidised and used up merely as a source of energy. The diet is so adjusted that a reduction of carbohydrates or fat will lead to increased oxidation of protein, which will be shown by a larger amount of nitrogen excreted. If now, instead of simply reducing the carbohydrate, a chemically equivalent amount of alcohol is substituted for it, and if it is found that there is no increase in the quantity of excreted nitrogen, the result is evidence that the alcohol has taken the place of the carbohydrate as an economiser of protein. Most of the experimenters are agreed that this is, in fact, the effect produced, though there are some exceptions, believed to be due to the fact that the subjects of the experiments in these cases were not accustomed to taking alcohol.
The conclusion is that alcohol has a certain food-value in a limited sense, namely, as a fuel or supplier of energy. This, however, is not the whole story. Its theoretical value as a food is in practice limited and counterbalanced by its other properties; and in any case it has, from the point of view of its food-value alone, no advantages over a foodstuff like sugar.