There is some inter-conversion between nutrients such as protein being converted to glucose (gluconeogenesis) for release into the bloodstream, but under normal conditions, the capacity to convert one nutrient into another for storage is very limited. Also, humans have little capacity to ‘waste’ extra energy by burning it off. This process, which has been termed luxuskonsumption, is common in animals such as rats but is very limited in humans. Therefore, the examination of each macronutrient as a separate entity is necessary and is summarised below.
Alcohol. There is an inconsistent relationship between reported alcohol intake and body mass index, with many studies showing that drinkers of large amounts of alcohol have lower body weights. This has led to hypotheses that alcohol calories may be subject to inefficient oxidation, thereby diminishing the impact of alcohol calories on energy balance. In heavy drinkers, there does seem to be an inefficient burning of alcohol because the addition of alcohol to a weight maintenance energy diet does not result in weight gain. Virtually all ingested alcohol is metabolised in the liver and this process produces energy. None of it is converted directly into fat. The main pathway is via the enzyme alcohol dehydrogenase and this should theoretically produce 7kcal/g of alcohol. However, alcohol also appears to induce another series of liver enzymes to burn it and this pathway produces far less energy for use than the usual one.
Alcohol, perhaps because of its toxic potential, is the top priority fuel for burning and, if present, displaces fat and, to a lesser extent, carbohydrate and protein as fuel sources for energy needs. Hence, while alcohol does not directly turn into body fat, it will certainly send any spare fat in the diet into storage. So it appears that the so-called ‘beer gut’ may not be full of beer calories, but rather full of the chips and nuts that accompanied the beer which have been sent to the fat cells for storage. A sobering fact though is that although alcohol does stimulate its own burning, it does not satisfy hunger, and indeed may stimulate appetite. The nature of beer is such that the type of food that is likely to be eaten with it will probably be high in fat (can you imagine beer and fruit rather than beer and nuts?). The sugar which is in the beer, wine and mixes may have an effect on reducing appetite and therefore counter the stimulation of appetite by the alcohol. Overall, we could consider alcohol as ‘half-balanced’ as a form of energy in the body and a potential promoter of weight gain.
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