About six weeks ago, Woodswalker asked me to write a post addressing the role of alcohol in a low-carb diet. This is a huge topic, but I will present a few thoughts for consideration. If you have questions or comments, please remember that I am a biochemist, not a psychiatrist.
Alcohol, also known as ethanol, contains seven calories per gram. That's somewhat less than fat at nine calories per gram, and quite a bit more than carbs and protein at four calories per gram. Pure grain alcohol contains zero carbs. It is not an essential food. The metabolism of ethanol is fairly straightforward.
The first pathway happens mainly in the liver and is constitutive. ADH is the enzyme alcohol dehydrogenase. ALDH is the enzyme acetaldehyde dehydrogenase, and TCA stands for the tricarboxylic acid (Krebs) cycle. The second pathway is also found in the liver and is inducible--that is, it can be upregulated if the body is required to detoxify large amounts of alcohol on a consistent basis. MEOS stands for mitochondrial ethanol oxidizing system.
If a meal is consumed that contains alcohol, carbs, protein and fat, the calories from the alcohol will be processed first. This means that fat will not be used for energy until all the calories from the ingested alcohol have been burned. If a signficant number of calories of alcohol are ingested, this will postpone or even prevent fat burning. Drinking hard (i.e., distilled) liquor by itself does not affect insulin secretion, but when hard liquor is consumed with food, it increases insulin resistance and insulin secretion. Hard liquor also contains quite a few calories per ounce. By contrast, an ounce of mixed drinks, wine or beer will have fewer calories from ethanol. However, mixed drinks, wine and beer all contain carbohydrates, and, if they are consumed in quantity, will result in insulin secretion and eventual weight gain.
According to Dr. Michael Eades (see the comment at 31 October 2008, 21:34), a single glass of dry wine per day can improve insulin sensitivity and can assist with weight loss. For those who can stop at one glass of wine, that's great. But remember that alcohol is a psychoactive drug, and as such, it lowers inhibitions. In the low-carb context, it is important to note that alcohol can lower inhibitions against consuming carbs, and inhibitions against consuming a second glass of wine as well.
Alcohol stops gluconeogenesis. Gluconeogenesis is the process used by the liver to keep blood glucose levels within normal limits. If a person consumes lots of carbohydrates, an alcohol-induced cessation of gluconeogenesis will probably not even be noticed. However, if a person consumes alcohol while doing very low-carb, he is likely to experience a fall in blood sugar followed by a compensatory release of adrenaline. This can lead to heart palpitations which will be relieved by drinking orange juice or eating a high-glycemic food. Unfortunately, this regimen is not conducive to longterm success on a low-carb diet. If a low-carber notices that alcohol consumption is followed by the symptoms of low blood sugar, it may be necessary for him to drink less than a full serving to minimize the undesirable side effects.