Researchers often define moderate drinking alcohol consumption as drinking a given number of units of alcohol over a given period of time. According to one popular definition, moderate drinking means two units of alcohol per day for men and one for women. What, then, is one unit of alcohol? For beer or wine we can more or less imagine what one unit means but, when it comes to spirits, things are suddenly much less clear. Much depends on the quantity poured into the glass. After all, a pub measure is not the same as a home measure.

For the purposes of scientific alcohol research, one unit of alcohol is commonly defined on the basis of the alcohol content per consumption, such that one glass of beer is equivalent to one glass of wine or one measure of spirits. It seems simple enough in theory, but practice is an altogether different story. After all, not all beers contain the same amount of alcohol, the content can vary considerably from one beer to the next. The same applies for wines and spirits. Beer contains an average of 4.5 percent alcohol by volume, but the alcohol content of light beers may be less than 3 percent, while some strong beers may contain as much as 9 percent alcohol or even more. Similar variations are encountered in the categories of wine and spirits. Not to mention quantity. If you, the consumer, suppose that one unit of alcohol equals one glass of beer or a good old pint, what do you make of the ‘kingsize’ bottles that sometimes hold three times as much beer as normal bottles? The difficulty of speaking in terms of a standard alcohol consumption is apparent from the difference from country to country. In Great Britain, for example, a standard consumption will contain 8 g alcohol, while a standard consumption in Japan contains 19.75 g alcohol. The United States has another standard again: 12 g alcohol.
An analysis of 125 international epidemiological surveys (Turner, 1990) researching the relations between the total number of alcohol consumptions per day and the effects on health revealed that three units of alcohol per day could vary, from one study to the next, from 24 g to 48 g alcohol per day! Anyone reading these studies must always keep sight of how a unit of alcohol is defined. Different researchers, different definitions.

There are different research methods for quantifying the amount of alcohol a person drinks. You can go to your target group and use in-depth interviews to map out their drinking levels. Or you can phone people and ask them questions about their drinking levels. The latter method is quicker and less expensive, without being any less efficient than the former, provided always that the questions are intelligently framed. The number of questions is also important. Questions such as ‘how many units of alcohol did you consume per day last year on average?’ will naturally seldom elicit a precise answer. The best questionnaires are broader-fronted and begin, for example, with ‘how many glasses did you drink last week?’, ‘do you sometimes drink more than five glasses in a session?’, and so forth. The wider the questionnaire is cast, the more accurate a picture is obtained of drinking levels.
It goes without saying that persons who are dependent on alcohol will hardly volunteer the real facts of their problematic condition and will, if anything, tend to understate.
To measure the effect of alcohol consumption on health, researchers must have suitable methods at their disposal to quantify drinking levels. Subjects must then, necessarily, be placed in different categories: total abstainers, light drinkers, moderate (‘social’) drinkers and heavy drinkers. In the absence of any international standard to carve these categories as it were in marble, different researches sometimes use different definitions, thus rendering comparison difficult.

Not only quantity, but quality is probably also the key. The effect on health is different when a moderate drinker drinks two glasses per day than a moderate drinker who downs 14 glasses on a Saturday night and does not drink during the rest of the week. Moreover, moderate drinking can produce different effects in different circumstances. Four glasses on top of a good dinner is not excessive, but driving home afterwards simply should not be attempted. Finally, the physical constitution of the drinker makes a difference. Women have proportionally more body fat and less body water than men. Because alcohol is a water-soluble molecule that readily dissolves in the bodily fluids, the concentration of blood alcohol increases faster in a woman than in a man, even if both subjects weigh the same and drink the same volume of alcohol.
The body fat / body water ratio changes with advancing age because body fat increases with the years. Given the same volume of alcohol and the same body mass, a man of 60 will nevertheless have more alcohol in the blood than will a man of 20 after drinking the same number of alcohol units.

It is especially difficult to draw a clear line between risk-free drinking, with its beneficial effect on health, and excessive drinking whereby the positive influence is turned inside-out to reveal a negative effect. Any such line must always be dependent on the circumstances and on the drinker in question.

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