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It would be useful if the above definition of moderate drinking were bolstered by numerical estimates of "safe" drinking limits. However, the usefulness of quantitative definitions of moderate drinking is compromised by the likelihood that a given dose of alcohol may affect different people differently. Adding further complexity, the pattern of drinking is also an important determinant of alcohol-related consequences. Thus, while epidemiologic data are often collected in terms of the "average number of drinks per week," one drink taken each day may have different consequences than seven drinks taken on a Saturday night (1).

Despite the complexity, numerical definitions of moderate drinking do exist. For example, guidelines put forth jointly by the U.S. Department of Agriculture and the U.S. Department of Health and Human Services (2) define moderate drinking as no more than one drink a day for most women, and no more than two drinks a day for most men. A standard drink is generally considered to be 12 ounces of beer, 5 ounces of wine, or 1.5 ounces of 80-proof distilled spirits. Each of these drinks contains roughly the same amount of absolute alcohol--approximately 0.5 ounce or 12 grams (3).

These guidelines exclude the following persons, who should not consume alcoholic beverages: women who are pregnant or trying to conceive; people who plan to drive or engage in other activities that require attention or skill; people taking medication, including over-the-counter medications; recovering alcoholics; and persons under the age of 21 (2). Although not specifically addressed by the guidelines, alcohol use also is contraindicated for people with certain medical conditions such as peptic ulcer.

The existence of separate guidelines for men and women reflects research findings that women become more intoxicated than men at an equivalent dose of alcohol (4). This results, in part, from the significant difference in activity of an enzyme in stomach tissue of males and females that breaks down alcohol before it reaches the bloodstream. The enzyme is four times more active in males than in females (5). Moreover, women have proportionately more fat and less body water than men. Because alcohol is more soluble in water than in fat, a given dose becomes more highly concentrated in a female's body water than in a male's (6).

Since the proportion of body fat increases with age, Dufour and colleagues recommend a limit of one drink per day for the elderly (7).

Psychological benefits of moderate drinking. A review of the literature (8) suggests that lower levels of alcohol consumption can reduce stress; promote conviviality and pleasant and carefree feelings; and decrease tension, anxiety, and self-consciousness. In the elderly, moderate drinking has been reported to stimulate appetite, promote regular bowel function, and improve mood (7).

Cardiovascular benefits of moderate drinking. There is a considerable body of evidence that lower levels of drinking decrease the risk of death from co ronary artery disease (CAD). This effect has been demonstrated in a broad range of older epidemiologic studies (9). More recently, Boffetta and Garfinkel (10) found that white American men who reported in 1959 that they consumed an average of fewer than three drinks per day were less likely to die during the next 12 years than men who reported abstinence. This finding was due primarily to a reduction in CAD. In a similar study using a wide range of ethnic groups, De Labry and colleagues (11) found that rates of overall mortality were lowest for men who consumed fewer than three drinks per day over a 12-year period.

Similar results have been obtained with female subjects. Stampfer and colleagues (12) analyzed data on middle-aged women and determined that consumption of approximately one drink per day decreases the risks of coronary heart disease. Razay and colleagues (13), using a random population sample, found consumption of up to two drinks per day to be associated with lower levels of cardiovascular risk factors in women. In postmenopausal women, the apparent protective effect of alcohol may be explained in part by an alcohol-induced increase in estrogen levels (14).

Various researchers have suggested that moderate drinking is not protective against CAD, arguing that higher mortality among abstainers results from including among them people who have stopped drinking because of ill health. Higher mortality among these "sick quitters" would explain the comparative longevity of moderate drinkers (15,16,17). However, studies investigating the "sick quitter" effect do not support that conclusion; including "sick quitters" in the abstinent category cannot completely explain the apparent protective effect of moderate drinking against CAD (10,18,19,20).

There are risks that might offset the benefits of moderate drinking. Research shows that adverse consequences may occur at relatively low levels of consumption (1).

Stroke. A review of epidemiologic evidence concludes that moderate alcohol consumption increases the potential risk of strokes caused by bleeding, although it decreases the risk of strokes caused by blocked blood vessels (21).

Motor vehicle crashes. While there is some evidence to suggest that low blood alcohol concentrations (BACs) bear little relationship to road crashes, impairment of driving-related skills by alcohol has been found to begin at 0.05 percent BAC or lower, with rapidly progressing deterioration as the BAC rises (22). A man weighing 140 pounds might attain a BAC of 0.05 percent after two drinks.

Interactions with medications. Alcohol may interact harmfully with more than 100 medications, including some sold over the counter (23). The effects of alcohol are especially augmented by medications that depress the function of the central nervous system, such as sedatives, sleeping pills, anticonvulsants, antidepressants, antianxiety drugs, and certain painkillers. There is a consequent increased danger of driving an automobile after even moderate drinking if such medications are taken (24). In advanced heart failure, alcohol may not only worsen the disease, but also interfere with the function of medications to treat the disease (25).

Cancer. Although most evidence suggests an increased risk for certain cancers only among the heaviest drinkers, moderate drinking may be weakly related to female breast cancer. In one study (26), breast cancer was approximately 50 percent more likely to develop in women who consumed three to nine drinks per week than in women who drank fewer than three drinks per week. Although evidence concerning large bowel cancer is conflicting, one study suggests the possibility of a weak relation to consumption of one or more drinks per day (27).

Birth defects. Several ongoing studies are exploring the fetal risks associated with low levels of alcohol consumption. In one study (28), chil dren whose mothers reported consuming an average of two to three drinks per day during pregnancy were smaller in weight, length, and head circumference and had an increased number of minor physical anomalies when examined at intervals through the age of 3. In addition, mothers' self-reported consumption of as few as two drinks per day during pregnancy was found to be related to a decrease in IQ scores of 7-year-old children (29).

The question of whether moderate drinking is a risk factor for the fetus is not altogether settled, because mothers' self-reports of alcohol consumption may be underestimates (30). However, animal research provides additional evidence for adverse fetal effects from low levels of drinking. Nervous system abnormalities occurred in monkeys whose mothers were exposed weekly to low doses of alcohol. An effect occurred at a maternal BAC as low as 0.024 percent (31). A 120-pound woman might attain this BAC after one drink. Similarly, low prenatal alcohol doses produced biochemical and physiological changes in rat brains (32,33).

Shift to heavier drinking. Recovering alcoholics, as well as people whose families have alcohol problems, may not be able to maintain moderate drinking habits (2). Once a person progresses from moderate to heavier drinking, the risks of social problems (for example, drinking and driving, violence, trauma) and medical problems (for example, liver disease, pancreatitis, brain damage, reproductive failure, cancer) increase greatly (34).