You might think the one-drink-per-day limit for women versus two for men is because women tend to be smaller, and you'd be partly right. A more surprising factor is that women are deficient in an enzyme (called alcohol dehydrogenase) that breaks down alcohol in the stomach and liver.

"For genetic reasons, women have considerably less of this enzyme than men. If a man and woman weighed the same and drank the same amount of alcohol, the woman would have higher blood levels of alcohol than the man for a longer period of time," explains preventive medicine specialist David Katz, MD, MPH, founding director of the Yale-Griffin Prevention Research Center.

"Whether alcohol and its metabolic breakdown products have toxic effects depends on how high blood levels go and how long they stay elevated." One drink can vary widely in both calorie and alcohol range, so make smart choices and sip slowly.

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