Moderate drinkers are wealthier, more educated and less likely to be disabled than teetotalers, which explains some, but not all, of the association between moderate alcohol consumption and longer life, according to a new study.

Having one drink a day halved a person's risk of dying over the next four years, Dr. Sei J. Lee of San Francisco VA Medical Center and colleagues found. After accounting for several factors that could influence alcohol use and mortality, the effect was weakened, but moderate drinkers were still 28% less likely to die than non-drinkers.

The first study to show that moderate drinkers live longer than either teetotalers or heavy drinkers was published in 1923. But the jury is still out on whether moderate drinkers are simply healthier overall than non-drinkers, or if alcohol itself used in moderation does benefit health, Lee and colleagues note.

They investigated the role of two risk factors associated with mortality that, to their knowledge, have not been studied together: functional disability and socioeconomic status (SES).

They looked at 12,519 men and women, aged 55 and older, enrolled in the Health and Retirement Study. During four years of follow-up, 14% of the non-drinkers died, compared to 7% of moderate drinkers and 12% of people who consumed three or more drinks daily.

According to Lee and colleagues, people who had a drink a day had a significantly higher socioeconomic status than non-drinkers, as measured by income, wealth, and years of education. For example, 37% of drinkers had a college education, compared to 14% of non-drinkers, and 52% of drinkers had $300,000 in assets, while 21% of non-drinkers did.

Non-drinkers also were more likely to have functional disabilities, for example difficulties in completing self-care activities like getting dressed or going to the bathroom, as well as problems with more complex activities such as making meals or managing their finances.

Forty-one percent of non-drinkers had trouble walking for several blocks, compared to 18 percent of moderate drinkers.

Overall, the moderate drinkers were half as likely to die as the non-drinkers, according to a report of the study in the Journal of the American Geriatrics Society.

After the researchers adjusted for traditional risk factors such as illness, smoking and obesity, the moderate drinkers were still 43% less likely to die during follow up. Once the researchers adjusted for SES and disability, the lower death risk for moderate drinkers compared to non-drinkers shrank to 28 percent.

"The results significantly strengthen the evidence that moderate drinking leads to lower rates of overall mortality," the researchers write. But, they add, moderate drinkers could have yet more beneficial characteristics not examined in the study, and it's possible "that adjustment for these characteristics could fully explain the alcohol-mortality relationship."

The only way to truly answer the question of whether moderate drinking is, in itself, beneficial would be to do a randomized, controlled trial, the researchers argue. Such a trial would present serious "logistical and ethical" hurdles, they note, but given the lack of consensus on the overall benefits and risks of moderate alcohol use, "the time has come to perform such a trial."