Of 54 studies reviewed, 47 included in the "abstainer" category individuals who were not long-term abstainers but had only recently stopped drinking or cut down to once per month or less, said Kaye M. Fillmore, Ph.D., of the University of California San Francisco School of Nursing.
Because many older people abstain from or cut down on drinking for health reasons such as disability, frailty, or medication use, the abstemious group in these studies likely included many people in poorer health than those who continued to drink moderately, Dr. Fillmore and colleagues argued online in Addiction Research and Theory.
In fact, when the investigators pooled the results of only the seven studies that did not commit this so-called "abstainer error," they found no significant protective effect against all-cause mortality associated with light or moderate alcohol use.
Focusing on a subset of 35 studies that also examined mortality from coronary heart disease, the significant results favoring alcohol consumption again disappeared when only the two studies not committing the abstainer error were considered.
Furthermore, introducing the abstainer error into the formerly error-free studies produced data that appeared to indicate a significant protective effect of moderate drinking on all-cause and coronary heart disease mortality, the investigators reported.
Because of lack of access to original data, the review team could not go back and "fix" the abstainer error in the other studies to see if that changed the results, Dr. Fillmore said.
Of interest, even the error-free studies still showed the familiar "J-shaped" association indicating reduced mortality among light and moderate drinkers, heightened mortality among heavy drinkers, and no effect on abstainers. However, the results were not statistically significant, with all 95% confidence intervals crossing unity.
This finding may support the idea that many of the abstainers quit drinking relatively recently because of poor or failing heath, the authors said.
"In other words, regular light drinking may be a marker for good health among middle aged and older people, not a cause of it," they explained. "As a consequence, estimates of the extent of the impact of cardiac benefits from light alcohol consumption on mortality risk may have been greatly over-estimated in previous meta-analyses."
The analysis may have implications for other recent studies claiming to find protective effects of moderate drinking against dementia, type 2 diabetes, active Helicobacter pylori infection, and the common cold, the authors said.
And clinicians who have been recommending moderate drinking for its health benefits should rethink their position, Dr. Fillmore said. Her own physician recommended moderate drinking to her, and anecdotal evidence suggested the recommendation is common-"at least in California where we love our wine," she added.
The meta-analysis included only prospective mortality studies on the association between alcohol use and all-cause or coronary heart disease mortality published from 1974 to 2004. Studies that also examined morbidity were excluded. The authors cautioned that their analysis can not disprove the idea that light drinking is good for health. Only further, properly designed studies could do that, they said.
"Future prospective studies should use far greater precision in their assessment of drinking behavior and abstinence," they concluded.
- Reviewed by Robert Jasmer, MD Assistant Professor of Medicine, University of California, San Francisco