Doctors have been telling us for years that a glass of red wine at night may be good for our hearts. But they still can't tell us why, exactly.Does the answer lie in the antioxidants known as polyphenols, which may or may not boost blood-vessel function, improve cholesterol levels, and fight inflammation? Or does alcohol play the more important role?A small new study may provide a clue. To compare the effect of polyphenols and alcohol on blood pressure, researchers instructed 67 older men at risk for heart disease to consume the same beverage every day for one month at a time: red wine (about two glasses), non-alcoholic red wine, or gin (about two shots).The daily doses of alcoholic red wine and of gin—which doesn't contain polyphenols—had no discernible effect on blood pressure. By contrast, when the men drank the non-alcoholic red wine, theirsystolic anddiastolic blood pressure fell by an average of 6 and 2 points, respectively, over the course of the month. (Systolic pressure is the top number in a blood pressure reading.)Though modest, blood-pressure declines in the same range have been shown to reduce the risk of heart disease and stroke by up to 20%, according to the study, which was published today in the American Heart Association journal Circulation Research."Our opinion is that [these] blood pressure-lowering...effects should be attributed to the polyphenols contained in wine," says senior author Dr. Ramon Estruch, M.D., Ph.D., a researcher at the University of Barcelona, in Spain."Alcohol," he adds, "seems to counteract the effects of the non-alcoholic fraction in red wine."The researchers suspect that polyphenols lowered blood pressure by raising blood levels of nitric oxide, which relaxes the arteries and allows blood to circulate more freely. In the study, only non-alcoholic red wine was associated with an increase in nitric oxide levels.Despite the growing evidence of red wine's heart benefits, doctors generally don't recommend it to their patients because of the hazards associated with alcohol. Non-alcoholic red wine might be an option for people who want to consume polyphenols without the alcohol, Estruch and his colleagues say.The study, however, left several questions unanswered that will need to be addressed in future research.It's not clear, for instance, whether the same experiment would produce similar results in healthy people or in women. And the researchers didn't measure the exact polyphenol content of the wines, raising the possibility that some other property of the non-alcoholic wine helped lower blood pressure.Another important drawback is that the researchers measured blood pressure on one day only, at the end of each month-long period, says Dr. Sharonne Hayes, M.D., founder of the Women's Heart Clinic at the Mayo Clinic in Rochester, Minnesota."Whether this [effect] would be sustained with longer-term use is unknown," Hayes says.