Does a drink a day keep heart attacks away? Over the past 20 years, numerous studies have found that moderate alcohol consumption, that's to say, one or two beers, glasses of wine or cocktails daily helps to prevent coronary heart disease. Last week a report in the New England Journal of Medicine added strong new evidence in support of that theory. More important, the work provided the first solid indication of how alcohol works to protect the heart. In the study, researchers from Boston's Brigham and Women's Hospital and Harvard Medical School compared the drinking habits of 340 men and women who had suffered recent heart attacks, with those of healthy people of the same age and sex. The scientists found that people who sip one to three drinks a day are about half as likely to suffer heart attacks as nondrinkers are. The apparent source of the protection:those who drank alcohol had higher blood levels of high-density lipoproteins  , the so-called good cholesterol  , which is known to prevent heart disease. As evidence has mounted, some doctors have begun recommending a daily drink for patients of heart diseases. But most physicians are not ready to recommend a regular happy hour for everyone. The risks of teetotalling are nothing compared with the dangers of too much alcohol, including high blood pressure, strokes and liver troubles--not to mention violent behavior and traffic accidents. Moreover, some studies suggest that even moderate drinking may increase the incidence of breast and colon cancer. Until there is evidence that the benefits of daily dose of alcohol outweigh the risks, most people won't be able to take a doctor's prescription   to the neighborhood bar or liquor store. The main theme of this passage is   _  .
the change in recent drinking habits the connection between cancer and alcohol whether moderate drinkers outlive nondrinkers whether alcohol may be good for one's health