But observational studies can show only correlations, not causation. And they have other limitations. One major confounding factor is that socioeconomic status is a strong predictor of health and life span — and it tracks closely with drinking levels. Studies show that compared to heavy drinkers and abstainers, people who drink moderately tend to be wealthier and have higher levels of education. They tend to have better health care, exercise more, eat healthier diets, and have less obesity.
One study that compared nondrinkers to moderate drinkers — defined as having two drinks daily for men and one for women — found that 27 out of 30 well established risk factors for heart disease were “significantly more prevalent” among nondrinkers. Rather than causing better health, in other words, moderate drinking may be a marker for higher socioeconomic status and other lifestyle factors that promote a longer life.
Another problem with observational studies is selection bias. In some large studies, people categorized as “nondrinkers” may actually be former heavy drinkers, or they may have health issues that cause them not to imbibe. Studies have found that nondrinkers have higher rates of physical disabilities, psychiatric problems and pre-existing illnesses. When rigorous studies take these factors into account, they find that the protective effect of moderate drinking disappears.
“The appearance of protection vanishes like the mist on an autumn day as the sun comes up,” said Timothy Stockwell, an alcohol researcher and director of the Canadian Institute for Substance Use Research at the University of Victoria. “All of these thousands of studies, when you do a forensic examination of them, most of them have these horrendous flaws and are open to these systematic biases.”
One way to get around these limitations is through genetic studies. Some people carry a genetic variant that disrupts their ability to metabolize alcohol, causing them to develop skin flushing, irritation and other unpleasant symptoms when they drink alcohol. As a result, they tend to abstain or drink very little. If alcohol was good for heart health, these people should in theory have more heart disease compared to others. Instead, as one large analysis published in BMJ in 2014 found, they have “a more favorable cardiovascular profile and a reduced risk of coronary heart disease than those without the genetic variant.”
The study concluded: “This suggests that reduction of alcohol consumption, even for light to moderate drinkers, is beneficial for cardiovascular health.”
Not everyone agrees that the health benefits of moderate drinking are illusory. Alcohol has blood-thinning properties, and red wine in particular contains polyphenols that have beneficial effects on the microbiome, said Dr. Erik Skovenborg, a family doctor and member of the International Alcohol Forum, an international group of scientists who study alcohol and health. Alcohol also raises HDL cholesterol, often referred to as the “good” kind, though recent studies have cast doubt on it being cardioprotective.