By Deidre McPhillips and Brenda Goodman of CNN
Researchers say they may be able to explain how light drinking benefits the heart, and its main effect doesn't stem from changes in the blood - as scientists once thought - but from its actions in the brain.
But because alcohol also raises the risk of cancer at any amount, however, researchers say they aren't advising people to imbibe. Instead, understanding this mechanism may point to healthier ways to tap into the same benefit, such as through exercise or meditation.
For decades, large epidemiological studies have shown that people who consume moderate amounts of alcohol - less than one drink a day for women, and one to two drinks a day for men - have lower risks of major cardiovascular events like heart attacks and strokes compared with people who abstain from alcohol completely as well as those who drink more.
Scientists have never been able to tease out exactly why this is the case, however. Alcohol seems to increase levels of HDL, the "good" cholesterol, and drinkers have lower levels of a sticky protein called fibrinogen in their blood, which may reduce the risk of dangerous clots. And in small amounts, alcohol may increase insulin sensitivity. But these don't seem to fully explain the benefit.
So a Boston-based team of cardiologists decided to look somewhere else: the brain.
Dr Ahmed Tawakol, senior author of the study and co-director of the Cardiovascular Imaging Research Center at Massachusetts General Hospital, noted that after you have a little alcohol, before you feel buzzed, you feel relaxed.
"If you think of short-term alcohol, the first effect that people get … is a little bit of a destressing response," he said.
For the study, Tawakol and his team analysed the drinking habits of thousands of people who were enrolled in the Mass General Brigham Biobank. They found that those who had one to 14 drinks per week were less likely to have a heart attack or stroke than those who had less than one drink per week, even after adjusting for genetic, lifestyle and other risk factors.
They also analysed brain scans of hundreds of these people and found that those who were light to moderate drinkers had reduced stress responses in the amygdala, the part of the brain that processes fear and threats - along with fewer heart attacks and strokes.
"We found that the brain changes in light to moderate drinkers explained a significant portion of the protective cardiac effects," Tawakol said.
The benefits were especially prevalent among people with a history of anxiety.
"Alcohol was twice as effective at reducing major adverse cardiac events among individuals with stress and anxiety," Tawakol said. "It was about 20 percent in most patients but 40 percent relative risk reduction among individuals with prior anxiety."
Tawakol studies something called the stress neural network, which is centred around a part of the brain called the amygdala. When the amygdala is overexcited, it turns up the sympathetic nervous system, setting the body up for a fight-or-flight response. This causes blood pressure to go up and increases inflammation. Certain neurons also become activated in the process and direct the bone marrow to release more pro-inflammatory cells.
The endocrine system is activated and pumps out cortisol, telling the body to store fat, which increases the risk of diabetes and high blood pressure, and adrenaline, which further turns up blood pressure. Over time, this cascade of effects may drive up the risk for heart attacks and strokes.
The brain scans of light drinkers showed markedly less activity in the amygdala than both non-drinkers and heavy drinkers, Tawakol said, even though they had fasted before their scans, so there was no alcohol in their system, indicating that light drinking may have an effect even after the buzz wears off.
But the researchers found that any amount of drinking raised the risk of cancer, and finding alternative methods to reduce stress is important.
"At the same amount of alcohol that was 'protective' of cardiovascular disease, we saw a similar increased risk of cancer, so we're not suggesting that there is an attractive quantity of alcohol for improving health," Tawakol said.
Two good candidates to replace light drinking for reducing stress are meditation and exercise, he said. It may be one day that there's a pill that could turn down the effects of stress on the body, too.
"There are lots of studies looking at meditation and meditation absolutely impacts the stress neural network systems," he said. Studies are looking at whether meditation reduces enough of the downstream components and will in turn lower the risk of heart attacks and strokes.
"Exercise has well-known effects on the brain but in particular really has a very nice, dose-related effect on the stress neural network," he said.
Some experts who weren't involved in the study were critical of its methods and message.
"This complex paper tries to work out why moderate alcohol may be associated with lower heart attack risks," said Naveed Sattar, a professor of metabolic medicine at the University of Glasgow.
"The issue is we know any amount of alcohol is associated with more strokes and heart failure, and with increases in cancer and deaths from cardiovascular causes," Sattar said in a statement.
"So to concentrate only on one small aspect, even if true, gives the wrong impression and the title of better heart health with light to moderate alcohol is misleading and perpetuates old myths we really need to move on from."
Petra Meier, a professor of public health at the University of Glasgow, pointed out that the study can show only associations. It can't prove that alcohol was the reason light drinkers seemed to have less stress in their brains.
"There are a number of explanations including that light-to-moderate drinkers are different from abstainers in relation to a number of personal characteristics. These differences explain why low-level alcohol consumption appears to be associated with beneficial health outcomes, but without alcohol consumption being the causative factor," Meier said in a statement.
CNN
