By Maggie Fox, Health and Science Correspondent
WASHINGTON (Reuters) - Heavy social drinkers show a pattern of brain damage similar to that seen in hospitalized alcoholics -- enough to impair day-to-day functioning, U.S. researchers said on Wednesday.
Brain scans show clear evidence of damage, and tests of reading, balance and other functions show people who drink more than 100 drinks a month have problems, the researchers said.
"Oftentimes alcoholics are the last ones to know they have a problem," said Dr. Peter Martin of Vanderbilt University in Tennessee, who wrote a commentary on the report.
"I think this is the first study of its kind that has looked at brain functioning in individuals who are heavy social drinkers who have not gone to get treatment for their alcoholism," added Martin, a professor of psychiatry who specializes in addiction.
Dieter Meyerhoff of the University of California San Francisco and colleagues examined 46 chronic, heavy drinkers and 52 light drinkers recruited using newspaper ads and flyers.
They used magnetic resonance imaging to look at physical brain structures and measured various brain chemicals associated with healthy brain function.
"The enrollment criterion for heavy drinkers was the consumption of more than an average of 100 alcoholic drinks per month for men over 3 years before the study (80 drinks for women)," the researchers wrote in their report, published in the journal Alcoholism: Clinical & Experimental Research.
One drink is usually defined as a serving of spirits, a glass of wine or a can or bottle of beer.
Standard tests of verbal intelligence, processing speed, balance, working memory, spatial function, executive function, and learning and memory were given to the volunteers.
"Our heavy drinkers sample was significantly impaired on measures of working memory, processing speed, attention, executive function, and balance," the researchers wrote.
Measures of brain chemicals and structures showed some of the same damage seen in alcoholics who were in the hospital or treatment centers, although with a slightly different pattern in the brain, they said.
The study is unusual in that most studies of brain damage from alcohol are done in people who have undergone treatment.
"What our findings indicate is that brain damage is detectable in heavy drinkers who are not in treatment and function relatively well in the community," Meyerhoff said in a statement.
Martin noted the volunteers in the study had gone without a drink for 12 hours and could thus be showing evidence of alcohol withdrawal rather than actual permanent brain damage.
"The problem of studying people who are out there drinking is you are never sure whether these are enduring effects or acute effects," Martin said in a telephone interview.
"Would these people, if they dried for a period of three or four weeks, would they have these abnormalities?"
Martin said it was most likely the damage was real and long lasting. "My personal experience is that there is an awful lot of evidence ... showing that the more people drink and the longer they drink, the more likely they are to have cognitive impairments."
Meyerhoff said moderate alcohol use for most adults translated to up to two drinks a day for younger men and one drink a day for women and older people.
"Our message is: Drink in moderation. Heavy drinking damages your brain ever so slightly, reducing your cognitive functioning in ways that may not be readily noticeable. To be safe, don't overdo it," Meyerhoff said.