Genes appear to play a major role in whether a person experiences a blackout after heavy drinking, according to a new study from alcoholism researchers at Washington University School of Medicine in St. Louis and the Queensland Institute of Medical Research in Brisbane, Australia. Blackouts are periods of time that cannot be remembered later.
Elliot C. Nelson, M.D., associate professor of psychiatry at Washington University, led the study examining genetic influences on blackouts in general and on the incidence of having three or more blackouts in one year. The study appears in the March issue of The Archives of General Psychiatry. "Our results suggest more than 50 percent of the risk for having a blackout at some point in life seems to be controlled by genetic factors," Nelson says. "The same is true for the risk of having three blackouts in a single year. Genetic factors account for almost 58 percent of that risk."
Nelson and colleagues also found a surprisingly large percentage of those who report having had blackouts when drinking heavily. Surveying 2,324 identical and fraternal twin pairs who were part of the Australian Twin Register, the researchers found that more than 39 percent of women and 52 percent of men reported having a blackout. They also discovered that more than 11 percent of the women and 20 percent of the men had three or more blackouts in a year.
Twins surveyed for the study were born between Jan. 1, 1964 and Dec. 31, 1971. Because identical twins share identical genes while fraternal twins share about half of the same genes, researchers study twin pairs to learn whether certain traits are inherited, result from shared experiences or occur following environmental experiences that one twin had but the other did not.
To determine whether genetic or environmental factors play a more significant role, researchers compare identical and fraternal twin pairs to see whether a trait is shared more often by the identical twins — with identical DNA — or occurs just as frequently in fraternal twins. If it's common for both twins to be affected by a trait whether they are identical or fraternal, then researchers assume the trait is related to environmental exposures they might share. However, if something more commonly affects identical twins than fraternal twins, then the assumption is that genes play a substantial role.
Based on those assumptions, structural equations are developed that allow the estimation of genetic and shared environmental contributions to the variance in blackouts seen among the more than 4,500 men and women surveyed.
Using those equations, Nelson's team found that much of the risk for blackouts was genetic. The researchers also found that blackouts were more common in those who were alcohol-dependent.
"Alcoholics certainly have a higher rate of blackouts," he says, "but blackouts also are common among non-alcoholics."
How do genes contribute to risk for blackouts? The study did not address that issue directly, but Nelson says other research has suggested some targets.
"People who drink on an empty stomach or gulp their alcohol have higher rates of blacking out," he says. "Therefore, less efficient variants of genes involved in metabolizing alcohol could predispose people to blackouts. We also know that alcohol has effects on systems in the brain that are involved in memory formation, so it is likely that genes whose products are components of those systems may be contributing to the risk for blackouts."
As is the case for alcohol-dependence, Nelson says that many genes probably contribute to the risk for alcohol-related blackouts. He also expects there will be overlap between genes that contribute to risk for becoming alcoholic and genes that predispose a person to blackouts.
What's certain is that the more a person drinks, the more likely they are to suffer blackouts or to become alcohol-dependent. Those who reported drinking to intoxication and binge drinking also were more likely to report blackouts.
"If you drink enough, you are likely to either black out or pass out," Nelson says. "It has been reported that individuals who can drink large amounts of alcohol without passing out are predisposed to becoming alcohol-dependent and to having blackouts."
Nelson EC, Heath AC, Bucholz KK, Madden PAF, Fu Q, Knopik V, Lynskey MT, Whitfield JB, Statham DJ, Martin NG. Genetic Epidemiology of Alcohol-Induced Blackouts. Archives of General Psychiatry, vol. 61, pp. 257-263, March, 2004.
This research was funded by the National Institute on Alcohol Abuse and Alcoholism and National Institute on Drug Abuse of the National Institutes of Health and the Australian National Health and Medical Research Council.