A study of more than 6,000 adult twins from Australia suggests that there seems to be a “greater genetic influence in those who took their first full drink at a younger age,” says first author Arpana Agrawal, putting younger drinkers at greater risk for alcohol dependency.

WASHINGTON-ST. LOUIS (US)—The age at which a person takes a first drink may influence genes linked to alcoholism, making the youngest drinkers the most susceptible to severe problems, a new study finds.

Scientists at the Washington University in St. Louis School of Medicine say that genetic factors contributed to risk for alcohol dependence at rates as high as 90 percent in the youngest drinkers.

“There seemed to be a greater genetic influence in those who took their first full drink at a younger age,” says first author Arpana Agrawal, assistant professor of psychiatry. “That’s very consistent with what has been predicted in the literature and in the classification of types of alcohol dependence, but we present a unique test of the hypothesis.”

Agrawal and her colleagues examined previously collected data from 6,257 adult twins from Australia, including identical and fraternal, male and female twins, using statistical methods to measure the extent to which age at first drink changed the role of heritable influences on symptoms of alcohol dependence.

Using the twin model, they were able to tease out genetic influences, shared environmental influences and non-shared environmental factors.

While genetic factors played an important role for twins who started drinking early, for those who started drinking at older ages, environmental factors that make twins different from each other, such as unique life events, gained prominence.

The twins in the study were 24 to 36 years old when they were interviewed, but some reported taking their first drink as young as age 5 or 6.

Those who were 15 or younger when they started drinking tended to have a greater genetic risk for alcohol dependence. Those who were 16 or older before they took their first drink later became alcohol dependent, but their dependence was related more to environmental factors.

“We don’t have actual gene expression data in this study, but we could hypothesize that exposure to early-onset drinking somehow modifies the developing brain,” Agrawal says.

“Particularly frequent or heavy early drinking may influence gene expression and contribute to more severe outcomes. Our research cannot prove that, but it’s something that neuro-imaging and gene expression studies certainly should investigate.”

Another possibility is that early drinking exposes adolescents to certain environment influences, such as their peer groups, that somehow enhance genetic influences that contribute to risk for alcohol dependence.

“Something about starting to drink at an early age puts young people at risk for later problems associated with drinking,” Agrawal says. “We continue to investigate the mechanisms, but encouraging youth to delay their drinking debut may help.”

Age is a factor, but not the only factor in determining whether a person will become an alcoholic, Agrawal stresses.

“Some early-onset drinkers do not develop alcohol problems and some late-onset drinkers do—we are working on why that is the case, but it is important to note that this is one risk factor among many and does not determine whether a person will, or will not, develop alcohol dependence.”

Agrawal says there have been two main hypotheses for why age at first drink is a well-known risk factor. “One has been that common genetic and environmental factors contribute both to the risk for alcohol dependence and to the likelihood a person will be younger when consuming their first drink.

“A second hypothesis suggests starting to drink at a younger age exerts an influence on alcohol dependence that is independent of these shared factors. Our findings suggest there may be some truth to both hypotheses.”

Studying twins offers advantages when attempting to learn about genetic and environmental influences on alcohol dependence, Agrawal notes. Because identical twins share 100 percent of their DNA, differences in drinking behavior must come from environmental factors. Similarities between identical twins tend to be influenced by genes and family environment.

“Particularly identical twins offer us the opportunity to study the perfect natural experiment of genetically identical individuals whose drinking trajectories are modified by their shared and unique life experiences,” she explains. “They are important assets in the study of complex behaviors, such as alcohol consumption.”

The study will be published in the December issue of Alcoholism: Clinical & Experimental Research, but is available online through the journal’s Early View.

The study was supported by the National Institute of Alcohol Abuse and Alcoholism of the National Institutes of Health and the ABMRF/Foundation for Alcohol Research.

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