Researchers say that chronic alcohol and marijuana consumption during youth is associated with compromised neurocognitive abilities in later adolescence and adulthood.
The findings of the new study, in which researchers examined fibre tract integrity affected by adolescent alcohol and marijuana use for 1.5 years, support previous findings of reduced white-matter integrity in these youth.
Chronic use of alcohol and marijuana during youth is associated with poorer neural structure, function, and metabolism, as well as worsened neurocognitive abilities into later adolescence and adulthood.
This may be due to biological and psychosocial transitions occurring during adolescence that impart increased vulnerability to neurotoxic influences.
A study of longitudinal changes in fibre tract integrity associated with adolescent alcohol and marijuana use during 1.5 years supports previous findings of reduced white-matter integrity in these youth.
"Research has shown differences in the brains of teens who use alcohol and marijuana as compared to teens who do not use these drugs or report only very infrequent, minimal use," Joanna Jacobus, corresponding author of the study from the University of California, San Diego, said.
"Alcohol and marijuana may have a negative impact by altering important cellular communication in the brain, preventing development of new healthy cells, and/or causing inflammation, which can adversely impact healthy brain development in many ways. For example, the results can lead to changes in brain structure such as volume, and function such as activity."
"The teen brain is continuing to develop, so many neural systems are not yet fully matured, as compared to adults' brains."
"Brain connections important for inhibiting risky behaviours are still forming, and some youth are more likely to choose immediate effects, such as alcohol or marijuana use, over long-term benefits," Jacobus said.
For 18 months, the researchers followed 92 adolescents (63 males, 29 females), ages 16 to 20 years, divided into two groups: 41 with extensive alcohol and marijuana use histories by mid-adolescence, and 51 with consistently minimal if any substance use.
Participants were part of an ongoing longitudinal study of substance use in adolescence with teens recruited from local schools from 2005 to 2007.
Both groups received diffusion tensor imaging and detailed substance use assessments, along with toxicology screening, at baseline and 18-month follow-ups - 182 scans in all - as well as interim substance-use interviews every six months.
"We found evidence for poorer white matter tissue health in teens who engage in heavy alcohol and marijuana use compared to those who abstain," Jacobus said.
She noted that white matter, the "information highway of the brain", allows for quick and efficient communication between brain regions. Compromised white matter can mean slower cognitive processing and poorer cognitive performance such as memory, attention, and decision-making.
"As to whether there were differences in these teens before they began using alcohol and marijuana is difficult to determine, but we found that increasing alcohol use over 1.5 years in late adolescence was related to a decline in white matter health 18 months later, supporting a negative effect of alcohol use on the brain despite potential pre-existing differences."
"Our findings underscore that early initiation of alcohol and marijuana use can have negative implications on the brain."
"We hope this information can be communicated to teens to help them understand why drinking during adolescence is discouraged. In the future, biomarkers such as tissue health may help identify teens that are particularly vulnerable for engaging in riskier behaviours such as drinking," she added.
The study will be published in a special online issue of Alcoholism: Clinical and Experimental Research.