Adolescence is a period of dramatic transformation in the healthy human brain, leading to both regional and general brain volume changes. Recent high-resolution Magnetic Resonance Imaging (MRI) studies emphasize the effects of ongoing myelination, indicating a substantial maturation process (see Figure 1). The period of adolescence is often defined as spanning the second decade of life, although some researchers expand their definition of adolescence to include the early 20s as well. Research into brain maturation in adolescence is particularly important, given that it is normally considered the peak period of neural reorganization that contributes to normal variation in cognitive skills and personality Additionally, it is seen as the period of major mental illness onset, such as schizophrenia. Despite growing evidence for pronounced changes in both the structure and function of the brain during adolescence and early adulthood, few studies have explored this relationship directly using in vivo imaging methods. Thus, little is still known about the relationship between adolescent behaviour and outcomes, and maturational effects on morphological and functional aspects of the brain.
Figure 1 – Brain development during adolescence
The psychological and social changes of that occur during adolescence include a higher level or orientation towards and identification with peers, group socialism and personality consolidation. A main social behavioural change is the tendency to use alcohol and other stimulants. In European countries and especially Denmark, 60% of all adolescents report having had their first alcoholic whole drink before age 15, the majority reporting a debut at around age 12 (see Figure 2). Today, alcohol is considered a normal part of adolescent culture.
Figure 2 – Age of alcohol use debut, as measured by the first consumption of a full alcoholic unit. Note that the majority of debuts are around 12 years. Furthermore, note the relatively high number (8%) reporting a <9 year old debut. (x-axis numbers indicate age of reported first full drink of alcoholic beverage; “yngre end 9″ = “younger than 9″; “jeg har ikke drukket” = “I have not yet tried alcohol”)
The effects of prolonged regular alcohol consumption in adults are today considered well documented. Studies of extreme cases such as Wernicke’s encephalopathy and foetal alcohol syndrome have shown that alcohol at critical periods or over time can have severe effects on significant modification and damage of brain structure, physiology and function. In adults, heavy alcohol consumption results in atrophy of grey and white matter, particularly in the frontal lobes, cerebellum, and limbic structures. Heavy drinking also raises the risk of ischemic and hemorrhagic stroke.
Adolescents tend to drink larger quantities on each drinking occasion that adults, possibly a combination of lower sensitivity to some of the unpleasant effects of intoxication and altered patterns of social interaction, including a willingness to indulge in more risky activities. However, it has been suggested that adolescents may be more sensitive to some of alcohol’s harmful effects on brain function. Research now suggests that, even over the shorter time frame of adolescence, drinking alcohol can harm the liver, bones, endocrine system, and brain, and interfere with normal growth. Studies in humans have found that alcohol can lower the levels of growth- and sex hormones in both adolescent genders.
Recent studies using animal models have demonstrated that the adolescent brain is even more sensitive to alcohol consumption. Alcohol inhibits normal neurogenesis, the process in which neurons are created. The magnitude of this effect has been related to the level of consumption; higher levels of consumption (e.g. binge consumption) had the most severe effects on the brain. Furthermore, alcohol impairs spatial memory function in adolescent animals more than adults, a result that is thought to be mediated by a larger inhibitory effect of alcohol on neural transmission in adolescence.
The long-term effects of alcohol consumption on brain maturation are yet poorly understood in human adolescence. Studies report that a history of high alcohol consumption in adolescence has been associated with reduced hippocampal volumes (see Figure 3), and with subtle white-matter microstructure abnormalities in the corpus callosum. In order to understand the effects of alcohol on the brain these findings must be compared to several factors pertaining to alcohol consumption in young adults, including 1) onset of alcohol use; 2) amount of alcohol consumed regularly; 3) type of alcohol consumption (regular use or binge drinking). In addition, a number of confounding factors need to be addressed and controlled between different study groups, including 1) general cognitive function; 2) the effects of gender and pubic stage; 3) gender-related preferences for alcoholic beverages; and 4) the effects of gene-related differences in neurotransmitter function.
Figure 3 – Hippocampus volume differences in adolescents with alcohol use disorder (AAU, right) compared to healthy adolescents (left). Volume estimation is corrected for general brain size.
So who says alcohol is not damaging? We know it is in adults. We know it is in prenatal development. Why not during adolescence? Even more so; during this period of brain development so much is happening; the continuation of brain development; pruning and consolidation of brain, cognition and personality; all combined with the coctail of changes resulting from hormonal changes. Add alcohol, and in heavy doses in binge drinking, and you’ve got a recipe for brain dysfunction and detrimental brain development.
In most cultures, alcohol is seen as acceptable, even for adolescents. Alcohol is possibly even more problematic in countries such as Denmark, since it is not illegal for children and adolescents to drink alcoholic beverages (although it is strangely enough illegal for them to buy it; i.e. they have to get it from their parents or another >15 year old). Statistically, Denmark ranges among the countries that has the earliest alcohol debut and the highest mean weekly/monthly alcohol consumption in adolescence. In general, alcohol is more socially and culturally accepted in the Western world (or all cultures?). But given that alcohol has such damaging effects should we allow it if it turns out to have detrimental effects on brain development and cognitive functions?
You can put it another way: given that you know this, would you allow your teenage son or daughter to drink alcohol? If you knew that her or his brain would respond negatively (both long and short term) to the alcohol, would you let it happen? Thick twice