The present study found an association between the consumption of caffeinated drinks and clinical depression in children. However, a cause-effect relationship cannot be determined due to the correlational nature of the data. In fact, very few empirical studies have previously addressed this issue. We cannot determine whether the high consumption of caffeine in our study population was actually "causing" depression or if high consumption was used to relieve some symptoms of depression. It is possible that children meeting diagnostic criteria for major depression use caffeine to "self-medicate" to ease the symptoms associated with depression. Studies have shown that adults often use substances to alleviate psychiatric symptoms [18
]. In addition, caffeine usage has been moderately associated with a genetic predisposition for major depression, anxiety and substance use disorders [19
]. It is therefore possible that the depressed youth in the current study may ingest caffeine and sweets for temporary relief [7
The issue of a link between externalizing problems and depression, may be less intuitive. However, studies have shown that depressive symptoms in children are present somewhat differently than in adults. Children frequently show "irritability and aggressiveness", which may be perceived as "externalizing behaviors" by parents and teachers; thus their CBCL-T-scores on the externalizing scale may be high [20
The primary vehicle for caffeine consumption in children is soft drinks, which generally also contain a large amount of sugar [22
]. Accordingly, in this study we also found a correlation between depression and the consumption of sweets and colas. Studies have shown that sugar, like caffeine, activates dopaminergic reward circuits in a manner that is similar to drugs of abuse such as cocaine [23
]. Therefore caffeine in soft drinks may act synergistically on the dopaminergic system, with the high levels of sugar in the drinks.
It is well documented that caffeine consumption in adults increases alertness and leads to high cognitive performance [26
]. However, studies have also shown that high caffeine consumption can trigger anxiety and depressive-like conditions [28
]. It has been postulated that caffeine at nontoxic doses acts as a competitive antagonist to the adenosine A1 and A2 receptors [30
] and studies have shown that adenosine interacts with dopamine DRD2 receptors [31
] as well as with glutamatergic neurotransmission [33
]. Both the dopaminergic and gluatmatergic systems have been implicated in psychiatric disorders (mood disorders). The adenosine A1 receptor subtype is inhibitory while the A2 subtype is facilitatory. Since caffeine acts at both adenosine receptor subtypes, some authors have argued that caffeine at different concentrations may have opposing effects by acting at different receptors subtypes [34
]. Other studies suggest a J-shaped caffeine dosage curve as an independent risk factor for suicide [35
]. In fact these researchers have shown that caffeine consumption at low to moderate doses may have protective effects. For example, they found that moderate caffeine consumers had a lower risk of suicide, while heavy coffee drinkers (> 8 cups/day) had a greater risk for suicide [35
]. Furthermore, other researchers have found that daily caffeine consumption of up to 140 mg is associated with a reduced risk of depression [36
Studies in school children have shown that high caffeine consumers (> 50 mg/day) had significantly more negative-effect symptoms than low consumers (< 10 mg/day) [37
]. More recent studies have also shown that caffeine-dependent adolescents had a significantly higher score on self-reports of anxiety and depression than a non-dependent group [6
]. Interestingly, some studies have also linked the abuse of other drugs with caffeine consumption. Bernestein and colleagues (2002) showed that adolescents dependent on marijuana consumed significantly more caffeine than non-marijuana dependent controls. Earlier studies have shown that alcohol, marijuana, and amphetamine abusers have a greater likelihood of coffee drinking by 12 years of age when compared to non-abusers [38
]. It is therefore important for parents to closely monitor their children's consumption habits as a possible indicator of mood, behavior, and/or addictive problems on the rise.
The methods used in the current study present a number of limitations. First, we did not measure the actual average daily intake of caffeine or carbohydrates. We estimated consumption based on self-reported items on the NBI which is based on a likert scale. Second, our results should be considered preliminary due to the small sample size. Third, more male subjects than female subjects participated in the study and thus we cannot generalize our results to the young female population.