The present study investigated the association between excessive smartphone use and dietary habits in Korean adolescents using a cross-sectional design with a large sample. Most participating adolescents (87.7%) reported that they had used a smartphone in the last 30 days, and 66.5% reported using a smartphone for over two hours per day. These findings indicate the pervasive use of smartphones by adolescents and are comparable with U.S. studies that found that 92% of teens reported going online daily and 45% were online “almost constantly” [10][9].
The major findings of the present study are the significant associations between specific dietary patterns and smartphone usage time. Adolescents who consume more soda, sweetened beverages, drinks with a high level of caffeine, fast food, instant noodles, and snacks, which could be labeled as addictive food, spent more time using a smartphone. In contrast, frequent intake of fruits and vegetables was linearly associated with lower levels of smartphone usage. Intake of milk was also associated with lower smartphone usage, although the association was less strong compared to fruits and vegetables. To our knowledge, our study presents novel findings, in that there have been no previous studies of the association between smartphone use and dietary habits in adolescents. In particular, the linearity of the associations between the consumption frequency of specific foods and smartphone usage time is interesting, although the effect size of the associations is small. Consistent with our findings, previous studies have reported associations between psychiatric symptoms and dietary patterns in disorders characterized by impulsivity and vulnerability to addiction, such as Attention-Deficit/Hyperactivity Disorder (ADHD) [26][27][28], binge-eating disorder [29], and gambling [30]. For instance, Kim et al reported linear associations between scores on ADHD symptoms (hyperactivity and inattention) and the consumption frequency of specific foods in the general population of school-aged children [27].
The fifth edition of the Diagnostic and Statistical Manual of Mental Disorders has included internet gaming disorder in the section “Conditions for Further Study” [31], and gaming disorder was listed as a formal diagnosis in the International Classification of Diseases 11th Revision, released in 2018 [32]. Although excessive smartphone use has not yet been agreed upon as a formal mental disorder, the addictive potential of excessive smartphone use should be studied carefully, considering that internet gaming is one of the major uses of smartphones and that smartphones are widely accessible at any time. Impulsivity and altered reward circuit activation in the brain are the shared neurobiological features in the disorders vulnerable to addiction, such as ADHD and substance abuse [33][34]. These are also the major predictive dimensions of excessive smartphone use [35][13]. Dietary patterns are also known to affect impulsivity and reward circuit activation [18].
Although the concept of food addiction is a controversial topic and there is no consensus on whether humans show addictive-like eating [37], previous animal studies indicate that high consumption levels of high-fat and high-sugar foods induce neurobiological changes, such as dysfunction of the reward system and downregulation of dopamine receptors observed in other addictive disorders [37][38][18]. Human studies have also suggested evidence for the addictive potential of hyperpalatable food via neural activation in addictive-like eating behavior and substance dependence, such as altered activation in the reward circuit [39][40][37][19].
In contrast to addictive high-fat, high-sugar, and highly processed foods, the Mediterranean diet, which is rich in fruits and vegetables, has been reported to be negatively associated with impulsivity and sensation-seeking [41]. Food addictions are also associated with a lower intake of the Mediterranean diet [36]. These results are consistent with the findings of the present study. Despite multiple studies suggesting a protective association of fruit and vegetable intake with impulsivity and vulnerability to addiction, the underlying mechanism has not yet been identified clearly. However, studies have hypothesized some mechanisms, including phytochemicals, short chain fatty acids, and the antioxidant properties in fruit and vegetables, and have reported positive evidence for their neuroprotective effect [42].
Even though dietary patterns had a significant linear association with smartphone usage time in our study, dietary patterns do not necessarily cause smartphone “addiction” directly. For “addiction,” in addition to excessive use, other elements (such as social impairment, tolerance, and failure to control behavior) also must be present [13]. Our study found relatively inconsistent associations for the odds ratios for the problems caused by smartphone use among the food intake frequency groups, compared to a linear association between dietary habit and smartphone usage time. These findings might suggest that the effect of dietary patterns on smartphone use is not so strong as to elicit the label of smartphone “addiction”; however, it may be that diet can contribute to the vulnerability to addiction by changing the underlying neural mechanism of the reward circuit activation. Previous studies also have reported consistent findings regarding the addictive potential of foods. Twenty-one studies and twelve studies included in a systematic review about “food addiction” reported evidence of brain reward system dysfunction and impaired control according to food intake, respectively, which indicates the existence of a change in neurobiological function caused by dietary habits [19]. However, only two studies reported evidence for social impairment caused by “food addiction” [19]. These studies suggest that addiction involves more social factors in addition to neurobiological change, which is in line with our findings.
Limitations
There are some limitations to this study that should be noted. First, it used a cross-sectional design, which limits the ability to determine causal relationships. Children and adolescents who are vulnerable to addiction and whose sensation-seeking and impulsive temperament are high might share the common features of smartphone overuse and frequent intake of addictive food. However, previous studies reported that dietary intervention can change the core symptoms in children with ADHD. In addition, Lumley et al. reported that greater impulsivity is associated with consuming a Western-style diet, with bidirectional causation [43]. Considering these previous studies, smartphone usage might be influenced by long-term dietary patterns. However, longitudinal or experimental studies are needed to confirm the exact causal relationship.
Second, our study was based only on an anonymous, self-reported questionnaire completed by participants, and it is possible that adolescents inaccurately reported their dietary patterns and smartphone use. In addition, although we included items to evaluate the problems caused by smartphone use, validated tools to assess the severity of smartphone abuse in terms of addiction criteria were not adopted in our study. Thus, future studies that include multi-informant reports and objective measures assessing children’s behavior are needed to confirm the findings of our study. Importantly, a previous study examining the correspondence between adolescent and parent reports for psychiatric symptoms found that the adolescents’ reports for substance abuse were more accurately correlated with the real diagnosis than the parents’ reports [44]. This suggests that reports from adolescents can be viewed as reliable.
Finally, our study was performed using a typically developing population, which limits the generalizability of the findings to the populations with addictive psychopathology, such as ADHD, substance abuse, and eating disorders. To confirm the present findings, future studies in groups with psychiatric disorders are warranted.