In this study, we found that more than half of the participants reported being ED consumers. We also found that male gender, upper secondary school level, low physical activity, increased leisure screen time, rural residency and low SES were determinants for daily ED consumption and for being a high consumer of EDs.
The proportion of adolescents who reported to consume EDs is in line with the results of a large European survey from 2013 (48–82%) [1]. Several large-scale studies on adolescents from other parts of the world have given reports on the proportion of ED consumers; these proportions included, for instance, 35.2% in a study from New Zealand [18], 17.9% in a study from the US [13], 18.2% in a Canadian study [22] and 12% in a Korean study [15]. Overall, there seem to be some differences in rates of consumption in different countries, and the proportions found in our study do not stand out from the rest of the world. It should be noted that any discrepancies in the findings, in addition to reflecting cultural differences, may be a result of methodological differences between the studies [18]. While some of the studies assess the amount of EDs consumed during the last week, others assess ED consumption over the last month or over an even longer period; and for this reason one should be careful when making direct comparisons between the findings reported by single studies. It should also be noted that our study used a rather wide definition of being an ED consumer, as the responses included in this category ranged from consuming EDs daily to once a month or less. This definition corresponds to those applied in other studies [1, 9].
We found higher ED consumption in upper secondary school (grades 8–10, ages 15–19 years) compared to lower secondary school (grades 11–13, ages 12–15 years). Recent studies have shown that children as young as 10 years of age are consuming EDs [9, 10]. It is a cause for concern that the regular consumption of beverages containing high levels of sugar and caffeine, in addition to a variety of other constituents, is being found in younger age groups in many countries. Adolescents have consistently been found to be the age group consuming the highest amounts of EDs [1, 9].
As has been demonstrated in other large-scale cross-sectional studies, we found in our study that male adolescents had a markedly higher level of ED consumption than females; on average, they consumed twice the amount. This finding seems to be largely consistent across studies [1, 9, 10, 13, 22,23,24]. Independent associations between frequent and high ED consumption on the one hand, and low SES and rural residency on the other, were found in our study. Similarly, findings that reveal associations between ED consumption and rural residency, as well as between ED consumption and the educational background of parents, have been suggested in previous research [9, 24].
Regarding the lifestyle factors examined in this study, we found that there was an inverse association between ED consumption and the frequency of physical activity. In contrast, a study of Saudi adolescents found that those who performed over 60 min of physical activity daily were 1.4 times more likely to consume EDs at least 3 days per week [25]. However, in several studies from the US and Saudi Arabia, no association has been found between ED consumption and level of physical activity [23, 30, 31]. This variation in the results may reflect cultural differences; however, inconsistent findings may also be influenced by methodological differences between the studies, such as the number of participants and differing ways of assessing physical activity.
Furthermore, our results are in line with previous findings concerning an association between ED consumption and time spent in front of a screen [23, 25, 31]. The current findings suggest that adolescents who are heavy screen users have a higher risk of excessive ED consumption. Since screen time has been found to be adversely associated with sleep outcomes [32], as well as related to the symptoms of depression, psychological distress and low self-esteem [33], it is noteworthy that a relatively large proportion (12.4%) of the adolescents in our study reported that they spent at least 6 hours of their daily leisure time in front of a screen.
Our finding that high ED consumption is more common among adolescents with a lower SES, who spend more leisure time in front of a screen and who are less physically active, suggests that for some adolescents, ED consumption may be an additional factor leading to potentially adverse effects on somatic and mental health and development. Even making no inferences on causality, it can be argued that it probably would be beneficial for these adolescents to reduce their ED intake.
Strengths and limitations
There are some strengths and limitations to our study that should be taken into account.
There is always the risk of recall and reporting bias with self-reported data. However, in order to attain a large number of participants in this kind of cross-sectional study, the use of questionnaires is necessary. Our calculations of daily ED consumption are based on self-reported average frequency and volume, making this measure a crude semi-quantitative estimate. Non-participation is a challenge in epidemiologic studies [34] and it should be noted that eligible schools, classes or students who did not participate in the survey may have influenced the findings presented here. The average response rate for upper secondary schools was 51.2%, while it was 78.2% for lower secondary schools.
One asset of this study is that it covered a large geographical area, including both rural and urban areas. Furthermore, it employed a well-established survey [27, 35] in which all schools are invited to participate. The sample size is another major advantage of this study.
Directions for future research
In the future, we suggest that researchers might consider using more continuous registration methods, such as daily registrations of ED consumption, physical activity and media use over a predefined period of time. We recommend that further experimental studies should be conducted to investigate the effects of ED consumption. There seems to be a lack of longitudinal studies in this research field, and systematic reviews are also warranted.
Significance
The finding of widespread ED consumption among children and adolescents, as well as the characteristics of high-consumption groups, must be considered by national public health authorities in the perspective of a possible need for introducing regulations for the marketing and sale of these products. In addition, a focus on adolescents at risk of increased ED consumption should be established in primary and secondary health care, as well as in schools.