Adolescence is considered a significant period for establishing independent diet patterns from parents that last for a lifetime (1). Adolescents in high school spend the day at school, where they might be prone to consuming sugar-sweetened beverages (SSBs, e.g., soda, fruit-flavored drinks, sports drinks) and fast foods (e.g., hamburgers, pizza) via cafeterias and vending machines (2). In addition, high school students tend to eat at fast-food restaurants with their friends as a convenient place to spend their leisure time (3). Studies have reported that 63% of adolescents in the USA consumed SSBs at least once a day, while approximately 94% of Korean high school students consumed SSBs at least once a week, and approximately 40% of them consumed SSBs more than once a day (4, 5). In addition, according to a global school-based survey, adolescents consumed fast foods an average of 1.05 times a week (6). Based on national data in 2019, 82.5% of Korean adolescents consumed fast foods more than once a week (7). Thus, frequent SSB and fast-food consumption is a widespread diet pattern among adolescents (2, 8).

Regarding physical health, frequent SSB and fast-food consumption was significantly associated with developing obesity and metabolic syndromes, such as type 2 diabetes and dyslipidemia (9–11). In addition, SSB and fast-food consumption were associated with adolescents' poor mental health, including stress, depression, and suicidality (12–15). Adolescence is a developmental period with more mental health problems than in other developmental periods. Thus, dietary behaviors that are potentially associated with mental health are important in preventing mental health problems in adolescents (12, 16). Moreover, Korean high school students may have poor mental health with decreased resilience against stressors due to increased academic competition and a social climate that demands higher academic performance (17). In this context, reducing the consumption of SSBs and fast foods as unhealthy diet behaviors may be important for improving Korean adolescents' mental health. However, in a previous study of adolescents from 32 countries, the significance of the association between SSB and fast-food consumption and mental health varied (13). Liu et al. (18) proposed that the inconsistent findings were associated with ethnicity and the definitions of consumption of diet and mental health. Thus, the association between SSB and fast-food consumption and mental health should be evaluated in Korean adolescents.

Furthermore, SSB and fast-food consumption might form clustered diet patterns with significant positive associations in adolescent high school students (2, 19). According to Kang et al. (20), combining lifestyle behaviors may present neutralized or synergistic effects on health. Thus, the combined consumption of SSBs and fast foods may have more negative effects on mental health with synergetic effects than the sum of their independent consumption. Thus, hypothesis of this study was that combined consumption of SSBs and fast foods would have more negative effects on mental health of adolescent high school students than their independent consumption. However, considering the multi-dimensional effects of covariates, identifying the effects of the combined consumption of SSBs and fast foods on mental health in adolescents was limited. Nevertheless, the significance of their effects on mental health differed according to controlled covariates in adolescents (21, 22).

According to the biopsychosocial model proposed by Engel (23), individuals' health was influenced by biological (e.g., age, sex), social (e.g., socioeconomic status of the family), and psychological factors (e.g., mood, health behaviors). The biopsychosocial model is an appropriate framework for understanding the interactive influence of these three factors on individual health. It encourages a comprehensive understanding of individuals' unique characteristics associated with their health statuses (23, 24). In the literature review, as covariates, associated factors of adolescents' mental health were: sex (25) and sleep satisfaction (25) in biological factors; grade (25), academic achievement (25), living with family members (25), family's socioeconomic status (25), type of school (25), and area of residential location in social factors (25); and perceived health status (26), perceived body shape (27), skipping breakfast (28), screen-based sedentary time (29), moderate and vigorous physical activity (30), current smoking consumption (25), current alcohol consumption (25), the experience of sexual intercourse (31), and experience of substance use (25) in psychological factors. Thus, this study aimed to identify the effects of combining the consumption of SSBs and fast foods on mental health, including stress, depressive symptoms, and suicidal ideation of Korean high school students, who participated in the 17th Korea Youth Risk Behavior Web-based Survey (KYRBS), after controlling for covariates.

This study identified the effects of combining the consumption of SSBs and fast foods on stress, depressive symptoms, and suicidal ideation in adolescents in Korea. The results of this study indicate that more than medium consumption of SSBs or fast foods may lead to increased stress, depressive symptoms, and suicidal ideation in adolescents.

Previous studies also reported that frequent SSB and fast-food consumption were associated with increased stress, depressive symptoms, and suicidality in adolescents (12, 32–34). In a previous study of Korean adolescents (32), consumption of SSBs and fast foods more than 3–4 times per week was associated with increased stress. Similarly, Yim et al. (34) reported that SSB consumption was associated with a 1.04-fold increase in stress. In addition, depressive symptoms increased with the consumption of SSBs and fast foods more than 1–2 times a week (32). Furthermore, daily consumption of SSBs and fast foods was associated with a 1.37–1.4 and 1.24–1.50 times increase in the depression of Iranian adolescents, respectively (15). Regarding suicidal behaviors, consumption of SSBs more than thrice a day (13) and more than seven times a week was associated with increased suicide attempts (35). Frequent fast-food consumption was associated with a 1.31-fold increase in suicide attempts among adolescents (13). Thus, SSB and fast-food consumption may significantly affect adolescents' mental health. According to Oliver et al. (36), higher emotional stress tends to have a significant association with increased consumption of sweeter and high-fat foods. Similarly, more severe depression might be associated with increased consumption of sweet and high-fat foods (34, 37, 38). Jacob et al. (13) proposed that SSB and fast-food consumption might promote suicidal ideation with increased vulnerability via high emotional stress and depression.

Regarding the relation between eating and emotions, individuals with emotional stress seek coping methods to minimize negative feelings due to tension. Eating may be the most common, simple, and least conscious behavior to cope with stressful situations (39); therefore, stress might induce increased appetite via activation of the nervous system (40). Arnow et al. (41) also reported that emotional triggers (negative emotions) could induce eating, regardless of physical hunger. Furthermore, as stress and depression are usually associated with eating (42), emotional eaters consume sweet and high-fat foods more frequently in response to stressors (43). Finally, emotional eating due to negative emotions could worsen mental health status via poor psychological wellbeing and emotion regulation (44). Thus, SSB and fast-food consumption and mental health could be interrelated. In this context, along with the decreased consumption of SSBs and fast foods, healthy coping methods to relieve negative emotions are suggested to prevent emotional eating, including frequent and increased consumption of SSBs and fast foods.

Combining more than medium consumption of SSBs and fast foods had a more significant association with increased stress, depressive symptoms, and suicidal ideation than their independent consumption in adolescents. In addition, combining high consumption of SSBs and low to high consumption of fast foods may have dose-dependent negative effects on stress, depressive symptoms, and suicidal ideation in adolescents. In a previous study, combining the consumption of caffeinated energy drinks and junk foods was more significantly associated with poor mental health than their independent consumption (14). Xu et al. (12) also reported that 40% of Chinese adolescents' psychological symptoms may be associated with the combined consumption of SSBs and fast foods. Thus, combined consumption of SSBs and fast foods may present more negative dose-dependent effects with synergetic effects. In this context, health education in schools and communities should be provided to adolescents and their parents to restrict SSB and fast-food consumption. In addition, creating a school environment that restricts the sale of SSBs and fast foods, such as limits on installing SSB vending machines and fast-food sales in cafeterias, must be considered. Moreover, health drinks and snacks that can replace SSBs and fast foods should be suggested by healthcare providers in school. According to Rocha et al. (45), school-based nutrition education and feeding program targeting the prohibition of purchasing SSBs and fast foods and offering healthy foods can be a tool to promote a secure school food environment for adolescents' health. Thus, for promoting the mental health of adolescents, the results of this study might provide evidence for developing school-based feeding programs targeting the restriction of SSB and fast-food consumption in schools and communities. In the field of public health, the independent and combined negative effects of SSB and fast-food consumption on mental health should be widely publicized, with policy proposals for decreased consumption to promote both the physical and mental health of adolescents. Accordingly, it is expected that the results of this study will serve as a starting point for social concerns and efforts to limit the consumption of SSBs and fast foods for preventing mental health problems in adolescents.

This study may contribute to strengthening the evidence for developing interventions aimed at decreasing SSB and fast-food consumption in adolescents. Nevertheless, this study had several limitations. First, SSB and fast-food consumption were assessed via drinking or eating days, without considering the total amount of consumption. Thus, further studies should evaluate adolescents' total amount of SSB and fast-food consumption from all sources. Second, most variables in the 17^th KYRBS, 2021, including variables in this study, were assessed using single questions. Thus, further studies should apply systematically developed instruments with sufficient validity and reliability to assess these variables more comprehensively. Third, due to its cross-sectional study design, this study could not identify the causal relationship between independent and outcome variables. Causal relationships among these variables may be verified using a longitudinal study. Fourth, this study involved only Korean high school students as participants; however, eating behaviors may differ with sociocultural background according to country. Thus, future studies should be performed with adolescents from various countries and cultures to draw relevant comparisons.

This study identified the effects of combining consumption of SSBs and fast foods on stress, depressive symptoms, and suicidal ideation among adolescents in Korean high schools. Combining more than medium consumption of SSBs and fast foods was associated with greater stress, depressive symptoms, and suicidal ideation than their independent consumption. In addition, high combined consumption of SSBs, with low to high consumption of fast foods, may have dose-dependent negative effects on stress, depressive symptoms, and suicidal ideation in adolescents. Based on the results of this study, healthcare providers in schools and communities should prioritize developing school/community-based feeding programs (for improving nutrition knowledge and secure food environments) and policies targeting the restriction of SSB and fast-food consumption to promote adolescent mental health.

The raw data supporting the conclusions of this article will be made available by the authors, without undue reservation.

JSR planned and carried out the entire research process, including the preparation of the article, data analysis, interpretation of results, and writing the manuscript.