Mental well-being in adolescence and eight years of follow-up for mental illness, risky behaviours, and mortality in 67,945 15-19-year-olds: a prospective cohort study.

Background: Adolescence is a sensitive developmental period during which complaints of poor mental well-being increase drastically. We investigated how social determinants associated with self-reported mental well-being and how self-reported mental well-being associated prospectively from adolescence to young adulthood with mental illness, risky behaviours, and mortality. Methods: We used data on 67,945 Danish students aged 15-19, surveyed in 2014 (baseline). We employed a person-centred approach using Latent Class Analysis to define mental well-being subgroups with nine indicators of mental well-being: life satisfaction, self-esteem, irritability, low mood, stress, loneliness, self-efficacy, and confidence in parents and friends. By linkage to national health, social, and mortality registers, we obtained information on social determinants at baseline and the following outcomes during young adulthood: Mental illness, self-harm and suicide attempts, alcohol-related and substance use-related hospital contacts, emergency room contacts due to interpersonal violence, and all-cause mortality. Descriptive statistics were produced to illustrate associations between social determinants and mental well-being, and Poisson and Cox regression were used to estimate incidence rates and hazard ratios over the 8.2-year follow-up period. Findings: We identified four distinct groups of mental well-being: Good (32%), Moderate (35%), Poor (19%), and Very Poor (14%). We observed pronounced gradients of social and familial disadvantage such as financial difficulties and parental alcohol problems across mental well-being groups, with greater disadvantage linked to poorer mental well-being. The risk of mental illness, self-harm and suicide attempts, alcohol-related and substance use-related hospital contacts, emergency room contacts due to interpersonal violence, and all-cause mortality was consistently lowest in the Good, at intermediate levels in the Moderate and Poor, and substantially higher in those with Very Poor mental well-being. For example, hazard ratios for mental illness were 1.51 (95% CI = 1.39, 1.64), 2.61 (2.41, 2.82) and 5.32 (4.92, 5.76) in the Moderate, Poor and Very Poor, respectively, corresponding to incidence rate differences of 27.2 (21.5, 32.8), 85.5 (77.4, 93.5) and 230 (214, 246) per 10,000, as compared to the Good mental well-being group. The higher risk of all outcomes remained across 8.2 years of follow-up. The external validity of findings was confirmed by repeating all analyses in independent data. Interpretation: Our study reveals that poor adolescent mental well-being, as operationalised multidimensionally from self-reported information, predicts adverse outcomes and even mortality during young adulthood. These findings urgently call for mental health complaints in adolescents to be taken seriously and prioritised for identification and intervention. Funding: The study was funded by the Tryg Foundation.