From loneliness to depression: A longitudinal diagnostic study among Norwegian university students.

Purpose: Loneliness is an increasing public health concern among young adults. There is insufficient prior research on the association between loneliness and depressive disorder among students in higher education.

Methods: This prospective population-based cohort study from Norway invited all full-time students aged 18-35 to participate in the 2022 Students' Health and Wellbeing Study (SHoT). Of the 53,362 respondents, a subgroup of 16,418 students was randomly selected for diagnostic follow-up, with valid data from 10,460 participants. Loneliness was assessed in 2022 using the Three-Item Loneliness Scale (T-ILS) and Major Depressive Episodes (MDE) were assessed in 2023 using a self-administered electronic version of the Composite International Diagnostic Interview, version 5.0 (CIDI 5.0).

Results: A clear dose-response relationship was observed: students in the highest loneliness quintile had a substantially increased risk of MDE one year later. After adjusting for age and baseline anxiety and depression symptoms, the relative risk (RR) for MDE in the highest versus lowest T-ILS quintile was 2.02 (95% CI: 1.58-2.63) for females and 2.64 (95% CI: 1.63-4.49) for males-representing a ~ 70-75% reduction from unadjusted estimates. The overall prevalence of MDE was 21.1% in females and 11.2% in males. Formal interaction analyses indicated a statistically significant multiplicative interaction by sex, but no evidence of additive interaction.

Conclusions: Loneliness is a strong and independent predictor of MDE in young adults, even after accounting for baseline mental health. Targeting loneliness may be important for preventing depression in university populations.