Loneliness as a transdiagnostic risk factor for mental and physical health in adulthood — state of the evidence, research gaps, and ways forward

Loneliness has been increasingly recognized as a transdiagnostic risk factor impacting both mental and physical health. This review synthesizes current meta-analyses, systematic reviews, and the most recent empirical studies to assess loneliness as a determinant of psychological symptoms and disorders, including depression, anxiety, and suicidal thoughts and behaviors, as well as its role in physical health outcomes such as cardiovascular disease, diabetes, and neurocognitive disorders. Loneliness contributes to these health risks through multiple pathways, including stress, physiological dysregulation, and maladaptive behavioral patterns. This review also outlines research gaps and future directions: Despite consistent associations across samples and analytical strategies, methodological challenges persist — particularly in establishing causality. It remains unclear whether loneliness is a cause, correlate, or consequence of poor health, largely due to the reliance on cross-sectional studies and insufficient attention to confounders, mediators, and colliders. Addressing these limitations requires theory-driven data collection and evaluation and interdisciplinary collaboration. • Loneliness predicts depression, anxiety and suicidal ideation across designs. • Strong links to cardiovascular, metabolic and neurocognitive health risks. • Multiple pathways: stress, dysregulation, inflammation and health behavior. • Causality unclear due to cross-sectional designs and overadjustment issues. • Future work needs causal designs, mechanisms, and intensive longitudinal data.