Loneliness is a public health concern associated with increased morbidity and mortality. Adverse health behaviours and a higher body mass index (BMI) have been proposed as key mechanisms influencing this association. The present study aims to examine the relationship between loneliness, adverse health behaviour and a higher BMI, including daily smoking, high alcohol consumption, physical inactivity, unhealthy dietary habits, and obesity in men and women and across different life stages.We conducted a cross-sectional study using data from the 2017 Danish National Health Survey (entitled "How are you?"). Loneliness was assessed using the Three-Item Loneliness Scale. Logistic regression models were employed to analyse the association between loneliness, health behaviour and obesity in a sample of 122,258 individuals (16 + years). The models were adjusted for sex, age, educational attainment, country of origin, and partnership status. Stratified analyses were conducted to investigate differences by sex and life stages.Loneliness was associated with an increased risk of daily smoking (Adjusted odds ratio (AOR) = 1.30; 95% CI: 1.21-1.40), physical inactivity (AOR = 1.87; 95% CI: 1.75-1.99), unhealthy diet (AOR = 1.58; 95% CI: 1.47-1.70), and obesity (AOR = 1.60; 95% CI: 1.49-1.72). Conversely, loneliness was associated with a reduced risk of high alcohol consumption in men (AOR = 0.82; 95% CI: 0.74-0.0.91).Our study provides evidence that loneliness is associated with adverse health behaviour and obesity in both men and women and across the lifespan. These findings suggest that health behaviours and obesity may influence the association between loneliness and poor health outcomes. Longitudinal studies are needed to clarify the causal relationships underlying these associations.