Multilevel assessment of small-area influences on unhealthy behaviors among adults in Quebec, Canada

National and provincial estimates of unhealthy behaviors may obscure meaningful small-area heterogeneity. Using aggregate dissemination areas (ADAs), this study aims to capture these small-area variations and evaluate their capacity to meaningfully discriminate individuals according to major unhealthy behaviors and their co-occurrence among adults in Quebec, Canada. We conducted a cross-sectional analysis of 18,383 adults from the Trajectoire de Soins–Données Enrichies (TorSaDE) cohort, linking the 2015–2016 Canadian Community Health Survey to neighborhood-level data defined by ADAs. Unhealthy behaviors included insufficient fruit and vegetable intake, physical inactivity, heavy drinking, and smoking. Multilevel logistic regression models with individuals nested within ADAs were fitted, adjusting for age, education, income, material deprivation, and urbanicity. Neighborhood effects were evaluated using measures of discriminatory accuracy, including the variance partition coefficient (VPC), median odds ratio (MOR), and area under the receiver operating characteristic curve (AUC). Unhealthy behaviors were highly prevalent, particularly insufficient fruit and vegetable intake (61.9%) and physical inactivity (43.4%). Heavy drinking and smoking affected 22.9% and 14.1% of participants, respectively. Nearly half of adults (45.7%) exhibited at least two co-occurring unhealthy behaviors, with higher prevalence among men and socioeconomically disadvantaged groups. Overall, between-neighborhood variation was limited. Smoking (VPC = 4.7%) and heavy drinking (VPC = 2.9%) showed the greatest neighborhood-level heterogeneity and achieved moderate discriminatory accuracy after adjustment (AUC > 70%). For other behaviors, neighborhood-level variance was minimal (VPC < 1.2%) and discriminatory accuracy remained low (AUC < 70%). ADA-level context showed limited and behavior-specific discriminatory power for unhealthy behaviors in Quebec. While place-based interventions may still be relevant for certain behaviors, these findings highlight the limitations of using ADAs as proxies for neighborhoods. They have important implications for interpreting small-area effects and informing proportionate, behavior-specific public health strategies for health equity.