Association of Dietary Animal and Plant Protein Composition with All-Cause Mortality: 24-Year Population-Based Cohort Study

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Background: This study examined the associations of dietary animal (AP) and plant protein (PP) with all-cause mortality in an Italian population and assessed the potential effect modification by sex, smoking status, and overweight/obesity (defined as BMI ≥ 25 kg/m2). Methods: This longitudinal population-based study included 1350 adults (50.2% females), aged 40–74 years, recruited between 1991 and 1995, who were followed for all-cause mortality through the regional mortality registry until 2015. Dietary data were collected using a food frequency questionnaire, and protein composition was analysed within a compositional data analysis framework, modelling the balance of AP and PP within the overall macronutrient composition. The associations of protein balances with all-cause mortality were estimated using Cox proportional hazards models adjusted for potential confounders. Effect modification was evaluated through stratified analyses. Results: During follow-up, 405 deaths occurred. A greater AP relative to other macronutrients was associated with higher mortality overall (hazard ratio (HR): 1.37; 95% confidence interval (CI): 1.00–1.87) and in males (HR: 1.57; 95% CI: 1.05–2.33). In stratified analyses, these associations were restricted to ever smokers overall (HR 2.06, 95% CI 1.32–3.20), males (HR 1.90, 95% CI 1.18–3.06), females (HR 3.29, 95% CI 1.03–10.54), and to participants with normal weight (HR 1.91, 95% CI 1.07–3.41). No overall association was observed for PP. Among females, PP was associated with lower mortality in those with normal weight. Conclusions: The associations between AP and PP and mortality differed by sex, smoking status, and adiposity, supporting more tailored dietary recommendations.