HEALTHCARE COSTS ACROSS THE BODY MASS INDEX SPECTRUM: THE ROLE OF AGE AND MULTIMORBIDITY — A SYSTEMATIC REVIEW

Background: Body mass index (BMI) is widely used to classify nutritional status and assess health risk at the population level. Both low and high BMI values are associated with adverse health outcomes; however, the economic consequences of abnormal BMI across the full BMI spectrum have not been comprehensively synthesized in recent literature. Objective: To systematically review evidence published since 2015 on healthcare costs associated with BMI categories ranging from underweight to severe obesity, with particular attention to the modifying roles of age and multimorbidity. Methods: A systematic literature search was conducted in PubMed to identify studies published between January 2015 and March 2025 reporting healthcare costs stratified by BMI. Observational studies, cohort studies, registry-based analyses, and cost-of-illness studies were included. Due to substantial methodological heterogeneity, a narrative synthesis was performed. Results: Across healthcare systems, a consistent non-linear association between BMI and healthcare costs was observed. Normal BMI was associated with the lowest expenditures, while both underweight and obesity were linked to increased costs. Severe obesity generated the highest costs, largely driven by chronic disease burden and hospitalization. Age and multimorbidity substantially modified the BMI–cost relationship, with underweight particularly associated with prolonged hospitalization among older adults. Conclusions: Healthcare costs vary significantly across the BMI spectrum. Both extremes of BMI are associated with increased economic burden, underscoring the importance of age-sensitive and multimorbidity-informed prevention and care strategies.