Examining Age-Adjusted Associations between BMI and Comorbidities in Mongolia: Cross-Sectional Prevalence

(1) Background: This study investigated the association between body mass index (BMI) categories and comorbidities in the context of a developing country, utilizing data from a nationwide health screening in Mongolia. (2) Methods: The study included 181,080 individuals (mean age 47.0 ± 15.3, 42.0% male) from the population-based general health screening. We counted the number of diseases from participants' medical records based on ICD-10 codes, excluding those categorized under Z00-Z99 and codes indicating acute disorders, as well as individuals classified as underweight. (3) Results: Among study participants, the prevalence of two or more comorbidities was 4.2%. The weight distribution comprised 40.4% normal weight; 37.1% overweight; and 16.9%, 4.4%, and 1.2% in the Class I, II, and III obesity categories, respectively. Comorbidities increased with BMI: normal weight (0.222); overweight (0.255); and Class I (0.290), Class II (0.302), and Class III obesity (0.303), suggesting a dose-dependent likelihood of having multiple diseases. Adjusted linear regression (beta coefficients, 95% CIs) showed increased comorbidity risks in overweight (0.017, 0.013-0.021) and obesity (0.034, 0.030-0.039). Interaction analysis with age revealed a significant effect (p < 0.001). While comorbidities tend to increase with higher BMI categories in all age-tertile groups, this association was notably stronger among younger individuals. (4) Conclusions: Obesity is associated with a twofold increase in the prevalence of multiple comorbidities compared to normal weight. Our findings also highlight the critical role of age in the development of multiple diseases, with BMI remaining a significant factor across various age groups, encompassing both younger and older adults.