Low Body Mass Index Increases Frailty Risk in Old–Old Adults: Findings From a 3‐Year Longitudinal Study

ABSTRACT Objectives Frailty risk increases with age. It is important to understand how body mass index (BMI) relates to changes in frailty risk among adults aged 75 years and older (old–old adults). We examined the longitudinal association between baseline BMI and changes in frailty risk over 3 years. Methods This 3‐year retrospective cohort study included 515 old–old adults (median age 79.3 years; 45.0% male) who participated in the Latter‐Stage Older Persons Health Checkup in Amami City, Japan. Participants were categorized into three BMI groups: < 21.5, 21.5–24.9, and ≥ 25.0 kg/m 2 . Frailty risk was assessed using the Old–Old Questionnaire for Medical Checkups, a validated tool for assessing multidimensional frailty. Thirteen items were analyzed, excluding smoking and social support domains, to better examine the association between BMI and frailty risk. Higher scores indicate a greater frailty risk. We used linear mixed‐effects models to analyze the associations between BMI categories at baseline and changes in frailty risk. Results The BMI < 21.5 kg/m 2 group participants reported a greater annual increase in frailty risk compared with those in the BMI 21.5–24.9 kg/m 2 group ( β = 0.17, Standard error = 0.07, p = 0.014). The BMI ≥ 25.0 kg/m 2 group participants reported no significant change in frailty risk compared with those in the BMI 21.5–24.9 kg/m 2 group ( β = −0.01, Standard error = 0.06, p = 0.821). Conclusion Old–old adults with a BMI of ≤ 21.5 kg/m 2 experienced a more pronounced increase in frailty risk over the 3‐year follow‐up period.