Low Body Mass Index Is Associated with a 5-Year Decline in Accelerometer-Measured Physical Activity in Patients with COPD

Background: /Objectives: Reduced physical activity (PA) and increased sedentary behavior (SB) are associated with adverse outcomes in chronic obstructive pulmonary disease (COPD). However, the factors associated with their long-term change remain unclear. We investigated the baseline factors associated with a longitudinal decline in PA and increase in SB. Methods: Stable outpatients with COPD who participated in both the SPACE (baseline) and E-PAC (follow-up) studies in Japan were analyzed retrospectively. At baseline, 29 clinical variables were assessed, including demographic and anthropometric data, nutritional indices (body mass index [BMI], upper arm circumference), pulmonary function, blood tests, and the lowest percutaneous oxygen saturation (SpO2) during the 6-min walk test (6MWT). PA and SB were measured with a waist-worn triaxial accelerometer and expressed in metabolic equivalents (METs). Annual percentage changes were analyzed using Spearman’s rank correlation and multiple linear regression. Analyses were exploratory, with 95% confidence intervals reported. Results: Thirty-two men (median age 71.0 years; median follow-up 5.0 years) were analyzed. Greater annual declines in PA were associated with a poorer baseline nutritional status (lower body weight, BMI, and upper arm circumference), lower pulmonary function, and a lower lowest SpO2 during the 6MWT (i.e., exercise-induced desaturation). A greater annual increase in SB was associated with a lower lowest SpO2 and a higher baseline hemoglobin concentration. In multivariable models, BMI remained independently associated with the declines in total PA and in the duration at ≥3.0 METs. Conclusions: In this exploratory analysis, nutritional status, pulmonary function, and exercise-induced desaturation were associated with long-term behavioral changes in COPD, with BMI showing the most consistent association with the decline in PA. These hypothesis-generating findings require confirmation in a larger cohort.