Daily Step Count Is Associated With Plasma C-Reactive Protein and IL-6 in a US Cohort With COPD

Background: Physical activity is an important clinical marker of disease status in COPD. COPD is also characterized by low-grade systemic inflammation. However, the relationship between physical activity and systemic inflammation in COPD is unclear. Methods: We monitored daily step count, a directly measured physical activity, using the StepWatch Activity Monitor, an ankle-worn accelerometer, in 171 people with stable COPD. Exercise capacity was assessed with the 6-min walk test (6MWT). We measured plasma C-reactive protein (CRP) and IL-6 levels. Linear regression models examined the cross-sectional associations of daily step count and 6MWT distance with CRP and IL-6 levels. Results: Subjects had a mean age 72 +/- 8 years and mean FEV1 1.5 +/- 0.57 L (54 +/- 20% predicted). Median daily step count was 5,203 (interquartile range [IQR], 3,627-7,024], CRP level was 2.4 mg/L (IQR, 1.2-5.0), and IL-6 level was 2.9 pg/mL (IQR, 2.0-5.1). Each 1,000-step increase in daily step count was associated with a 0.94 mg/L and 0.96 pg/mL decrease in CRP (P = .020) and IL-6 (P = .044) levels, respectively, adjusting for age, FEV1 % predicted, pack-years smoked, cardiac disease, current statin use, history of acute exacerbations, and season. There was a significant linear trend of increasing daily step count by quartiles and decreasing CRP (P = .0007) and IL- 6 (P = .023) levels. Higher 6MWT distance was also significantly associated with lower CRP and IL-6 values. Conclusion: People with COPD who walked the most had the lowest plasma CRP and IL- 6 levels. These results provide the conceptual basis to study whether an intervention to promote walking will reduce systemic inflammation in people with COPD.