Background
COPD patients with similar BMI or classified into the same BMI category (underweight [UW], normal weight [NW], overweight [OW] and obese [OB]) may present different levels of fat-free mass index (FFMI). No study investigated whether the impact of a lower FFMI on functional outcomes is similar among COPD patients stratified in different BMI categories.

Methods
In this retrospective study, we used baseline data from the COSYCONET cohort. Assessments included lung function, body composition (by bioelectrical impedance analysis), six-minute walk distance (6MWD) and health-related quality of life (HRQL). Patients were classified as UW, NW, OW, OB and as presenting low, normal or high FFMI. Between-group comparisons were performed using GLM with adjustment for sex and age. Multiple linear regression was used to investigate the independent associations between body composition and functional outcomes in each BMI category.

Results
2137 COPD patients (61% males, age: 65±8 years, FEV₁: 52.5±18.8 %pred) were included. The proportions of patients in UW, NW, OW and OB categories were 12.3%, 31.3%, 39.6%, 16.8%, respectively.  The frequency of low FFMI increased from higher to lower BMI categories (OB:39%, OW:42% NW:53% UW:81%). Patients with low FFMI, irrespective of the BMI category, and OB patients, irrespective of the FFMI classification, presented worse exercise capacity and HRQL compared to NW patients with normal-to-high FFMI (P<0.05 for all). In UW and NW, fat-free mass (FFM) was positively associated with 6MWD (P<0.05), however, in OW, FFM was not associated with 6MWD after adjustment for lung function. Finally, in OB, fat mass was negatively associated with 6MWD (P<0.05 for all).

Conclusion
Low FFMI is associated with poor functional outcomes in UW, NW and OW patients with COPD. OB patients independently of FFMI classification were impaired compared to NW patients with normal-to-high FFMI. Depending on the BMI category, body composition is differently associated with 6MWD.