Chronic obstructive pulmonary disease (COPD) is complicated by malnutrition in 20-70% of cases. In COPD, low fat-free mass is associated with a higher risk of morbidity and mortality. L-citrulline is a non-protein amino acid that has a direct effect on muscle protein synthesis. Oral supplementation with L-citrulline (10 g/day) in malnourished patients has shown to increase fat-free mass. The effects of L-citrulline supplementation in malnourished COPD patients are unknown. The main objective of this prospective, single-centre, randomised, double-blind, placebo-controlled study will be to determine the impact of 45 days of nutritional supplementation with L-citrulline (10 g/day) on lean body mass (fat-free mass index (in kg.m-2) measured by bioelectrical impedance analysis) in malnourished patients (BMI \< 20 kg/m2 if \< 70 years old or \< 22 kg/m2 if ≥ 70 years old) with COPD at stages 3-4 of the GOLD classification.