Chronic obstructive pulmonary disease (COPD) is a very common and chronic lung condition and is a leading cause of morbidity and death. These patients have persistent breathlessness and exercise intolerance, affecting their ability to carry out routine daily tasks. Standard COPD treatments include medicines/puffers as well as participation in a Pulmonary Rehabilitation (PR) program. PR programs are delivered by a diverse team of healthcare experts in exercise and nutrition. It is possible that an emerging nutritional oral supplement could target the muscular dysfunction seen in patients with COPD in part by promoting better working mitochondria, the energy 'engine' of muscle. A series of recently published studies in sedentary adults and in older adults have demonstrated the safety, tolerability, and potential clinical effectiveness of this supplement. In this regard, the investigators plan to lead a large randomized controlled trial (RCT) to test whether oral supplementation in patients with COPD who are also participating in a standard PR program will improve overall exercise performance. The investigators will also test muscle strength, cognition, body composition, and other clinically important outcomes such as quality of life. Lastly, the investigators will use muscle tissue from a subgroup of volunteers to investigate the effect on muscle/mitochondrial structure/function. The focus is actually the critical 'first step' before the larger RCT: a pilot and feasibility study on a smaller number of participants with COPD, as an important proof-of-concept that the larger study can, and should, be conducted.