Nutritional supplementation during pulmonary rehabilitation in COPD: Do not expect an elixir of life but keep the hunger for more robust evidence

Pulmonary rehabilitation (PR) is a complex interdisciplinary intervention based on a thorough patient assessment, followed by patient-tailored therapies. These include, but are not limited to, exercise training, dietary counseling, education, and behavior change to improve the physical and psychological condition of people with chronic respiratory disease. Promoting long-term adherence to health-enhancing behaviors is at the essence. PR for patients with chronic obstructive pulmonary disease (COPD) has proved its merits in terms of improving many domains including dyspnea, fatigue, exercise capacity, selfmanagement, and health status. Also, studies have suggested lower exacerbation frequency and healthcare utilization after completion of PR. Further optimizing components of the intervention is an important focus of current research in the field of PR. In the interpretation of (synthesized) data from different PR programs, it is important to bear in mind that a large heterogeneity exists in many aspects of PR programs around the globe, as illustrated by Spruit et al. in a survey among representatives of 430 centers from 40 countries. Programs vary particularly in setting (home-based, outpatient, inpatient, or combinations), duration, and composition of the multidisciplinary team. However, with respect to the latter, a commonality in this survey is that the dietician is the most represented discipline in PR teams after the chest physician. This underscores the global recognition of the importance of nutritional management in patients undergoing PR. Indeed, in COPD, common clinically relevant nutritional traits include involuntary weight and muscle mass loss, decreased skeletal muscle oxidative capacity, (central and sarcopenic) obesity, and bone decalcification. While nutritional management in COPD should not be delayed until inclusion in a PR program, it follows a careful assessment consisting of a dietary history, anthropometric and body composition measurements, and calculations of energy and protein requirements. Goals are formulated to optimize dietary quality and habits, supplement nutritional deficiencies, and potentially nutritional interventions may be installed to enhance PR outcomes. Exercise training, alone or as part of PR, may contribute to improving some of the nutritional traits. However, ventilatory constraints during exercise in patients with COPD may limit the level of improvement. It is therefore that the appealing concept of nutrition as an ergogenic aid to enhance the effect of exercise training in COPD has attracted the attention of researchers for many years. In 2012, this specific topic was reviewed by Van de Bool et al. narratively summarizing the evidence for supplementation of carbohydrates, protein and amino acids, polyunsaturated fatty acids (PUFAs), vitamin D, creatine, and antioxidants. At that time, it was concluded that although among investigated nutritional supplements PUFAs appear promising to improve exercise capacity, possibly through improving mitochondrial function, solid evidence to support a performance-enhancing effect of nutritional supplementation in COPD was lacking. As it was commented later in a European Respiratory