GDF15 and chronic obstructive pulmonary disease: The data are consistent with the suggestion that GDF‐15 may contribute directly to the loss of muscle mass that occurs in patients with COPD based on three main observations: firstly, that serum levels of GDF‐15 are inversely proportional to RFCSA in patients; secondly, that muscle GDF‐15 expression is elevated in COPD patients; and thirdly, that over‐expression of GDF‐15 leads to a reduction in fibre size in mice.