Plasma orexin-A levels and BMI, FFM, and FM values were highest in the control group and lowest in UW patients with COPD, indicating that plasma orexin-A levels might influence body composition in patients with COPD. However, a reduction in plasma orexin-A levels and BMI, FFM, and FM values may not be a reflection of impaired pulmonary function because none of these parameters were found to be related to FEV1 percentage of predicted in patients with COPD. Although a significant relationship was found between plasma orexin-A levels and BMI and FM values, no relationship was found between orexin-A levels and FFM (a measure of functional tissue depletion). These findings suggest that orexin-A might influence the body composition of patients with COPD through regulation of FM, rather than by directly altering their global body composition. The mechanism by which orexin-A regulates fat distribution in patients with COPD is still uncertain. Leptin, a hormone produced mainly by adipocytes, has been found to play an important role in the regulation of energy metabolism, and has been shown to participate in the regulation of orexin levels in animals22,25–27
; and it has been suggested that leptin might provide negative feedback in the regulation of FM in patients with COPD.28–29
Plasma leptin levels have been shown to relate to BMI and percentage of fat in patients with COPD,28–
as well as to the FM values of patients with emphysema.29
Leptin, orexin-A, and TNF-α are well-known and important factors relevant to body weight loss. However, the relationship among these factors in patients with COPD is still unknown. In the present study, we showed that there was no correlation between plasma levels of orexin-A and those of TNF-α. Although plasma leptin levels were not measured in the present study, it is possible that interactions between orexin-A and leptin might be responsible for alterations in the body composition in patients with COPD. The interaction of these two factors will be clarified in the near future.