With respect to causality, does the use of CPAP improve abnormal glucose metabolism parameters?
Yes. This is seen in subjects without diabetes, in nonobese patients with diabetes, and in patients with poorly controlled diabetes. Caveat: In this body of work, the reader must discern from the trial whether or not there was a published measure of CPAP compliance and, when subjects were compliant with CPAP treatment, how effective the treatment was in normalizing events. It is likely that the study group that will get maximum benefit is the one with normalized sleep (duration > 7 hours, with residual AHI < 5 events/hr). Trials reporting definitions for CPAP treatment adherence and those demonstrating no change in BMI during the study period do show improvement. A study of patients without diabetes who had moderate-to-severe OSA reported that CPAP significantly improved insulin sensitivity after only 2 days of treatment and that the improvement persisted at the 3-month follow-up with no significant changes seen in body weight. Interestingly, this benefit was most pronounced in nonobese subjects. In contrast, the same research laboratory showed no improvement in insulin sensitivity in obese patients with T2DM. In other trials, postprandial blood glucose levels improved most significantly after CPAP use in patients with T2DM and OSA. Overall OSA treatment in patients with diabetes translated to a mean decrease in HbA 1c of 0.4% (from 8.4% to 8%). The CPAP influence was larger in those with higher initial HbA 1c values. In one trial of consecutive patients with poorly controlled T2DM (HbA 1c > 8.5%) and moderate or severe OSA (undiagnosed/or known but not untreated), normalization of respiratory events translated to a mean decrease in HbA 1c of 0.9%. This is respectable T2DM therapy because most current T2DM medications lower HbA 1c by roughly 0.5% to 1.5%. Therefore, one can say that for the price of a good night of sleep the patient with diabetes is also likely to achieve glycemic improvement. Naysayers would argue that although this benefit is reported, it is likely offset by weight gain resulting from CPAP therapy. However, in the randomized controlled trials that addressed this question, there was no change in body composition; furthermore, daytime sleepiness improved and physical activity increased over a 3-month period. It is reasonable to conclude that the use of CPAP is at least weight neutral, but this issue remains under investigation. There remain concerns that the decreased work of breathing with CPAP and the improved GH secretion associated with restoration of sleep architecture may increase the tendency to gain weight