On one hand, some studies have shown that CPAP therapy could improve diabetes. A 2012 study looked at 50 people with moderate to severe sleep apnea to see if CPAP could actually help control their diabetes. They gave half the participants CPAP treatment and the other half a sham CPAP for eight weeks. The team found that the therapy didn’t bring blood glucose levels back to normal completely, but it did improve insulin sensitivity in people with severe sleep apnea. 

    In a different six month trial, 50 patients with sleep apnea and diabetes were split into a group that received CPAP and a group that did not. Measures of insulin resistance and sensitivity as well as glucose tolerance improved in the treated group, in step with better nocturnal blood oxygen levels. 

   On the other hand, however, a study in Australia that also gave 298 people real CPAP or a placebo didn’t find improvement in blood glucose control^7. To explain their results, the Australian team wrote that "it is possible that the impact of obstructive sleep apnea is mainly relevant for the development of diabetes, but not for control of established diabetes."

   Gagnadoux's own work has tried to detangle the reasons that CPAP may or may not help diabetes in patients who have both conditions.  He and his colleagues studied 762 people who were diagnosed with sleep apnea after their first polysomnography. The participants included 497 people already diagnosed with diabetes. The remaining 265 weren't diagnosed with diabetes before the study, but were selected because they had elevated blood glucose levels. 

   What the research team found may help explain why experts have struggled to understand if CPAP can help relieve diabetes symptoms. 

   Overall, they found that people with more severe sleep apnea had lower glucose control, meaning that their blood glucose was more likely to be elevated^8. However, this pattern was not obvious in the group of people who had previously diagnosed diabetes that they were treating. "Potentially, this means that treating obstructive sleep apnea has a greater effect on blood glucose control in newly diagnosed patients," Gagnadoux explains. Patients who are managing their diabetes already may not see improvements when they start CPAP. That doesn't mean that CPAP therapy isn't worth it, however.

   "When you are treating a patient with both sleep apnea and diabetes, you have to treat the sleep apnea but you also have to do your best to treat the diabetes," says Gagnadoux. Thus far, studies do not show that treating sleep apnea alone will help with diabetes. 

   Still, Mostafavi advises that physicians consider CPAP therapy for sleep apnea patients, but especially for those who have both sleep apnea and diabetes. “The data on CPAP treatment and glucose controls is mixed, I can’t claim that putting people on CPAP is going to make their diabetes go away,” he says. “Some do see an improvement, some don’t. But sleep apnea is a significant condition in itself that requires treatment.”