Metabolic syndrome occurs when three or more of the following are present: Elevated blood pressure, elevated fasting blood sugar, increased waist circumference (think belt size), low HDL level (the good cholesterol), and high triglycerides. The metabolic syndrome increases risk for heart disease, stroke, and diabetes. In the December 15th, 2011 edition of the New England Journal of Medicine is an article about the effects of CPAP on reducing the metabolic syndrome. The study authors explain that metabolic syndrome is present in 70-80% of patients with obstructive sleep apnea (OSA). Of course CPAP treats OSA, but its effect on metabolic syndrome is not clear.
This study is high quality, as it was a double-blind, placebo-controlled, randomized, crossover design in 99 patients who received three months of either CPAP or sham CPAP, with a one month washout period between interventions. The results showed that real CPAP, when compared to sham CPAP, was able to significantly reduce body mass index, waist circumference, blood pressure, glycosolated hemoglobin (a marker of blood sugar), triglycerides, total cholesterol, and LDL (the bad cholesterol). The author explains that OSA causes dips in oxygen levels that can cause the release of certain fat-inducing hormones and damage the lining of blood vessel walls. These changes could increase the risk for metabolic syndrome, and CPAP appears to prevent this based on the study results.
I wonder if the improvement in many of the factors was due to the weight loss, rather than just the CPAP. In other words, reducing weight can improve blood pressure, lipid values, and blood glucose values. CPAP does not directly cause weight loss, but it can improve energy enough to allow patients to exercise more. They also might not eat as much food, since some patients use food to increase energy. With more energy from CPAP use, they might not eat as much.