Vitamin D and obstructive sleep apnea: a systematic review and meta-analysis

There was a relative insufficiency in serum 25(OH)D levels among OSA patients compared to control patients, which was incrementally exacerbated with increasing severity of sleep apnea. It was unclear whether a low 25(OH)D was a risk factor for OSA or if OSA was a risk factor for 25(OH)D. It was also possible that the association between 25(OH)D and OSA was due to body mass index (BMI).