A study published by the journal American Thoracic Society found that low vitamin D status is not linked with obstructive sleep apnea.
Sleep apnea is a disorder characterized by disrupted breathing patterns during sleep. Obstructive sleep apnea (OSA) is the most common type of sleep apnea, resulting from obstruction of the upper airway due to a temporary relaxation of the muscles lining the throat. There are several risk factors that contribute to the onset of OSA, including but not limited to excess weight, hypertension, large neck circumference, diabetes and the male gender.
Individuals with OSA experience repetitive periods of shallow or paused breathing, despite the attempt to continue breathing during sleep. This condition may be dangerous as a result of the lack of oxygen during these episodes, which usually last between 20 – 40 seconds. As a result, those with OSD often feel fatigued, and experience an impaired memory and ability to complete daily tasks.
Past research has determined a moderate increase in OSA prevalence and severity during the winter months. Some researchers hypothesize that this variation may be related to the seasonality of vitamin D. Since vitamin D has been shown to improve muscle function, researchers theorize that low vitamin D status may play a role in the development and severity of OSA.
In the current study, researchers aimed to determine if vitamin D status is associated with the incidence and severity of OSA, independent of common OSA risk factors. Therefore, they performed a cross-sectional analysis of the Outcomes of Sleep Disorders in Older Men study. This provided the researchers with data from the sleep study (polysomnography), serum 25(OH)D measurement, demographic and comorbidity information of the 2,827 older males included in the analysis.
Did the researchers find a relationship between vitamin D and OSA? Here is what they found:
- The mean 25(OH)D concentration of the subjects in this study was 28.8±8.8 ng/ml.
- Those with the lowest quartile of 25(OH)D (6-23ng/ml) had a 45% increased odds of severe sleep apnea when compared to those in the highest quartile.
- However, when adjusting for BMI and neck circumference, the two main risk factors of OSA, this association decreased to only a 5% increased likelihood of severe sleep apnea.
- Low vitamin D status was not independently associated with increased odds of mild or moderate sleep apnea.
The researchers summarized their findings,
“Our findings suggest that the association between lower 25(OH)D levels and a higher odds of OSA was due in large part to greater BMI and larger neck circumference among participants with lower 25(OH)D levels. These results indicate that low 25(OH)D may simply be a marker of larger BMI and neck circumference, rather than directly contributing to OSA pathogenesis.”
The results from this study recently received attention from the media, with headlines stressing the lack of relationship between vitamin D and sleep apnea. However, there are several limitations to this study that many authors have neglected to acknowledge.
As with any study which conducts a cross-sectional analysis of a large cohort, results indicate correlation only, not causation. Vitamin D deficiency is known for being closely linked with obesity. Since vitamin D is stored in fat and muscle cells, in obese individuals, the stored vitamin D is distributed over more body weight and diluted. Therefore, when evaluating vitamin D status in a condition affecting a mainly overweight population, adjusting for a high BMI would skew these results.
The researchers recommended that longitudinal studies need to be conducted in order to determine if vitamin D is associated with OSA, aside from acting as a marker for a high BMI as they hypothesized.
Sturges, M. & Cannell, JJ. Closer look: Is low vitamin D status associated with sleep apnea? The Vitamin D Council Blog & Newsletter, January 2016.