vitamin d is a fat - soluble secosteroid prohormone ingested in the diet and produced in the skin following exposure to ultraviolet rays in sunlight , and conversion to active forms of vitamin d occurs in the liver and kidneys.1 the metabolic processes regulated by vitamin d include serum calcium and phosphate homeostasis , bone remodeling , neuromuscular function , inflammation , and transcription of proteins involved in cell growth and apoptosis.2 nonspecific musculoskeletal pain and weakness are prominent features of many diseases linked to vitamin d deficiency.36 vitamin d deficiency may increase the risk of developing obstructive sleep apnea mediated by inflammatory rhinitis7,8 and/or tonsillar hypertrophy.9,10 vitamin d deficiency may also contribute to the development of daytime sleepiness via central inflammatory mediators.11,12 patients seeking care in a sleep medicine clinic typically complain of sleep disruption , fatigue , and sleepiness , sometimes in association with chronic nonspecific musculoskeletal pain . low levels of serum vitamin d have been reported in patients who report such pain,4,13 but it is not clear if these data apply to patients whose pain is discovered incidentally during evaluation for another complaint . we were interested in the possibility that the presence of pain might indicate low vitamin d levels contributing to poor sleep quality and/or daytime symptoms of impairment in patients undergoing evaluation for a suspected sleep disorder . traditional risk factors for low vitamin d include race ( with darker skin tone conferring higher risk ) , obesity , and advancing age.1 our primary aim was to test the hypothesis that subnormal levels of vitamin d exist in patients with sleep disorders who admit to chronic nonspecific musculoskeletal pain , and to evaluate whether predictive risk factors were present . our secondary aim was to formulate a biomarker for vitamin d deficiency based on the risk factors observed . between april , 2008 and october , 2010 , consecutive patients seen during routine consultation visits at an academic sleep medicine clinic were interviewed within the context of a full sleep history and underwent a physical examination . the patients also completed a questionnaire screening for the presence of various symptoms potentially linked to sleep disruption , including the presence of moderate to severe musculoskeletal pain contributing to sleep disruption or daytime discomfort , or both . all questionnaire responses were subsequently reviewed and verified in a face - to - face interview by a physician board - certified in internal medicine and sleep medicine ( dm ) . for some patients , pain symptoms were initially denied , but were disclosed after specific questioning about regions of the body where pain might be experienced ( eg , do you have pain in your back , legs , or arms that you notice during sleep ?  ) . patients who admitted to the presence of functionally significant pain and agreed to undergo venous blood sampling for 25-hydroxyvitamin d ( n = 153 ) were included in the analysis ( table 1 ) . all research - related procedures were approved by the institutional review board for human research at our institution . serum 25-hydroxyvitamin d levels were determined by immunoassay according to the specifications of the manufacturer ( diasorin liason , saluggia , italy ) . a 25-hydroxyvitamin d level below 20 ng / ml was considered to indicate vitamin d deficiency.1 the study design is shown in figure 1 . the unpaired t - test was used to compare mean 25-hydroxyvitamin d levels , with reported representative mean values for normal subjects and for groups formed on the basis of a primary complaint of somatic pain . the relationship between vitamin d deficiency and known risk factors the risk factors considered were race ( black versus white / hispanic ) , obesity ( body mass index  30 kg / m versus > 30 kg / m ) , age ( 60 years versus > 60 years ) , gender , and season ( blood draw months during summer [ may - june - july ] versus winter [ november - december - january ] ) . mean 25-hydroxyvitamin d values were analyzed for the entire cohort and according to significant risk factors . a biomarker function for predicting vitamin d deficiency was derived using linear discriminant analysis.14 its predictive accuracy was assessed by calculating sensitivity , specificity , and area under the receiver - operator characteristics curve ( auroc).15 the likelihood that the results would be able to be generalized was estimated by computing a biomarker function using data from 90% of the patients and evaluating it using data from the remaining 10% of patients . the procedure was repeated ten times using different groups of ten patients to evaluate the predictive accuracy . between april , 2008 and october , 2010 , consecutive patients seen during routine consultation visits at an academic sleep medicine clinic were interviewed within the context of a full sleep history and underwent a physical examination . the patients also completed a questionnaire screening for the presence of various symptoms potentially linked to sleep disruption , including the presence of moderate to severe musculoskeletal pain contributing to sleep disruption or daytime discomfort , or both . all questionnaire responses were subsequently reviewed and verified in a face - to - face interview by a physician board - certified in internal medicine and sleep medicine ( dm ) . for some patients , pain symptoms were initially denied , but were disclosed after specific questioning about regions of the body where pain might be experienced ( eg , do you have pain in your back , legs , or arms that you notice during sleep ?  ) . patients who admitted to the presence of functionally significant pain and agreed to undergo venous blood sampling for 25-hydroxyvitamin d ( n = 153 ) were included in the analysis ( table 1 ) . all research - related procedures were approved by the institutional review board for human research at our institution . serum 25-hydroxyvitamin d levels were determined by immunoassay according to the specifications of the manufacturer ( diasorin liason , saluggia , italy ) . a 25-hydroxyvitamin d level below 20 ng / ml was considered to indicate vitamin d deficiency.1 was used to compare mean 25-hydroxyvitamin d levels , with reported representative mean values for normal subjects and for groups formed on the basis of a primary complaint of somatic pain . the relationship between vitamin d deficiency and known risk factors the risk factors considered were race ( black versus white / hispanic ) , obesity ( body mass index  30 kg / m versus > 30 kg / m ) , age ( 60 years versus > 60 years ) , gender , and season ( blood draw months during summer [ may - june - july ] versus winter [ november - december - january ] ) . mean 25-hydroxyvitamin d values were analyzed for the entire cohort and according to significant risk factors . a biomarker function for predicting vitamin d deficiency was derived using linear discriminant analysis.14 its predictive accuracy was assessed by calculating sensitivity , specificity , and area under the receiver - operator characteristics curve ( auroc).15 the likelihood that the results would be able to be generalized was estimated by computing a biomarker function using data from 90% of the patients and evaluating it using data from the remaining 10% of patients . the procedure was repeated ten times using different groups of ten patients to evaluate the predictive accuracy . the mean 25-hydroxyvitamin d level in the study population was 19.8  11.1 ng / ml , with 54% of the patients having vitamin d deficiency ( 25-hydroxyvitamin d level < 20 ng / ml ) . this 25-hydroxyvitamin d level was significantly lower than that commonly seen in clinically normal populations , and the prevalence of vitamin d deficiency was consistently higher ( table 2 ) . compared with the results of published studies involving patients for whom pain was a primary complaint , the mean level in our cohort was either significantly lower or statistically indistinguishable , with only one exception ( table 2 ) . the risk for vitamin d deficiency was 50 times higher in black than in white / hispanic subjects , and an increased risk was also seen in subjects who were obese or younger than 60 years ( table 3 ) . increased risk was not observed in association with gender or winter months . in subjects from whom blood was collected during the summer months , 23 of 41 ( 56% ) had vitamin d deficiency compared with 18 of 31 subjects ( 58% ) from whom blood was collected during the winter months . mean 25-hydroxyvitamin d levels in patients with and without obstructive sleep apnea were not significantly different ( table 4 ) . risk factors for vitamin d deficiency ( table 3 ) were generally found to be predictive in patients with and without obstructive sleep apnea ( table 4 ) . the biomarker function formed on the basis of the risk factors found to be associated with vitamin d deficiency was b(x ) = 0.207x1 + 0.011x2  0.007x3 + 0.092 , where x1 is race ( 1 = white / hispanic , 2 = black ) , x2 is body mass index ( kg / m ) , and x3 is age ( years ) . the auroc was 0.78 , and the mean auroc for ten 90:10 cross - validation determinations was 0.75  0.02 . at a biomarker threshold value of 0.45 , the performance of the biomarker function in classifying each patient in the cohort is shown in figure 2 . 25-hydroxyvitamin d levels were not significantly different between patients with and without obstructive sleep apnea . the biomarker function was found to be applicable for both groups ( table 4 ) . vitamin d is an ubiquitous metabolic regulator , and sleep is a physiologic process that can be impaired as a consequence of dysregulation of many different metabolic processes . we found that the presence of nonspecific functionally significant pain ( revealed on direct questioning , but not necessarily mentioned a priori by the patient ) is a reliable marker for the presence of low vitamin d in patients undergoing specialist evaluation for a suspected sleep disorder . although low 25-hydroxyvitamin d has been previously reported in patients with complaints of chronic pain , patients in the cohort described in our study were incidentally discovered to have pain symptoms during consultation for another reason . in some cases , pain symptoms were disclosed only after specific questioning . nevertheless , the 25-hydroxyvitamin d values found mirror the low values reported in cohorts defined on the basis of pain as a chief complaint , suggesting that 25-hydroxyvitamin d may play a role in the pathophysiology of diseases typically classified as sleep disorders . one possibility is that chronic nonspecific musculoskeletal pain contributes to subjective sleep disturbances and/or a magnification of perceived daytime impairment , and both symptoms frequently lead to specialist evaluation for a suspected sleep disorder . vitamin d deficiency has the biologic potential to contribute mechanistically to obstructive sleep apnea via myopathy,16 facilitating development of chronic rhinitis,7,8 or contributing to tonsillar hypertrophy.10 in our cohort , mean 25-hydroxyvitamin d values were not significantly different between those with and without obstructive sleep apnea ( p = 0.17 ) , and the risk factors identified for vitamin d deficiency were generally valid for both groups ( table 4 ) . we have previously described a patient presenting with a syndrome clinically indistinguishable from idiopathic central nervous system hypersomnia whose excessive daytime sleepiness symptoms resolved following treatment of severe vitamin d deficiency,17 and we have also recently reported a relationship between epworth sleepiness scale scores and 25-hydroxyvitamin d levels.18 taken together , the evidence supports the notion that the presence of nonspecific pain in a patient with a suspected sleep disorder may be predictive of vitamin d deficiency , a problem which could act as a cofactor for perceived symptoms of a broad array of sleep disorders . although the present evidence does not warrant routine testing of all sleep clinic patients for vitamin d deficiency , a biomarker that reliably predicts vitamin d deficiency might form a useful basis for selecting patients for testing , ultimately leading to improved diagnosis and treatment . in the present study , the vitamin d status of a considerable number of our patients ( vitamin d deficiency present or absent ) was correctly identified based solely on standard demographic data ( figure 2 ) . the presence or absence of obstructive sleep apnea did not appreciably alter the predictive value of the biomarker ( table 4 ) . an unexpected finding in this cohort was that younger age appeared to confer a higher risk for vitamin d deficiency . typically , older age is associated with decreased ability of the skin to produce active vitamin d. our current data do not provide a satisfactory explanation for this paradox . age - dependent sun exposure habits , use of supplements , and use of medications ( such as anticonvulsants or steroids ) are known to decrease circulating 25-hydroxyvitamin d , and may be responsible . it is also possible that the nonspecific physical symptoms associated with vitamin d deficiency prompt younger patients to seek specialist care for sleep disorders . it might be argued that a potential limitation of this study was the absence of a control group without chronic nonspecific musculoskeletal pain . because our main purpose was to compare our cohort with a healthy adult population and with populations formed on the basis of musculoskeletal pain as a chief complaint , we consider that use of historical controls was reasonable and clinically relevant for comparison purposes . notwithstanding the putative limitation of historical control groups , the consistency and size of the observed differences ( table 2 ) the reliability of this inference is further supported by our finding that two risk factors strongly associated with vitamin d deficiency , ie , race and obesity , were also associated with vitamin d deficiency in the cohort ( table 2 ) . sleep clinic patients who incidentally disclosed chronic nonspecific musculoskeletal pain during specialist evaluation at our sleep medicine clinic had significantly lower serum levels of 25-hydroxyvitamin d and a higher prevalence of vitamin d deficiency than historical controls without pain , and had 25-hydroxyvitamin d levels and vitamin d deficiency rates similar to patient groups defined on the basis of chronic pain as a presenting complaint . the likelihood of vitamin d deficiency in this cohort was estimated reliably using a biomarker derived from common demographic data .