the prevalence of    vitamin d insufficiency in    the us , as measured by serum concentrations of 25-hydroxyvitamin d ( 25(oh)d ) , has been classified as an  epidemicaffecting approximately 25%35% of the adult population . globally , the prevalence is reported to be even higher , affecting up to 80% of high - risk populations such as dark - skinned individuals or those with limited / no sun exposure . the importance of vitamin d in skeletal muscle ( skm ) function has been demonstrated by the presence of the vitamin d receptor ( vdr ) in both the nucleus and plasma membrane of human skm fibers   and by defective muscle development and function in vdr knockout mice     . the biologically   active form of vitamin d , 1,25-dihydroxyvitamin d ( 1,25(oh)2d ) , acts through genomic control to regulate protein synthesis in muscle cells . rapid responses to 1,25(oh)2d in skm are mediated by the membrane - bound vdr , which triggers several key cell signal cascades , especially those involved in calcium flux and mitogenic processes . reports of severe adult - onset vitamin d deficiency , known as osteomalacic myopathy , provided the first evidence of a link between vitamin d and skm in humans .   in this condition of soft , undermineralized bone , remarkably , muscle weakness can even exist during deficiency without biochemical signs of bone involvement     . several large cross - sectional studies have described diminished muscle strength and physical performance in individuals with lower serum 25(oh)d     [ 810 ] . moreover , studies have shown that when vitamin d supplementation is sufficient to raise serum 25(oh)d concentrations , outcomes related to muscle function improve   risk of falls , relative size and number of type ii muscle fibers , and lower limb and grip strength     [ 11 , 12 ] . ninety - three percent of ambulatory outpatients with chronic , nonspecific musculoskeletal pain were found to be deficient in vitamin d     .    in a separate report involving chronic low back pain patients , 83% had low vitamin d status ; following vitamin d therapy , 95% of these patients showed improvement in their symptoms .   to further clarify the importance of vitamin d to muscle , the present study examined the association of vitamin d status and exercise - induced muscle pain and weakness in healthy people . the muscle pain and weakness were induced in the elbow flexors by vigorous eccentric exercise . we tested the hypothesis that lower serum concentrations of 25(oh)d are associated with greater muscle weakness and pain after eccentric exercise in young , healthy volunteers . the relationship between vitamin d status and muscle pain / strength after eccentric exercise ( ee ) was explored using a secondary analysis of data collected from an investigation of exercise - induced muscle damage that was designed to examine sensitization to pressure and activity after lengthening contractions . the study was approved by the university of missouri health sciences institutional review board ; all participants gave informed written consent prior to data collection . twenty - seven males and 21 females , aged 18 to 45 years , were recruited by public advertisement from the university of missouri columbia campus and community . exclusion criteria were   arm strength training two to three times per week within the previous six months ; injury to the shoulder , arm , wrist , or hand in the previous six months ; consumption of any performance enhancing drugs or medications that could affect pain perception or response to eccentric exercise ; illness within one week of participation ; chronic medical conditions affecting pain perception ; hypercholesterolemia or thyroid dysfunction ; history of heat illness , kidney or liver dysfunction , cardiac condition , or muscle disease ; family history of muscle disease or sickle cell disease ; allergy to aspirin or nonsteroidal anti - inflammatory medications ; lack of regular menstruation for females . prior to the initial two sessions , participants were instructed to refrain from smoking for three hours , alcohol for 12 hours , and taking pain medication for 24 hours . after the initial session , five more sessions were conducted , all of which were held during the follicular phase of the menstrual cycle for females . throughout the five subsequent sessions , self - care behaviors such as massage , consumption of any pain relieving medications , and the application of heat , cold or pain relieving cream , were prohibited . during the initial session , descriptive data were collected by questionnaire , including sex , race , ethnicity , date of last menstrual period , and   medications , including birth control , and dietary supplements consumed . baseline isometric strength of the nondominant elbow flexors was measured as peak force , using the biodex system 3 ( biodex medical systems , shirley , ny ) . five maximal voluntary isometric contractions at elbow flexion of 90 were performed , each lasting 5 seconds and separated by a 2-minute rest . baseline pain ratings for the nondominant arm were obtained during complete flexion and extension of the elbow in supination . numerical ratings for both pain intensity and pain unpleasantness were recorded , using a validated scale of 0100 , in which 0 indicated  no pain  or  not at all unpleasant  and 100 meant the  most intense pain sensation imaginable  or  most unpleasant pain imaginable ,    respectively . at least 24 hours after the first session , the volunteers returned for a second session during which muscle strength and pain measures were repeated , followed by eccentric strength testing of the nondominant arm . then an ee protocol was completed that consisted of 3 sets of 12 repetitions with strong verbal encouragement to produce 75% of maximal eccentric peak torque on each repetition . one hour later , and then daily for 4 days , participants returned for muscle pain and isometric strength measurements . this induced muscle pain has been documented to last up to 10 days , while the muscle weakness has been reported to last over 30 days in some participants . serum 25(oh)d was assayed in serum two days after ee using a commercially   available radioimmunoassay ( diasorin , stillwater , mn ; intra - assay cv = 10.8% ) . linear regression of all outcome variables against serum 25(oh)d was carried out using the statistical package there is evidence of sex - specific variations in muscle strength , ( as well as other phenotypic traits ) , according to specific vdr polymorphisms      [ 20 , 21 ] . participant characteristics and serum 25(oh)d are listed in table 1 .   according to the recommended vitamin d status classifications of grant and holick . 10.4% of all subjects were deficient ( < 50  nmol / l ) , 54.2% were insufficient ( 5080 nmol / l ) , and 33.3% were sufficient ( 80250  nmol / l ) ; one male participant had a very high 25(oh)d serum concentration ( 269.5  nmol / l ) . as expected , pain measures for both sexes increased after ee , peaking 2 days after ee . figure 1 depicts the time course for pain intensity during unloaded flexion and extension . also as expected , peak torque for both sexes decreased after ee , reaching a nadir at 1 hour for females and 2 days for males , and then gradually increasing , as shown in figure 2 . linear regression of pain measures against serum 25(oh)d showed that vitamin d status did not predict either of the pain outcomes for males or females , at any time after ee . additionally , vitamin d status did not predict muscle strength expressed as percent of baseline strength . , we plotted ( figure 3 ) the mean pain intensity rating ( scale 0100 ) during unloaded movement through full range of motion of the elbow versus serum 25ohd concentrations in female participants ( the plot for males was not different ) . to further clarify the importance of vitamin d to muscle , this study examined the association of vitamin d status and exercise - induced muscle pain and weakness in healthy people . we tested the hypothesis that lower serum concentrations of 25(oh)d are associated with greater muscle weakness and pain after eccentric exercise in young , healthy volunteers . the pattern of elbow flexor strength after eccentric exercise was as expected , with a decrease in strength in the 48-hour period after eccentrics , followed by a gradual increase in torque values . vitamin d status did not predict this outcome at any time point after eccentric exercise , nor did it predict baseline strength . although the basis for the lack of effects observed in our study is unknown , we offer two possible explanations : age- or genotype - related differences in muscle vdr , or vitamin d threshold effects in muscles may be lower than other tissues . the number   of vdrs present in skeletal muscle declines with age ; thus , strength   in young adults , such as the participants in this study , may be less sensitive to vitamin d insufficiency because of the abundance of vdr in their myofibers . moreover , specific vdr polymorphisms are associated with variations in muscle strength     [ 20 , 24 , 25 ] . no study has yet investigated the interplay in muscle between vitamin d status and vdr variants . it is possible that individuals with certain vdr genotypes may be more sensitive to vitamin d insufficiency or may respond differently to supplementation as is the case in bone      and prostate     . heaney has written extensively on the threshold effect of vitamin d in intestinal calcium absorption and calcium homeostasis outcomes ; results from these studies put the threshold concentration of serum 25(oh)d at 80 in contrast , bischoff - ferrari et al . based a vitamin d threshold estimate on the risk of falls and fractures , which partly reflects muscle function . this inquiry put the threshold concentration of serum 25(oh)d at 40  nmol / l . two recent studies investigating the association between vitamin d status and muscle strength in young girls also support a lower threshold for muscle function outcomes . foo et al .       found greater handgrip strength in girls with lower serum 25(oh)d concentrations , and ward et al .    reported that vitamin d status was positively associated with lower body muscle power and force in adolescent girls . the mean serum 25(oh)d concentrations were 34 and 21  nmol / l , respectively , for the cohorts in these studies , in contrast to 77  nmol / l for our participants . nmol / l , the association of improved muscle strength and serum 25(oh)d would only be observed in groups with poorer status . further investigation of the vitamin d threshold effect as it pertains to mechanisms of hypertrophy following exercise training is required ; and indeed , it may point to improved response to therapeutic exercise by patients from populations at greatest risk for vitamin d deficiency , especially those with darker - pigmented skin or those who are not given regular access to sunlight ( e.g. ,   hospitalized inpatients or nursing home residents ) . although there is a clear link between low vitamin d status and the pain of osteomalacia , there are no data available for a role of vitamin d in the delayed pain associated with eccentric exercise .   in our study , the only known attempt to link the two , participants ' ratings of pain intensity and unpleasantness were collected during unloaded movement ; our results showed that serum 25(oh)d concentrations did not predict either of the pain measurements .   despite our belief that vitamin d status would be negatively associated with muscle pain and weakness , at least some of the ensuing pain following eccentric exercise is thought to be caused by an inflammatory response which is   essential for muscle tissue remodeling and hypertrophy . this inflammatory response is quite similar to the initial nonspecific immune response to microbial challenge     [ 33 , 34 ] .   in its immunomodulatory role it is therefore conceivable that adequate vitamin d status may support a robust inflammatory response to eccentric exercise   and thus contribute to a high level of delayed soreness . furthermore , the active form of vitamin d , 1,25(oh)2d , influences the differentiation and proliferation of muscle cells . when those actions are considered along with encouragement of the nonspecific immune response , it appears possible that postexercise hypertrophy could indeed be improved with adequate vitamin d.   the logical question , consequently , is what serum 25(oh)d concentration is adequate for an optimal postexercise inflammatory response ? just as discussed for the muscle strength outcomes , it may be that the vitamin d threshold for eccentric exercise - associated pain is relatively low , and which might explain why we did not see a relationship between vitamin d status and pain ratings in our participants . the exclusion criteria of the primary analysis disallowed anyone with pre - existing pain or muscle disease so it is conceivable that individuals with muscle pain due to hypovitaminosis d would not be included in our sample and may have restricted ( especially at the lower end ) the range of serum 25(oh)d concentrations observed for participants in our study . we assumed that serum 25(oh)d remained fairly stable throughout the 6-day intervention as it has a serum half - life of ~3 weeks . this assumption may be invalid in the unlikely event that a participant spent a day in the sun after the second day ; however , the effect of this event  if it occurred  can be considered rather inconsequential as it takes at least four days to obtain 80% photoconversion of dermal precursors to vitamin d3 and even longer to affect 25(oh)d concentrations   .   in conclusion , our data indicates that vitamin d status does not predict outcomes of muscle pain and weakness in young , healthy volunteers after eccentric exercise . whether the threshold effect of vitamin d in the muscle   or some other factor can explain our observed lack of association remains to be elucidated . regardless , further investigations into exercise training processes are necessary to yield insight into the plausible relationship between vitamin d status and the changes in the benefits and risks of resistance training . there are numerous research groups investigating the phenomenon of exercise - induced muscle damage ; serum 25ohd assays could be easily included to yield insight into the effects of vitamin d status within those contexts .
vitamin d receptors have been identified in skeletal muscle ; and symptoms of vitamin d deficiency include muscle weakness and pain . moreover , increased serum 25-hydroxyvitamin d ( 25(oh)d ) concentrations have been associated with improved muscle function . to further clarify the importance of vitamin d to muscle 
 , we examined the association between vitamin d status and exercise - induced muscle pain and weakness in healthy people . 
 muscle damage to the elbow flexors was induced with eccentric exercise ( ee ) in 48 individuals ( 22.5  3.2  yrs ) . 
 muscle pain ratings following unloaded movement and peak isometric force ( if ) were collected before ee and for 4 days post - ee . 
 linear regression was used to determine if serum 25(oh)d was a predictor of any outcome . in males , r2-values from 0.48 to 1.00 . 
 r2 for if ranged from 0 to 0.02 and p - values from 0.48 to 1.00 . in females , r2 for pain ratings ranged from 0.01 to 0.11 and p - values from 0.14 to 0.59 . 
 r2 for if ranged from 0 to 0.04 and p - values from 0.41 to 0.90 . in conclusion , vitamin d status did not predict muscle pain or strength after ee - induced muscle damage in young healthy men and women .