CONCLUSIONS Vitamin D supplementation positively affected lower limb muscle strength in athletes , but not upper limb muscle strength or muscle power .
Different muscle groups and functions may respond differently to vitamin D supplementation .
BACKGROUND Vitamin D may play a role in skeletal muscle because of the discovery of VDR in skeletal muscle .
However , vitamin D deficiency is a global problem , including athletes .
Studies examining the effect of vitamin D supplementation on muscle function in athletes have inconsistent results .
Therefore , we aim ed to quantitatively summarize the evidence for the effect of vitamin D supplementation on skeletal muscle strength and explosive power of athletes using a meta- analysis .
Improving the vitamin D status of vitamin D deficient adults is associated with improved mitochondrial oxidative function in skeletal muscle.
OBJECTIVE Suboptimal mitochondrial function has been implicated in several disorders in which fatigue is a prominent feature . Vitamin D deficiency is a well-recognized cause of fatigue and myopathy . The aim of this study was to examine the effects of cholecalciferol therapy on skeletal mitochondrial oxidative function in symptomatic , vitamin D-deficient individuals . DESIGN This longitudinal study assessed mitochondrial oxidative phosphorylation in the gastrosoleus compartment using phosphorus-31 magnetic resonance spectroscopy measurements of phosphocreatine recovery kinetics in 12 symptomatic , severely vitamin D-deficient subjects before and after treatment with cholecalciferol .  All subjects had serum assays before and after cholecalciferol therapy to document serum 25-hydroxyvitamin D ( 25OHD ) and bone profiles . Fifteen healthy controls also underwent (31)P-magnetic resonance spectroscopy and serum 25OHD assessment . RESULTS The phosphocreatine recovery half-time ( τ1/2PCr ) was significantly reduced after cholecalciferol therapy in the subjects indicating an improvement in maximal oxidative phosphorylation ( 34.44 ± 8.18 sec to 27.84 ± 9.54 sec , P < .001 ) . This was associated with an improvement in mean serum 25OHD levels ( 8.8 ± 4.2 nmol/L to 113.8 ± 51.5 nmol/L , P < .001 ) . There was no difference in phosphate metabolites at rest . A linear regression model showed that decreasing serum 25OHD levels was associated with increasing τ1/2PCr ( r = -0.41 , P = .009 ) . All patients reported an improvement in fatigue  after cholecalciferol therapy . CONCLUSIONS Cholecalciferol therapy augments muscle mitochondrial maximal oxidative phosphorylation after exercise in symptomatic , vitamin D-deficient individuals . This finding suggests that changes in mitochondrial oxidative phosphorylation in skeletal muscle could at least be partly responsible for the fatigue experienced by these patients . For the first time , we demonstrate a link between vitamin D and the mitochondria in human skeletal muscle
CONCLUSIONS Vitamin D supplementation positively affected lower limb muscle strength in athletes , but not upper limb muscle strength or muscle power .
Different muscle groups and functions may respond differently to vitamin D supplementation .
BACKGROUND Vitamin D may play a role in skeletal muscle because of the discovery of VDR in skeletal muscle .
However , vitamin D deficiency is a global problem , including athletes .
Studies examining the effect of vitamin D supplementation on muscle function in athletes have inconsistent results .
Therefore , we aim ed to quantitatively summarize the evidence for the effect of vitamin D supplementation on skeletal muscle strength and explosive power of athletes using a meta- analysis .