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 .
Supplementation with High Doses of Vitamin D to Subjects without Vitamin D Deficiency May Have Negative Effects: Pooled Data from Four Intervention Trials in Tromsø
Data were pooled from four r and omized clinical trials with vitamin D performed in Tromsø with weight reduction , insulin sensitivity , bone density , and depression scores as endpoints . Serum lipids , glycated hemoglobin ( HbA1c ) , and high sensitivity C-Reactive Protein , ( HS-CRP ) were measured at baseline and after 6–12 months of supplementation with vitamin D 20 000 IU–40 000 IU per week versus placebo .  A total of 928 subjects who completed the interventions were included . At baseline the mean serum 25-hydroxyvitamin D ( 25(OH)D ) level in those given vitamin D was 55.9 ( 20.9 ) nmol/L and the mean increase was 82.4 ( 40.1 ) nmol/L. Compared with the placebo group there was in the vitamin D group at the end of the studies a slight , but significant , increase in HbA1c of 0.04 % , an increase in HS-CRP of 0.07 mg/L in those with serum 25(OH)D < 50 nmol/L , and in those with low baseline HDL-C and serum 25(OH)D < 50 nmol/L a slight decrease serum HDL-C of 0.08 mmol/L ( P < 0.05 ) . No serious side-effects were seen . In conclusion , in subjects without vitamin D deficiency , there is no improvement in serum lipids , HbA1c , or HS-CRP with high dose vitamin D supplementation . If anything , the effect is negative
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 .