The evidence shows that vitamin D supplementation provides some benefit in fracture prevention ( likely ∼10–15 % relative reduction ) , particularly at a dose ≥800 IU and with calcium ; a likely benefit in the rate of falls , though it is less clear whether the number of fallers changes ; and a possible small ( ∼5 % ) relative reduction in mortality .
Evidence does not support the use of vitamin D supplementation for the prevention of cancer , respiratory infections or rheumatoid arthritis .
Similarly , evidence does not support vitamin D supplementation for the treatment of multiple sclerosis and rheumatoid arthritis or for improving depression/mental well-being .
Regular testing of 25-hydroxyvitamin D is generally not required , and mega-doses ( ≥300,000 IU ) appear to increase harms .
Over the past decade , a large body of observational evidence has suggested an association between  lower vitamin D status ( 25-hydroxyvitamin D ) and multiple acute and chronic disorders , including cancer , multiple sclerosis , depression and respiratory tract infections .
This evidence has fostered the hypothesis that increasing vitamin D intake may treat and prevent such disorders .
Our objective was to perform a critical analysis of the highest-level evidence for ten common beliefs regarding vitamin D for the prevention of falls , fractures and respiratory tract infections , the reduction of cancer incidence/mortality and overall mortality , and the prevention or treatment of depression/mental well-being , rheumatoid arthritis and multiple sclerosis , as well as maximum dosing and regular testing .
Randomized Trial of Vitamin D Supplementation and Risk of Acute Respiratory Infection in Mongolia
OBJECTIVE : Observational studies suggest that serum levels of 25-hydroxyvitamin D ( 25[OH]D ) are inversely associated with acute respiratory infections ( ARIs ) . We hypothesized that vitamin D supplementation of children with vitamin D deficiency would lower the risk of ARIs . METHODS : By using cluster r and omization , classrooms of 744 Mongolian schoolchildren were r and omly assigned to different treatments in winter ( January – March ) . This analysis focused on a subset of 247 children who were assigned to daily ingestion of unfortified regular milk ( control ; n = 104 ) or milk fortified with 300 IU of vitamin D3 ( n = 143 ) . This comparison was double-blinded . The primary outcome was the number of parent-reported ARIs over the past 3 months . RESULTS : At baseline , the median serum 25(OH)D level was 7 ng/mL ( interquartile range : 5–10 ng/mL ) . At the end of the trial , follow-up was 99 % ( n = 244 ) , and the median 25(OH)D levels of children in the control versus vitamin D groups was significantly different ( 7 vs 19 ng/mL ; P < .001 ) . Compared with controls , children receiving vitamin D reported significantly fewer ARIs during the study period ( mean : 0.80 vs 0.45 ; P = .047 ) , with a rate ratio of 0.52 ( 95 % confidence interval : 0.31–0.89 ) . Adjusting for age , gender , and history of wheezing , vitamin D continued to halve the risk of ARI ( rate ratio : 0.50 [ 95 % confidence interval : 0.28–0.88 ] ) . Similar results were found among children either below or above the median 25(OH)D level at baseline ( rate ratio : 0.41 vs 0.57 ; Pinteraction = .27 ) . CONCLUSIONS : Vitamin D supplementation significantly reduced the risk of ARIs in winter among Mongolian children with vitamin D deficiency
The evidence shows that vitamin D supplementation provides some benefit in fracture prevention ( likely ∼10–15 % relative reduction ) , particularly at a dose ≥800 IU and with calcium ; a likely benefit in the rate of falls , though it is less clear whether the number of fallers changes ; and a possible small ( ∼5 % ) relative reduction in mortality .
Evidence does not support the use of vitamin D supplementation for the prevention of cancer , respiratory infections or rheumatoid arthritis .
Similarly , evidence does not support vitamin D supplementation for the treatment of multiple sclerosis and rheumatoid arthritis or for improving depression/mental well-being .
Regular testing of 25-hydroxyvitamin D is generally not required , and mega-doses ( ≥300,000 IU ) appear to increase harms .
Over the past decade , a large body of observational evidence has suggested an association between  lower vitamin D status ( 25-hydroxyvitamin D ) and multiple acute and chronic disorders , including cancer , multiple sclerosis , depression and respiratory tract infections .
This evidence has fostered the hypothesis that increasing vitamin D intake may treat and prevent such disorders .
Our objective was to perform a critical analysis of the highest-level evidence for ten common beliefs regarding vitamin D for the prevention of falls , fractures and respiratory tract infections , the reduction of cancer incidence/mortality and overall mortality , and the prevention or treatment of depression/mental well-being , rheumatoid arthritis and multiple sclerosis , as well as maximum dosing and regular testing .