INTERPRETATION Results indicate that vitamin D has a protective effect against RTI , and dosing once-daily seems most effective .
BACKGROUND Low levels of 25-OH vitamin D are associated with  respiratory tract infection ( RTI ) .
However , results from r and omized controlled trials are inconclusive .
Therefore , we performed a systematic review and meta- analysis to assess the preventive effect of vitamin D supplementation on RTI .
Cholecalciferol plus calcium suppresses abnormal PBMC reactivity in patients with multiple sclerosis.
CONTEXT The active metabolite of vitamin D , 1,25-dihydroxyvitamin D [ 1,25(OH)(2)D ] , is a potent modulator of immune cells in vitro . OBJECTIVE Our objective was to determine whether the sun-dependent nutrient , cholecalciferol , can alter disease-associated cellular immune abnormalities in patients with multiple sclerosis ( MS ) . DESIGN This was an open-label , 12-month , r and omized controlled trial . SETTING Patients with MS were recruited from the MS Clinic at St. Michael 's Hospital , Toronto . PATIENTS Forty-nine patients were matched ( for age , sex , disease duration , disease-modifying drug , and disability ) and enrolled ( treated n = 25 ; control n = 24 ) . Four patients were lost to follow-up ( n = 2 from each group ) . INTERVENTION Treated patients received increasing doses of cholecalciferol ( 4,000 - 40,000 IU/d ) plus calcium ( 1200 mg/d ) , followed by equilibration to a moderate , physiological intake ( 10,000 IU/d ) . Control patients did not receive supplements . MAIN OUTCOME MEASURES At enrollment and at 12 months , peripheral blood mononuclear cell ( P BMC ) proliferative responses to disease-associated , MS-relevant , and control antigens were measured , along with selected serum biochemical markers . RESULTS At 12 months , mean serum 25-hydroxyvitamin D [ 25(OH)D ] concentrations were 83 ± 35 nmol/liter and 179 ± 76 nmol/liter in control and treated participants , respectively ( paired t , P < 0.001 ) . Serum 1,25(OH)(2)D did not differ between baseline and 1 yr . In treated patients , 12-month P BMC proliferative responses to neuron antigens myelin basic protein and exon-2 were suppressed ( P = 0.002 ) . In controls , there were no significant changes in disease-associated P BMC responsiveness . There were no significant differences between groups in levels of selected biomarkers . INTERPRETATION MS-associated , abnormal T cell reactivities were suppressed in vivo by cholecalciferol at serum 25(OH)D concentrations higher than 100 nmol/liter
INTERPRETATION Results indicate that vitamin D has a protective effect against RTI , and dosing once-daily seems most effective .
BACKGROUND Low levels of 25-OH vitamin D are associated with  respiratory tract infection ( RTI ) .
However , results from r and omized controlled trials are inconclusive .
Therefore , we performed a systematic review and meta- analysis to assess the preventive effect of vitamin D supplementation on RTI .