Table 3

Studies of functions of vitamin D

ORGAN OR SYSTEMEFFECT OF SUFFICIENT VITAMIN DEFFECT OF INSUFFICIENT OR DEFICIENT LEVELS OF VITAMIN DOPTIMAL VITAMIN D INTAKE FOR HEALTH
Jejunum and ileumIncreases absorption of calcium and magnesium to 30%1Absorption of calcium and magnesium reduced to 10%85 nmol/L allows maximum absorption64,65; with adequate VTD levels, >800 mg of calcium might be unnecessary66
BoneMaintains calcium and phosphate homeostasis and is required for proper mineralization59Rickets or osteomalacia;62 short- latency diseaseRickets and osteomalacia are prevented when VTD levels are >25 nmol/L67
ParathyroidRegulates calcium and phosphate levels, controls conversion of 25-hydroxyvitamin D to 1,25-dihydroxyvitamin D59Parathyroid hormone excretion increases as levels of VTD decrease resulting in secondary hyperparathyroidism, which in turn results in resorption of calcium from bone and exacerbates osteoporosisParathyroid hormone levels are dramatically suppressed when VTD levels are maintained at >50 nmol/L67; levels begin to rise when 25-hydroxyvitamin D levels fall <78 nmol/L
Cardiovascular system via VDRInhibition of vascular smooth-muscle proliferation; suppression of vascular calcification; down-regulation of pro- inflammatory cytokines; up-regulation of anti-inflammatory cytokines. VTD acts as a negative endocrine regulator of the renin-angiotensin system68Might contribute to congestive heart failure2,69; deficiency results in loss of calciotropic effect in long- latency diseaseCurrently unknown, but 2000–4000 IU of vitamin D3 are being suggested70
Muscle via VDRModulates calcium transport, protein synthesis, and kinetics of muscle contraction71Muscle weakness, limb pain, and impaired physical function72; loss of calciotropic effectMaximum neuromuscular performance achieved with VTD levels of 125 nmol/L33
Skin via VDRProduction of calcitrol that regulates cellular function in keratocytesAntiproliferative, immunosuppressive, and prodifferentiating effectsVTD analogues are used for psoriasis73
Islet cells via VDRImprovement in insulin sensitivity4Negative effect on beta cell function with reduced insulin secretion; loss of immune modulatory effectRaising VTD levels from 25 to 75 nmol/L improves sensitivity by 60%; optimal level has not been determined
Certain cancer cell types mediated via VDRSuppressed growth and increased apoptosis74,75; stabilized chromosomal structure and prevented DNA breakdown76Loss of antiproliferative effectOptimal level undetermined
Immune system modulatorStimulated expression of potent anti- microbial peptides, increased “oxidative burst” potential of macrophages77Increased susceptibility to influenza77 and tuberculosis78Optimal level undetermined; summer levels of 125 nmol/L likely required77
Innate immune functionIncreased production of cathelicidins effective against Escherichia coli, methicillin-resistant Staphylococcus aureus, Pseudomonas aeruginosa, and CandidaDecreased wound barrier functionOptimal dose undetermined7982
  • VTD—vitamin D, VDR—vitamin D receptor.