Studies of functions of vitamin D
| ORGAN OR SYSTEM | EFFECT OF SUFFICIENT VITAMIN D | EFFECT OF INSUFFICIENT OR DEFICIENT LEVELS OF VITAMIN D | OPTIMAL VITAMIN D INTAKE FOR HEALTH |
|---|---|---|---|
| Jejunum and ileum | Increases absorption of calcium and magnesium to 30%1 | Absorption 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 |
| Bone | Maintains calcium and phosphate homeostasis and is required for proper mineralization59 | Rickets or osteomalacia;62 short- latency disease | Rickets and osteomalacia are prevented when VTD levels are >25 nmol/L67 |
| Parathyroid | Regulates calcium and phosphate levels, controls conversion of 25-hydroxyvitamin D to 1,25-dihydroxyvitamin D59 | Parathyroid hormone excretion increases as levels of VTD decrease resulting in secondary hyperparathyroidism, which in turn results in resorption of calcium from bone and exacerbates osteoporosis | Parathyroid 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 VDR | Inhibition 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 system68 | Might contribute to congestive heart failure2,69; deficiency results in loss of calciotropic effect in long- latency disease | Currently unknown, but 2000–4000 IU of vitamin D3 are being suggested70 |
| Muscle via VDR | Modulates calcium transport, protein synthesis, and kinetics of muscle contraction71 | Muscle weakness, limb pain, and impaired physical function72; loss of calciotropic effect | Maximum neuromuscular performance achieved with VTD levels of 125 nmol/L33 |
| Skin via VDR | Production of calcitrol that regulates cellular function in keratocytes | Antiproliferative, immunosuppressive, and prodifferentiating effects | VTD analogues are used for psoriasis73 |
| Islet cells via VDR | Improvement in insulin sensitivity4 | Negative effect on beta cell function with reduced insulin secretion; loss of immune modulatory effect | Raising VTD levels from 25 to 75 nmol/L improves sensitivity by 60%; optimal level has not been determined |
| Certain cancer cell types mediated via VDR | Suppressed growth and increased apoptosis74,75; stabilized chromosomal structure and prevented DNA breakdown76 | Loss of antiproliferative effect | Optimal level undetermined |
| Immune system modulator | Stimulated expression of potent anti- microbial peptides, increased “oxidative burst” potential of macrophages77 | Increased susceptibility to influenza77 and tuberculosis78 | Optimal level undetermined; summer levels of 125 nmol/L likely required77 |
| Innate immune function | Increased production of cathelicidins effective against Escherichia coli, methicillin-resistant Staphylococcus aureus, Pseudomonas aeruginosa, and Candida | Decreased wound barrier function | Optimal dose undetermined79–82 |
VTD—vitamin D, VDR—vitamin D receptor.