The Role of Vitamin D in the Body
Vit D effects:
- Enhances absorption of calcium, phosphate, and magnesium from the small intestine (when stimulated by PTH) by increased synthesis of calcium binding protein
- Enhances reabsorption of calcium and phosphate in the kidneys (with the help of estrogen and PTH)
- Enhances resorption of calcium and phosphorus from the bones (when stimulated by PTH)
- Stimulates insulin production in the pancreas
Dietary Sources of Vitamin D
Dietary sources:
- Fish oil
- Salt water fish
- Fortified milk, orange juice, morning cereals
Metabolism of Vitamin D
Vit D2 (ergocalciferol or ergosterol) is obtained by diet, from plants.
Vit D3 (cholecalciferol) is produced in the skin from 7-dehydrocholesterol under UV influence.
Calcidiol or calciferol or 25-hydroxycholecalciferol [25(OH)D] or 25-D is synthesized in the liver from vit D2 and D3. Half-life of calcidiol is several weeks. 25-D is in vitamin D supplemments.
All above forms of vitamin D are inactive.
Calcitrol or 1,25-dihydroxycholecalciferol [1,25(OH)D] is synthesized in the kidneys from calcidiol by the help of PTH and is the only active form of vitamin D. Half-life of calcitrol is few hours. Synthesis of calcitrol is enhanced in kidneys by hormone PTH and low blood phosphate.
Causes of Vitamin D Deficiency
- Low vitamin D intake - but only in inadequate sun exposure (less than 30 minutes a week)
- Inadequate exposure to ultraviolet light (winter, bedridden persons)
- Anticonvulsant-drug therapy (Phenytoin) (1)
- Renal dialysis
- Renal disese: nephrotic syndrome
- Hypertension
- Diabetes mellitus, insulin resistance
- High serum concentrations of parathyroid hormone and alkaline phosphatase
- Low serum concentrations of ionized calcium and albumin
What Blocks Activation of Vitamin D?
Vitamin D has to be chenged into vit 25D in the liver and furtherly into 1,25D in the kidneys to become active. What can block this activation:
- Vit D to 25D conversion may be blocked by liver disease
- 25D to 1,25D conversion may be blocked by hormone calcitonin or kidney disease
Symptoms of Vitamin D Deficiency
Symptoms:
- Musculoskeletal pain
- Rickets in children
- Osteomalacia in adults (non-adequate mineralization of bones)
Vitamin D deficiency can contribute to development of multiple sclerosis or inflammatory bowel disease (3).
References:
- Phenytoin supposedly caused osteomalacia (ncbi.nlm.nih.gov/pubmed)
- Vitamin D (ods.od.nih.gov)
- Low vitamin D causes MS and IBD (ncbi.nlm.nih.gov/pubmed)
- Vit D stimulates insulin production (merck.com)