Chemical characterstics of vitamin D
There are two biological precursors of vitaminD 7-dehydrocholestrol and ergosterol. Under the influence of uv-rays each undergoes a chemical change yielding vitamin D3 (cholecalciferol) and vitamin D2 (ergocalciferol).
Vitamin D is stable over long periods in oil solutions but quite unstable in the presence of mineral salts.
Digestion and absorption of Vitamin D
Dietary Vitamin D is absorbed along with dietary fats in small intestines and transported to the lymph system. Bile is essential for the absorption of this vitamin.Excess vitamin is stored in the body.
Functions of vitamin D
Major part of the vitamin circulating in the blood is neither D2 or D3 but the metabolite 25, hydroxy cholecalciferol to which the ingested vitamin is converted in the liver.
A more potent metabolite 1,25- dihydroxycholecalciferol or calcitriol is synthesized in the kidney.
It appears in the intestinal mucosal cells, bones and skeletal muscles for regulating calcium absorption and metabolism.
VitaminD is therefore, a precursor of true hormone 1,25-dihydroxycholecalciferol which performs a vital function.
Vitamin D through the action of these active metabolites performs the following functions:
- Absorption of calcium from digested food.
- Reabsorption of phosphate in the renal tube.
- Calcification of osteoblast cells of growing skeletal structures.
Deficiency symptoms of Vitamin D
- Deficiency causes rickets in children and osteomalacia in adults. Both these reprsents the failure of the process of calcification.
- Skeletal malformations occur because of the inability of soft bones to withstand the stress of bodyweight.
- Delayed closures of fontanelles and softening of skull in infants,soft fragile bones with bowing of legs,spinal curvature,swelling of wrists,knee and ankle joints, poorly developed muscles, restlessness and nervous irritability are some of the other symptoms.
- Rickets is difficult to diagnose in the early stages.
Sources of vitamin D
The major sources of vitamin D are food and exposure to sunlight. The major food sources are:
- Fortified foods are the major dietary sources of vitamin. Milk in the United States is fortified with 10 micrograms (400 IU) of vitamin D per quart.
- Fatty fish and fish oils
Exposure to sunlight is an important source of vitamin D. Ultraviolet (UV) rays from sunlight trigger vitamin D synthesis in the skin.