There are two nutritionally important forms of Vitamin D in man, namely
- Calciferol/Ergocalciferol (vitamin D2): It is produced by irradiation of ergosterol which was isolated from ergot. Thus, it is of plant origin.
- Cholecalciferol (vitamin D3): It is of animal origin and can be produced in the malpighian layer of human epidermis by ultraviolet rays which convert pro-vitamin D3 present in the skin to cholecalciferol.
Vitamin D is stored largely in the fat deposit. It is absorbed by the help of bile salt. It is then carried in the plasma and is excreted in the bile; some of the vitamin D is inactivated by the hepatic chromosomal drug metabolizing system in the liver.
It is unique hormone because it is derived both from sunlight and foods.
- Sunlight: Vitamin D is synthesized on the body by the action of ultraviolet rays of the sunlight on 7-dehydrocholesterol which is stored in large abundance in the skin. Exposure to UV rays is critical; these can be filtered off by air pollution. Dark-skinned races such as Negroes also suffer from this disadvantage because black skin can filter off nearly 95% of UV rays.
- Foods: Vitamin D is found to occur only in the food of animal origin, which include liver, egg yolk, butter, cheese and some species of fish. Fish liver oil is also the richest source of vitamin D. These days foods are fortified with this vitamin and these are milk, vanaspati margarine, and infant foods.
- Function in Intestine: Vitamin D promotes intestinal absorption of calcium and phosphorus.
- Effects on Bones: It stimulates normal mineralization, enhances bone resorption and affects the collagen maturation.
- Effects on Kidney: Vitamin D increases tubular resorption of phosphate and it has a variable effect on reabsorption of calcium.
- Other functions of this vitamin include enhancement of normal growth.
In growing children a deficiency of vitamin D causes rickets while in adults mostly in women its deficiency causes osteomalacia.
- Rickets: This is a disease primarily due to vitamin D intake deficiency but also an inadequate supply of calcium, phosphorous and sunlight may also play a part. This disease is observed in children between the ages of 6 months to 2 years. There is reduced calcification of growing bones. This disease is characterized by growth failure, muscular hypotony, bone deformity, tetany, and convulsion as a result of hypocalcemia. There is also an increasing level of alkaline phosphate in the serum. The bony deformities include curved legs pigeon chest, deformed pelvis, Harrison’s sulcus, kyphoscoliosis and rickety rosary.
- Osteomalacia: In adults, osteomalacia is a result of decreased vitamin D intake. It is mostly seen in women of poor economic status or has undergone repeated cycles of pregnancy and lactation. Lactation puts relatively more burden on the mother than pregnancy. The bones become even softer than in rickets. Due to the bone-softening body weight causes bowing of legs, shortening of vertebra and flattening of the pelvic bone.
It may occur due to
- Decreased intake of vitamin D in the diet
- Decreased formation of Vitamin D in the skin in those living in areas without ample sunlight or in the population with dark skin or with thick skin.
- Malabsorption of vitamin D due to gastric surgery, chronic pancreatitis or chronic diarrhea etc.
- Increased catabolism of vitamin D
- Renal Disease.
It may lead to a condition called Hypervitaminosis D. An excessive intake of vitamin D over prolonged periods results in a raised serum calcium level, nausea, vomiting, anorexia and digestive disturbances. There may arise kidney stone, calcification of body tissues especially renal arterioles, heart stomach and the aorta and irreversible kidney damage. In case of infants, growth is retarded. Excess intake of vitamin D by the pregnant women has been found to produce supra-valvular aortic stenosis in the fetus.
- Adults: 2.5mcg
- Infants and children: 50. Mcg
- Pregnancy and lactation: 10 mcg