Vitamin D (Calciferol)

 Vitamin D (Calciferol)

(Antirachitic factor)

Introduction

In 1782 it was found out that cod-liver oil cures Rickets. In 1918 Mellanby produced experimental rickets in animals and confirmed the conception which was so far prevalent that rickets was due to deficiency of vitamin D. Since then further advancement has been made and it is now established that synthesis of vitamin D occurs in the body under the influence of the ultra-violet rays of the sunlight. The amount of vitamin D thus formed is greatest during summer months and lesser in the winters.

Chemistry

This is a group of vitamin D of which D₂ and D₃ are important for nutritional purposes. All the members are sterol compounds with D₂, or activated ergosterol or ergocalciferol or viosterol, is an isomer of ergosterol. This vitamin is very thermo stable. The structure of vitamin D₃ or cholecalciferol, also known as activated 7-dehydrocholesterol is the same as that of D₂ except that the side chain on position 17 is that of cholesterol.

Synthesis

D₂ is obtained when plant sterol, ergosterol, is activated by irradiation with ultra-violet rays. The epidermal cells of the skin of human contain large amounts of provitamin D (7-dehydrocholesterol) which can be converted to vitamin D₂ by exposure of the skin to sun light.

D₃ is also made by ultra-violet irradiation of 7-dehydrocholesterol and is the form that occurs in nature. Human can synthesise provitamin D₃ in the body.

Absorption

It is absorbed in the intestine in presence of bile and fatty acids.

Properties

Both D₂ and D₃ are

·        white, crystalline material

·        soluble in fat and fat-solvent like ether, chloroform, and acetone

·        stable in heat, acids and alkalis

·        resistant to oxidation

Distribution

Animal Sources

Fish-liver oils are the richest sources.

·        cod-liver oil-100 i.u. per gram

·        halibut-liver oil-1,000 i.u. per gram

·        Mammalian liver negligible amounts

·        Butter

·        milk  

·        eggs

Plant sources

·        Vegetable oils- negligible

Functions

(1) Helps in calcium absorption from the intestine. The rate of active transport of calcium across the intestinal wall is increased by vitamin D.

(2) It promotes the absorption of phosphate if there is increased absorption of calcium.

(3) As Calciferol increases indirectly the resorption of calcium from fully calcified bone so it helps in the calcification of new bone. Vitamin D maintains normal structure of bone and assists to govern the equilibrium between bone calcium and blood calcium.

(4) Calciferol in small amounts increases the excretion of calcium by kidneys and in large doses excretion of phosphate also.

(5) Helps in the development of the normal teeth. In deficiency of vitamin D the formation of the teeth becomes defective and leads to the development of dental caries.

(6) It influences the handling of phosphate by the kidney. It may act in connection with alkaline phosphatase, liberating Inorganic phosphate which influences the deposition of calcium phosphate in ossification process.

(7) It is necessary for proper bone growth by promoting endochondral growth of long bones.

(8) Vitamin D controls the retention of calcium and parathyroid regulates the level of blood calcium by controlling mobilization of calcium from the bones. Thus vitamin D and parathyroid help each other in Calcium metabolism and bone formation.

(9) It lowers the pH in the colon, cecum, ileum, etc., and increases the urinary pH simultaneously. This may be a secondary effect due to absorption of Calcium.

(10) In physiological doses, it increases the citrate content of bone, blood and other tissues as well as urinary excretion.

(11) It counteracts the inhibitory effect of calcium ions on the hydrolysis of phytates (inositol hexaphosphate).

Mode of action

The mode of action is mainly by promoting the transport of calcium and secondarily phosphate in the blood stream. On the whole the function of vitamin D is to cause increased absorption, longer retention and better utilization of calcium and phosphorus in the body.

Deficiency signs

The primary sign in this vitamin deficiency is an increased loss of calcium and phosphate in the feces. This leads to a fall in their blood level, and hence not available for bone formation. Due to this reason, the children suffer from rickets (wrickken =to twist), and adults from osteomalacia.

·        In rickets the bones remain soft due to less deposition of calcium salts. So the long bones remain cartilaginous and easily bend under the weight of the body. There is defective ossification so there is malformation of the chest and pelvis, and changes in the spinal curvature (scoliosis), softness in skull bones.

·        Osteomalacia is a form of adult ricket. It is due to deficiency of vitamin D and calcium salts in the diet. It occurs in women during pregnancy and lactation when a large amount of calcium is depleted from the pregnant woman (mother).

International unit

Equivalent to activity of 0.025 micro-gram of Calciferol

Daily requirement

·        Infants below the age of 1 year	400-800 i.u.
·        Children and adolescents up to 20 years	400 i.u.
·        Adults	400-800 i.u.

This should simultaneously be supplemented with adequate intake of calcium and phosphorus.

 Hypervitaminosis D

·        loss of weight

·        reduced excretion of calcium and phosphorus

·        increased blood calcium

·        nausea

·        vomiting

·        headache

·        drowsiness

·        Extensive deposit of calcium in the soft regions which are not normally calcified, such as kidney, heart, artery, etc

·        The signs of renal failure may appear