VITAMIN - D , Strucutre,activation,sorces,RDA,functions,deficiency symptoms - EASY NOTES

Vitamin D is a fat soluble vitamin 

Vitamin D is also known as Cholecalciferol & anti- rachitic factor.

STURCTURE OF VITAMIN D

• there are two forms of vitamin D
• Cholecalciferol - 
• also called as Vitamin D2
• naturally occurring 
• obtained from animal sources by the action of UV rays of sunlight on 7 dehydrocholesterol
• Ergocalciferol - 
• also called as Vitamin D3
• artificially occurring 
• obtained from plant sources by the action of UV rays on ergosterol

ABSORPTION,TRANSPORT & ACTIVATTION OF VITAMIN D

• Dietary vitamin D is absorbed in the duodenum along with lipids. 
• It is transported to the liver through chylomicron

ACTIVE FORM OF VITAMIN D

• Calcitriol (1,25 dihydroxy cholecalciferol )is the active form of vitamin D
• Cholecalciferol gets activated to calcitriol
• two steps are involved in activation of cholecalciferol - in liver & in kidney

STEP 1  -ACTIVATION IN LIVER -

• Cholecalciferol is hydroxylated at 25th position to form 25 hydroxycholecalciferol in the presence of the enzyme 25-hydroxylase
• the activated compound formed is 25 hydroxycholecalciferol

STEP 2  -ACTIVATION IN KIDNEY -

• 25 hydroxycholecalciferol is transported to kidney
• in Kidney 25 hydroxycholecalciferol  is hydroxylated at 1st position to form 1,25 hydroxycholecalciferol in the presence of the enzyme 1,α--hydroxylase
• the activated compound formed is 1, 25 hydroxycholecalciferol

SOURCES

• Sunlight, cod liver oil, fish liver oil, Egg Yolk & Liver

FUNCTIONS

• Vitamin D helps in the regulation of calcium & phosphorous by acting at three levels.

I-ACTION OF CALCITRIOL ON INTESTINE

• Increases the synthesis of calcium binding protein - calbindin
• calbindin increases the calcium uptake in the intestine 
• hence it increases the plasma calcium & phosphorous concentration 

II-ACTION OF CALCITRIOL ON KIDNEY

• decreases the excretion of calcium & phosphorous from kidney
• increases the reabsorption of calcium & phosphorous

II-ACTION OF CALCITRIOL ON BONE

• increases the deposition of calcium & phosphorous and so increases mineralization 
• calcitriol and PTH(para-thyroid hormone) together stimulate the synthesis of Osteocalcin(calcium binding protein in bone) 
• Osteocalcin increases the mobilization of calcium & phosphorous and causes increase in the levels of plasma calcium & phosphorous

RDA(recommended dietary allowance)

• 200-400 IU/day

DEFICIENCY MANIFESTATIONS

• deficiency of vitamin D causes  Rickets in children & Osteomalacia in Adults

RICKETS

• Poor mineralization of bones causing soft & pliable bones
• Weight bearing bones are bent & deformed
• Other clinical features of children with rickets are 
• large head with protruding forehead, 
• pigeon chest, 
• bow legs(curved legs) 
• knock knees  
• abnormal curvature of the spine (kyphosis)
• low plasma levels of calcium & phosphorous

OSTEOMALACIA

•  also called as adult rickets.
• Demineralization of previously formed bones which makes the bone soft and easily prone to fractures,