Vitamin D
Metabolism and biochemical functions
N.Revanth
House surgeon
Introduction
• It is a fat soluble vitamin.
• Resembles sterols in structure and functions like a
hormone
• Synthesized by the body after exposure to UV rays or
obtained from food sources
Chemical form
• Ergocalciferol (vitamin D2) is formed from ergosterol and
present in plants.
• Cholecalciferol (vitamin D3) is found in animals
• Ergocalciferol and cholecalciferol are the sources for
vitamin D activity and are referred to as provitamins.
• Both ergocalciferol and cholecalciferol are similar in
structure except that ergocalciferol has an additional
methyl group and double bond
• 25 hydroxy vit D3(calcidiol)
• 1,25 dihydroxy vit D3(calcitriol )
Absorption,Distribution,Metabolism
• Absorption:
Vitamin D absorbed from the small intestine for which bile
is essential
• Distribution:
Stored in adipose tissue
• Metabolism:
Transport to the liver,then kidneys,where it is activated
Excretion
• Excreted in bile
• Metabolized to water soluble metabolites and then
excreted in urine as well
Biochemical functions
• Calcitriol is a biologically active form of vitamin D
• It regulates the plasma levels of calcium and phosphate
• It acts at 3 different levels
1. Action of calcitriol on intestine
2. Action of calcitriol on bone
3. Action of calcitriol on kidney
Functions
• Cell differentiation
inhibits proliferation and stimulates the normal differentiation of
cells
• Immunity
The inactive form of vitamin D aids in the formation of active
vitamin D which enhances immunity and inhibits the
development of autoimmune disease.
• Calcium balance
Calcitriol restores normal calcium levels by increased
absorption of dietary calcium, increased mobilization of calcium
from bone, and increased reabsorption from kidneys
Functions
• Heart disease
Renin,important in regulation of blood pressure and heart
health,is associated with vitamin D levels
• Maintaining normal phosphorus levels
Bone cannot be properly mineralized without phosphorus
De iciency
• During vitamin D de iciency calcium absorption cannot
be increased to meet body’s need
• Therefore calcium is taken from bones to maintain normal
serum levels
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De iciency of vitamin D
The de iciency diseases are rickets in children and
osteomalacia in adults. Hence, vitamin D is known as
antirachitic vitamin.
Causes of vitamin D de iciency
Nutritional de iciency of vitamin D is the most common
cause. This can occur in people who are not exposed to
sunlight properly, e.g. inhabitants of northern latitudes,
in winter months. Malabsorption of vitamin (obstructive
jaundice and steatorrhea). Abnormality of vitamin
D activation. Liver and renal diseases may retard the
hydroxylation reactions.
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• Clinical features of rickets
Rickets is seen in children.There is insuf icient mineralisation of
bones become soft and pliable. The bone growth is markedly
affected
• The classical features of
rickets are bone deformities. Weight-bearing bones are
bent
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• Clinical features of osteomalacia
The term is derived from Greek “osteon” = bone; and
“malakia” = softness. The bones are softened due to
insuf icient mineralization and increased osteoporosis.
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• Requirements of vitamin D
Children = 10 microgram (400 IU)/day; Adults = 5 to
10 microgram (200 IU)/day; Pregnancy, lactation = 10
microgram/day; Senior citizens above the age of 60 = 600
IU per day.
• Sources of vitamin D
Exposure to sunlight produces cholecalciferol. Moreover
ish liver oil, ish and egg yolk are good sources of the
vitamin. Milk contains moderate quantity of the vitamin.
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• Hypervitaminosis D
Doses above 1500 units per day for very long periods may
cause toxicity. Symptoms include weakness, polyuria,
intense thirst, and calci ication of soft tissues (metastatic
calci ication), especially in renal tissues.
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Thank you
