Vitamin D is a fat-soluble vitamin that is important for calcium absorption and bone health. It can be synthesized in the skin upon exposure to sunlight or obtained from dietary sources. The biologically active form, calcitriol, increases intestinal calcium absorption and maintains calcium and phosphate levels in the blood through actions in the intestine, kidney, and bone. A daily intake of 400 IU is recommended, and deficiency can lead to rickets in children or osteomalacia in adults.

1. VITAMIN-D BY
P.KUSUMA
MSc-1

2. INTRODUCTION TO
• Vitamins are organic compounds required in small amount in
the diet of animals in order to ensure, healthy growth.
• Vitamins are essential nutrients because animal cannot
synthesize such compounds inadequate amount for its daily
needs.
• Historically the name vitamins arose from the fact that they are
vital amines, hence were named vitamins.
• The alphabet ‘e’ from vitamins was removed when it was found
that all such compounds were not amines, and named vitamins.
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3. CLASSIFICATION OF VITAMINS
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4. VITAMIN-D
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INTRODUCTION:-
• Vitamin D is a group of fat-soluble secosteroids responsible for increasing intestinal
absorption of calcium, magnesium, and phosphate, and many other biological
effects.
• In humans, the most important compounds in this group are vitamin D3 (also
known as cholecalciferol) and vitamin D2 (ergocalciferol).
• It resembles sterols in structure and functions like a hormone.
• Vitamin D has a significant role in calcium homeostasis and metabolism.
• It is also called as sunshine vitamin.

5. CHEMISTRY OF VITAMINE-D
• Ergocalciferol (vitamin D2) is formed from ergosterol and is
present in plants. Cholecalciferol (vitaminD3) is found in
animals. Both the sterols are similar in structure except that
ergocalciferol has an additional methyl group and a double
bond. Ergocalciferol and cholecalciferol are sources for
vitamin D activity and are referred to as provitamins.
• During the course of cholesterol biosynthesis, 7-
dehydrocholesterol is formed as an intermediate. On
exposure to sunlight, 7-dehydrocholesterol is converted to
cholecalciferol in the skin (dermis and epidermis).
• The synthesis of vitamin D3 in the skin is proportional to the exposure to sunlight. Dark skin
pigment(melanin) adversely influences the synthesis of cholecalciferol.

6. Absorption, transport and storage
• Vitamin D is absorbed in the small intestine for which bile is essential.Through
lymph, vitamin D enters the circulation bound to plasma α2-globulin and is
distributed throughout the body. Liver and other tissues store small amounts
of vitamin D
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METABOLISM AND BIOCHEMICAL FUNCTIONS
Vitamins D2 and D3, as such, are not biologically active.They are
metabolized identically in the body and converted to active forms of vitamin
D.

7. Synthesis of 1,25-DHCC
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• Cholecalciferol is first hydroxylated at 25th position to 25-hydroxycholecalciferol (25-OH D3) by a specific hydroxylase
present in liver.
• 25-OH D3 is the major storage and circulatory form of vitamin D.
• Kidney possesses as specific enzyme, 25-hydroxycholecalciferol (calciol) l -hydroxylase which hydroxylates 25
hydroxycholecalciferol at position 1 to produce 1,25-dihydroxycholecalciferol (1,25-DHCC).
• 1,25-DHCC contains 3 hydroxyl groups (1,3 and 25carbon) hence referred to as calcitriol.
• Both the hydroxylase enzymes (of liver and kidney) require cytochrome P450,NADPH and molecular oxygen for the
hydroxylation process.
Regulation of the synthesis of 1,25.-DHCC
• The concentration of 1,25-DHCC is regulated by plasma levels of calcium and phosphate.
• They control hydroxylation reaction at position 1.
• Low plasma phosphate increases the activity of 25-hydroxycholecalicferol 1- hydroxylase.
• Low plasma calcium enhances the production of parathyroid hormone which in turn activates 1-hydroxylase.
• Thus the action of phosphate is direct while that of calcium is indirect on kidney 1-hydroxylase.

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9. Biochemical functions
1. Calcitriol( 1,25-DHCC)is the biologically active form of vitamin-D .lt regulates the plasma Levels of
calcium and phosphate. Calcitriol acts at 3 different levels( intestine, kidney and bone) to
maintain plasma calcium( normal rang 9-11 mg/dl).
2. Action of calcitriol on the intestine : Calcitriol increases the intestinal absorption of Calcium and
phosphate. In the intestinal cells, calcitriol binds with a cytosolic recept0r to form a calcitriol-
receptor complex.This complex then approaches the nucleus and interacts with a specific DNA
leading to the synthesis of a specific calcium binding protein.This protein Increases the calcium
uptake by the intestine.The mechanism of action of calcitriol on the target issue( intestine )is
similar to the action of a steroid hormone.
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10. BIOCHEMICAL FUNCTIONS
3. Action of calcitriol on the bone : In the Osteoblasts of bone, calcitriol
stimulates calcium Uptake for deposition as calcium phosphate ,Thus Calcitriol
is essential for bone formation.The bone is an important reservoir of calcium
and phosphate. Calcitriol along with parathyroid hormone Increases the
mobilization of calcium and Phosphate from the bone .This causes elevation in
the plasma calcium and phosphate levels.
4. Action of calcitriol on the kidney :Calcitriol is also involved in minimizing the
excretion of calcium and phosphate through the kidney, by decreasing their
excretion and enhancing reabsorption.The sequence of events that take place
in response to low plasma calcium concentration and the action of calcitriol on
intestine, kidney and bone, ultimately leading to the increase in plasma calcium
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11. BIOCHEMICAL FUNCTIONS
• 24 ,2 5 - Dihydroxycholecalciferol (24,25- DHCC) is
another metabolite of vitamin D, lt is also
synthesized in the kidney by 24-hydroxylase .The
exact function of 24,25-DHCC is not known . lt is
believed that when calcitriol concentration is
adequate,2 4-hydroxylase acts leading to the
synthesis of a less important compound 2 4, 25-
DHCC. In this way, to maintain the Homeostasis of
calcium synthesis 0f 24,25-DHCC is also important.
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15. vitamin D is a hormone and
not a vitamin-justification
• Calcitriol (1,25-DHCC) is now considered an important calciotropic hormone,. While, cholecalciferol
is the prohormone.The following characteristic features of vitamin D (comparable with hormone)
justify its status as a hormone.
1 .Vitamin D3 (cholecalciferol) is synthesized in the skin by ultra-violet rays of sunlight.
2 .The biologically active form of vitamin-D , calcitriol is produced in the kidney.
3. Calcitriol has target organs-intestine, bone and kidney, where it specifically acts
4. Calcitriol action is similar to steroid hormones . lt binds to a receptor in the cytosol and the complex
acts on DNA to stimulate the synthesis of calcium binding protein.
5. Actinomycin D inhibits the action of calcitriol.This supports the view that calcitriol exerts its effect
on DNA leading to the synthesis RNA (transcription).
6. Calcitriol synthesis is self-regulated by a feedback mechanism i.e., calcitriol decreases its own
Synthesis.
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16. RECOMMENDED DIETARY ALLOWANCE (RDA)
•The daily requirement of vitamin
D is 400 International Units (IU)
or 10 mcg of cholecalciferol
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17. DIETARY SOURCES
• Good sources of vitamin D include fatty fish, fish liver oils, egg yolk etc. Milk is not a good source of
vitamin D.Vitamin D can be provided to the body in three ways
1. Exposure of skin to sunlight for synthesis of vitamin D.
2. Consumption of natural foods;
3. By irradiating foods (like yeast) that contain precursors of vitamin D and fortification of foods (milk,
butter etc.).
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18. DEFICIENCY OF VITAMIN-D
• Vitamin D deficiency is relatively less common since this vitamin can be synthesized in the body
• However, insufficient exposure to sunlight and consumption of diet lacking vitamin D results in its
deficiency.
• Vitamin D deficiency occurs in strictVegetarians, chronic alcoholics, individuals with Liver and kidney
diseases or fat malabsorption syndromes.
• Deficiency of vitamin D causes rickets in children and osteomalacia in adults.
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19. CONTINUATION…
•Hypervitaminosis D
• Vitamin D is stored mostly in liver and slowly metabolized.
• Among the vitamins ,vitamin D is the most toxic in overdoses (10-100times RDA).
• Toxic effects of hypervitaminosis D include demineralization of bone (resorption) and
Increased calcium absorption from the intestine, Ieading to elevated calcium in plasma
(hypercalcemia).
• Prolonged hypercalcemia is associated with deposition of calcium in many soft tissues
such as kidney and arteries.
• Hypervitaminosis D may lead to formation of stones in kidneys (renal calculi).
• High consumption of vitamin D is associated with loss of appetite, nausea ,increased thirst,
loss of weight etc.
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