Vitamin D can be obtained through dietary sources like fish, eggs, and fortified foods, but it is also synthesized in the skin upon exposure to sunlight, making it both a vitamin and a hormone. Vitamin D is absorbed in the small intestine and transported through the bloodstream bound to vitamin D binding protein. In the liver and kidneys, vitamin D undergoes hydroxylation reactions to produce calcidiol and the active form calcitriol, which regulates calcium and phosphate levels in the blood by increasing their absorption in the intestine and mobilization from bone.

1. Vitamin ‘D’

2. Learning objectives
• Introduction
• Absorption/ transport and storage
• Formation in body
• Action/ function
• Regulation
• Deficiency
• hypervitaminosis

3. • Fat soluble vit.
• Resembles sterols in structure and fxn like hormoneà CHOLESTEROL.
üDietary source
• Ergocalciferol, aka vit D2(plant)
• Cholecalciferol, aka vit D3( animal)
üIn our body:
• BUT, vit D is also synthesized in on exposure to sunlight à sunshine
vitamin
• So, NOT STRICTLY VITAMIN bcozà synthesized in the skin; against
the criteria of vitamin.
Introduction

4. üDietary sources
• Fish, fish liver oils, egg yolk etc.
• fortified foods (milk, butter, other dairy products etc.)
üRecommended dietary allowance (RDA)
ü 400 IU or 10 mg of cholecalciferol daily. BUT,
ü In countries with good sunlight (like India) : 200 IU (or 5 mg of
cholecalciferol)
ü🌞🌝

5. • Dietary vit D absorbed ( bile salt)à fat soluble.
• In intestinal mucosal cellà incorporated in chylomicronsà enters in
circulation via lymph.
• In circulationà bind with vit D binding protein ( DBP)à COMPLEX
• Further, Vit D3 synthesized in skinà also binds with DBP.
• Taken up by different tissueà liver & adipose tissue stores it.
• > 90% is bound by DBP, < 1% IS FREE.
Absorption, transport and storage

6. 👉 👉Formation
of
Vitamin D
In our body 🌝🌞

7. 1. starts in Skin ( dermis & epidermis)
• 7-Dehydrocholesterol (intermediate of cholesterol met, skin),
• On exposure to UV light, à previtamin D à by breaking the bond betn
9 and 10 positionà B ring is opened.
• In a period of Hours à cholecalciferol ( calciol)à by photochemical
process.
• Melanin competes 7-Dehydrocholesterol in absorbing UV rays ( black
skin à less synthesis)

8. 2.Reaches to Liver
üIn the liver,
• cholecalciferol ( skin & food) , calciol
25-OH D3 ( calcidiol) ( ½ life 15 D)
Rate is FAST
üMAJOR STORAGE AND CIRCULATORY FORM OF VITAMIN D.
üReleased in circulation bound with DBP
Hydroxylated at 25th position By 25-hydroxylase

9. 3. Finally reaches to kidney
üIn the kidney
• Calcidiol (25-OH D) à 1-hydroxylation à to form
1,25-dihydroxyvitamin D (calcitriol) by 1-⍺ hydroxylase (Slow,rate
limiting.)
• Biologically most active vitamin D. (1000 times more potent>
calcidiol)
• ½ life : 12-14 hrs.
üBoth hydroxylase ( kidney & liver):for the hydroxylation process
• cytochrome P450,
• NADPH and
• molecular oxygen.

10. 24, 25-hydroxy cholecalciferol
üIn kidney,
üAnother metabolite- 24, 25-hydroxy
cholecalciferol is also formed.
• when calcitriol is adequateà no
more extra is requiredà a less imp
compound 24,25-DHCC à by 24
hydroxylase.
• The exact function of 24,25-DHCC
is NOT KNOW

12. Functions
üCalcitriol ( 1,25-DHCC) is the biologically most active.
• Principal fxn is to maintain plasma calcium level.
qCalcitriol acts on 3 different organs.
1. intestine
2. kidney
3. bone
To Maintain plasma calcium.
( normal =9 -11 mg/dl).

13. • In intestine calcitriol binds with cytosolic receptor à form calcitriol
receptor complex.
• Complex reaches to nucleus à interacts with DNAà synthesis of
calcium binding protein called calbindin.
• ↑↑ calcium absorption
• ↑↑ phosphate absorption
1)Action on the intestine.
calbindin.

14. 2. Action on bone: Diverse
üMobilizes bone mineral
• In the osteoblast of bone à ↑↑es ca++ uptake à deposited as calcium
phosphateà bone formation.
üAt the same time,
• Vitamin D is also believed to promote bone resorption & calcium
mobilization to raise the levels of Ca and P in blood in association with
PTH.
• bone resorption by ↑↑ osteoclastic activity.
• So, overall bone remodeling.

15. 3. Action on the kidney

16. In one diagram: action of calcitriol.

17. Regulation of vit D
üRegulated by
1) PTHà direct
2) PHOSPHATEà direct
3) CALCIUMà indirect
4) 1,25 (OH)2 D ( calcitriol)à self regulated by feedback
mechanism
PTH &
hypophosphatemia
Hypocalcemia
↑↑ activity of 1-⍺ hydroxylaseà
↑↑ synthesis of calcitriol.
DIRECT
At first, it ↑↑es PTH levelà then
only ↑↑ activity of 1-⍺
hydroxylaseà↑↑ synthesis of
calcitriol.
INDIRECT

18. Deficiency
• Relatively less common, since can be synthesized in body.
üBUT,
• Insufficient exposure to sunlight+ low vit D dietà deficiency
• Occurs in
• Strict vegetarians
• Chronic alcoholics
• Liver or kidney disease
• Fat malabsorption syndrome
• Rickets in children
• Osteomalacia in adults
• Osteoporosis in old people

19. Renal rickets ( how kidney causes rickets ?)
(renal osteodystrophy)
↓↓ed 1-⍺ hydroxylase

20. Hypervitaminosis D- toxicity
• Among the vitamins, vit D is the most toxic in overdoses (10-
100 times RDA).
üLeads to
• Demineralization of bone ( resorption)+ increased calcium absorp
from intestineà hypercalcemia
• Prolonged hypercalcemiaà deposition in soft tissue like kidney &
arteriesà calcinosis
• Stones in kidney ( calculi) & rise in BP.
üIn hypervitaminosis: toxicity
• ↑↑ed calcitriolà↑↑es bone resorption BM stem cell à form more
osteoclast cell & ↑↑ osteoclastic activity.

21. TASK FOR YOU….. 📝
JUSTIFY- VIT D is also hormone.