Vitamin DDr.S.Chakravarty ,MD

Vitamin D

• (yeast) Vitamin D2 : Ergosterol (pro D2) Ergocalciferol (D2) added to milk and dairy.

• (Human) Vitamin D3 : Pro (7-dehydrocholesterol) Pre (9,10-secosterol) (D3) cholecalciferol.

• 7-Dehydrocholesterol is present in epidermis and dermis.

• Short exposure of UV light 290-315nm converts pro to pre vitD3 enters blood circulation and equilibrates to D3.

Sunlight • Prolonged exposure to sunlight leads to conversion of previtamin

D3 to Lumisterol and Tachysterol.

• Seasonal variation – more UV at the end of winter in cold countries.

• Geographical variations

• Other Considerations :- Sunscreens :-In labs tests with Sunscreen lotions block UV and decrease

production of vitamin D3.– But because people who use more sunscreen spend more time in the

sun, so naturally they will have higher vitamin D levels.

Activation of vitamin D3 :

skin

7-Dehydrocholesterol

CHOLECALCIFEROL

SKIN :-UV Light

25 hydroxycholecalciferol

LIVER :- 25 hydroxylase

KIDNEY :- 1 α hydroxylase(induce )

Dietary source required if insufficient exposure to UV light.

1,25 dihydroxy cholecalciferol

+PARATHYROID

HYPOCALCEMIA

(+)

X CIRRHOSIS , LIVER FAILURE

X CRF, END STAGE RENAL DIASEASEc

Bone :- Osteoclasts ; Ca+2 Mineralization(with PTH)

Duodenum: Increase calcium uptake from intestine

Vit DKidney :-Vit D : Retention of Ca+2

PTH :- Conserves Ca+2 at expense of PO4

-3

Calcium metabolism:

• Calcium is stored as calcium-hydroxy-appetite crystals in bone.

• 99% present in bone – which maintains serum calcium levels of (9-10.5 mg/dl)

• 5 mg is ionized calcium – physiologically important

• 3.5 -4 mg is bound to serum albumin (0.8mg /gm of albumin)

• 1mg exists as bound to anions (citrate).

Functions of calcium:

Intracellular Extracellular

• Second messenger

• Muscle contraction

• Membrane excitability

• Degranulation and release of hormones.

• Co-factor in enzyme activity.

• Nerve impulse transmission

• Coagulation cascade

• Maintains intracellular conc.

• Deposition of calcium in bone from diet.

What happens in Hypocalcemia :

Increase in PTH secretion and decrease calcitonin.

1. Increased distal tubular re-absorption of calcium and excretion of phosphate:

2. Increase 1α-hydroxylase activity of kidney.

3. Increase 1,25 dihydroxy cholecalciferol increased absorption in the small intestine.

4. Increased Osteoclastic activity bone resorption and increased calcium in circulation.

During Hypercalcemia :

• Release of calcitonin inhibits the activity of osteoclasts

• Decrease in PTH and Decreased 1,25 -Dihydroxycholecalciferol decreased renal and intestinal absorption.

• Bone formation due to supersaturation of calcium and phosphate.

Vitamin D deficiency:Hypocalcemia and hypophosphatemia :

poor mineralization and stunted growth

Rickets : children• Widening of growth plate – rachitic

rosary at the costchondral junction.

• Craniotabes – ping pong skull

• Bow legs or knocked knees – bending of long bones due to poor mineralization.

• Reduced radio-opacities on long bones.

• Lax muscles – protruberant abdomen.

Osteomalacia • Seen in adults

• Looser’s zone or pseudofractures seen in scapula, pelvis and femoral neck.

• Soft, flexible, brittle, deformed bones

• Progressive weakness

• Pain in pelvis, lower back, and legs

Other functions of VIT D:

• Regulation of cell growth – Cancer prevention

• Regulation of immune function – prevents autoimmune disease

• Regulation of blood pressure – prevents heart disease

Hypervitaminosis D:

Similar to features of hypercalcemia:

• Constipation, polyuria, polydipsia, confusion

• Calcinosis of soft tissues – metastatic calcification.

• Renal stones

Thank you