Vitamin D and its Effects on Health

Lecture 41: Importance of Vitamin D

• Vitamin D is a group of sterols with hormone-like functions.
• Vitamin D is also called the "sunshine vitamin" due to its synthesis in the skin in response to sunlight.
• Vitamin D deficiency is more common during winter months due to decreased sunlight exposure.
• The active form of vitamin D, 1,25-dihydroxycholecalciferol (Calcitriol), binds to intracellular receptor proteins.

Specific Objectives

• Students will understand the synthetic pathway of vitamin D in the body.
• Students will be able to explain the activation process of vitamin D.
• Students will be able to explain the main functions of vitamin D.
• Students will discuss the clinical effects of vitamin D deficiency.

Distribution of Vitamin D

• Diet:
  • Ergocalciferol (vitamin D2) is found in plants.
  • Cholecalciferol (vitamin D3) is found in animal tissues.
  • Both are preformed vitamin D sources.
• Endogenous precursor:
  • 7-Dehydrocholesterol, an intermediate in cholesterol synthesis, is converted to cholecalciferol in the dermis and epidermis when exposed to sunlight.

Synthesis and Regulation of Calcitriol

• Diagram demonstrating the synthesis and regulation of calcitriol.
• Vitamin D3 (cholecalciferol) formation from 7-dehydrocholesterol in the skin,
• 25-hydroxylation in the liver,
• conversion to the active form, calcitriol (1,25-dihydroxycholecalciferol) in the kidneys.
• PTH (parathyroid hormone) and serum phosphate levels affect the pathway.

Function of Vitamin D

• The primary function of calcitriol is to maintain adequate plasma calcium levels.
• Aids in calcium and phosphorus absorption and retention for bone building.

Effect of Vitamin D on the Intestine

• 1,25-dihydroxyvitamin D3 (calcitriol) stimulates intestinal absorption of calcium and phosphate.
• Increased calcium binding protein (calbindin) synthesis enhances calcium uptake.
• Mechanism of action is typical of steroid hormones.

Effect of Vitamin D on Renal Tubules

• Calcitriol increases calcium and phosphorus reabsorption in renal tubules, conserving both minerals.

Effect of Vitamin D on Bone

• Calcitriol stimulates osteoblasts which in turn secrete alkaline phosphatase.
• Increased local phosphate concentration is a result.
• Increased ionic product of calcium and phosphorus results in bone mineralization.

Other Functions of Vitamin D

• Enhances immune function.
• Possible cancer prevention effects.
• Aids in infection control.
• Reduces inflammation.
• Boosts mood.
• Potential aid in weight loss.
• Promotes rheumatoid arthritis reduction.
• Lowers risk of type II diabetes.
• Might reduce heart disease risk.

Deficiency of Vitamin D

• Nutritional Deficiency: Insufficient sunlight exposure or inadequate vitamin D intake.
• Malabsorption: Conditions like obstructive jaundice or steatorrhea.
• Activation Abnormalities: Liver or kidney disease hindering hydroxylation reactions.

Clinical Indications

• Vitamin D deficiency causes bone demineralization leading to rickets (children) and osteomalacia (adults).
• Rickets characterized by collagen matrix formation without complete mineralization, resulting in pliable bones.
• Osteomalacia involves pre-existing bone demineralization, increasing fracture risk.

Toxicity of Vitamin D

• Vitamin D is a fat-soluble vitamin and is stored in the body, metabolized slowly.
• High doses (e.g., 100,000 IU for weeks/months) cause loss of appetite, nausea, thirst, and stupor.

Reference

• Vasudevan, DM, Sreekumari, S, and Kannan, V. (2011). Textbook of biochemistry for medical students, 6th Edition.