Vitamins 1

Metabolism of Vitamin D

• Dietary vitamin D is absorbed from the small intestine and transported to the liver via chylomicrons.
• In the liver, vitamin D is activated and metabolized to 25-hydroxycholecalciferol (calcifediol).
• Calcifediol is transported to the kidney, where it undergoes further hydroxylation to form the hormone 1,25-dihydroxycholecalciferol (calcitriol).
• Calcitriol is transported to target organs (intestine and bone) via vitamin D-binding protein.

Functions of Vitamin D

• Increases and regulates calcium and phosphate absorption from the small intestine.
• Stimulates the biosynthesis of calcium-binding protein, which transports calcium to the blood.
• Essential for bone mineralization (calcification).
• Inhibits calcium and phosphate excretion in the urine.

Vitamin D Deficiency

• Occurs due to inadequate sun exposure, inadequate dietary intake, or defective renal tubular phosphate reabsorption.
• Leads to rickets (in children) and osteomalacia (in adults), causing softening and deformities of bones.

Vitamins

• Organic substances essential for health and growth, but not synthesized in the body in sufficient quantities.
• Needed in small amounts and must be obtained through diet.
• Do not enter body structure or tissue structure.
• Act as catalysts and are not oxidized to produce energy.

Types of Vitamins

• Fat-soluble vitamins (A, D, E, and K) and water-soluble vitamins (B and C).
• Fat-soluble vitamins are nonpolar, require fat and bile salts for absorption, and are stored in the body.
• Water-soluble vitamins are polar, easily absorbed, and not stored in the body (except for vitamin B12).

Vitamin A

• Present in animal products (fish, liver, oils, meats, eggs, cheese, and butter) and plant products (carotenes in carrots, tomatoes, and sweet potatoes).
• Converted to vitamin A in the intestine and liver.
• 95% is stored in the liver.
• Has two forms: vitamin A1 (in animal products, high activity) and vitamin A2 (in fish and liver, 40% activity of A1).
• Provitamin is carotene.

Metabolism of Vitamin A

• Absorbed from the intestine in the presence of fat and bile salts.
• Transported to the liver through chylomicrons and lipoproteins.
• Converted to retinol, which can be oxidized to retinal or retinoic acid, both having vitamin activity.

Functions of Vitamin A

• Required for visual function, particularly for rhodopsin in the retina.
• Essential for health and growth, reproduction, mucus secretion, and maintaining differentiated epithelia.
• Essential for bone formation and calcification.

Deficiency of Vitamin A

• Leads to night blindness, keratomalacia, xerophthalmia, and premalignant conditions.
• Causes keratinization of the skin, blocking sebaceous glands, and squamous metaplasia.

Toxicity of Vitamin A

• Excess intake can cause drowsiness, headache, abdominal pain, vomiting, and skin desquamation in children.
• Chronic toxicity leads to hair loss and skin fissuring.