WATER-SOLUBLE VITAMINS

General Absorption and Transport

  • All water-soluble vitamins (except vitamin B12) are absorbed mainly in the jejunum and ileum.
  • Absorption mechanisms:
    • Passive diffusion
    • Na+ (sodium)-coupled active transport pump
  • Once in enterocytes, vitamins exit to portal circulation via a Na+-coupled ATP-dependent pump.

VITAMIN B1 (THIAMINE)

  • Biologic half-life: 10–20 days
  • Function: Key cofactor in amino acid and carbohydrate metabolism
  • Food sources: Yeast, brown rice, whole-grain cereals, legumes, pork
  • Storage: Limited tissue storage

VITAMIN B2 (RIBOFLAVIN)

  • Function: Flavin and component of flavin-adenine dinucleotide (FAD) → essential in mitochondrial redox reactions
  • Dietary sources: Meats, fish, milk, eggs, yeast, green veggies, enriched foods

VITAMIN B3 (NIACIN)

  • Two forms: Nicotinic acid and Nicotinamide
  • Function: Form NAD and NADP (crucial for redox reactions in carb, fatty acid, and protein metabolism)
  • Food sources: Meat (especially liver), yeasts, cereals, legumes, seeds
  • Can be synthesized from tryptophan in humans

VITAMIN B5 (PANTOTHENIC ACID)

  • Function:
    • Key role in acetylation reactions (tricarboxylic acid cycle)
    • Converted to coenzyme A after ATP-dependent phosphorylation
  • Dietary sources: Egg yolk, liver, broccoli, milk, chicken, beef, potatoes, whole grains

VITAMIN B6 (PYRIDOXINE)

  • Absorption: Passive diffusion in jejunum and ileum
  • Function:
    • Critical in amino acid decarboxylation
    • Gluconeogenesis
  • Food sources: Meats, whole grains, vegetables, nuts

BIOTIN

  • Function:
    • Cofactor in carboxylase enzymes
    • Essential for carbohydrate & lipid metabolism (acts as CO2 carrier)
  • Food sources: Liver, egg yolk, soybean, yeast

VITAMIN C (ASCORBIC ACID)

  • Function:
    • Cofactor/cosubstrate in reducing iron and copper (redox reactions)
    • Provides electrons for molecular oxygen reduction
  • Dietary sources: Citrus fruits, tomatoes, Brussels sprouts, potatoes, cauliflowers, broccoli, strawberries, cabbage, spinach

VITAMIN B12 (COBALAMIN)

  • Absorption:
    • Binds intrinsic factor (IF) in small intestine
    • IF–B12 complex binds cubilin in the ileum → requires energy
    • Enters plasma bound to transcobalamins (TCs) (especially TCII)
  • Function: Required for DNA synthesis, especially in rapidly dividing cells
  • Food sources: Meat, dairy products (only animal/dairy sources)

FAT-SOLUBLE VITAMINS

General Absorption and Transport

  • Released from proteins via gastric and pancreatic proteolysis
  • Bile salts solubilize vitamins in micelles for enterocyte uptake
  • Packaged into chylomicrons → enter intestinal lymphatics & portal circulation → transported to liver

VITAMIN A

  • Family of compounds: Retinols, β-carotenes, carotenoids (“retinoic acids”)
  • Function:
    • Phototransduction (vision)
    • Cellular differentiation, especially in eyes
  • Dietary sources:
    • Retinols: Liver, egg yolk, butter
    • β-carotene: Green leafy vegetables

VITAMIN D (CALCIFEROL)

  • Structure: Lipid-soluble with a four-ringed cholesterol backbone
  • Function:
    • Calcium absorption & bone metabolism
  • Sources:
    • Sunlight (ultraviolet photoisomerization of provitamin D → vitamin D3/cholecalciferol)
    • Diet: Fortified milk, fatty fish, cod liver oil, eggs

VITAMIN E

  • Function:
    • Free radical scavenger
    • Protects polyunsaturated fatty acids from peroxidation
  • Dietary sources: Oils, meat, eggs, leafy vegetables

VITAMIN K

  • Function:
    • Cofactor for coagulation factors (VII, IX, X, prothrombin)
    • Essential for γ-carboxylation of certain proteins in coagulation
  • Dietary vitamin K1 (phylloquinone): Green leafy vegetables
  • Vitamin K2 (menaquinone): Synthesized by gut microflora

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