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