🧈 Lecture 3: Part 1 - Fat-Soluble Vitamins

Vitamins

Definition
  • Organic compounds with a carbon skeleton

Solubility
  • Can be lipid-soluble or water-soluble
  • Solubility is important

Functions
  • Perform a variety of functions
  • Metabolic regulation
  • Coenzymes
  • Antioxidants

Essential Vitamins
  • May or may not be considered essential
  • Depends on the species

Fat-Soluble Vitamins

Vitamin A
  • Retinol
  • Retinal (retinaldehyde)
  • Retinoic acid (retinol metabolite)
  • Retinyl esters (esterified retinol)
  • Beta-carotene (provitamin A carotenoid)

Vitamin E
  • Tocopherols: alpha, beta, gamma, delta
  • Tocotrienols: alpha, beta, gamma, delta

Vitamin K
  • K₁ (phylloquinone)
  • K₂ (menaquinone)
  • K₃ (menadione)

Vitamin D
  • D₂ (ergocalciferol)
  • D₃ (cholecalciferol)

Why is it important to know which vitamins are fat-soluble?
  • They can be stored in body fat, so they can accumulate to toxic levels

Vitamin A

Functions
  • Vision
  • Growth
  • Tissue maintenance
  • Immune function
  • Embryonic development

Biologically Active Forms
  • Retinol
  • Retinal (retinaldehyde)
  • Retinoic acid

Storage Forms — Inactive
  • Retinyl ester
    • From animal products
    • Stored in the liver
  • Beta-carotene (mostly)
    • From plant products
    • Associated with red/orange pigmentation

Vitamin A Forms
  • Retinyl esters → Retinol → Retinaldehyde → Retinoic acid

Vitamin A – Vision

Photoreceptors
  • Retinal + opsin (light-sensitive protein) = rhodopsin and iodopsin

Rhodopsin
  • Photoreceptor found in rod cells of the retina
  • Responsible for black and white vision
  • Works in dim light → “night vision”

Iodopsin
  • Photoreceptor found in cone cells of the retina
  • Responsible for colour vision
  • Works in bright light

Vitamin A – Toxicity and Deficiency

Toxicity
  • Hypervitaminosis A → too much vitamin A in the body
  • Caused by excess preformed vitamin A, not beta-carotene

Deficiency
  • Fetal abnormalities
  • Impaired growth
  • Night blindness

Vitamin A – Essentiality

Preformed Vitamin A is Essential For
  • Felids
  • Marine birds
  • Most carnivorous mammals
  • These animals lack sufficient amounts of 15,15′-dioxygenase, an enzyme needed to convert beta-carotene into vitamin A

Not Essential For
  • Canids → foxes, coyotes, wolves
  • Most herbivores
  • Most omnivores
  • These animals can readily convert beta-carotene into vitamin A

Vitamin D

Functions
  • Homeostatic regulation of calcium and phosphorus → keeps calcium and phosphorus levels balanced
  • Growth
  • Bone remodeling → changes and maintains bone tissue
  • Eggshell formation

Sources

Vitamin D₂ – Ergocalciferol
  • Very few natural sources
  • Foods are often fortified with D₂

Vitamin D₃ – Cholecalciferol
  • Sunlight → UV-B exposure
  • Phytoplankton
  • Zooplankton
  • Marine fish

Vitamin D – Activation

Inactive Forms
  • D₂ → obtained from the diet
  • D₃ → obtained from sunlight and diet
  • D₂ and D₃ are biologically inactive

Active Form
  • Calcitriol → 1,25-dihydroxycholecalciferol
  • Also called 1,25-dihydroxyvitamin D₃ or 1,25(OH)₂D
  • Calcitriol is the biologically active form

Vitamin D Activation
  1. UV-B radiation reaches the skin
  2. UV-B initiates conversion of 7-dehydrocholesterol → vitamin D₃ (cholecalciferol)
  3. In the liver, vitamin D₃ → 25-hydroxyvitamin D₃ (calcidiol or 25(OH)D₃)
  4. In the kidney, 25-hydroxyvitamin D₃ → 1,25-hydroxyvitamin D₃ (calcitriol or 1,25(OH)₂D₃)

Main Function of Calcitriol
  • Calcitriol regulates calcium and phosphorus (Ca/P)

Vitamin D – Dietary Requirement

Mammals
  • Vitamin D is only required in the diet for mammals with insufficient sun exposure
  • Polar regions
  • Fossorial/subterranean → live or spend most time underground
  • Nocturnal → active at night

Vitamin D₃
  • Essential for amphibians, birds, and reptiles
  • These animals cannot use vitamin D₂

Mammals
  • Can use vitamin D₂ or D₃

Vitamin D – Toxicity and Deficiency

Toxicity
  • Hypervitaminosis D → too much vitamin D, causing hypercalcemia
  • Calcium builds up in soft tissues and organs

Deficiency
  • Rickets → soft bones in young animals
  • Osteomalacia → brittle bones in adults

Vitamin E

Function
  • Antioxidant
  • Inhibits or prevents oxidative damage
  • Scavenges free radicals → unstable or reactive atoms

Forms

Tocopherols
  • Alpha, beta, gamma, delta
  • Organic compounds with various methylated phenols
  • Alpha-tocopherol = most active form of vitamin E

Tocotrienols
  • Alpha, beta, gamma, delta

Vitamin E – Toxicity and Deficiency

Toxicity
  • Hypervitaminosis E → too much vitamin E
  • Can counteract vitamin K actions → affects blood coagulation

Deficiency
  • Muscular dystrophy → muscle weakness and paralysis
  • Nerve damage
  • Infertility in rats and birds

Vitamin K

Function
  • Coagulation → blood clotting

Sources

Vitamin K₁ – Phylloquinone
  • Found in leaves of green plants

Vitamin K₂ – Menaquinone
  • Found in animal tissue and gut bacteria
  • Animal tissue and gut bacteria can convert K₁ → K₂

Vitamin K₃ – Menadione
  • Synthetic precursor
  • Inactive in its synthetic form
  • Plants and bacteria can activate it
  • Animals cannot activate it

Vitamin K – Toxicity and Deficiency

Toxicity
  • K₁ and K₂ are innocuous → not harmful, even in very large amounts
  • K₃ can be toxic
  • K₃ is no longer used as a supplement

Deficiency
  • Increased risk of hemorrhage or excessive bleeding