Lecture 6: Fat Soluble Vitamins (Lipid) Soluble Vitamins (A, D, E, K)

Page 1: Lecture Overview: Fat Soluble Vitamins

  • Vitamins covered: A, D, E, K, which are essential nutrients crucial for maintaining overall health.

  • Focus: Unique properties compared to water-soluble vitamins, including their absorption mechanisms, essential functions, deficiency symptoms, and potential for toxicity. Fat-soluble vitamins are stored in the body's fatty tissue and liver, leading to their longer retention compared to water-soluble vitamins.

Page 2: Vitamin A: Retinoids and Carotenoids

  • Retinoids (Animal sources): Active forms include retinol, retinal, and retinoic acid. Retinol is a vital vitamin necessary for various biochemical processes, retinal is crucial for vision as it combines with opsin to form rhodopsin in the retina, and retinoic acid functions as a signaling molecule regulating gene expression.

  • Carotenoids (Plant sources): Pro-vitamin A precursors, primarily beta-carotene, which can be converted into Vitamin A in the intestine depending on the body's needs. Other carotenoids also possess antioxidant properties that contribute to cellular health.

  • Storage: Approximately 90\text{\text%} of Vitamin A is stored in the liver, where it can be mobilized as needed to support bodily functions.

Page 3: Absorption and Metabolism of Vitamin A

  • Mechanism: Absorbed with dietary fat through micelles formed in the intestine and transported to the liver in chylomicrons. The efficiency of absorption can be influenced by the presence of dietary fat and the health of the digestive system.

  • Conversion: The conversion process involves Retinol ⇌\rightleftharpoons Retinal →\rightarrow Retinoic acid, which illustrates the dynamic nature of Vitamin A metabolism in response to biological needs.

  • Efficiency: Retinoids have a higher absorption rate of 70-90\text{\text%}; however, carotenoid absorption is less efficient and may decrease with excessive intake of carotenoid-rich foods.

  • Storage: Liver-bound retinol can be released into the bloodstream as needed by specific transport proteins, ensuring a steady supply for various physiological functions.

Page 4: Functions of Retinoids

  • Vision: Retinal is required for the visual cycle, particularly for night and color vision, playing a key role in the conversion of light into visual signals in the retina.

  • Differentiation: Retinoic acid regulates gene expression, crucial for cell differentiation, particularly in developmental processes (e.g., bone, tooth enamel formation) and maintaining epithelial tissue health.

  • Immunity: Retinoids help maintain mucous membrane integrity, crucial for immune defense, and play a role in the differentiation of immune cells.

  • Deficiency Symptoms: Symptoms of Vitamin A deficiency include Xerophthalmia, characterized by keratinization of eye cells and replacement of mucous-secreting cells, leading to severe vision problems.

  • Reproduction: Retinol is essential for steroid hormone synthesis, influencing reproductive health and development at various life stages.

Page 5: Carotenoids and Dietary Requirements

  • Carotenoid Function: Besides being precursors to Vitamin A, carotenoids function as lipid-soluble antioxidants, helping to neutralize free radicals and protect against oxidative damage in cellular membranes.

  • RDAs (Adults):

    • Males: 900 \, \text{\textmu g} (2,970 I.U.2,970 \, \text{I.U.})

    • Females: 700 \, \text{\textmu g} (2,333 I.U.2,333 \, \text{I.U.})

  • Tolerable Upper Intake Level (UL): 3,000 \, \text{\textmu g}, above which there is an increased risk of adverse effects.

  • RAE (Retinol Activity Equivalencies): 12 \, \text{\textmu g} of food beta carotene is equivalent to 1 \, \text{\textmu g} of retinol, demonstrating the conversion efficiency important for dietary planning.

Page 6-7: Vitamin A Deficiency

  • Skeleton: Cessation of bone growth and potential malformations owing to the essential role of Vitamin A in bone development and maintenance.

  • Eyes: Common deficiency manifestations include night blindness (an early sign), Xerosis (dry eyes), Bitot’s spots (small white patches on the conjunctiva), and irreversible Xerophthalmia which can lead to blindness.

  • Skin: Affected by hyperkeratosis, characterized by excessive keratin production leading to follicle plugging and rough skin.

  • Immunity: Deficiency results in lowered resistance to infections; changes in the urogenital and digestive lining may heighten infection susceptibility.

  • Note: Zinc is necessary for the mobilization of Vitamin A from the liver, influencing its bioavailability and function.

Page 8: Toxicity and Food Sources

  • Hypervitaminosis A: Can manifest acutely (symptoms like nausea and blurred vision) or chronically (liver damage, osteoporosis, birth defects) when intake exceeds 1010 times the RDA.

  • Hypercarotenemia: Caused by excessive carotenoid intake; results in a harmless orange tint to skin without health implications.

  • Sources:

    • Retinoids: Eggs, liver, full-fat milk.

    • Carotenoids: Various plant foods including carrots, sweet potatoes, and leafy greens, emphasizing a balanced diet for adequate intake.

Page 9: Vitamin D: Synthesis and Metabolism

  • Nature: Functions more as a hormone than a vitamin; synthesized from cholesterol upon exposure to ultraviolet light in the skin, highlighting the importance of sun exposure for adequate levels.

  • Forms: Vitamin D exists primarily as D3D_3 (Cholecalciferol) from animal sources or self-synthesis and D2D_2 (Ergocalciferol) from plant sources.

  • Activation Pathway: The synthesis pathway is complex, starting from skin/diet, proceeding to the liver for conversion to 25-hydroxy vitamin D25\text{-hydroxy vitamin D}, and ultimately to the kidneys where it is converted into the active form (Calcitriol/1,25-dihydroxy cholecalciferol1,25\text{-dihydroxy cholecalciferol}).

  • Regulation: The synthesis process is regulated by parathyroid hormone (PTH) which amplifies synthesis in response to low blood calcium levels, emphasizing the hormone-like characteristics of Vitamin D.

Page 10: Vitamin D: Functions and RDAs

  • Blood Calcium Regulation: Critical for maintaining calcium homeostasis by increasing intestinal absorption of calcium, decreasing renal excretion, and working synergistically with PTH to stimulate bone resorption and utilization.

  • Neuromuscular Function: Required for proper nerve and muscle function, illustrating the vitamin's role beyond bone health.

  • Adequate Intake (AI): 10 μg/day10 \, \text{μg/day} (400 I.U.400 \, \text{I.U.}), highlighting the importance of sufficient intake for health.

Page 11-12: Vitamin D: Deficiency, Toxicity, and Sources

  • Deficiency: Manifestations include Rickets in children (leading to bone malformations) and Osteomalacia in adults, characterized by soft and brittle bones, underlining the critical role of Vitamin D in skeletal health.

  • Toxicity: Hypervitaminosis D can lead to dangerously high blood calcium levels, causing heart damage and kidney stones; however, toxicity is not associated with sunlight exposure.

  • Sources: Fortified milk, fatty fish such as salmon and mackerel, cod liver oil, and egg yolks, emphasizing the need for dietary sources alongside sunlight.

Page 13: Vitamin E: Forms and Absorption

  • Forms: Comprises 44 tocopherols and 44 tocotrienols, with the d-isomer (natural) forms being biologically active. d-alpha tocopherol is the primary focus due to its potent antioxidant properties.

  • Metabolism: Vitamin E is absorbed with dietary fat, stored in the liver and adipose tissues, and can be regenerated by Vitamin C, highlighting the interplay between these antioxidants.

Page 14: Vitamin E: Functions and RDAs

  • Antioxidant Function: Protects cell membranes (lipids), lungs, red blood cells (RBCs), low-density lipoprotein (LDL) cholesterol, and nervous tissue from oxidative damage, thus safeguarding cellular integrity and health.

  • RDA: 15 mg/day15 \, \text{mg/day} (75 I.U.75 \, \text{I.U.}), reinforcing the need for adequate dietary intake.

Page 15: Vitamin E: Deficiency and Sources

  • Deficiency: Can lead to hemolytic anemia (breakage of RBCs), nerve degeneration, and manifest as leg cramps, indicating the vitamin's pivotal role in maintaining cellular functions.

  • Toxicity: High doses may interfere with blood clotting and are contraindicated with anticoagulant medications, demonstrating the importance of dosage and medical context.

  • Sources: Include nuts, seeds, wheat germ, and various plant oils, highlighting the importance of a diverse diet.

Page 16: Vitamin K: Forms and Functions

  • Forms: Divided into K1 (Phylloquinone from plants), K2 (Menaquinone produced by bacteria), and K3 (Menadione - synthetic and potentially toxic).

  • Functions: Acts as a coenzyme required for the synthesis of blood clotting proteins (such as prothrombin) and is essential for bone mineralization processes, indicating its critical role in both hemostasis and skeletal health.

Page 17: Vitamin K: Requirements and Deficiency

  • AI: Males 120 μg/day120 \, \text{μg/day}; Females 90 μg/day90 \, \text{μg/day}, reflecting gender-based dietary recommendations.

  • Deficiency: Can lead to hemorrhaging, particularly in newborns with sterile intestines unable to produce K2, and poses a risk for developing osteoporosis later in life due to impaired bone mineralization.

  • Interactions: Vitamin K can interfere with anti-clotting medications like Coumadin, signifying the need for careful monitoring in patients on such medications.

  • Sources: Richly found in green leafy vegetables and produced in the gut by bacteria, reinforcing the importance of dietary variety for maintaining adequate levels.