Fat- soluble: vitamin A, D, E, and K

• Fat-soluble vitamins


  • Vitamin A
  • Vitamin D
  • Vitamin E
  • Vitamin K

  
• Fat-soluble vitamins


  Require bile for their digestion and absorption, travel through the lymphatic system within chylomicrons before entering the bloodstream, many require protein carriers for transport, excesses are stored primarily in the liver and adipose tissue
• Vitamin A


  The first fat-soluble vitamin to be recognized, has three active forms in the body: retinol, retinal, and retinoic acid, collectively known as retinoids
• Retinol-binding protein (RBP)


  • Special transport protein that picks up vitamin A from the liver and carries it in the blood
• Roles of Vitamin A in the body


  • Promoting vision
  • Participating in protein synthesis and cell differentiation, thereby maintaining the health of epithelial tissues and skin
  • Supporting reproduction and growth
• Retinol


  Supports reproduction and is the major transport and storage form of the vitamin
• Retinal


  Active in vision and is also an intermediate in the conversion of retinol to retinoic acid

  
• Retinoic acid


  Acts like a hormone, regulating cell differentiation, growth, and embryonic development
• Conversion of Vitamin A Compounds


  Retinol to retinal is reversible, whereas the pathway from retinal to retinoic acid is not
• Photosensitive cells of the retina


  • Rods contain the rhodopsin pigment and respond to faint light, cones contain the iodopsin pigment and function in color vision
• Rhodopsin


  Retina contain pigment molecules
• Opsin


  Each rhodopsin molecule is composed of a protein bonded to a molecule of retinal
• Cell differentiation


  The process by which immature cells develop specific functions different from those of the original that are characteristic of their mature cell type
• Epithelial cells


  Cells on the surface of the skin and mucous membranes
• Epithelial tissue


  The layer of the body that serves as a selective barrier between the body's interior and the environment
• Mucous membranes


  The membranes, composed of mucus-secreting cells, that line the surfaces of body tissues

  
• Vitamin A helps to protect against skin damage from sunlight


  Vitamin A promotes differentiation of epithelial cells and goblet cells, on glands that synthesize and secrete mucus

  
• In men, retinol participates in sperm development


  In women, vitamin A supports normal fetal development during pregnancy
• Children lacking vitamin A


  Fail to grow
• Vitamin A participates in the dismantling of bone


  The cells that break down bone contain sacs of degradative enzymes, with the help of vitamin A, these enzymes eat away at selected sites in the bone, removing the parts that are not needed
• Osteoclasts


  The cells that destroy bone during growth
• Osteoblasts


  The cells that build bone
• Beta-carotene


  Serves primarily as a vitamin A precursor, some beta-carotene may act as an antioxidant
• Key antioxidant nutrients


  • Vitamin C
  • Vitamin E
  • Beta-carotene
  • Selenium
• Vitamin A deficiency


  Leads to infectious diseases and blindness, about 1 to 2 percent of those with deficiency become blind every year, half of them dying within a year of losing their sight

  
• Infectious diseases associated with vitamin A deficiency


  • Measles
  • Pneumonias
  • Severe diarrhea
  • Malaria
  • HIV (Human Immunodeficiency virus, causes IDS)
• Night blindness


  One of the first detectable signs of vitamin A deficiency, caused by the retina not receiving enough retinal to regenerate the visual pigments bleached by light

  
• Xerophthalmia


  Blindness due to vitamin A deficiency, develops in stages from the cornea becoming dry and hard, to softening of the cornea that leads to irreversible blindness

  
• Xerosis


  Abnormal drying of the skin and mucous membranes, a sign of vitamin A deficiency
• Keratomalacia


  Softening of the cornea that leads to irreversible blindness, seen in severe vitamin A deficiency
• Keratin


  A water-insoluble protein, the normal protein of hair and nails
• Vitamin A deficiency


  Affects the epithelial cells, causing them to change shape and secrete the protein keratin, leading to dry, rough, and scaly skin

  
• Beta-carotene overconsumption


  May turn the skin yellow (not harmful), in excess it may act as a prooxidant, promoting cell division and destroying vitamin A
• Excessive vitamin A during pregnancy


  Leads to abnormal cell death in the spinal cord, increasing the risk of birth defects, most damaging in the first trimester
• Massive doses of vitamin A have no beneficial effect on acne, the prescription medicine Accutane is made from vitamin A but is chemically different and effective against the deep lesions of cystic acne
• Vitamin D


  Not an essential nutrient, as ultraviolet rays from the sun can convert a precursor in the skin to pre-vitamin D3, the active form is actually a hormone
• Vitamin D's role in bone health


  It assists in the absorption of calcium and phosphorus, helping to maintain blood concentrations of these minerals
• Vitamin D deficiency


  Creates a calcium deficiency and increases the risks of severe chronic diseases, can prevent adolescents from reaching their peak bone mass, and can trigger seizures due to low blood calcium
• Rickets


  Bones fail to calcify normally, causing growth retardation and skeletal abnormalities, with bowed legs and beaded ribs as signs
• Osteomalacia


  Poor mineralization of bone results in a painful bone disease, causing the bones to be soft, flexible, brittle, and deformed