nutrtion

• Vitamin

  Essential organic compounds needed in small amounts
• Characteristics of vitamins as a group

  • Not a source of energy
  • Needed for energy metabolism, growth, development, maintenance
• Types of vitamins

  • Fat-soluble vitamins (A, D, E, K)
  • Water-soluble vitamins (B-vitamins and Vitamin C)
• Food sources for fat-soluble vitamins

  • Retinoid sources: liver, fish, fish oils, fortified milk, eggs
  • Carotenoid sources: dark-green and yellow-orange vegetables and fruits
• Fat-soluble vitamins

  • Dissolve in organic solvents
• Water-soluble vitamins

  • Dissolve in water
• Vitamins cannot be synthesized by the body or are synthesized in inadequate amounts
• Vitamins are supplied by both plants and animals
• If a vitamin deficiency develops, health declines
• Some vitamins may be useful as pharmacological agents
• Niacin is used as blood-cholesterol-lowering treatment in megadoses
• Absorption of fat-soluble vitamins

  Absorbed with dietary fats, adequate absorption depends on bile and pancreatic lipase, 40 to 90% absorbed under optimal conditions
• Absorption of water-soluble vitamins

  Not dependent on dietary fats, 90 to 100% absorption rate
• Vitamins must be efficiently absorbed from the small intestine
• Malabsorption of vitamins

  • If absorption is decreased, more must be consumed
  • Fat malabsorption may cause poor absorption of fat-soluble vitamins
  • Alcohol abuse may lead to malabsorption of some B-vitamins
  • Some diseases may require vitamin supplements to prevent deficiencies
• Digestion and absorption of vitamins

  1. Fat-soluble vitamins are packaged and delivered with dietary fats in chylomicrons and lipoproteins

  2. Water-soluble vitamins are delivered to the bloodstream and then distributed throughout the body
• Storage of fat-soluble vitamins

  Stored in liver and adipose tissue, with the exception of vitamin K (large intestine)
• Storage of water-soluble vitamins

  Have limited storage, should be consumed daily, the exceptions are vitamin B-12 and B-6
• Toxic effects of any vitamin are possible
• Vitamin toxicity


  • Most likely from vitamin A and D taken in amounts 5 to 10 times greater than DRI guidelines
  • Supplements usually supply less than twice the Daily Value of vitamins and minerals
• Retinoids


  Biologically active form of vitamin A, exist in 3 forms that can be interconverted: retinol, retinal, retinoic acid
• Carotenoids

  Yellow-orange pigment in fruits and vegetables, some are provitamins that can be converted into vitamin A
• Retinoid sources

  • Liver, fish, fish oils, fortified milk, eggs
• Carotenoid sources


  • Dark-green and yellow-orange vegetables and fruits
• Beta-carotene has the greatest amount of provitamin A activity
• 70% of vitamin A in North American diets comes from animal sources
• Retinal Activity Equivalents (RAE)


  Accounts for different biological activity of retinol and carotenoids
• One RAE = 1 mg retinol, 12 mg beta-carotene, 24 μg of other 2 provitamin A carotenoids
• RDA for vitamin A

  900 μg RAE for men, 700 μg RAE for women
• Average intakes of adult men and women in North America meet DRI guidelines for vitamin A
• Absorption, transport, storage, and excretion of preformed vitamin A

  1. Retinyl ester needs to be separated into retinol and fatty acid by enzymes to be absorbed

  2. 90% of retinol is absorbed in small intestine by protein carrier

  3. Retinyl ester is reformed and packaged into chylomicron
• Absorption, transport, storage, and excretion of dietary carotenoids

  1. Attached to proteins that must be removed by enzymes prior to absorption

  2. 5 to 60% is absorbed by passive diffusion

  3. Carotenoids are cleaved to form retinal or retinoic acid in intestinal cells

  4. Retinal is converted to retinol

  5. Retinol attaches to fatty acid to form retinyl ester and is packaged into a chylomicron
• 90% of body's vitamin A is found in the liver, which stores enough retinyl ester to last several months
• Functions of vitamin A (retinoids)

  • Growth, development, cell differentiation, vision, immune function
• Role of vitamin A in growth and development

  • Involved in the development of eyes, limbs, cardiovascular system, nervous system, epithelial cells and mucous-forming cells
  • Lack of vitamin A during early stages of pregnancy can result in birth defects and fetal mortality
• Role of vitamin A in cell differentiation

  Retinoids bind to retinoid receptors in the cell nucleus, regulating gene expression and directing cell differentiation
• Role of vitamin A (as retinoic acid) in the target cell

  Retinoic acid binds to retinoid receptors in the cell nucleus, regulating gene expression and directing cell differentiation
• Role of vitamin A in vision

  • Retinal is needed in the retina to turn visual light into nerve signals to the brain
  • Rods are responsible for translating objects into black-and-white images and detecting motion
  • Cones are responsible for translating objects into color images
• Bleaching and regeneration of rhodopsin

  1. In rods, 11-cis-retinal binds to opsin to form rhodopsin

  2. Absorption of light changes 11-cis-retinal to all-trans-retinal

  3. Opsin separates from all-trans-retinal via a bleaching process, triggering a change in ion permeability of photoreceptors

  4. To keep visual process functioning, all-trans-retinal must be converted back to 11-cis-retinal
• Role of vitamin A in immune function

  Vitamin A helps maintain epithelium, the barrier that protects the body against entry of pathogens