中农考研生物化学-第十一章 维生素 -辅酶.pptVIP

How vitamins differ from the energy-yielding nutrients Structure Vitamins are individual units, not long chains Function Do not yield energy but assist in releasing energy from CHO, fats and proteins Food content Small amts needed daily (micrograms and milligrams) Vitamin Precursors Compounds that can be converted into active vitamins Also known as provitamins Must measure both active vitamin as well as provitamin when assessing intake Other Characteristics Organic Nature Can be destroyed so food needs to be handled with care to avoid loss in storage or cooking Solubility Hydrophilic: water soluble B vitamins and vitamin C Hydrophobic: fat soluble Vitamins A,D,E and K Fat Soluble Vitamins Active Forms of Vitamin A Three active forms of vitamin A called retinoids: retinol, retinal, retinoic acid Retinol made from beta-carotene (vitamin A precursor) , stored in liver, released into bloodstream when needed Retinal and Retinoic Acid are made from retinol as needed by the body Vitamin A as a Steroid Hormone Serious Vitamin A Deficiency Can progress to thickening of the cornea keratomalacia Leading to permanent blindness xerophthalmia Deficiency can be reversed if appropriate vitamin A is given early enough Cataracts Vitamin A Food Sources of Vitamin A Preformed vitamin A found only in animal products milk liver butter Vitamin D Roles of Vitamin D Promotes bone mineralization Maintains blood calcium levels by: absorbing Ca from the GI tract drawing it from the skeleton retaining it through the kidneys Many other functions throughout the body Vitamin D Deficiency Bone fail to calcify; bone abnormalities are the most obvious sign Rickets vitamin D deficiency disease of children characterized by abnormal bone growth bowed legs or knock knees, outward bowed chest and knobs on the ribs How much sun exposure in needed ? Depends on darkness of skin and degree of exposure: exposure a few times/wk should be sufficient 10 to 15 minutes for fair skin ~ up to 1.5 hou