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Lecture

 

Chapter 13 The Water-Soluble Vitamins
(In your syllabus, this is listed for Wk 13.)

Vitamins are essential organic nutrients that are needed in small amounts in the diet to promote and regulate body processes needed for growth, reproduction, and tissue maintenance.  Vitamins do not provide energy.

Vitamins are naturally present in foods, added to foods by fortification and enrichment, and contained in supplements.  The amount of a vitamin that is available to the body is regulated by vitamin absorption, transport, activation, storage, and excretion.

Vitamin deficiencies remain a major health problem worldwide.  In industrialized countries, marginal dietary deficiencies and toxicities from supplements are a growing concern.

Recommended intakes for vitamins are established by evaluating the results from many different kinds of research.  The DRIs express recommended intakes as RDAs or AIs. See the inside cover of your textbook.

Thiamin in its active coenzyme form is required for the generation of energy from carbohydrate, fat, and protein. The best food sources are lean pork, legumes, and whole or enriched grain products.  The thiamin-deficiency disease, beriberi, causes nervous system abnormalities.  Deficiencies are common in alcoholics.  No toxicity has been identified.

Riboflavin coenzymes are needed for the generation of energy.  Riboflavin deficiency is rarely seen alone because food sources of riboflavin are also sources of other B vitamins and because riboflavin is needed to activate several other vitamins.  Milk, meat, and enriched grain products are the best food sources.  No toxicity has been identified.

Niacin coenzymes are important in the breakdown of carbohydrate, fat, and protein and in the synthesis of fatty acids and sterols.  A deficiency results in pellagra, which is characterized by dermatitis, diarrhea, and dementia, and death if untreated.  Beef, chicken, turkey, fish, and enriched grain products are the best food sources.  The amino acid tryptophan can be converted into niacin, so dietary tryptophan can meet some of the niacin requirement.  Supplements of the nicotinic acid form of niacin can lower elevated blood cholesterol but frequently cause toxicity symptoms such as flushing, tingling sensations, nausea, and a red skin rash.

Biotin is needed for the synthesis of glucose and fatty acids and the metabolism of certain amino acids.  It is difficult to set a dietary requirement for biotin because some of our biotin need is met by bacterial synthesis in the GI tract.  Therefore, an AI rather than an RDA has been established.  Liver and egg yolks are good sources.  Toxicity has not been reported.

Pantothenic acid is part of coenzyme A (CoA), which is required for the production of energy from carbohydrate, fat and protein and the synthesis of cholesterol and fat.  It is abundant in the food supply, and deficiency is rare,  There is no RDA, but an AI has been established.

Pyridoxal phosphate, the coenzyme form of vitamin B-6, is needed for the activity of more than 100 enzymes involved in the metabolism of carbohydrate, fat, and protein.  Vitamin B-6 is particularly important for amino acid metabolism.  Adequate amounts are necessary to keep levels of homocysteine low and reduce the risk of heart disease.  Food sources include chicken, fish, liver, eggs, and whole grain. Large doses of vitamin B-6 can cause nervous system abnormalities.

Folate is necessary for the synthesis of DNA, so it is especially important for rapidly dividing cells.  Folate deficiency results in macrocytic anemia.  Low levels of folate before and during early pregnancy are associated with an increased incidence of neural tube defects.  It is recommended that women of childbearing age consume 400 micrograms of folic acid from fortified foods and supplements in addition to the folate found in a varied diet.  Food sources include liver, legumes, oranges, leafy green vegetables, and fortified grains.  Low folate can increase blood homocysteine levels; a high intake of folate can mask the early symptoms of vitamin B-12 deficiency.

Vitamin B-12 is needed for the metabolism of folate and fatty acids and to maintain the insulating layer of myelin surrounding nerves.  Deficiency results in anemia and nerve damage.  Vitamin B-12 is found almost exclusively in animal products.  Its absorption from food requires adequate levels of stomach acid, intrinsic factor, and pancreatic secretions.  Marginal deficiency is a concern to vegans, who consume no animal products, and in older individuals in whom stomach acid secretion is reduced.  Vitamin B-12 supplements or fortified foods are recommended for these groups.

Choline is a substance necessary for metabolism and is not currently classified as a vitamin.  It may be required in the diet at certain stages of life, so an AI has been established.

Vitamin C is a water-soluble antioxidant that is needed for the synthesis and maintenance of connective tissue.  A deficiency of vitamin C results in the disease scurvy.  Large doses can result in GI symptoms.

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Study Guide

1. How do vitamins differ from energy-yielding nutrients with regard to structure,
   function, and amounts found in foods? What two factors affect the availability of
   vitamins from foods? Define bioavailability. Upon what does bioavailability depend? Why
   should care be taken when storing or cooking foods that contain water-soluble
   vitamins? How does solubility affect vitamin absorption, transport, and storage? How 
   and why did the Committee on Dietary Reference Intakes address high doses of
   vitamins?

2. How do the B vitamins function in general in the body? How do B vitamins act as
    coenzymes?

3.  What disease is associated with thiamin deficiency, and how was it discovered? What
    symptoms are associated with thiamin deficiency disease? What foods are exceptional
    sources of thiamin?

4. What are the two coenzyme forms of riboflavin? How do riboflavin's coenzymes
    function in the body? What systems in the body are affected by riboflavin deficiency?
    What food group provides the greatest contribution of riboflavin to the diet? How do
    light and heat affect riboflavin?

5. What role does niacin play in metabolism?
    Name the amino acid that can be used in the body to make niacin. Define niacin
    equivalents. What disease is associated with niacin deficiency? What type of diet will
    produce niacin deficiency? What are the "four Ds" associated with niacin deficiency?
    What is niacin flush? What disease or medical condition is sometimes treated with
    niacin? What foods are the best sources of preformed niacin? What types of foods are
    best for niacin equivalents in the diet?

6. How does biotin generally function in energy production? How can a biotin deficiency
    be induced in animals and humans, and what are the effects? What are the sources of
    biotin for humans?

7. What is the coenzyme form of panothenic acid? Generally speaking, how does the
    coenzyme function? What are particularly good food sources of panothenic acid?

8.  What is the coenzyme form and function of vitamin B-6? How does B-6 affect amino
     acid metabolism? Where is B-6 stored in the body? How does alcohol intake affect B-6
     status? How does the medication INH affect B-6 status? What are the symptoms of
     B-6 deficiency? How did researchers discover that B-6 had toxic levels? What are the
     toxic effects of B-6? What are the best food sources for B-6?

9.  What is the coenzyme form of folate? What is the main function of folate's coenzyme?
     With what other B vitamin does folate interact? How does the body handle excess
     folate? What is the effect of alcohol abuse on folate status? Define dietary folate
     equivalents. Why are folate recommendations increased for pregnant women? Explain
     the importance of folate in reducing the risks of neural tube defects. What is the
     recommendation for folate supplementation prior to and during pregnancy? What
     connection exists between folate and heart disease? What are the two first
     symptoms of folate deficiency? Describe how anti-cancer drugs, aspirin, oral
     contraceptives, and smoking interact with folate. What foods have an abundant
     amount of folate?

10. Describe the relationship between B-12 and folate. What is intrinsic factor and how
     does it affect B-12 absorption? What is the most likely reason for developing a B-12
     deficiency? Define atrophic gastritis. What is the condition caused by B-12
     deficiency?  What other B vitamin deficiency results from a B-12 deficiency? In what
     foods is B-12 found almost exclusively? How does B-12 intake possibly impact vegan
     vegetarians?

12. What is the disease caused by vitamin C deficiency? Define anti-scorbutic factor
     and antioxidant. How does vitamin C act as an antioxidant? How does vitamin C
     function with regard to iron? How does vitamin C function with regard to collagen
     formation? In what compounds does vitamin C serve as a cofactor in their synthesis?
     What conditions or stressors are known to increase the need for vitamin C? What is
     the RDA for vitamin C for men and women? Cite the recommendations for vitamin C for
     each of the following: scurvy, smokers, and people who have undergone surgery or
     extensive burns. Why is there an increased RDA for people who smoke or are
     recovering from surgery or burns? What are the earliest signs and symptoms of
     vitamin C deficiency? What are the symptoms associated with vitamin C toxicity? Cite
     several instances in which large amounts of vitamin C can produce toxic symptoms.
     What is the upper level of vitamin C intake for adults? What are the foods that
     provide generous amounts of vitamin C?

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