Copyright © 2010 Wolters Kluwer Health | Lippincott Williams & Wilkins Vitamins Chapter 5

Copyright © 2010 Wolters Kluwer Health | Lippincott Williams & Wilkins

Vitamins

Chapter 5

Vitamins

Chapter 5


VitaminsVitamins

• In 1913 thiamin was discovered as the first vitamin, the “vital amine” necessary to prevent the deficiency disease beriberi

• Today, 13 vitamins have been identified as important for human nutrition


Understanding Vitamins Understanding Vitamins

• Organic compounds made of carbon, hydrogen, oxygen, and sometimes nitrogen or other elements

• Facilitate biochemical reactions within cells

• Essential to life

• Individual molecules

• Do not provide energy (calories), but they are needed for metabolism of energy

• Needed in microgram or milligram quantities


Understanding Vitamins (cont’d)Understanding Vitamins (cont’d)

• Vitamins are chemically defined

– Extremely complex chemical substances

– Absorption rates of natural and synthetic vitamins sometimes differ because of different chemical forms of the same vitamin

• Vitamins are susceptible to destruction

– Vitamins in food are susceptible to destruction and subsequent loss of function


• Vitamins may exist in more than one form

– Different forms perform different functions in the body

– Provitamins

• Vitamins are essential

– With few exceptions, the body cannot make vitamins

• Some vitamins are coenzymes

– Many enzymes cannot function without a coenzyme, and many coenzymes are vitamins




• Some vitamins are antioxidants

– Free radicals are produced continuously in cells as they burn oxygen during normal metabolism

o Oxidize body cells and DNA in their quest to gain an electron and become stable

– Antioxidants protect body cells from being oxidized (destroyed) by free radicals

– Major antioxidants are vitamin C, vitamin E, and beta-carotene



• Some vitamins are used as food additives

– Some foods have vitamins added to them simply to boost their nutritional content

• Vitamins as drugs

– In megadoses, vitamins function like drugs, not nutrients


QuestionQuestion

• Vitamins do not provide calories. What do they do?

a. Help metabolize energy

b. Function like drugs at normal doses

c. Provide essential nutritional needs at gram or higher doses

d. Prevent biochemical reactions in the body


AnswerAnswer

a. Help metabolize energy

Rationale: Vitamins do not provide energy but are needed for metabolism of energy.


Vitamin Classifications Based on Solubility

Vitamin Classifications Based on Solubility

• Fat-soluble vitamins

– Characteristics

o Sources are the fat and oil portion of foods

o Absorption: fat encased in chylomicrons that enter the lymphatic system before circulating in the bloodstream

o Transportation through the blood occurs by attaching to protein carriers because fat is not soluble in watery blood

o When consumed in excess stored primarily in liver and adipose tissue


Vitamin Classifications Based on Solubility (cont’d)


• Fat-soluble vitamins

– Characteristics (cont’d)

o Can be toxic if consuming high intakes through supplements

o Generally do not have to be consumed daily because the body can retrieve them from storage as needed




• Vitamin A

– Preformed vitamin A is found only in animal sources

– Also includes provitamin A carotenoids

o Natural plant pigments found in deep yellow and orange fruits and vegetables and most dark-green leafy vegetables

o Beta-carotene, lutein, and lycopene are among the most commonly known carotenoids




• Vitamin A (cont’d)

– Best known for its roles in normal vision, reproduction, growth, and immune system functioning

– Body can store up to a year’s supply of vitamin A

– Only preformed vitamin A is toxic in high doses

– Beta carotene is nontoxic

– Hypercarotenemia


Vitamin Classifications Based On Solubility (cont’d)

Vitamin Classifications Based On Solubility (cont’d)

• Vitamin D

– Unique in that the body has the potential to make all of the vitamin D it needs if exposure to sunlight is optimal and liver and kidney function are normal

o Cholecalciferol

– Another distinctive feature of vitamin D is that it acts like a hormone because it is synthesized in one part of the body (skin) and stimulates functional activity elsewhere




• Vitamin D (cont’d)– Primary function of vitamin D is to maintain

normal blood concentrations of calcium and phosphorus

– Vitamin D is important for immune function – It is possible to fulfill the vitamin D requirement

by taking a daily 15-minute walk in the sun under optimal conditions

– Winter, living in northern latitudes, and being of black race and older age are associated with low vitamin D synthesis




• Vitamin D (cont’d)

– Dietary source is considered essential because few people meet optimal conditions

o Vitamin D occurs naturally in only a few foods

– Many experts believe the AI for vitamin D is set too low

– Overt deficiency of vitamin D causes poor calcium absorption

o Rickets and osteomalacia

– Current upper limit for vitamin D is set at 50 micrograms (2,000 IU)




• Vitamin E

– Generic term that describes a group of at least 8 naturally occurring compounds

– Alpha-tocopherol is considered the most biologically active form of vitamin E

– Functions as the primary fat-soluble antioxidant in the body

– Protects polyunsaturated fatty acids (PUFAs) and other lipid molecules, such as LDL cholesterol, from oxidative damage




• Vitamin E (cont’d)– Need increases as the intake of PUFA increases – Megadoses may help protect against atherosclerosis,

some types of cancer, cataracts, age-related macular degeneration, Parkinson’s disease, and Alzheimer’s disease

– Deficiency can occur in very specific instances o Premature infants o May occur secondary to a genetic abnormality or

malabsorption syndromes such as cystic fibrosis and short bowel syndrome




• Vitamin E (cont’d)

– Signs and symptoms of deficiency include peripheral neuropathy, ataxia, and ultimately death

• Vitamin K

– Occurs naturally in 2 forms

o Phylloquinones, found in plants

o Menaquinones, which are synthesized in the intestinal tract by bacteria




• Vitamin K (cont’d)

– Coenzyme essential for the synthesis of prothrombin

– Clinically significant vitamin K deficiency is defined as vitamin-K–responsive hypoprothrombinemia

o Characterized by an increase in prothrombin time

– Deficiency does not occur from inadequate intake but may occur secondary to malabsorption syndromes or the use of certain medications that interfere with vitamin K metabolism or synthesis, such as anticoagulants and antibiotics


QuestionQuestion

• What is one of the functions of vitamin D?

a. Synthesis of prothrombin

b. Protects polyunsaturated fatty acids (PUFAs) and other lipid molecules

c. Aids in reproduction

d. Helps immune function


AnswerAnswer

d. Helps immune function

Rationale: Vitamin D is important for immune function.




• Water-soluble vitamins

– Thiamine

o Coenzyme in the metabolism of carbohydrates and branched-chain amino acids

o Beriberi has been virtually eliminated

o Deficiency is usually seen only in alcoholics

o No adverse effects have been noted from high intakes of thiamin




• Water-soluble vitamins (cont’d)

– Riboflavin

o Vitamin B2 is an integral component of the coenzymes flavin adenine dinucleotide (FAD) and flavin mononucleotide (FMN) that function to release energy from nutrients in all body cells

o Milk and dairy products contribute the most riboflavin to the diet





– Riboflavin (cont’d)

o Biochemical signs of inadequate riboflavin status can appear after only a few days of poor intake

o Elderly and adolescents are at greatest risk for riboflavin deficiency

o Deficiency symptoms include sore throat, cheilosis, stomatitis, glossitis, and dermatitis

o Certain diseases, such as cancer, heart disease, and diabetes, precipitate or exacerbate riboflavin deficiency





– Niacin

o Vitamin B3 exists as nicotinic acid and nicotinamide

o Body can make it from the amino acid tryptophan

o Niacin requirements are stated in niacin equivalents (NEs)

o Part of the coenzymes nicotinamide adenine dinucleotide (NAD) and nicotinamide adenine dinucleotide phosphate (NADP), which are involved in energy transfer reactions in the metabolism of glucose, fat, and alcohol in all body cells





– Niacin (cont’d)

o Pellagra, the disorder caused by severe niacin deficiency, is rare in the United States and usually is seen only in alcoholics

o Niacin deficiency may be treated with niacin, or tryptophan, or both

o Large doses of niacin in the form of nicotinic acid (1 g to 6 g/d) are used therapeutically to lower total cholesterol and LDL-cholesterol and raise high-density lipoprotein (HDL)–cholesterol





– Vitamin B6

o Vitamin B6 and pyridoxine are group names for 6 related compounds that include pyridoxine, pyridoxal, and pyridoxamine

o Helps plays a role in the synthesis, catabolism, and transport of amino acids and in the conversion of tryptophan to niacin




•Water-soluble vitamins (cont’d)

– Vitamin B6 (cont’d)

o Deficiencies of vitamin B6, folic acid, and vitamin B12 lead to an increase in blood homocysteine levels

o Supplements of vitamin B6 have been used for a variety of other conditions, although supportive evidence is lacking

o High intake of vitamin B6 from food does not pose any danger






o Deficiencies of vitamin B6 are uncommon but are usually accompanied by deficiencies of other B vitamins

o Secondary deficiencies are related to alcohol abuse and to other drug therapies such as isoniazid, the antituberculosis drug that acts as a vitamin B6 antagonist





– Folate

o Generic term for this B vitamin that includes both synthetic folic acid found in vitamin supplements and fortified foods and naturally occurring folate in food such as green leafy vegetables, dried peas and beans, seeds, liver, and orange juice





– Folate (cont’d)

o Major function is in the synthesis of DNA

o Folate is recycled through the intestinal tract

o Folate deficiency impairs DNA synthesis and cell division and results in macrocytic anemia and other clinical symptoms





– Folate (cont’d)

o Adequate intake of folate before conception and during the first trimester of pregnancy reduces the risk of neural tube defects

o Upper limit for folic acid is 1,000 micrograms/ day

o Consistently high intakes of folate can mask vitamin B12 deficiency





– Vitamin B12

o Cobalamin

o Vitamin B12 has important role in maintaining the myelin sheath around nerves

o Large doses of folic acid can alleviate the anemia caused by vitamin B12 deficiency (a function of both vitamins), but folic acid cannot halt the progressive neurologic impairments that only vitamin B12 can treat






o Only water-soluble vitamin that does not occur naturally in plants

o Requires an intrinsic factor

o Deficiency symptoms may take 5 to 10 years or longer to develop

o People with pernicious anemia require parenteral injections of vitamin B12






o Recommended that people over 50 obtain most of their requirement from fortified foods or supplements

– Other B vitamins

o Pantothenic acid

o Biotin





– Non-B vitamins

o Inositol

o Choline

o Carnitine





– Vitamin C

o Ascorbic acid

o Found in citrus fruits and juices

o Prevents scurvy

o Water-soluble antioxidant that protects vitamin A, vitamin E, PUFA, and iron from destruction




• Water-soluble vitamins (cont’d)– Vitamin C (cont’d)

o Newest RDA for vitamin C represents an increase from the previous recommendation

o No clear and convincing evidence that large doses of vitamin C prevent colds

– Phytochemicalso Literally plant chemicals o When eaten in the “package” of fruit, vegetables,

whole grains, or nuts, these chemicals work together with nutrients and fiber to promote health


QuestionQuestion

• Which B vitamin can cause permanent neurologic damage and may not demonstrate signs or symptoms for 10 years?

a. B6

b. B12

c. Folate

d. B2


AnswerAnswer

b. B12

Rationale: Large doses of folic acid can alleviate the anemia caused by vitamin B12 deficiency (a function of both vitamins), but folic acid cannot halt the progressive neurological impairments that only vitamin B12 can treat. Vitamin B12 deficiency symptoms may take 5 to 10 years or longer to develop, because the liver can store relatively large amounts of B12 and the body recycles B12 by reabsorbing it.


Vitamins in Health Promotion Vitamins in Health Promotion

• Vitamins A (as carotenoids), C, and E are of concern for American adults based on dietary intake data or evidence of public health problems

• A food group approach rather than actual intake calculations is “good enough” to assess vitamin adequacy

• “Vitamins of concern” are found almost exclusively in plants


Vitamins in Health Promotion (cont’d)Vitamins in Health Promotion (cont’d)

• What about supplements?

– People who choose to take an all-purpose multivitamin should select one that provides 100% of the daily value (DV) for vitamins with an established DV

– The USP stamp ensures the quality of supplements, but not the safety or benefits

– High-cost supplements are not necessarily superior to lower-cost ones


Vitamins in Health Promotion (cont’d)Vitamins in Health Promotion (cont’d)

• Can supplements be used as insurance against poor food choices?

– Supplements are not substitutes for healthy food: “supplement” means “add to,” not “replace”

• A word of caution

– Supplements taken to augment an intake that is already adequate are unnecessary

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Copyright © 2010 Wolters Kluwer Health | Lippincott Williams & Wilkins Vitamins Chapter 5