Download Perspectives in Nutrition, 8th Edition

Perspectives in Nutrition, 8th Edition
Chapter 13 Outline: The Water-Soluble Vitamins
After studying this chapter, you will be able to:
1.
2.
3.
4.
5.
Identify the water-soluble vitamins.
List important food sources for each water-soluble vitamin.
Describe how each water-soluble vitamin is absorbed, transported, stored, and excreted.
List the major functions of and deficiency symptoms for each water-soluble vitamin.
Describe the toxicity symptoms from the excess consumption of certain water-soluble
vitamins.
6. Distinguish between vitamins and non-vitamins, such as carnitine and taurine.
13.1
Water-Soluble Vitamin Overview
A.
General
1.
Essential organic compounds needed in small amounts for normal function,
growth, and maintenance of body tissues
2.
General functions of vitamins (see Figure 13-1)
a.
Energy metabolism
i.
Thiamin
ii.
Riboflavin
iii.
Niacin
iv.
Pantothenic Acid
v.
Biotin
b.
Blood Formation and Clotting
i.
Vitamin B-6
ii.
Vitamin B-12
iii.
Folate
iv.
Vitamin K
c.
Protein and Amino Acid Metabolism
i.
Vitamin B-6
ii.
Folate
iii.
Vitamin B-12
iv.
Vitamin C
v.
Choline
vi.
Riboflavin (indirect)
d.
Antioxidant Defenses
i.
Vitamin E
ii.
Vitamin C (likely)
iii.
Carotenoids
iv.
Riboflavin (indirect)
e.
Gene Expression
B.
i.
Vitamin A
ii.
Vitamin D
f.
Bone Health
i.
Vitamin A
ii.
Vitamin D
iii.
Vitamin K
iv.
Vitamin C
3.
Water-soluble vitamins are not well-stored and pose low risk for toxicity, except
for:
a.
Vitamin B-6
b.
Vitamin B-12
4.
Water-soluble vitamins are more easily destroyed than fat-soluble vitamins
a.
Heat
b.
Light
c.
Air
d.
Alkaline
e.
Metals
5.
Tips for preserving the vitamin content of fruits and vegetables (see Table 13-1)
a.
Keep cool until eaten
b.
Refrigerate fruits and vegetables (except bananas, onions, potatoes, and
tomatoes) in moisture-proof, airtight containers or in the vegetable
drawer
c.
Trim, peel, and cut fruits and vegetables minimally
d.
Microwave, steam, or stir-fry vegetables
e.
Minimize cooking time
f.
Avoid adding fats to vegetables during cooking if you plan to discard the
liquid
g.
Do not add baking soda to vegetables to enhance green color
h.
Store canned and frozen fruits and vegetables carefully
Coenzymes: A Common Role of B-Vitamins
1.
Coenzyme: small, organic molecules, type of cofactor
2.
Cofactor: substance that combines with an inactive form of an enzyme to activate
it (e.g., metals, vitamins)
3.
Apoenzyme: inactive form of enzyme
4.
Holoenzyme: active form of enzyme (i.e., apoenzyme + cofactor)
5.
B-vitamin needs may increase with increased physical activity due to their role in
energy metabolism, but increased food intake usually supplies requirements
6.
Examples of coenzyme forms of B-vitamins (see Table 13-2 and Figure 13-3)
a.
Thiamin pyrophosphate (thiamin)
b.
Flavin adenine dinucleotide (riboflavin)
c.
Flavin mononucleotide (riboflavin)
d.
Nicotinamide adenine dinucleotide (niacin)
e.
Nicotinamide adenine dinucleotide phosphate (niacin)
C.
13.2
f.
Coenzyme A (pantothenic acid)
g.
N-carboxylbiotinyl lysine (biotin)
h.
Pyridoxyl phosphate (vitamin B-6)
i.
Tetrahydrofolic acid (folic acid)
j.
Methylcobalamin (vitamin B-12)
7.
In foods, B-vitamins exist as free vitamins and coenzymes, sometimes bound to
proteins
a.
Digestion breaks down coenzymes and protein-bound vitamins to free
vitamins
b.
Free vitamins are absorbed
c.
There is no benefit to consuming supplemental vitamins in coenzyme
form over free form
Grains: One Important Source of B-Vitamins
1.
Milling of grains (refinement) removes germ, grain, and husk, which contain
many vitamins and minerals (see Figure 13-5)
2.
Bread and cereal enrichment
a.
Thiamin
b.
Riboflavin
c.
Niacin
d.
Folic acid
e.
Iron
3.
Nutrients still low in enriched, refined grain products
a.
Vitamin B-6
b.
Potassium
c.
Magnesium
d.
Zinc
e.
Fiber
f.
Phytochemicals
Thiamin
A.
General
1.
Devastating effects of beriberi were seen in Asian countries where milled white
(polished) rice is the staple food
2.
Also known as vitamin B-1
3.
Easily destroyed by cooking (heat) and alkaline
B.
Thiamin in Foods
1.
Pork
2.
Sunflower seeds
3.
Legumes
4.
Whole and enriched grains and cereals
5.
Green peas
6.
Asparagus
7.
Organ meats
C.
D.
E.
13.3
8.
Peanuts
9.
Mushrooms
Thiamin Needs and Upper Level
1.
RDA
a.
Adult men: 1.2 mg
b.
Adult women: 1.1 mg
2.
DV: 1.5 mg
3.
Average U.S. intake
a.
Men: 2 mg/d
b.
Women: 1.2 mg/d
4.
No UL has been set
Functions of Thiamin
1.
TPP is required for metabolism of carbohydrates and branched-chain amino acids
a.
Decarboxylation reactions: remove CO2
b.
Transketolase: enzyme in pentose phosphate pathway that forms DNA
and RNA
Thiamin Deficiency
1.
Develops after 14 days on thiamin-deficient diet
2.
Beriberi
a.
“I can’t, I can’t” in Sinhalese - leads to weakness due to impaired
nervous, muscle, gastrointestinal, and cardiovascular function
b.
Symptoms, many due to disruption of glucose metabolism
c.
Wet beriberi: affects cardiovascular system, leading to congestive heart
failure
d.
Dry beriberi: neurological symptoms only
3.
Wernicke-Korsakoff Syndrome
a.
Cerebral beriberi
b.
Mainly due to alcoholism
c.
Symptoms
i.
Vision changes (e.g., double vision, crossed eyes, rapid eye
movements)
ii.
Ataxia: inability to coordinate voluntary muscle movement
iii.
Impaired mental function
Riboflavin
A.
General
1.
Vitamin B-2
2.
“Yellow enzyme” due to yellow-green fluorescence
3.
Susceptible to destruction by light; should be stored in plastic containers
B.
Riboflavin in Foods
1.
Milk products
2.
Enriched white bread, rolls, and crackers
3.
Eggs
C.
D.
E.
4.
Meat
5.
Liver
6.
Mushrooms
7.
Green leafy vegetables
8.
Broccoli
9.
Asparagus
Riboflavin Needs and Upper Level
1.
RDA
a.
Adult men: 1.3 mg
b.
Adult women: 1.1 mg
2.
DV: 1.7 mg
3.
Average intake
a.
Men: 2.1 mg
b.
Women: 1.5 mg
4.
No UL has been set
Functions of Riboflavin
1.
Coenzyme forms (flavins) participate in many oxidation/reduction reactions
a.
Flavin mononucleotide
b.
Flavin adenine dinucleotide (FAD is oxidized form, FADH2 is reduced
form)
2.
Energy Metabolism
3.
Other B-Vitamin Functions
a.
Tryptophan  niacin requires FAD
b.
Formation of PLP requires FMN
c.
Riboflavin participates in folate metabolism
4.
Antioxidant Function
Riboflavin Deficiency
1.
Ariboflavinosis
2.
Difficult to separate from deficiencies of other B-vitamins
3.
Develops after 2 months of riboflavin-deficient diet
4.
High-risk populations
a.
Adolescent girls
b.
Elderly
c.
Cancer
d.
CVD
e.
Diabetes
f.
Alcoholism
g.
Malabsorption disorder
h.
Poor diet
i.
Long-term phenobarbital use increases breakdown of riboflavin and
other nutrients in the liver
j.
Avoidance of milk or milk products
13.4
Niacin
A.
General
1.
Pellagra is the only dietary deficiency disease to ever reach epidemic proportions
in the U.S. (early 1900s in southeastern states)
2.
Vitamin B-3
3.
Two forms
a.
Nicotinic acid (niacin)
b.
Nicotinamide (niacinamide)
4.
Two coenzyme forms
a.
Nicotinamide adenine dinucleotide (NAD+)
b.
Nicotinamide adenine dinucleotide phosphate (NADP+)
5.
Very heat stable
6.
Little cooking losses
B.
Niacin in Foods
1.
Obtained from food as preformed niacin
a.
Poultry
b.
Meat
c.
Fish
d.
Enriched bread products
e.
Coffee and tea
f.
Mushrooms
g.
Wheat bran
h.
Peanuts
2.
Synthesized by the body from tryptophan
3.
Niacin equivalents take into consideration preformed niacin and contribution of
tryptophan
C.
Niacin Needs and Upper Level
1.
RDA
a.
Adult men: 16 mg NE
b.
Adult women: 14 mg NE
2.
DV: 20 mg
3.
UL: 35 mg (based on flushing; applies only to niacin supplements and fortified
foods)
D.
Functions of Riboflavin
1.
Coenzyme forms participate in 200+ oxidation/reduction reactions, especially
those that produce ATP
2.
NAD+ is an electron and hydrogen acceptor; required for catabolism of
carbohydrates, proteins, and fats
3.
NAD+ regenerated by pyruvate  lactate under anaerobic conditions
4.
In electron transport chain, NADH + H+ donates electrons and H+
5.
Alcohol metabolism requires niacin coenzymes
6.
Synthetic pathways require NADPH + H+ (reduced form)
a.
Fatty acid synthesis
E.
F.
13.5
b.
Liver and mammary glands have high [NADPH + H+]
Niacin Deficiency
1.
Pellagra (“mal de la rosa,” “red sickness”), once thought to be an infectious
disease, was found to be a dietary deficiency
a.
Prevalent in areas with corn-based diets (low niacin bioavailability, low
tryptophan content)
b.
Nicotinic acid was found to cure black tongue (similar to pellagra) in
dogs
c.
Pellagra has been eradicated by enrichment of grains and protein-rich
diets
2.
High-risk populations
a.
Severe malabsorption
b.
Chronic alcoholism
c.
Hartnup’s disease (conversion of tryptophan  niacin is blocked)
d.
Africa (famine or refugee camps)
Pharmacological Use of Niacin
1.
1 - 2 g/d nicotinic acid (60 x RDA; controlled time-release preparation) may be
prescribed to lower LDL cholesterol and increase HDL
2.
In combination with diet, exercise, and other medications, nicotinic acid lowers
risk of heart attack
3.
Side effects
a.
Flushing
b.
GI tract upset
c.
Liver damage
Pantothenic Acid
A.
General
1.
“From every side” - supplied by a wide variety of foods
2.
Part of coenzyme A (combination of pantothenic acid, derivative of ADP, and
part of cysteine)
3.
Sulfur atom (from cysteine) is functional end of CoA
B.
Pantothenic Acid in Foods
1.
Meat
2.
Milk
3.
Many vegetables
4.
Mushrooms
5.
Peanuts
6.
Egg yolks
7.
Yeast
8.
Broccoli
9.
Soy milk
10.
Unprocessed foods are better sources than processed foods (milling, refining,
freezing, heating, and canning reduce pantothenic acid content)
C.
D.
E.
13.6
Pantothenic Acid Needs and Upper Level
1.
AI: 5 mg
2.
Average intake exceeds AI
3.
DV: 10 mg
4.
No UL has been set
Functions of Pantothenic Acid
1.
CoA essential for formation of acetyl-CoA from metabolism of carbohydrate,
protein, fat, and alcohol; enters citric acid cycle
2.
CoA is building block for fatty acids, cholesterol, bile acids, and steroid
hormones
3.
Pantothenic acid forms acyl carrier protein, shuttles fatty acids through metabolic
pathways to increase chain length
4.
CoA donates fatty acids to proteins
Pantothenic Acid Deficiency
1.
Rare; only experimentally induced
Biotin
A.
General
1.
“Egg white injury:” development of severe rash, fur loss, and paralysis in rats fed
large amounts of egg whites
B.
Sources of Biotin: Food and Microbial Synthesis
1.
Two forms in food
a.
Free vitamin
b.
Biocytin: protein-bound form
2.
Food sources
a.
Whole grains
b.
Eggs
c.
Nuts
d.
Legumes
3.
Nutrient databases are incomplete with respect to biotin content of many foods
4.
Bacteria in the large intestine synthesize biotin; bioavailabilty from this sources
is unknown because most efficient absorption occurs in small intestine
C.
Biotin Needs and Upper Level
1.
AI: 30 µg
2.
Average intake meets AI
3.
DV: 300 µg
4.
No UL has been set
D.
Functions of Biotin
1.
Coenzyme for carboxylase enzymes that add CO2 to various compounds;
required for metabolism of carbohydrates, proteins, and fats
a.
Pyruvate  oxaloacetate
b.
Catabolism of threonine, leucine, methionine, and isoleucine for energy
c.
Acetyl-CoA  malonyl CoA in fatty acid synthesis
E.
Biotin Deficiency
1.
2.
13.7
Rare
High-risk populations
a.
1/112,000 infants has genetic defect that results in low biotinidase
b.
Ingestion of large amounts of raw egg whites (avidin binds biotin;
denatured by cooking)
Vitamin B-6
A.
General
1.
Vitamin B-6 coenzyme required for metabolism of amino acids
2.
Susceptible to destruction by heat and other processing
B.
Vitamin B-6 in Foods
1.
Animal sources are most bioavailable
a.
Meat
b.
Fish
c.
Poultry
2.
Plant sources
a.
Whole grains (lost during refinement, not replaced by enrichment)
b.
Carrots
c.
Spinach
d.
Potatoes
e.
Bananas
f.
Avocados
g.
Fortified breakfast cereals
C.
Vitamin B-6 Needs and Upper Level
1.
RDA
a.
Adult men: 1.7 mg
b.
Adult women: 1.3 mg
2.
DV: 2 mg
3.
Average intakes exceed RDA
4.
UL: 100 mg, based on development of irreversible nerve damage
D.
Functions of Vitamin B-6
1.
PLP is coenzyme in 100+ reactions, mostly involving nitrogen-containing
compounds
2.
Synthesis of Compounds
a.
In RBCs, PLP catalyzes a step in heme synthesis
b.
Neurotransmitter synthesis
i.
Tryptophan  serotonin
ii.
Tyrosine  dopamine or norepinephrine
iii.
Histadine  histamine
iv.
Glutamic acid  gamma-aminobutyric acid (GABA)
c.
Vitamin formation
i.
Tryptophan  niacin
E.
Vitamin B-6 Deficiency
1.
Rare
2.
F.
13.8
Symptoms
a.
Seborrheic dermatitis
b.
Microcytic hypochromic anemia: small, pale RBCs that lack sufficient
hemoglobin and have reduced O2-carrying capacity
c.
Convulsions
d.
Depression
e.
Confusion
3.
High-risk populations
a.
Poor diets
b.
Alcoholics: acetaldehyde decreases formation of PLP and may reduce its
biological activity
c.
Use of L-DOPA for Parkinson’s disease
d.
Use of isoniazid for tuberculosis
e.
Use of theophylline for asthma
Pharmacological Use of Vitamin B-6
1.
Carpal tunnel syndrome (50 - 300 mg/d): may repair damaged nerves or reduce
perception of pain; use should be supervised by physician
2.
Premenstrual syndrome: weak evidence; not recommended
3.
Nausea during pregnancy (30 - 75 mg/d): safe and likely to be effective; use
should be discussed with physician
Folate
A.
General
1.
“Folium:” leaf
2.
Folate: various, naturally-occurring forms in foods
3.
Folic acid: synthetic form found in supplements and fortified foods
4.
50 - 90% destroyed by food processing and preparation
a.
Heat
b.
Oxidation
c.
UV light
d.
Vitamin C is protective
B.
Folate in Foods
1.
Liver
2.
Legumes
3.
Green leafy vegetables
4.
Avocados
5.
Oranges, orange juice, grapefruit juice
6.
Fortified grains and cereals
7.
Milk (low content, but high consumption)
8.
Potatoes (low content, but high consumption)
C.
Folate Needs and Dietary Folate Equivalents
1.
RDA: 400 µg
2.
DV: 400 µg
3.
D.
E.
F.
13.9
Dietary folate equivalents (DFE) reflect differences in absorption from natural
and synthetic sources
a.
1 DFE = 1 µg food folate = 0.6 µg folic acid taken with food = 0.5 µg
folic acid taken on an empty stomach
b.
DFE = µg food folate + (µg folic acid x 1.7)
Upper Level for Folate
1.
UL = 1000 µg; applies only to synthetic folic acid
2.
Excess folate may mask vitamin B-12 deficiency
3.
FDA limits folic acid in non-prescription vitamin supplements
Functions of Folate
1.
Exchange of single carbon groups in metabolic pathways
2.
Central coenzyme form: tetrahydrofolic acid (THFA)
3.
DNA Synthesis
4.
Amino Acid Metabolism and Other Functions
a.
Formation of neurotransmitters; supplementation may enhance action of
antidepressant medications
b.
Maintenance of normal blood pressure
c.
Reduced risk of colon cancer
Folate Deficiency
1.
High-risk populations
a.
Low intake
b.
Inadequate absorption (e.g., alcoholism)
c.
Increased need (e.g., pregnancy increases needs to 600 µg)
d.
Compromised utilization (e.g., vitamin B-12 deficiency)
e.
Chemotherapy medications
f.
Anti-convulsant medications
g.
Excessive excretion (e.g., chronic diarrhea)
2.
Consequences
a.
Megaloblastic (macrocytic) anemia: RBCs cannot divide normally to
become mature red blood cells; large, immature RBCs
b.
Large, immature cells in the GI tract; decreased absorptive capacity leads
to diarrhea
c.
Compromised immune function due to disruption of WBC synthesis
d.
Neural tube defects
Vitamin B-12
A.
General
1.
Animal products are the only reliable sources
2.
Contains cobalt as part of complex, multi-ring structure
3.
Two active coenzymes
a.
Methylcobalamin
b.
5-deoxyadenosylcobalamin
4.
B.
C.
D.
E.
Research linking vitamin B-12 deficiency to pernicious anemia was worthy of 6
Nobel Prizes
Vitamin B-12 in Foods
1.
Synthesized by microorganisms
a.
Grazing animals acquire vitamin B-12 through soil
b.
Bacteria synthesize vitamin B-12 in ruminant animals
2.
Meat (especially organ meats)
3.
Poultry
4.
Seafood
5.
Eggs
6.
Dairy products
7.
Fortified foods
8.
Algae and fermented soy products may contain vitamin B-12 analogs, which may
not function as vitamin B-12 in the body
Vitamin B-12 Needs and Upper Level
1.
RDA: 2.4 µg
2.
DV: 6 µg
3.
Average intake exceeds RDA 2 - 3 X, providing 2 -3 years’ worth of storage in
liver
4.
No UL has been set
Functions of Vitamin B-12
1.
Homocysteine  methionine requires methionine synthase and methylcobalamin
(vitamin B-12 coenzyme)
a.
Deficiency of vitamin B-12 or folate leads to decreased methionine and
SAM synthesis, increased [homocysteine]
2.
Metabolism of fatty acids with odd numbered carbon chain requires
methylmalonyl mutase and 5-deoxyadenosylcobalamine (vitamin B-12
coenzyme)
Vitamin B-12 Deficiency
1.
Pernicious (“leading to death”) anemia can result from inadequate production of
intrinsic factor
2.
Macrocytic (megaloblastic) Anemia
a.
Identical to anemia from folate deficiency
b.
Due to disruption of normal DNA and RBC synthesis
3.
Neurological Changes
a.
Paresthesia: burning, tingling, prickling, and numbness in the legs
b.
Mental problems: loss of concentration and memory, disorientation,
dementia
c.
Loss of bowel and bladder control
d.
Visual disturbances
e.
GI tract problems: sore tongue, constipation
f.
Neurological complications often precede development of anemia
4.
Elevated Plasma Homocysteine Concentrations
a.
b.
c.
d.
5.
13.10
Risk factor for heart attack and stroke
Associated with cognitive dysfunction
Associated with osteoporotic fractures
Supplementation with folate, B-12, and B-6 can lower blood
homocysteine, but evidence that supplementation lowers risk for diseases
associated with high blood homocysteine is weak
Persons at Risk of Vitamin B-12 Deficiency
a.
Affects ~20% of older Americans
i.
Mostly due to atrophic gastritis
ii.
Not severe enough to produce anemia, but may lead to
neurological problems and elevated homocysteine
iii.
Supplementation with crystalline vitamin B-12 improves vitamin
status
b.
Malabsorption syndromes
i.
Monthly injections to bypass GI tract
ii.
Use of vitamin B-12 nasal gel to bypass GI tract
iii.
Very high oral doses (1 - 2 mg/d), some of which is passively
absorbed
c.
Vegetarians
i.
Liver stores from previous omnivorous diet can delay onset for
years
ii.
Infants born to or breastfed by vegetarian or vegan mothers may
develop anemia, neurological problems, diminished brain
growth, degeneration of spinal cord, poor intellectual
development; supplementation and use of fortified foods is
advised
Choline
A.
General
1.
Choline is not yet considered a B-vitamin because it can be synthesized by the
liver, although deficiency can lead to liver and kidney problems
2.
No coenzyme form
3.
High concentrations in the body
4.
Composed of phospholipids, such as phosphatidylcholine (lecithin)
B.
Choline in Foods
1.
Milk
2.
Liver
3.
Eggs
4.
Peanuts
5.
Lecithin added to foods during processing
6.
Nutrient content data is incomplete for many foods
C.
Choline Needs and Upper Level
1.
AI
D.
E.
13.11
a.
Adult men: 550 mg
b.
Adult women: 425 mg
2.
Dietary needs throughout the lifespan are not known; requirements may be met
by body synthesis alone
3.
Average intake exceeds AI; no need for supplementation
4.
UL: 3.5 g based on fishy body odor, low blood pressure, vomiting, salivation,
sweating, and GI tract effects
Functions of Choline
1.
Component of cell membranes
2.
Component of blood lipoproteins
3.
Precursor for acetylcholine, a neurotransmitter associated with attention,
learning, memory, and muscle control
4.
Liver export of VLDL
5.
Methyl donor for homocysteine  methionine
6.
May protect against cardiovascular disease based on association of high intakes
of choline with low [C-reactive protein], a marker of inflammation
Choline-Deficiency Diseases
1.
Only observed in humans fed choline-deficient TPN
2.
Fatty liver, liver damage
Vitamin C
A.
General
1.
Humans, guinea pigs, fruit bats, and some birds and fish are unique in their
inability to synthesize vitamin C
2.
Ascorbic acid and dehydroascorbic acid (oxidized form)
3.
Electron donor for many body processes
4.
Least stable vitamin, easily lost in processing and cooking (up to 40%)
B.
Vitamin C in Foods
1.
Citrus fruits
2.
Peppers
3.
Green vegetables
4.
Tomatoes
5.
Fortified fruit drinks
6.
Potatoes (due to high consumption)
C.
Vitamin C Needs
1.
RDA
a.
Adult men: 90 mg
b.
Adult women: 75 mg
2.
DV: 60 mg
3.
Average intakes exceed RDA, but 14% of U.S. men and 10% of women have
poor vitamin C status
4.
Smokers require 35 mg/d extra vitamin C
D.
Upper Level for Vitamin C
1.
2.
E.
F.
2 g based on adverse GI effects (e.g., bloating, stomach inflammation, diarrhea)
High doses slightly increase risk of kidney stone formation and excess iron
absorption in high-risk individuals
3.
High doses may give false results for blood in the stool
Functions of Vitamin C
1.
Electron donor in oxidation/reduction reactions
2.
Cofactor role for several metalloenzymes (e.g., keeps iron in reduced ferrous
form, allowing enzymes that require iron as a cofactor to remain active)
3.
Collagen Synthesis
4.
Synthesis of Other Vital Compounds
a.
Aids in biosynthesis by keeping iron or copper in metalloenzymes in
reduced state (Fe2+ or Cu+)
b.
Tyrosine
c.
Thyroxine
d.
Carnitine
e.
Neurotransmitters (e.g., norepinephrine, epinephrine, serotonin)
f.
Conversion of cholesterol to bile acids
g.
Corticosteroids
h.
Aldosterone
5.
Antioxidant Activity
a.
Recycling of vitamin E
b.
Some research indicates that vitamin C increases oxidative stress (e.g., in
diabetes)
c.
High concentrations in eye tissue and neutrophils (type of WBC) suggest
that vitamin C does have antioxidant actions
6.
Iron Absorption
a.
With meals, vitamin C modestly facilitates intestinal absorption of nonheme iron by conversion from ferric to ferrous forms
b.
Counters action of food components that inhibit iron absorption
7.
Immune Function
a.
Highest concentration of vitamin C in WBCs, which may protect against
oxidative damage
b.
Supplementation beyond RDA may is not recommended for improving
immune function
Vitamin C Deficiency
1.
Scurvy is due to impaired synthesis of collagen
a.
Fatigue
b.
Pinpoint hemorrhages
c.
Bleeding gums and joints
d.
Impaired wound healing
e.
Bone pain
f.
Fractures
g.
Diarrhea
G.
13.12
h.
Psychological problems (e.g., depression)
i.
Fatal if untreated
2.
Vitamin C, Cancer, and Heart Disease
a.
Based on roles of vitamin C in antioxidant and immune functions
b.
Best evidence for prevention of cancers of mouth, esophagus, stomach,
and lung, but healthy diet is advocated rather than vitamin C
supplementation
c.
Many but not all studies suggest that good vitamin C status protects
against heart disease, but clinical trials fail to provide evidence of the
connection
3.
High-risk populations
a.
Young and middle-aged adults, likely due to low consumption of
fortified breakfast cereals and/or vitamin supplements
b.
Smokers have increased vitamin C requirements due to increased
oxidative stress
c.
Women using oral contraceptives
d.
Burn and trauma patients, who require extra vitamin C for collagen
synthesis
e.
Poverty
f.
Alcoholics
Vitamin C Intake above the RDA
1.
Little research exists to compare various levels of intake
2.
Above 100 mg/d, excess is generally excreted in urine
3.
200 mg/d is suggested to be highest amount needed to maximize health benefits
of vitamin C; can be obtained through consumption of fruits and vegetables
4.
Vitamin C and the Common Cold
a.
Doses up to 1000 mg/d
b.
Any benefit is modest (e.g., reduce cold duration by 1 day)
c.
Not enough evidence to suggest megadoses of vitamin C for prevention
of common cold
Vitamin-Like Compounds
A.
General
1.
Necessary to maintain normal body functions
2.
Can be synthesized in the body, but biosynthesis is at expense of other nutrients
(e.g., amino acids)
3.
Needs increase during times of rapid tissue growth, but deficiencies do not exist
in otherwise healthy adults
4.
Additional research is required to determine whether vitamin-like compounds are
required in specific life stages or disease states
5.
Currently included in infant formulas
B.
Carnitine
1.
2.
3.
4.
C.
13.13
Needs are met from animal foods (meat and dairy products) and liver
biosynthesis from lysine and methionine
Average consumption: 100 - 300 mg/d
Functions
a.
Transports fatty acids from cytosol to mitochondria for beta-oxidation
b.
Removal of excess organic acids produced by mitochondrial metabolism
c.
Removal of toxic compounds in people with inborn errors of metabolism
d.
Improvements in people with progressive muscle disease and heart
muscle deterioration
e.
Research on use for weight loss or ergogenic aid is limited
May be conditionally essential
a.
Recovery from disease
b.
Malnutrition
c.
Serious trauma
d.
Cirrhosis
e.
Kidney dialysis
f.
Preterm birth
Taurine
1.
Synthesized from methionine and cysteine
2.
Abundant in muscle, platelets, and nerve tissue
3.
Attached to bile acids
4.
Dietary sources: animal products
5.
Average consumption: 40 - 400 mg/d
6.
Functions:
a.
Photoreceptor activity in the eye
b.
Antioxidant activity in WBCs and lungs
c.
Nervous system function
d.
Platelet aggregation
e.
Cardiac contraction
f.
Insulin action
g.
Cell differentiation and growth
7.
No evidence of utility for nervous system conditions or prevention of CVD or
cataracts
8.
Body synthesis is likely to meet needs in healthy adults, but may be conditionally
essential (likely for improved fat absorption)
a.
Children with cystic fibrosis
b.
Preterm infants
Medical Perspective: Neural Tube Defects
A.
Defect in early development of the neural tube leads to:
1.
Spina bifida: spinal cord or spinal fluid bulge through the back
a.
Paralysis
b.
Incontinence
B.
C.
D.
c.
Hydrocephalus
d.
Learning disabilities
2.
Anencephaly: absence of a brain; leads to death shortly after birth
Neural tube forms and closes during the first 21 - 28 days after conception
Ensuring good folate status among women of childbearing potential is important
Folic acid fortification of refined cereals and grains began in 1998
1.
Rates have decreased by 1/3
2.
Rates vary by race and ethnicity
3.
Movement to double rate of fortification is countered by those who are concerned
over the risk of masking vitamin B-12 deficiency
4.
Fortified foods supply 200 µg/d