Vitamins
•Organic (carbon-based) compounds that are required for many reactions in the body
Water-Soluble Vitamins
•B vitamins and vitamin C
•Absorbed from lumen of gut on their own
•Do not travel in lipoproteins
•With exception of B12, aren’t stored to a high degree in body
•Toxicity potential is low for most
Nutrient Bioavailability
•The rate at and extent to which a nutrient is absorbed and used by the body
Bioavailability is dependent on
•Digestion and transit time of food through gut
•Previous nutrient intake and status
•Other foods and nutrients consumed at the same time
•Method of food preparation
•Source of nutrient (food vs. supplement)
Water-Soluble Vitamins
•The 8B’s and a C
Most B vitamins are coenzymes
•Necessary for enzyme action
•Many are also involved in energy metabolism
–NAD, FAD, coenzyme A
•Flour and cereal products are now enriched/fortified with thiamin, riboflavin, niacin,
iron and folic acid
Thiamin
•The first water-soluble vitamin discovered
•Part of thiamin pyrophosphate
–Two reactions in citric acid cycle
–Transketolase in pentose phosphate pathway (produces NADPH and pentose sugars for
DNA and RNA synthesis)
Thiamin deficiency--beriberi
•First seen in Japanese sailors and chickens fed white rice
•Muscle weakness
•Edema (fluid accumulation)
•Nerve degeneration
Thiamin Deficiency in US
•Mostly seen in alcoholics
–Lousy diet
–Alcohol reduces thiamin absorption from food
•Wernicke’s encephalopathy--dementia due to thiamin deficiency
Thiamin Sources
•Pork
•Enriched products
•Nuts and seeds
•Soy milk
•Destroyed by heat and alkali (baking soda)
DRI’s for Thiamin
•RDA 1.2 mg for men, 1.1 mg for women
•No UL
Riboflavin
•Part of two coenzymes
–FAD (flavin adenine dinucleotide)
–FMN (flavin mononucleotide)
•Participate in citric acid cycle, fatty acid beta-oxidation
•Light sensitive
DRI for Riboflavin
•RDA--1.1 mg for women, 1.3 mg for men
•Riboflavin deficiency very rare, except in alcoholics
•No UL
Food Sources of Riboflavin
•Liver
•Dairy products
–Buy your milk in plastic or paper cartons
•Enriched cereals
Niacin
•Component of two coenzymes
–NAD (nicotinamide adenine dinucleotide)
–NADP (nicotinamide adenine dinucleotide phosphate)
•Can be made in limited quantities from the amino acid tryptophan
Niacin deficiency--Pellagra
•“A Plague of Corn”--D. Roe
•The 4 D’s of Pellagra
–Dermatitis
–Dementia
–Diarrhea
–Death
Why does corn cause niacin deficiency?
•Niacin in corn is bound to protein and is unavailable
•Remedy--treat corn with lime
–Masa to Mexicans
•Pellagra is still a threat in other parts of the world
–Africa
–Southeast Asia
Food sources of niacin
•Liver
•Other meats
•Peanut butter
•Fortified cereals
DRI for Niacin
•RDA--men 16 mg, women 14 mg
•Listed as Niacin Equivalents
–60 mg tryptophan = 1 mg niacin
–Protein is about 1% tryptophan
–UL = 35 mg
BUT…
•High doses of niacin (250-2000 mg/day) is used to reduce cholesterol in some people
–Reduces LDL cholesterol
–Increases HDL cholesterol
•Not recommended for people with diabetes
•Most common side effect is flushing of face
•Can cause liver disease
Vitamin B6
•Pyridoxal, pyridoxine, pyridoxamine
•And associated phosphates
•Active coenzyme form is pyridoxal phosphate
Pyridoxal phosphate-dependent reactions
•Protein--transamination, deamination
•Rate-limiting step in hemoglobin synthesis
•Homocysteine conversion to cysteine
•Carbohydrate-glycogenolysis and gluconeogenesis
•Neurotransmitter synthesis--serotonin, dopamine, norepinephrine
Vitamin B6 Deficiency
•Rare on its own
•Microcytic hypochromic anemia
•Dermatitis
•Neurologic symptoms
DRI’s for B6
•RDA for men and women aged 19-50: 1.3 mg
•Men > 51 years: 1.7 mg
•Women > 51 years: 1.5 mg
•UL: 100 mg/day
Food sources of Vitamin B6
•Liver
•Oatmeal
•Bananas
•Fortified cereals
•Chickpeas
B6 Toxicity
•Neurologic damage at doses > 1 g/day
•No evidence that high doses of B6 are useful in PMS or carpal tunnel syndrome
Folic acid and vitamin B12
•Folic acid is pteroylglutamic acid
•Folate can have 3-11 molecules of glutamate attached
•Vitamin B12 is largest vitamin
•Only known role for the mineral cobalt in biological systems
Folate absorption from diet
•Enzyme conjugase removes excess glutamate groups off folate
•Rest is similar to other water-soluble vitamins
B12 absorption from diet
•Salivary glands produce R protein, which binds to B12 in stomach
•Parietal cells (HCl-producing cells) in stomach produce intrinsic factor
•In small intestine, pancreatic proteases degrade R protein--B12 binds to intrinsic factor
B12 absorption, continued
•B12-intrinsic factor complex binds to receptor on cells in terminal ileum
•B12 circulates in blood bound to transcobalamin
B12 absorption affected by…
•Reduced intrinsic factor production with age and use of over-the-counter drugs like
Zantac
•Stomach surgery
•Surgery to the terminal ileum
B12 functions
•Methylmalonyl coA to succinyl coA
•Homocysteine to methionine
–Also requires folate
Folate functions
•As tetrahydrofolate, delivers 1-carbon units to various molecules, including
homocysteine
•Also required for DNA synthesis
Folate or B12 Deficiency
•Megaloblastic anemia
–Large red blood cells
–Short-lived
–Low hemoglobin level
B12 Deficiency alone
•Pernicious anemia--when elderly person has less acid (less IF), it results in lower B12
absorption
•Peripheral nerve problems
•Hypersegmentation of nuclei of neutrophils (most common white blood cell in blood)
Folate deficiency
•May be associated with increased incidence of spina bifida and neural tube defects
•Reason why flour is now fortified with folate
•ALL WOMEN OF CHILDBEARING AGE SHOULD TAKE 400 µg/day folate
Homocysteine
•A new risk factor for heart disease
•Folate, B12, and B6 are coenzymes in conversion of homocysteine to less harmful
amino acids
•Supplementation has some success in reducing plasma levels
Drugs that interfere with folate
•Alcohol
•Methotrexate (arthritis, cancer)
•Phenytoin (anti-seizure)
•Phenobarbital
Drugs that interfere with B12
•Inhibitors of stomach acid secretion
•Nitrous oxide (laughing gas)
DRI’s
•RDA for B12: 2.4 µg/day
•RDA for Folate: 400 µg/day
•UL’s
–Folate 1000 µg/day
–B12 none
Do NOT treat a B12 deficiency with folic acid
•Anemia will resolve
•BUT nerve problems will progress and irreversible damage can occur
Sources
•Folate: liver, spinach, asparagus, fortified cereals
•B12: animal sources only
Vitamin C
•Still controversial after all these years
Some animals can make their own vitamin C. Those that can’t…
•Humans
•Guinea pigs
•Chimps and other primates
Functions of Vitamin C
•Antioxidant--can protect vitamin E from being oxidized
•Synthesis of collagen (connective tissue protein in blood vessels, bone, tendons)
•Increases iron absorption from non-meat sources
•Synthesis of neurotransmitters
•Immune function in general
Vitamin C Deficiency--Scurvy
•Bleeding gums
•Bruise easily
•Poor wound healing
•Bone pain
•Irritability, depression
DRI’s for Vitamin C
•RDA: men 90 mg, women 75 mg
•Smokers need 35 mg/day on top of RDA
•UL= 2000 mg/day
Excess Vitamin C
•May boost iron absorption too much in people with iron storage disease
•“joggers’ trots”--diarrhea caused by unabsorbed vitamin C in the colon
•High doses of vitamin C DO NOT cure colds