Vitamins Flashcards
Definition of vitamins
Complex organic substances required in the diet in small amounts, compared to other dietary components like protein, carbohydrate and fat. Absence leads to deficiency
Definition of vitamer
Different structural form of a vitamin
Water soluble vitamins and properties
B
C
Not stored extensively
Needed regularly
Generally not toxic in excess (within reason)
Fat soluble vitamins and properties
A
D
E
K
Stored
Not absorbed/excreted easily
May be toxic in excess (A, D)
Overarching property of all B vitamins
All act as coenzymes in metabolic pathways
Sources of thiamine (B1)
Whole grain, pork, poultry, fish, vegetables, dairy
Requirement high if carbohydrate intake high
Thiaminases (raw fish) and antithiamine (tea, coffee) render it inactive
Biochemical role of B1
Deficiency
Thiamine pyrophosphate, essential cofactor for pyruvate dehydrogenase
Pyruvate =pyruvate dehydrogenase=> Acetyl CoA
Deficiency leads to lactate accumulation in muscles
3 forms of thiamine deficiency presentation
Infantile beri beri
sudden onset, cardiovascular symptoms
Acute cardiac beri beri
Congestive heart failure, enlarged heart, liver engorged neck veins with sudden heart failure
Chronic dry beri beri
Symmetrical ascending peripheral neuropathy
Weakness, numbness ataxic gait, painful extremeties
What is Wernicke’s encephalopathy
Where is it often seen and why
Cerebral beri beri
Confusion, ataxia, polyneuropathy, disorientation
Often seen in alcoholics
Inhibition of AT of B1 from intestines
Inhibition of enzyme that converts thiamin => TPP
Will develop into Korsakoff’s psychosis
Korsakoff’s psychosis
Loss of memory of recent events
Why do alcoholics often have vitamin B deficiencies
Inadequate vitamins, nutrients from alcohol
GI tract malfunctions
Cirrhotic liver affects storage, transport and metabolism of many vitamins
Storage and transport of fat soluble vitamins impaired
Riboflavin (B2) properties
UV sensitive, in milk, associated with proteins
Protein bound, if diet adequate in protein, adequate in B2
Low status in alcoholics, elderly, some adolescents
Not toxic in excess
Function of riboflavin
FAD, FMN in redox
Riboflavin deficiency
Rare, except in alcoholics
Symptoms, mild: cheilosis, angular stomatitis, cateracts
Niacin, vitamer of B3
Found in small amounts in cereals
High protein diets don’t need niacin, formed from tryptophan
Function of niacin B3
As NAD and NADP in redox reactions
Deficiency of B3
Pellagra
Affects maize eating people in EU, US. Fatal if severe
Photosensitive dermatitis
Diarrhoea
Dementia
Function of pyridoxine (B6)
Active form: pyridoxal phosphate
Essential for AA metabolism (transamination, deamination)
Haem synthesis
Pyridoxine (B6) deficiency
Caused by presence of antagonists
Isoniazid in TB combines with PP => inactive, B6 given
Therapeutic uses/toxicity
Used in a variety of settings (Downs, autism), doesn’t really help
Deficiency can lead to seizures, but not all seizures are caused by B6 deficiencies
OD can lead to peripheral neuropathy
Function of cobalamin (B12)
Carrier of methyl groups in mammalian metabolism
Function and structure of folate (B9)
Carrier of 1C units (not necasserily methyl)
Active form: tetrahydrofolate, maintained by dihydrofolate reductase
Source of folate
Mainly in green veg, liver, whole grain
Absorption of B12, B9
B12 only in animal tissue
Binds to IF secreted from gastric cells
Needed for absorption and transport of B12
Metabolic functions of folate
IC transfer reactions in
purine, pyramidine synthesis
AA metabolism (homocysteine => methionine)
Metabolic functions of B12
Conversion of homocysteine => methionine
In branched chain AA metabolism
The relationship between folate and B12
Folate
Dehydrofolate
(DHF reductase)
Tetrahydrofolate
Enters pool of ICTH derivatives, all interconvertible except MeTHF
MeTHF converted back to tertrahydrofolate via B12 (homocysteine => methionine)
From the pool of ICTHF derivatives, purines, pyrimidines and amino acids can be formed
What happens when there is a B12 deficiency in the pool of ICTHR derivatives system
MeTHR form is irreversible, it cannot be converted back into another form
B12 deficiency traps THF in the MeTHF form proving functional THF deficiency
Explains why hematological picture of B12 deficiency is identical to folate deficiency
Results in lack of 5, 10 methylene THF and DNA synthesis
Haemopoietic cells die in marrow immature
Neurological changes in inadequate myelin synthesis in B12 deficiency
Numbness in fingers hands and forearms
Tingling hand and feet
Loss of position sense
Unsteadiness, ataxia, confusion, moodiness, depression
Spinal cord, brain and peripheral nerve lesions
Causes of B12 deficiency
Absent, defective IF
Gastric atrophy
Gastrectomy
Coeliac, Crohns disease
Causes of folate (B9)
Topical sprue
Durgs
Ethanol
Link between folate and neural tube defects
Folic acid reduces incidence of neural tube defects
Function of panthenoic acid (B5)
Sources
Deficiency
Component of CoASH in metabolism and transfer of C chains (FA oxidation)
Ubiquitous, deficiency rare
Sources of biotin (B7)
Widely distributed in peanuts, chocolate, egg yolk
Normally sufficient quantities provided by intestinal bacterial synthesis
Function of biotin (B7)
Prosthetic group for carboxylations
Pyruvate => oxaloacetate
Acetyl CoA => malonyl CoA
Deficiency of biotin (B6)
Rare on normal diet unless eating raw egg whites
Long term antibiotic therapy wipes out intestinal flora
Sources of ascorbic acid (VC)
Citrus, tomatoes, berries
Function of ascorbic acid
Antioxidant nutrient
Hydroxylation of proline and lysine in collagen formation, needed to maintain Fe II needed for proline and lysine hydroxyls activity i reduced active state
Reduction of dietary Fe in stomach for absorption
Possibly in vivo as an antioxidant protecting vitamins A, E, K from oxidation
Vitamin C deficiency
Scurvy
Well fed human has a 6 month store
Signs of scurvy after 3 months on vitamin C free diet
Impaired wound healing, hemorrhages and anaemia, joint pain
Vitamin C status in the Uk
Low status in elderly, alcoholics, adolescents on junk food
Smokers need twice the normal intakes as the turnover is greatly increased by smoking
The risks of megadoses of vitamin C
Oxalate kidney stones in susceptible individuals
Diarrhoea
Systemic conditioning
Causes of deficiency of fat soluble vitamins
Dietary deficiency
Low fat diet
Usually by choice in developed societies
Fat malabsorption
Definition of primary deficiency
Inadequate intake in diet
Definition of secondary deficiency
Secondary causes that lead to the inadequate absorption of vitamins
Sources of vitamin A
As retinol in animal liver and fish liver oils, whole milk, egg yolk
As carotenoids in green, yellow, orange vegetables and fruit
Forms of vitamin A
Retinoic acid, hormone
Retinal, in vision
B carotene, antioxidant
Functions of vitamin A
Control of protein synthesis
In vision
Functions of vitamin A
Control of protein synthesis
Retinoic acid binds specific receptor protein in cytosol
Binds chromatin, affects synthesis of proteins involved in cell growth and differentiation
Acts similarly to steroid hormone
Function of vitamin A
In vision
At low intensity light, II cis retinal participates in conversion of light energy to impulses in optic nerve in rod cells in retina
Transport and storage of vitamin A
From the gut => liver in chylomicrons
From liver => tissues bound to a specific retinal binding protein and pre albumin
Vitamin A deficiency
Rarely seen in developed countries but more common in developing countries
Usually associated with inadequate protein diets
Affects synthesis of retinal binding protein and transport to tissue
Administrations of vitamin A does not often help
Night blindness as a result of vitamin A deficiency
Followed by progressive keratinization of cornea
xerophthalmia) and keratomalacia (irreversible blindness
Toxicity/teratogenicity of vitamin A
Dermatitis, hair loss, mucous membrane defects, hepatic disfunction, thinning and fracture of long bones
Unlikely with normal sources but possible with supplements
Sources of vitamin E
Vegetable oils, wheat germ oil, nuts, green vegetables
Canned and frozen foods severely depleted
Includes family of tocopherols
Natural occurring antioxidant
Function of vitamin E
Prevention oxidation of unsaturated/polyunsaturated FA (PUFA)
PUFA susceptible to free radical attack
Destruction disrupts membrane structures and cell integrity
Some PUFAs are precursors of prostaglandins and so PG metabolism also disturbed
Free radical scavenging in vitamin E
PUFA susceptible to free radical attack
Generates other radicals
Reaction of FA radical w O2 => peroxyl radical
Leads to chain reaction by attacking other PUFA
Reacts with Vitamin E, terminates chain reaction
Vitamin E = radical itself, stable
Can be reduced to original form by other antioxidants
Vitamin E deficiency
Premature, low birth weight infants, Vitamin E does not cross placenta easily
Human milk, poor source of Vitamin E
Found in formula milk
Hemolytic anaemia due to fragility of RBC
Vitamin D
Group of similar compounds
D3 (Cholecalciferol) sources and function in the body
Naturally occurring in animals
In skin by UV on 7-dehydrocholesterol
Functions by binding => intracellular receptors that eventually interact with DNA
Act like steroid hormones
D2 (ergocalciferol) sources and function in the body
Derived from ergosterol in plants, fungi and mould
Milk and dairy, fortified margarine, eggs
Formed from ergosterol by activation of UV
Acts to maintain correct levels of Ca, PO4 3- in blood, proper mineralization of the bone achieved
Vitamin D deficiency in children and adults
Rickets in children
Mineral:matrix ratio is low in bone
Bending of long bones and kyphosis
Tootheruption delayed
Osteomalacia in adults
Muscle weakness, bone pain, decalcification of long bones
Nearly always due to Vitamin D deficiency, rarely Ca deficiency
Vitamin D toxicity
Excessive consumption = toxic
Hypercalcaemia, GI tract disturbances and calcification of soft tissue
Fatal when severe
Sources of Vitamin K
Green leafy veg best, small amounts in milk, meat, eggs and cereals
Considerable amount from bacterial flora jejunum, ileum
Vitamin K deficiency
Defective blood clotting, depends on cascade system of interacting proteins
Deficiency resulting in increased clotting time, rare except in long term antibiotic theraoy
Hemorrhagic disease of new borns
Poor placental transfer, no fut flora
Some affected babies develop intracranial haemorrhages
50% die, 50% have neurological malfunction
