Fat-Soluble Vitamins

Question 1

Fat-soluble vitamins

Answer 1

. ADEK 
. Stored in liver or fatty depots 
. D endogenously synthesized by skin and K is by intestinal flora 
. A dm D are extensively metabolized
. E and K are not metabolized

Question 2

Vitamin A characteristics

Answer 2

. Retinoids
. Provitamin A belongs to carotenoids that are cleaved to produce 2 molecules of vit. A, most abundant is beta-carotene
. 1 retinol equivalent (RE) = 1ug of retinol (12 ug of other carotenoids)
. 11-cis retinal is component of visual pigment rhodopsin
. Sources: animal tissue and certain plants

Question 3

Transport of vit. A

Answer 3

. Dietary retinol transported as retinyl esters in chylomicrons
. Secreted by liver in assoc. w/ plasma retinol-binding proteins
. Retinol bound to retinol-binding protein
. RBP-retinol complex assoc. w/ protein transthyretin (made in liver) to prevent loss through kidney filtration

Question 4

Vit. A storage

Answer 4

. Stored as retinyl esters in liver and adipose then oxidized to retinoic acid that binds to nuclear receptors to stimulate genes
. Liver has 6-12 month supply

Question 5

Vit. A functions

Answer 5

. Vision
. Gene regulation
. Antioxidant

Question 6

Vit. A and vision

Answer 6

. All-trans retinal delivered to retina
. 11-cis retinal binds to protein Opsin to form rhodopsin
. When light strikes rhodopsin, the 11-cis retinal converts back to trans and dissoc. From opsin
. Generate nerve impulse mediated by cGMP
. All free trans coverts back to cis and cycle repeats

Question 7

Vit. A and gene regulation

Answer 7

. All-trans retinoic acid move to epithelial cells
. Bind to specific receptor proteins that are members of nuclear family of hormone receptors
. Acts w/ specific sequences on genes involved in growth and differentiation to affect expression
. Retinoic acid receptors (RAR) and its related receptors (RXRs) bind 9-cis-retinoic acid and genes are up or down regulated
. Suppresses synthesis of keratin

Question 8

Vit. A and anti-oxidant

Answer 8

. Anti-oxidant in lipid-rich environments
. Optimal level vit. A may reduce cancer
. Excess vit. A is toxic though

Question 9

Vit. A severe hypovitaminosis

Answer 9

. Xerophthalmia: progressive keratinization of cornea of the eye leading to blindness
. 5 million children have this
. Only seen in developing countries

Question 10

Vit. A mild deficiency

Answer 10

. Night blindness
. Follicular hyperkeratosis anemia
. Even mild deficiencies are rare in developed countries

Question 11

Special needs for vit. A

Answer 11

. Premature infants w/ low liver reserves
. Individuals w/ fat malabsorption syndromes
. Hepatobiliary tree disease
. Pancreatic insufficiency
. Intestinal mucosa dysfunction

Question 12

Vit. A sources

Answer 12

. Liver
. Dark green
. Yellow veggies

Question 13

Endogenous vit. D

Answer 13

. Skin contains cholesterol metabolite 7-dehydrocholesterol that is converted to vit. D3 when exposed to UV light (290-315 no)
. Cholecalciferol (vit. D3) converted to active form by 2 hydroxylations (1 in liver via CYP2R1 yielding 25-OH-cholecalciferol, 2 in kidney via CYP27B1 yielding 1,25-diOH-cholecalciferol)

Question 14

Dietary vit. D

Answer 14

. Fortified dietary products

. Ergocalciferol is obtained by irradiating ergostterol from yeast w/ UV light

Question 15

Vit. D function

Answer 15

. Acts alone in intestinal mucosa to induce synthesis of protein for Ca absorption
. Acts synergistically w/ PTH to promote bone resorption and inhibit Ca excretion in kidney by stimulating reabsorption

Question 16

Vit. D hypovitaminosis

Answer 16

. Deficiency in kids: rickets

. Deficiency in adults: osteomalacia

Question 17

Special needs for vit. D

Answer 17

. In fat malabsorption syndromes
. During corticosteroid use and anti-convulsants
. Excess can be deleterious

Question 18

Vit. D sources

Answer 18

. Sunlight (usually only major source unless no sun) 
. Saltwater
. Fish 
. Liver
. Egg yolk 
. Fortified foods

Question 19

Vit. E

Answer 19

. Exists in 8 forms (only alpha-tocopherol meets human requirements)
. Taken up in liver after going through small intestine
. Only resecretes alpha-tocopherol via hepatic alpha-tocopherol transfer protein
. Liver metabolizes and secretes all other vit. E forms
. Function: antioxidant
. Sources: widespread, but veggies oils are best sources

Question 20

Vit. E hypovitaminosis

Answer 20

. Inc. fragility of RBC membranes

Question 21

Vit, K

Answer 21

. Functions: cofactor in gamma-carboxylation of glutamate residues in certain proteins that include precoagulation factors and bone proteins
. Sources: widespread in foods, but intestinal flora produce about 1/2 RDA
. No toxicity for excess

Question 22

Vit. K hypovitaminosis

Answer 22

. Uncommon, but can cause hemorrhage
. Predisposing conditions: parenteral nutrition, malabsorption syndrome, and liver dysfunction
. Newborn infants have low liver stores and breast milk is low in Vit. K so it is common to give infants parenteral prophylactic dose
. Special needs: counteract overdoses of oral coumarin anticoagulants, when broad spectrum antibiotic therapy is used bc intestines become sterilized