Fat Soluble Vitamins Flashcards
What are the 2 Vitamin A classifications?
Retinoids = the biologically active form of vitamin A
Exist in 3 forms that can be interconverted:
- Retinol (an alcohol) or retinyl ester
- Retinal (an aldehyde)
- Retinoic acid
Tail of vitamin A molecule can be in cis or trans form: this determines the function of the vitamin
Carotenoids (~700 types found in food) = provitamin A (can be converted to retinol)
Have “non-vitamin A” activity too
Are the yellow-orange pigments in fruit and vegetables
The ones that can be converted to vitamin A are:
1. Alpha-carotene
2. Beta-carotene
3. Beta-cryptoxanthin
Describe the digestion of vitamin A
Digestion:
Rentinyl esters and retinol need to be freed from protein in food and retinyl esters must be split into retinol and FA
Provitamin A carotenoids usually bound to a protein molecule in food too
Split via enzymes activity in GIT to be absorbed
Formation of micelles with bile, and other FA
Describe the absorption of vitamin A
Absorption:
70-90% of retinol intake absorbed if at least 10g of fat in meal
5-60% of carotenoids intake absorbed (broad range => dependent on fat intake: form of the food – uncooked = less absorption)
Describe the activation of vitamin A
Activation:
Carotenoids cleaved to form retinal or retinoic acid. Retinal converted to retinol and retinoic acid
Takes place in the enterocytes and in the liver
Describe the transport of vitamin A
Transport:
In enterocytes:
- Retinyl esters are reformed, packaged in chylomicrons, transported via the lymphatic system
- Retinoic acid and carotenoids can enter bloodstream directly, transported to the liver or tissues bound to albumin
From the liver:
- Retinoids bound to retinol binding protein + transthyretin for transport to cells
- Carotenoids that are not transformed are carried out by lipoproteins
Describe the storage and excretion of vitamin A
Stores:
>90% of retinoid stores are found in stellate cells of the liver, as retinyl esters (enough to last several months), small amounts in adipose tissue (gives it a yellow tint), kidneys, bone marrow testicles, eyes
Some stores of carotenoids in adipose tissue, cell membrane, in the eye (lutein, zeaxanthin, beta carotene)
Excretion:
Small amount in urine; carotenoid excreted as part of bile
Describe the 2 primary functions of retinoids
Cell differentiation: growth and development of foetus
Vitamin A carried by RBP and transtheritin in blood
Enters the target cell and binds to a cellular retinoid binding protein
Is released, enters the nucleus and binds to nuclear retiod receptors (RAR and RXR)
Complex binds to DNA, activating gene transcription
mRNA codes for protein which provides a cellular response
Night vision adaptation:
Rhodopsin (11-cis-retinal + opsin) absorbs light, converting to a11-trans-retinal. A11-trans-retinal separates from opsin. Opsin triggers responses that send a signal to the brain. Enzymes reconvert trans form to cis form and combine with opsin
Describe the requirements for vitamin A
RDI for vitamin A as retinol equivalent
UL for retinol only
No UL for beta-carotene
What are international units used?
To allow for consistency in dosage information, especially when there is risk of toxicity in large doses, particularly relevant for fat soluble vitamins
What are the signs and symptoms of vitamin A deficiency?
Night blindness (cant adapt to change in light levels)
Xerophthalmia
Keratomalacia (involves the cornea, results in blindness)
Follicular hyperkeratosis
What are the signs and symptoms of vitamin A toxicity?
Acute = GIT upset/nausea; muscular incoordination, headaches
Chronic = liver damage, haemorrhages, dry skin and mucous membrane, hair loss, bone loss, coma
Teratogenic = if the pregnant woman consumes large amount => foetal malformation, spontaneous abortion risks
What are some food sources of vitamin A?
Retinoids (preformed vitamin A)
- Beef and cod liver - Eggs - Dairy butter, milk/cheese - Low fat dairy products are commonly fortified with vitamin A (voluntarily)
Carotenoids (provitamin A) - eat with fat to improve absorption
- Sweet potato - Pumpkin - Carrot - Mango - Spinach
Supplemental form: usually retinyl palmitate and acetate +/- beta carotene
What is the biologically active form of vitamin D?
Calcitriol (1,25-dihydroxy-cholecalciferol)
Which fat soluble vitamin can be synthesised endogenously? Describe this production
Ultraviolet B (UVB) converts 7-dehydrocholesterol to cholecalciferol (vit.D3)
Cholecalciferol travels to liver to undergo hydroxylation to produce 25-hydroxy-cholecalciferol (calcidiol)
25-hydroxy-cholecalciferol is taken to the kidneys for the second hydroxylation, to produce the bioactive form of vitamin D: 1,25-dihydroxy-cholecalciferol (or calcitriol)
Which vitamin has hormone-like activity
Calcitriol
VItamin D from food a pro-hormone = precursor of calcitriol