A vitamin is an organic compound and a vital nutrient that an organism requires in limited amounts. An organic chemical compound (or related set of compounds) is called a vitamin when the organism cannot synthesize the compound in sufficient quantities, and it must be obtained through the diet; thus, the term "vitamin" is conditional upon the circumstances and the particular organism.

1. Aamir Ali Khan
M.Phil Biochemistry
Head of pathology department
Northwest institute of health sciences

2. Vitamin A is a generic term for a group of lipid soluble
compounds related to retinol.
 Retinol is often referred to as preformed vitamin A.
It is found only in animal sources, mainly as retinyl esters and in
food supplements.
Beta-carotene and other carotenoids that can be converted to
vitamin A by an enzymatic process in the body are referred to as
provitamin A.
They are found only in plant sources

3. Retinal, the oxidized metabolite of retinol, is required
for the process of vision
Retinoic acid, another vitamin A metabolite, is
considered to be responsible for all non-visual
functions of vitamin A

4. Retinoic acid combines with specific nuclear receptor
proteins which bind to DNA and regulate the expression of
various genes, thereby influencing numerous physiological
processes
Retinoic acid is therefore classified as a hormone

5. Receptor cells in the retina of the eye (rod cells) contain a light-
sensitive pigment called rhodopsin, which is a complex of the
protein opsin and the vitamin A metabolite retinal .
 The light-induced disintegration of the pigment triggers a cascade
of events which generate an electrical signal to the optic nerve.
Rhodopsin can only be regenerated from opsin and vitamin A.
Rod cells with this pigment can detect very small amounts of light,
making them important for night vision.

6. In the pigment epithelium of the
retina, all-trans -retinol is
isomerizes to 11-cis–retinol and
oxidized to 11-cis-retinaldehyde.
This reacts with a lysine residue in
opsin, forming the holoprotein
rhodopsin

7. The absorption of light by rhodopsin causes isomerization of
the retinaldehyde from 11-cis to all trans, and a
conformational change in opsin. This results in the release of
retinaldehyde from the protein and the initiation of a nerve
impulse.
The key to initiation of the visual cycle is the availability of
11-cis retinaldehyde, and hence vitamin A .
In deficiency, both the time taken to adapt to darkness and
the ability to see in poor light are impaired.

8. The many different types of cells in the body perform highly
specialized functions. The process whereby cells and tissues
become “programmed” to carry out their special functions is called
differentiation. Through the regulation of gene expression, retinoic
acid plays a major role in cellular differentiation.
Vitamin A is necessary for normal differentiation of epithelial
cells, the cells of all tissues lining the body, such as skin, mucous
membranes, blood vessel walls and the cornea
In vitamin A deficiency, cells lose their ability to differentiate
properly

9. Retinoic acid plays an important role in reproduction
and embryonic development, particularly in the
development of the spinal cord and vertebrae, limbs,
heart, eyes and ears

10. Vitamin A is required for the normal functioning of the
immune system and therefore helps to protect against
infections in a number of ways. It is essential in maintaining
the integrity and function of the skin and mucosal cells, which
function as a mechanical barrier and defend the body against
infection .
Vitamin A also plays a central role in the development and
differentiation of white blood cells, such as lymphocytes,
killer cells and phagocytes, which play a critical role in the
defense of the body against pathogens

11. Absorption and body stores
Vitamin A is absorbed in the upper part of the small intestine. Pro-
vitamin A carotenoids can be cleaved into retinol via an enzymatic
process. Preformed vitamin A occurs as retinyl esters of fatty acids. They
are hydrolyzed and retinol is absorbed into intestinal mucosal cells (i.e.
enterocytes). After re-esterification it is incorporated into chylomicrons,
excreted into lymphatic channels, delivered to the blood and transported to
the liver.
Vitamin A is stored in the liver as retinyl esters; stores are enough for one
to two years in most adults.

12. Carotenes are not absorb 100 %
1ug of retinol is taken as equivalent to 6ug of beta carotene
For vitamin A absorption, cellular retinol binding protein II
is necessary
Vitamin is then reestirified within the mucosal cells of the
small intestine where absorb with chylomicrons
Bile salts help in carotene and vitamin A absorption

13. Eye changes
“Xerophthalmia” bitot’s spots of cornea
“Keratomalacia” cornea becomes dull and
insensitive.

14. Cilia are lost and respiratory tract infections take place
Vitamin A increases immunity.
Xerosis (dryness) and scaling of hair follicles (Hair loss
with skin shedding)
Growth retardation
Deformed skull bones may result nerve degeneration and
paralysis
Defective formation of the tooth enamel takes place

15. Lipocytes of liver cells store it in the form of esters
Retinoic acid is transported by albumin in plasma

16. Headache, nausea, vomiting, drowsiness, peeling of skin,
anorexia, dry itchy skin, alopecia, cracking of the lips and
painful areas over bones
May be hepatomegaly, splenomegaly, hypothyroidism,
leucopenia, anemia, bleeding tendency because of
hypoprothrombinemia, high ALP

17. Vitamin D is a fat-soluble vitamin. Vitamin D is not found in many
foods.
Foods that naturally contain significant amounts of vitamin D are fatty
fish and eggs.
To increase intake of vitamin D, foods have been fortified.
Milk is the most commonly consumed products that are fortified with
vitamin D. Juices, breakfast cereals, breads, and yogurt are also fortified
with vitamin D.
Cholecalciferol is a form of vitamin D made naturally by the body when
the skin is exposed to sunlight. However, most people do not get enough
sunlight to satisfy their daily recommended intake through sunlight,
especially in the winter.

18. 7-Dehydrocholesterol, undergoes a nonenzymic reaction on
exposure to ultraviolet light, yielding previtamin D.
This undergoes a further reaction over a period of hours to
form the vitamin itself, Cholecalciferol, which is absorbed
into the blood stream.
The recommendations are 200 IU from birth to 50 years of
age, 400 IUs from 51-70 years age, and 600 IUs for 71+ years
of age.
These recommendations meet the minimum requirements,
mostly based on bone health

19. In the liver, Cholecalciferol, which has been synthesized in
the skin or derived from food, is hydroxylated to form the 25-
hydroxy derivative calcidiol.
 This is released into the circulation bound to a vitamin D
binding globulin which is the main storage form of the
vitamin.
In the kidney, calcidiol undergoes either 1-hydroxylation to
yield the active metabolite 1,25-dihy-droxyvitamin D
(calcitriol) or 24-hydroxylation to yield an inactive
metabolite, 24,25-dihydroxyvitamin D(24-hydroxycalcidiol).
Ergocalciferol from fortified foods undergoes similar
hydroxylation's to yield calcitriol.

20. Fish, including salmon and tuna are excellent
sources.
Egg also contains vitamin D, the highest
concentration is in the yolk

21. The main function of vitamin D is in the control of calcium
homeostasis, and in turn vitamin D metabolism is regulated
by factors that respond to plasma concentrations of calcium
and phosphate
Calcitriol acts to reduce its own synthesis by inducing the
24-hydroxylase and repressing the 1-hydroxylase in the
kidney. Its principal function is to maintain the plasma
calcium concentration.

22. Calcitriol achieves this in three ways: it increases intestinal
absorption of calcium, reduces excretion of calcium, and
mobilizes bone mineral.
In addition, calcitriol is involved in insulin secretion,
synthesis and secretion of parathyroid and thyroid hormones,
inhibition of production of interleukin by activated T
lymphocytes and of immunoglobulin by activated B
lymphocytes, differentiation of monocyte precursor cells, and
modulation of cell proliferation. In its actions, it behaves like
a steroid hormone, binding to a nuclear receptor protein.

23. The two main forms of calcium in supplements are
carbonate and citrate.
Calcium carbonate is more commonly available and
inexpensive.
Both forms are absorbed well by the body, but individuals
with low levels of stomach acid may absorb citrate better.
Carbonate is absorbed better when taken with food but
citrate is unaffected by the presence of food.

24. There are two forms available for vitamin D supplements,
D2 (Ergocalciferol) and D3 (Cholecalciferol).
Vitamin D2 is made by the conversion of a sterol found in
plants and yeast.
Unlike calcium these two forms are not equal in absorption.
Vitamin D3 is metabolized differently than D2 and research
shows it is more effective in raising serum concentrations of
the active form of vitamin D and is also able to maintain
higher levels for longer.

25.  Probably the most well understood function that Vitamin D
has is in the absorption of calcium from the small intestine.
When vitamin D is inadequate, calcium from the diet is
poorly absorbed.
Another well characterized and important role of vitamin D
is to maintain appropriate levels of calcium and phosphate in
the blood to ensure proper bone formation, mineralization,
growth, and repair.
Vitamin D has other roles that have been less well
characterized. It is thought to improve muscle strength and
immune function and reduce inflammation.

26. Deficiency (Complication)
People at risk for vitamin D deficiency include breast fed
infants, older adults, people with limited sun exposure, people
with dark skin, and people with fat malabsorption.
People age 50 and older are at increased risk of developing
vitamin D deficiency because as they age the skin looses its
ability to synthesize vitamin D as efficiently and the kidneys
also convert less to its active form
People with darker skin have more pigment melanin which
reduces the skin’s ability to produce vitamin D after sun
exposure.

27. Prolonged inadequate intake of vitamin D will lead to impaired
bone metabolism.
In children, under mineralization of bone causes soft and
deformed bones and can lead to the condition known as rickets.
When children developed rickets, their legs were too weak to hold
their weight, resulting in bowed legs, which often persisted into
adulthood.
Osteomalacia is the adult form of rickets, and leads to impaired
mobility and bone fractures
Osteoporosis, a condition that leads to weak and porous bones.

28. In the vitamin D deficiency disease rickets, the bones of
children are under mineralized as a result of poor absorption
of calcium.
Similar problems occur in adolescents who are deficient
during their growth.
Osteomalacia in adults results from demineralization of bone
in women who have little exposure to sunlight, often after
several pregnancies.
Although vitamin D is essential for prevention and treatment
of Osteomalacia in the elderly, there is little evidence that it is
beneficial in treating osteoporosis.

29. Some infants are sensitive to intakes of vitamin D as low as
50 µg/d, resulting in an elevated plasma concentration of
calcium.
This can lead to contraction of blood vessels, high blood
pressure, and calcinosis, the calcification of soft tissues.
Although excess dietary vitamin D is toxic, excessive
exposure to sunlight does not lead to vitamin D poisoning
because there is a limited capacity to form the precursor 7-
dehydrocholesterol and to take up Cholecalciferol from the
skin