Tuesday, April 5, 2016
Vitamins definition
VITAMINS
Introduction The vitamins are a group of complex organic compounds required in small quantities by the body for the maintenance of good health. They are not normally synthesized in the body and hence they should be supplied by the diet. The vitamins are present in foods in small quantities. Unlike other groups of nutrients the vitamins are not chemically similar to each other. Each vitamin has a specific chemical structure and a specific function or functions in the living system. Most of the vitamins act as coenzymes in the body. Normally a well balanced diet will supply all the necessary vitamins in sufficient quantity.
Classification Vitamins are generally classified into two main groups. (i) fat soluble vitamins (ii) water soluble vitamins.
Fat soluble vitamins The members of this group are A,D,E and K. All these vitamins are not soluble in water and readily soluble in fat dissolving organic solvents. Inside the body for their transport and metabolism they need the presence of fats.
Vitamin
- A Vitamin A is found only in foods of
animal origin. It is present in almost all species of fish, birds and mammals.
The yellow plant pigments α, β and γ carotenes and cryptoxanthin are precursors
of vitamin A. The body has the ability to convert these carotenoid compounds present
in the diet into vitamin A. 148 The chemical structure of β - carotene is such
that it oxidizes to form two molecules of vitamin A, the other provitamins form
only one. β-carotene is more efficiently converted to vitamin A than α - or γ -
carotene or cryptoxanthin. There are two forms
of vitamin A:
Vitamin A
1 which
occurs in the liver of marine water fish and Vitamin A
2 found in the liver of fresh water fish. The vitamin A which contain alcoholic group in the side chain is called as retinol (Fig. 8.1) and which contain aldehyde group is known as retinal. Though the two vitamins differ slightly in their chemical structures their physiological functions are the same.
2 found in the liver of fresh water fish. The vitamin A which contain alcoholic group in the side chain is called as retinol (Fig. 8.1) and which contain aldehyde group is known as retinal. Though the two vitamins differ slightly in their chemical structures their physiological functions are the same.
Vitamin A is
essential i. for the growth and metabolism of all body cells ii. for the formation
of rhodopsin (visual purple) a complex substance formed from retinol and
protein. Rhodopsin, a pigment found in retina is necessary for vision in dim
light.
iii. for the maintenance of healthy skin, particularly mucous membrane of the cornea and the lining of respiratory tract.
iii. for the maintenance of healthy skin, particularly mucous membrane of the cornea and the lining of respiratory tract.
Sources The liver of any animal is a rich source of vitamin A. Fish liver
oil is an excellent source. Whole milk, egg yolk, dark green leafy vegetables
and deep yellow vegetables and fruits are rich in carotenes, which can be
converted into vitamin A by the intestinal wall.
Requirements Vitamin A requirement is based on the intake to maintain the
normal blood level. Adults placed on a vitamin A free diet are found to show no
change in the level for several weeks.The capacity of the body to store vitamin
A provides for an effective emergency supply. Recommended amount of Vitamin A
for different age group is as follows: Infants - 1500 IU / day Children -
2000-3000 IU / day Adults - 5000 IU / day Pregnant and lactating women - 6000-8000
IU / day (IU = International units)
Absorption
and storage Vitamin A and carotene are
absorbed from the small intestine into the lymph system. The maximum absorption
is reached 3 to 5 hours after consumption. The rate of absorption of vitamin A
is more rapid than that of carotene. In the human being about 95% of the
vitamin A stored in the body is found in the liver with small amount in the
lungs, adipose tissue and kidneys.
Deficiency The earliest sign of vitamin A deficiency is concerned with vision. Initially there is a loss of sensitivity to green light, followed by impairment to adapt to dim light. This condition leads to night blindness. More 150 prolonged or severe deficiency leads to the ulceration of cornea and this condition is known as xerophthalmia or keratomalacia.
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