Fat-soluble vitamins can only be absorbed in the presence of fat. Therefore, the presence of some fat in the diet is essential for their absorption. Fat-soluble vitamins can be stored in the body and hence occasional intake of very high sources may help the body tide over periods of low intake.
This was the first fat-soluble vitamin to be discovered. It has a number of important functions in the body. Vitamin A is found only in animal foods, mainly as retinol. Plants provide a source of vitamin A for animals in the form of orange-yellow pigments called the carotenoids. The chief source in human nutrition is beta-carotene, which the body converts to vitamin A in the intestinal mucosa during absorption.
Vitamin A activity in foods used to be expressed in International Units (I.U.), with the following equivalencies-
1 IU = 0.3 mcg retinol
1 IU = 0.6 mcg beta-carotene
1 IU = 1.2 mcg other provitamin A carotenoids
- Function in Vision-Vitamin A occurs in the retina of the eye and is required in the process of vision to adjust to light of varying intensity (dark adaptation). It occurs in the light receptor cells in the retina in combination with protein. This substance is known as visual purple (rhodopsin).
- Health of Epithelial Tissues- These tissues cover the outer surface of the body, line the major cavities and all the tubular systems in the body. These are specialised tissues, of which the outer covering is resistant; protective epidermis and the internal tissue is a secretory mucous membrane.
- Immune Response- Many of the epithelial tissues are important barriers to infection. Vitamin A deficiency impairs this function in a non-specific way. In addition, vitamin A in a more specific way helps to maintain the lymphocyte pool. Vitamin A also functions in T-cell-mediated responses
- Haemopoesis- Vitamin A deficiency in man and experimental animals is consistently associated with an iron deficiency type of anaemia
- Growth- Retinoic acid is known to play its hormone-like function in control of growth and development of tissues in the musculo-skeletal system.
- Energy Balance- It has been recently shown that an enzyme in mitochondria, which controls the local production of energy as heat is under the transcriptional regulation of retinoic acid.
- Central Nervous System- Retinoic acid plays a major role in the development of the foetal central nervous system.
- Gap Junctional Communication- Gap junctions are narrow, hydrophilic pores connecting the cytosol of two adjacent cells. The gap junctions are reported to be involved in regulation of morphogenesis, cell differentiation, secretion of hormones and growth
- Activity of Carotenoids- Carotenoids carry out several important functions in animals and plants.
Sources of Vitamin A-
Vitamin A is present in animal foods only. Liver is the richest source of vitamin A. Other sources include butter, ghee, milk, curds and egg yolk. Refined oils and vanaspati are good sources of vitamin A if these are fortified with vitamin A.
Recommended Daily Allowances
The daily requirement of an adult for vitamin A is of the order of 600 mcg of retinol or 2400 mcg of beta-carotene per day derived from foods of either animal or vegetable origin. The allowance for infants is 350 mcg (about 1400 mcg of beta carotene).
The need increases gradually as the child grows to adolescence. No increased allowance during pregnancy is recommended but the allowance is increased to 950 mcg or 3800 mcg of beta-carotene during lactation.
Effect of Deficiency
- The deficiency may result from dietary lack of vitamin A, the provitamin, or poor absorption of these. The deficiency results in growth failure, affects the vision, the skin and the immune function adversely.
- The earliest symptom is impaired ability to see in dim light (dark adaptation); the next stage is inability to see normally in dim light which is known as night blindness or nyctalopia.
- The next symptom is usually dryness of lining of eyelids and eyeball (conjunctiva). A later and more severe stage of deficiency is xerosis (dryness) of the cornea. The cornea becomes dry and loses its transparency (xerophthalmia). In the last stage of the disease, keratomalacia, the cornea becomes soft and results in permanent blindness.
- In early stages, treatment with vitamin A will restore full vision; however, if advanced changes have taken place, blindness is inevitable. The term xerophthalmia refers to clinical manifestation of vitamin A deficiency. The skin changes include dryness, wrinkling, slate gray discolouration and thickening of the outer layer (hyperkeratosis). The hair may lose lustre.
- There is a noticeable shrinking, hardening and progressive degeneration of epithelial cells, which increases susceptibility to severe infections of the eye, the nasal passages, the sinuses, middle ear, lungs and genitourinary tract.
Pure vitamin D was isolated in crystalline form in 1930 and was called calciferol. Vitamin D is now considered a pro-hormone than a vitamin. Vitamin D is sometimes called the ‘sunshine vitamin’ because the body is able to convert a precursor 7-dehydrocholesterol, a sterol present in the skin, to vitamin D in the presence of sunlight. It can be synthesised in the body in adequate amounts by simple exposure to sunlight even for five minutes per day.
- Absorption of calcium and phosphorus- calcitriol, a hormone, is an activated form of vitamin D. It acts with two other hormones (the parathyroid hormone and the thyroid hormone calcitonin) and stimulates the absorption of calcium and phosphorus in the small intestine. Without the presence of vitamin D formation of strong and rigid bones is not possible.
- Bone mineralisation-The bone tissue formation from calcium and phosphorus and other materials is regulated by calcitriol. It regulates the rate of deposit and resorption of these mainerals in bone. This balancing process helps to build and maintain bone tissue. Vitamin D hormone can be used to treat rickets in children and osteoporosis (bone loss) in older women.
- Irradiation of the skin with sunlight is the main source of vitamin D. The mid-day sun is rich in ultra-violet light and helps in synthesising this vitamin.
- Foods are not a good source of vitamin D. It is found in small quantities in liver, egg yolk, milk and milk fat (butter and ghee), obtained form animals fed on pastures exposed to sunlight.
Effect of Deficiency -
- When adequate quantities of vitamin D are not available, strong and rigid bones are not formed. This leads to a condition in children known as ‘rickets’ which is characterised by poor growth and bone deformation such as bowed legs, beaded ribs, enlarged joints and skull deformation.
- Osteomalacia—a condition similar to rickets may develop in adult women due to lack of vitamin D.
Vitamin E or alpha-tocopherol is a fat-soluble vitamin.
No definite proof of vitamin E deficiency in human beings has been established.
- The main function of vitamin E, tocopherol, is its ability to prevent tissue breakdown, by virtue of its antioxidant nature.
- Vitamin E acts as nature’s most powerful fat-soluble antioxidant. In the lipid membranes of body tissues, the polyunsaturated fatty acids present are easy targets for oxygen to break down. Vitamin E protects the cell membrane fatty acids from damage by interrupting this oxidation process.
Food Sources –
The richest sources of vitamin E are the vegetable oils. Vegetable oils are also the richest sources of polyunsaturated fatty acids, which vitamin E protects. This is nature’s unique arrangement to package the two together. Other food sources include cereals, leafy vegetables, milk, eggs, muscle meats and fish.
Phylloquinone is the major form of vitamin K found in plants. It is also the form found in our dietary.
The function of vitamin K-
The basic function of vitamin K is in the blood-clotting process.
It is essential for the formation of prothrombin by the liver. Prothrombin is a normal constituent of the blood and helps clotting of blood on contact with air.
Green leafy vegetables are the principal dietary source of vitamin K.
Deficiency of vitamin K
Deficiency of vitamin K prolongs clotting time and may result in excessive bleeding after an injury. Human babies do not have reserves of vitamin K at birth and in many hospital it is, therefore, routine practice to give vitamin K to the expectant mother to prevent excessive bleeding at child birth.