Vitamins A, D, E and K are known as the fat-soluble vitamins. These vitamins are therefore present in food in close association with fats. An interesting fact about fat-soluble vitamins is that after being used for specific functions, the excess amount of these vitamins is stored in the body. Let us now learn more about each of these vitamins.
Vitamin-A:
Vitamin A or retinol is found only in the foods of animal origin. Animal foods like milk, butter, ghee, egg, fish and liver are rich sources of vitamin A. Liver oils of fish like shark are the richest sources.
Plant foods do not contain retinol. They contain certain orange or yellow coloured pigments called carotenoids which can be converted to retinol in the body. In other words these carotenoid pigments are precursors of vitamin A. The word "precursor" of course, refers here to a substance which can be converted to the vitamin in the body.
Beta carotene is the most widely distributed carotenoid in plant foods. Most of the yellow and orange colour of vegetables and fruits is due to these carotenoid pigments as we mentioned earlier. Ripe fruits such as mango, papaya and yellow/orange vegetables like carrot and pumpkin are rich in beta carotene. Green leafy vegetables also contain carotenoid pigments. Here the yellow and the orange colour of the carotenoid pigments is masked due to the presence of another pigment called chlorophyll which as you may know is green in colour. Green leafy vegetables such as spinach, mustard leaves and fenugreek leaves are very rich sources of beta carotene.
Absorption and storage: You have read that vitamin A is ingested either in the form of retinol or carotene. After absorption from the intestinal cells, retinol forms a complex with some fat-containing particles called chylomicrons and is taken to the bloodstream. Carotene is absorbed in much the same way as-retinol. After absorption most of the carotene is converted to retinol in the cells of the small intestine itself.
Being a fat-soluble vitamin, the presence of bile and fats greatly aids in vitamin A absorption. Further proteins help in its transport from the liver to other body tissues.
Functions: What does vitamin A do in the body? This remarkable substance has in fact three important functions as listed below:
1)Maintaining normal vision: Vitamin A plays an important role in maintaining normal vision. To understand this better we must first be familiar with the structure of the eye.
The retina has two kinds of cells rods and cones. Both rods and cones are senstive to changes in light but they react differently and perform different functions. While rods are sensitive to dim light, the cones respond to bright light.
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| Structure of the eye |
Now, what is the significance of rhodopsin in maintaining normal vision? Rhodopsin helps us to see in dim light. Consider the following situation.There must have been many occasions when you walked into the dark from a brightly lit room. You would definitely remember not being able to see for a short while. Why does this happen? It's all because the rhodopsin has broken down into its two components. It is therefore, no longer able to perform its functions. How then did you begin to see in the dark? This happened because rhodopsin is regenerated once again in the dark. If you look at these events closely you would notice that we started with rhodopsin and now we have gone through a process by which it is broken down and then regenerated. This is an example of a "cycle" in metabolism and is called the visual cycle.
Vitamin E :
Vitamin E is present in almost all foodstuffs. Vegetable oils like groundnut, soya, cottonseed and safflower are rich sources of vitamin E. Other good sources are whole grain cereals, dark green leafy vegetables, pulses, nuts and oilseeds. Foods of animal origin are low in vitamin E. However, foods like egg yolk, butter and liver contain some amount of the vitamin.
Absorption and storage: Like the other fat-soluble vitamins, absorption of vitamin E also requires the presence of fat and bile. After absorption from the upper part of the small intestine, vitamin E (as part of chylomicrons) is carried to the liver through the blood stream and is distributed to various body tissues. Almost all the body tissues have a small amount of the vitamin but it is mainly stored in muscles and adipose tissue.
Functions: The main role of vitamin E in our body is the protection it gives to other substances like unsaturated fatty acids, vitamins A and C. It prevents their destruction in the body as well as in foods.
Vitamin K :
Among plant foods, green leafy vegetables like spinach, cabbage and lettuce are rich sources of vitamin K. Other good sources include animal foods such as egg yolk, milk and organ meats like liver. Vitamin K is also manufactured by certain helpful bacteria which are normally present in the small intestine. Approximately half of the vitamin K needed by us gets manufactured in the intestinal tract and the other half is obtained from animal and plant foods.
Absorption and storage: The absorption of vitamin K requires bile since vitamin K is fat-soluble. After absorption from the upper part of the small intestine the vitamin is distributed to various body tissues. Vitamin K is stored in very small amounts in the body and its concentration is not high in any particular tissue.
Functions: Have you ever observed what happens when you cut your finger? Your finger, of course, starts bleeding. But after a while blood stops oozing out. Why? This is because a clot is formed on the wound and seals it off. Vitamin K plays an important role in clotting of blood and is therefore also termed as the "antibleeding vitamin" (one which prevents uncontrolled bleeding). How does vitamin K help in clotting of blood? It helps in the formation of a protein called prothrombin which, in turn, is essential for blood clotting.
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| Simplified version of visual cycle |
2)Supporting growth: Vitamin A is essential for the growth of the skeleton and soft tissues. The exact role of the vitamin in the growth of the body is still not understood. Research studies in this area have indicated that with the deficiency of vitamin A in the body, bones do not grow to their full length, and the overall growth of the body is affected.
3) Protecting against disease: Vitamin A plays an important role in keeping epithelial tissues moist and healthy. Some examples of epithelial tissues are the skin, the lining of our eyes and the lining of organs like the intestine and lungs. Without vitamin A the epithelial tissue will become dry and cracks will appear in the skin or inner walls of the digestive tract or lungs. This makes it easy for the germs to enter and cause diseases like diarrhoea, respiratory infections and eye infections. Various research studies have supported this and shown that vitamin A plays a beneficial role in preventing common illnesses in young children. When body levels of vitamin A are low the chances are more that the young child will develop infectious diseases. If these diseases are sufficiently severe they can even cause death. This is the reason why the deficiency of vitamin A is associated with child death.
Vitamin D :
Vitamin D is also called the "sunshine vitamin". This is because it is manufactured from a substance present in our skin on exposure to sunlight. As a result of this we do not necessarily have to depend on dietary sources of vitamin D. The easiest way of obtaining the vitamin is, in fact, enough exposure to sunlight.
Which foods contain vitamin D? Foods of animal origin like eggs, liver and butter contain the vitamin in significant amounts. It is, however, the fish liver oils which are the richest sources. On the other hand, most of the commonly used foods of plant origin do not contain vitamin D.
Absorption and storage: What happens to vitamin D in the body? The vitamin D we take in is absorbed along with fats from the small intestine. Bile is essential for effective absorption of fats and therefore, of vitamin D. Once absorption is completed, vitamin D enters the bloodstream as a part of the chylomicrons. Vitamin D formed in the skin on exposure to sunlight also enters the bloodstream. Whatever the source, the vitamin is then taken to the liver. Some of it is stored there and the remaining is distributed by the blood to various body tissues.
Functions: You might have heard that vitamin D makes bones strong and healthy. This is absolutely correct. How does vitamin D help in this? Read on to find out. Minerals like calcium and phosphorus, when deposited in the bones make them strong and hard. The process of deposition of minerals in the bones is termed as mineralization of bones. Vitamin D aids the process of mineralization in two ways:
- by increasing the absorptioh of calcium and phosphorus and
- by helping in the deposition of caicium and phosphorus in bones.
Vitamin E is present in almost all foodstuffs. Vegetable oils like groundnut, soya, cottonseed and safflower are rich sources of vitamin E. Other good sources are whole grain cereals, dark green leafy vegetables, pulses, nuts and oilseeds. Foods of animal origin are low in vitamin E. However, foods like egg yolk, butter and liver contain some amount of the vitamin.
Absorption and storage: Like the other fat-soluble vitamins, absorption of vitamin E also requires the presence of fat and bile. After absorption from the upper part of the small intestine, vitamin E (as part of chylomicrons) is carried to the liver through the blood stream and is distributed to various body tissues. Almost all the body tissues have a small amount of the vitamin but it is mainly stored in muscles and adipose tissue.
Functions: The main role of vitamin E in our body is the protection it gives to other substances like unsaturated fatty acids, vitamins A and C. It prevents their destruction in the body as well as in foods.
Vitamin K :
Among plant foods, green leafy vegetables like spinach, cabbage and lettuce are rich sources of vitamin K. Other good sources include animal foods such as egg yolk, milk and organ meats like liver. Vitamin K is also manufactured by certain helpful bacteria which are normally present in the small intestine. Approximately half of the vitamin K needed by us gets manufactured in the intestinal tract and the other half is obtained from animal and plant foods.
Absorption and storage: The absorption of vitamin K requires bile since vitamin K is fat-soluble. After absorption from the upper part of the small intestine the vitamin is distributed to various body tissues. Vitamin K is stored in very small amounts in the body and its concentration is not high in any particular tissue.
Functions: Have you ever observed what happens when you cut your finger? Your finger, of course, starts bleeding. But after a while blood stops oozing out. Why? This is because a clot is formed on the wound and seals it off. Vitamin K plays an important role in clotting of blood and is therefore also termed as the "antibleeding vitamin" (one which prevents uncontrolled bleeding). How does vitamin K help in clotting of blood? It helps in the formation of a protein called prothrombin which, in turn, is essential for blood clotting.
Vitamin A also supports immune health and can be obtained from natural foods such as carrots, pumpkin, sweet potatoes, butternut squash, dark green leafy vegetables, and cantaloupe.
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