UNIT 4 THE MICRONUTRIENTS
VITAMINS
-I :
Structure ,
4.1 Introductioo
4.2 Fat-soluble Vitamins
4.2.1 Vitamin A
4.2.2 Vitamin D
4.2.3 Vitamin E
4.2.4 Vitamin K
4.3 Water-soluble Vitamins
4.3.1 Vitamins of the Bcomplex Group
4.3.2 Vitamin C
4.4 Let Us Sum Up
4.5 Glossary
4.6 Answers to Check Your Progress Exercises
4.1 INTRODUCTION
You learnt about the sources, functions and utilization of macronutrients in
Block I. We will now begin our discu~sionon micronutrients with the study of
vitamins in this unit.
The "vita" part of the word "vitamin" means "life". Vitamins are, in fact, vital
and essential for life and health. They regulate metabolism, help in the growth
and maintenance of our body and protect against disease. You will learn a great
deal about these functions in the unit.
Vitamins ( l i k ~carbohydrates, fats and proteins) are organic compounds. Unlike
these nutrients, however, vitamins are present in minute quantities in food. This
does not mean we need vitamin pills to meet our needs! We can meet our
requirements for vitamins quite easily by consuming the right types of foods.
4
Some of the vitamins are soluble in water while others are soluble in fat. They
are hence classified into two categories: water-soluble vitamins and fat-saluble
vitamins. This unit will introduce you to the different vitamins included in each
of these categories. You will learn about the food sources, functions and
absorption of these vitamins.
Objectives
After studying this unit, you will be able to:
differentiate between fatsoluble and water-soluble vitamins
identify the food sources of each vitamin
describe the mechanism of absorption of each vitamin
describe the storage of fat-soluble vitamins and
state the important functions of each vitamin in the body
4.2 FAT-SOLUBLE VITAMINS
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.
4.2.1 Vitamin A
Vitamin A or retinol is fowd only in the foods of animal origin. Animal foods
like'milk, butter, ghee, egg, fish and liver are rich sources of vitamin A. Liver
-.*. - r c - L1:1*- L-I:L... -,.A ..-A -h--L -:-ha..+ --..--..-
,Bmlc Concepts In Plant food< d o not contaln retinol. They contain certain orange o r yellow
Nutdtlon-11 coloured pigments called carotenoids which can be converted t o retinol in the
body. In other words these carotenoid pigments are precursor5 of vitamin A. The
word "precur\or". of cour$e, refers here to a substance which can be'converted to the
vltamin In the body
Beta carotet7e 1s the most widely distributed caroteno~din plant foods. Most of
the yellow and orange colour of vegetables and frults is due to these carotenoid'
pigments as we mentioned earlier. Ripe fruits such as mango, papaya and
yellow,iorange vegetables like carrot and pumpkin are rich in beta carotene. Green
leqfy vegetables also contain caroteno~dpigments. Here the yellow and the orange
colour of the carotenoid pigments is masked due to the pre<ence of another
pigment called chlorophyll whlch, 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.
We earlier mentio ed specific animal foods which are rich sources of vitamin A.
However, you know that animal foods are expensive Most Indians d o not
consume enough animal foods to meet the vitamln A needs of the body. Hence
they depend on plant foods to meet their vitamin A needs. Plant foods, as you
know, d o not have retinol. They contain beta carotene instead. The conversion of
beta carotene to vitamin A or retinol is not very efficient in the body. In fact
only half of the beta carotene absorbed is converted to retinol. Thus people who
consume less animal foods need to consume enough plant foods such as green
leafy vegetables and orange-yellow fruits to meet vitamin A needs.
Absorption and storage: You have read that vitamin A is ingested either in the
form of retinol o r carotene. After absorption from the intestinal cells, retinol
forms a complex .with some fat-containing particles called chylomicrons and is
taken to the bloodstream. C a r o t e n ~is .absorbed in much the same way as-retinol.
After ahsorption most of the carotene is converted to retinol in the cells of the
small intestine ~tself.
Xctinol (whether absorbed as such or formed in the body from carotene) is
transported by the.blood to the liver as part of the chylomicrons. Approximately
90 per cent of the total absorbed vitamin is stored in the liver and the remaining
10 per cent is distributed in other organs and glands like the kidneys, lungs and
adrenal glands.
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 t o other body
tissues. This 1s a n example of how nutrients interact with each other.
Functions: What does vitamin A d o in the body? This remarkable substance has,
in fact, three important functions a s listed below:
1) Maintaining normal vision: Vitamin A plays a n important role in maintaining
normal vision. T o understand this better we must first be familiar with the
structure of the eye. Look at Figure4.1. Can you spot the retina?
The retina has two kinds of cells - rods and cones. Both rods and cones are
senskive to changes in light but they react differently and perform different
functions. While rods are sensitive t o dim light, the cones re5pond to bright
light.
Let us take a closer look at the rods. The rods contain a pigment called
rhodopsin. Rhodopsin is formed by the combination of a specific form of .
vitamin A with a protein. The amazing thing about rhodopsin is that it
breaks down into its components when exposed t o bright light. In the dark
these components - vitamin A and protein - again ,combine to regenerate
rhodopsin.
Now, what is the significance of rhodowin in maintaining normal vision?
Rhodopsin helps us to see in dim light. Consider the following situation.
There must have bee-I 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 happen7 It's all because the rhodopsin has broken
, The Mlaonutflent6-I :
Vlhminr
Retina
Cornea
Light -
m
Lens \
Light
\
Light sensitive cells of retina
I
~ o d cone
, Fig. 4.1 Structure of th; eye
. -
.
Adapted from illustration in Essentials of Nurritton ur.d Diet 7herapy by Sue R . Williams, Times
Mirror/Mosby College Publishing, 1986 (4th edition)
down into its two components. It is, therefore, no longer a.ble 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 a n example of a "cycle" in metabolism and is called the
visual cycle (Figure 4.2).
r-------------
- - - - - - - --1
I
I
I I
Rhodopsin
'
I I
1 I
I I
I--- -- - - - - - - - - - - - - - - - - - - - - - I
I
I
I Protein I
I
I
'P
Fig. 4.2 ~irh~lifled
version of vlsunl 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 d o 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
, Bulc Concepts In the skin, the lining of our eyes and the lining of organs like the intestine and
Nutrition-I1 lungs. Without vitamin A the epithelial tissue will become dry and cracks will
appear in the skin or inner walls of the digestive 'tract o r 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 ,9 plays a beneficial role in'preventing common illne,ses
in young children. When body levels of vitamin A are low the chances are
more that the young child will develop infectious dise'ases. 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.
4.2.2 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 d o 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 brigin d o not contain vitamin D.
Absorption and storage: What happens t o 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 bloo!stream as a part of the chylotnicrons.
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 mi erals in the bones is
$
termed as mineralization of bones. Vitamin D aids the proce s of mineralization
in two wayg:
i) b y increasing t h e absorptioh of calcium and phosphorus and
-
ii) by helping in the deposition of caicium and phosphorus in bones. ,
Check Your Progreor Ererefst 1
I) \Vhar are caroceno~ds'lList the rich sources of beta carotene.
Indicate whether the following statements are true o r false. Correct the false
statements.
a) Bile is essential for effective absorption of vitamin A and D. (True/False)
b) We meet most of our vitamin D needs from dietary sourch. (True/False)
C) Vitamin A keeps the epithelial tissue dry. (True/ False)
....................................................................
VITAMINS
-I :
Structure ,
4.1 Introductioo
4.2 Fat-soluble Vitamins
4.2.1 Vitamin A
4.2.2 Vitamin D
4.2.3 Vitamin E
4.2.4 Vitamin K
4.3 Water-soluble Vitamins
4.3.1 Vitamins of the Bcomplex Group
4.3.2 Vitamin C
4.4 Let Us Sum Up
4.5 Glossary
4.6 Answers to Check Your Progress Exercises
4.1 INTRODUCTION
You learnt about the sources, functions and utilization of macronutrients in
Block I. We will now begin our discu~sionon micronutrients with the study of
vitamins in this unit.
The "vita" part of the word "vitamin" means "life". Vitamins are, in fact, vital
and essential for life and health. They regulate metabolism, help in the growth
and maintenance of our body and protect against disease. You will learn a great
deal about these functions in the unit.
Vitamins ( l i k ~carbohydrates, fats and proteins) are organic compounds. Unlike
these nutrients, however, vitamins are present in minute quantities in food. This
does not mean we need vitamin pills to meet our needs! We can meet our
requirements for vitamins quite easily by consuming the right types of foods.
4
Some of the vitamins are soluble in water while others are soluble in fat. They
are hence classified into two categories: water-soluble vitamins and fat-saluble
vitamins. This unit will introduce you to the different vitamins included in each
of these categories. You will learn about the food sources, functions and
absorption of these vitamins.
Objectives
After studying this unit, you will be able to:
differentiate between fatsoluble and water-soluble vitamins
identify the food sources of each vitamin
describe the mechanism of absorption of each vitamin
describe the storage of fat-soluble vitamins and
state the important functions of each vitamin in the body
4.2 FAT-SOLUBLE VITAMINS
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.
4.2.1 Vitamin A
Vitamin A or retinol is fowd only in the foods of animal origin. Animal foods
like'milk, butter, ghee, egg, fish and liver are rich sources of vitamin A. Liver
-.*. - r c - L1:1*- L-I:L... -,.A ..-A -h--L -:-ha..+ --..--..-
,Bmlc Concepts In Plant food< d o not contaln retinol. They contain certain orange o r yellow
Nutdtlon-11 coloured pigments called carotenoids which can be converted t o retinol in the
body. In other words these carotenoid pigments are precursor5 of vitamin A. The
word "precur\or". of cour$e, refers here to a substance which can be'converted to the
vltamin In the body
Beta carotet7e 1s the most widely distributed caroteno~din plant foods. Most of
the yellow and orange colour of vegetables and frults is due to these carotenoid'
pigments as we mentioned earlier. Ripe fruits such as mango, papaya and
yellow,iorange vegetables like carrot and pumpkin are rich in beta carotene. Green
leqfy vegetables also contain caroteno~dpigments. Here the yellow and the orange
colour of the carotenoid pigments is masked due to the pre<ence of another
pigment called chlorophyll whlch, 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.
We earlier mentio ed specific animal foods which are rich sources of vitamin A.
However, you know that animal foods are expensive Most Indians d o not
consume enough animal foods to meet the vitamln A needs of the body. Hence
they depend on plant foods to meet their vitamin A needs. Plant foods, as you
know, d o not have retinol. They contain beta carotene instead. The conversion of
beta carotene to vitamin A or retinol is not very efficient in the body. In fact
only half of the beta carotene absorbed is converted to retinol. Thus people who
consume less animal foods need to consume enough plant foods such as green
leafy vegetables and orange-yellow fruits to meet vitamin A needs.
Absorption and storage: You have read that vitamin A is ingested either in the
form of retinol o r carotene. After absorption from the intestinal cells, retinol
forms a complex .with some fat-containing particles called chylomicrons and is
taken to the bloodstream. C a r o t e n ~is .absorbed in much the same way as-retinol.
After ahsorption most of the carotene is converted to retinol in the cells of the
small intestine ~tself.
Xctinol (whether absorbed as such or formed in the body from carotene) is
transported by the.blood to the liver as part of the chylomicrons. Approximately
90 per cent of the total absorbed vitamin is stored in the liver and the remaining
10 per cent is distributed in other organs and glands like the kidneys, lungs and
adrenal glands.
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 t o other body
tissues. This 1s a n example of how nutrients interact with each other.
Functions: What does vitamin A d o in the body? This remarkable substance has,
in fact, three important functions a s listed below:
1) Maintaining normal vision: Vitamin A plays a n important role in maintaining
normal vision. T o understand this better we must first be familiar with the
structure of the eye. Look at Figure4.1. Can you spot the retina?
The retina has two kinds of cells - rods and cones. Both rods and cones are
senskive to changes in light but they react differently and perform different
functions. While rods are sensitive t o dim light, the cones re5pond to bright
light.
Let us take a closer look at the rods. The rods contain a pigment called
rhodopsin. Rhodopsin is formed by the combination of a specific form of .
vitamin A with a protein. The amazing thing about rhodopsin is that it
breaks down into its components when exposed t o bright light. In the dark
these components - vitamin A and protein - again ,combine to regenerate
rhodopsin.
Now, what is the significance of rhodowin in maintaining normal vision?
Rhodopsin helps us to see in dim light. Consider the following situation.
There must have bee-I 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 happen7 It's all because the rhodopsin has broken
, The Mlaonutflent6-I :
Vlhminr
Retina
Cornea
Light -
m
Lens \
Light
\
Light sensitive cells of retina
I
~ o d cone
, Fig. 4.1 Structure of th; eye
. -
.
Adapted from illustration in Essentials of Nurritton ur.d Diet 7herapy by Sue R . Williams, Times
Mirror/Mosby College Publishing, 1986 (4th edition)
down into its two components. It is, therefore, no longer a.ble 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 a n example of a "cycle" in metabolism and is called the
visual cycle (Figure 4.2).
r-------------
- - - - - - - --1
I
I
I I
Rhodopsin
'
I I
1 I
I I
I--- -- - - - - - - - - - - - - - - - - - - - - - I
I
I
I Protein I
I
I
'P
Fig. 4.2 ~irh~lifled
version of vlsunl 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 d o 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
, Bulc Concepts In the skin, the lining of our eyes and the lining of organs like the intestine and
Nutrition-I1 lungs. Without vitamin A the epithelial tissue will become dry and cracks will
appear in the skin or inner walls of the digestive 'tract o r 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 ,9 plays a beneficial role in'preventing common illne,ses
in young children. When body levels of vitamin A are low the chances are
more that the young child will develop infectious dise'ases. 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.
4.2.2 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 d o 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 brigin d o not contain vitamin D.
Absorption and storage: What happens t o 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 bloo!stream as a part of the chylotnicrons.
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 mi erals in the bones is
$
termed as mineralization of bones. Vitamin D aids the proce s of mineralization
in two wayg:
i) b y increasing t h e absorptioh of calcium and phosphorus and
-
ii) by helping in the deposition of caicium and phosphorus in bones. ,
Check Your Progreor Ererefst 1
I) \Vhar are caroceno~ds'lList the rich sources of beta carotene.
Indicate whether the following statements are true o r false. Correct the false
statements.
a) Bile is essential for effective absorption of vitamin A and D. (True/False)
b) We meet most of our vitamin D needs from dietary sourch. (True/False)
C) Vitamin A keeps the epithelial tissue dry. (True/ False)
....................................................................
