VITAMINS
VITAMIN A
Vitamin A is a fat-soluble vitamin found in two main forms in food: preformed vitamin A (retinol)
and provitamin A carotenoids, such as \beta-carotene.
1. Animal Sources (Preformed Vitamin A)
• Liver and fish oils: These are exceptionally rich sources of vitamin A.
• Dairy products: Milk, cheese, butter, and yogurt contain moderate amounts.
• Eggs: The yolk is a good source of vitamin A.
• Fish: Oily fish like salmon and mackerel are also good sources.
2. Plant Sources (Provitamin A Carotenoids)
The body converts these carotenoids into retinol. The deeper the color of the fruit or vegetable,
the higher the amount of carotenoids.
• Yellow, red, and orange fruits and vegetables: Carrots, sweet potatoes, pumpkin,
mangoes, papaya, and apricots.
• Green leafy vegetables: Spinach, kale, and other dark leafy greens.
• Fortified foods: Many cereals, margarine, and infant formulas are fortified with
vitamin A.
Recommended Dietary Allowance (RDA)
The RDA for vitamin A is expressed in Retinol Activity Equivalents (RAE) to account for the
different bioactivities of retinol and provitamin A carotenoids.
• Men: 900 \mug RAE per day.
• Women: 700 \mug RAE per day.
• Pregnancy: 770 \mug RAE per day.
• Lactation: 1,300 \mug RAE per day.
Note: Excessive intake of preformed vitamin A can be toxic, with a Tolerable Upper Intake Level
(UL) for adults set at 3,000 \mug per day.
Biochemical Functions
Vitamin A is crucial for a wide range of physiological processes, including:
1. Vision
, • Visual Cycle: In the retina's rod cells, 11-cis-retinal, a derivative of vitamin A,
binds to the protein opsin to form the light-sensitive pigment rhodopsin. When light
strikes rhodopsin, the 11-cis-retinal isomerizes to all-trans-retinal, triggering a
cascade of events that sends an electrical signal to the brain, allowing for vision,
particularly in low light. The all-trans-retinal is then converted back to 11-cis-retinal
to regenerate rhodopsin.
2. Gene Regulation and Cell Differentiation
• Retinoic Acid: The form of vitamin A known as retinoic acid acts like a steroid
hormone. It binds to nuclear receptors (RARs) that regulate the expression of
hundreds of genes. This process is essential for the proper development and
differentiation of many cell types, especially epithelial cells in the skin, lungs, and
digestive tract.
3. Growth and Reproduction
• Vitamin A is necessary for normal growth and development in children. It also
plays a vital role in male and female reproductive systems.
4. Immune Function
• Vitamin A is critical for maintaining a healthy immune system. It supports the
function of various immune cells and the integrity of mucosal barriers, which are the
body's first line of defense against infection.
Deficiency Manifestations
Vitamin A deficiency is a major public health problem worldwide, particularly in developing
countries. The clinical manifestations primarily affect the eyes, skin, and immune system.
1. Ocular Manifestations (Xerophthalmia)
• Night blindness (Nyctalopia): This is one of the earliest signs of deficiency. It is
the inability to see well in dim light due to a lack of rhodopsin.
• Xerosis: The conjunctiva (the membrane covering the white of the eye) becomes
dry and wrinkled.
• Bitot's spots: Foamy, white or gray patches appear on the conjunctiva.
• Keratomalacia: In severe cases, the cornea softens, ulcerates, and can lead to
permanent blindness.
2. Other Manifestations
• Hyperkeratosis: The skin becomes dry, scaly, and rough due to the defective
, • Hyperkeratosis: The skin becomes dry, scaly, and rough due to the defective
differentiation of epithelial cells.
• Impaired immune function: Increased susceptibility to infections, especially
respiratory and diarrheal infections, which can lead to higher morbidity and
mortality, particularly in children.
• Growth retardation: Vitamin A deficiency can impair normal growth and
development.
• Reproductive issues: Infertility and impaired reproductive function can occur in
both males and females.
Wald’s Visual Cycle
Wald's Visual Cycle, also known as the retinoid cycle, is a series of biochemical reactions that
regenerate the visual pigments in the eye. This process is essential for continuous vision and for
adapting to different light conditions.
Here are the key points of the cycle:
• Light Absorption: The cycle begins when a photon of light is absorbed by
rhodopsin, the light-sensitive pigment found in rod cells.
• Isomerization: This light absorption causes the 11-cis-retinal component of
rhodopsin to change its shape, or isomerize, into all-trans-retinal.
• Dissociation: The all-trans-retinal then detaches from the opsin protein, a process
known as "bleaching" of the rhodopsin.
• Transport and Reduction: The free all-trans-retinal is reduced to all-trans-retinol,
which is then transported to the retinal pigment epithelium (RPE), a layer of cells
adjacent to the photoreceptors.
• Regeneration: In the RPE, the all-trans-retinol is converted back into 11-cis-retinal
through a series of enzymatic steps.
• Return and Recombination: The newly formed 11-cis-retinal is transported back
to the photoreceptor cells, where it recombines with opsin to regenerate rhodopsin.
This cycle allows the photoreceptor cells to be ready to absorb another photon of light, thus
enabling sustained vision. The regeneration of rhodopsin is a slower process, which explains why
it takes time to adapt to changes in light, like when moving from a brightly lit room to a dark one.
The cycle's reliance on vitamin A highlights the importance of this vitamin for healthy vision.
,The cycle's reliance on vitamin A highlights the importance of this vitamin for healthy vision.
VITAMIN D
Vitamin D is unique among vitamins because it can be obtained from both dietary sources and
from the body's own synthesis.
• Sunlight (Endogenous Synthesis): This is the most significant source for most
people. The skin contains a precursor molecule called 7-dehydrocholesterol. When
exposed to ultraviolet B (UVB) rays from sunlight, this molecule is converted into
cholecalciferol (Vitamin D_3).
• Dietary Sources:
• Fatty Fish: Oily fish like salmon, sardines, mackerel, and herring are
excellent natural sources of Vitamin D.
• Egg Yolk: The yolk of eggs contains Vitamin D.
• Fortified Foods: In many countries, foods are fortified with Vitamin D to
prevent deficiency. Common fortified foods include milk, some breakfast
, cereals, and margarine.
• Mushrooms: Some mushrooms, especially those exposed to UV light, can
be a source of Vitamin D_2 (ergocalciferol).
Recommended Dietary Allowance (RDA)
The RDA of Vitamin D is 400 International Units (IU), which is equivalent to 10 micrograms (mcg).
The countries with good sunlight such as India the RDA is 200 IU
Biochemical Functions
Vitamin D, specifically its active form calcitriol (1,25-dihydroxycholecalciferol), functions more
like a hormone than a typical vitamin. It plays a critical role in calcium and phosphate
homeostasis.
1. Calcium and Phosphate Homeostasis: This is the primary and most well-
known function. Calcitriol regulates the serum concentrations of calcium and
phosphate by acting on three target organs:
• Intestines: Calcitriol stimulates the synthesis of a calcium-binding protein in
the intestinal cells. This protein enhances the absorption of dietary calcium
and phosphate, increasing their levels in the blood.
• Kidneys: Calcitriol, along with parathyroid hormone (PTH), promotes the
reabsorption of calcium from the renal tubules, preventing its loss in urine. It
also helps in phosphate reabsorption.
• Bones: Calcitriol is essential for normal bone formation and mineralization.
When blood calcium levels are low, it can work with PTH to mobilize calcium
and phosphate from bone, helping to maintain blood levels.
, 2. Gene Regulation: Calcitriol binds to a specific receptor called the Vitamin
D receptor (VDR). This complex then binds to DNA, acting as a transcription
factor to regulate the expression of various genes, including those
responsible for calcium-binding proteins.
3. Other Functions: The VDR is found in most cells of the body, suggesting
that Vitamin D has non-skeletal functions. It is involved in cell differentiation
and growth, and has been linked to the regulation of immune, endocrine, and
cardiovascular systems.
Deficiency Manifestations
A deficiency in Vitamin D leads to impaired calcium and phosphate absorption, which in turn
affects bone health. The most prominent deficiency manifestations are:
• Rickets (in children): Rickets in children is characterized by bone deformities due to
incomplete mineralization, resulting in soft and pliable bones and delay in teeth formation. The
,weight-bearing bones are bent to form bow-legs. In rickets, the plasma level of Calcitriol is
decreased and alkaline phosphatase activity is elevated. Alkaline phosphatase is concerned
with the process of bone formation. There is overproduction of alkaline phosphatase related to
more cellular activity of the bone.
• Osteomalacia (in adults): In adults, Vitamin D deficiency leads to the demineralization of
existing bone. This causes the bones to become soft and weak, leading to bone pain, muscle
weakness, and an increased risk of fractures.
• Secondary Hyperparathyroidism: A chronic Vitamin D deficiency can lead to low blood
calcium levels (hypocalcemia). This triggers the parathyroid glands to produce more parathyroid
hormone (PTH) in an attempt to raise blood calcium. This condition, known as secondary
hyperparathyroidism, leads to increased bone breakdown to release calcium, further worsening
bone health.
,
VITAMIN A
Vitamin A is a fat-soluble vitamin found in two main forms in food: preformed vitamin A (retinol)
and provitamin A carotenoids, such as \beta-carotene.
1. Animal Sources (Preformed Vitamin A)
• Liver and fish oils: These are exceptionally rich sources of vitamin A.
• Dairy products: Milk, cheese, butter, and yogurt contain moderate amounts.
• Eggs: The yolk is a good source of vitamin A.
• Fish: Oily fish like salmon and mackerel are also good sources.
2. Plant Sources (Provitamin A Carotenoids)
The body converts these carotenoids into retinol. The deeper the color of the fruit or vegetable,
the higher the amount of carotenoids.
• Yellow, red, and orange fruits and vegetables: Carrots, sweet potatoes, pumpkin,
mangoes, papaya, and apricots.
• Green leafy vegetables: Spinach, kale, and other dark leafy greens.
• Fortified foods: Many cereals, margarine, and infant formulas are fortified with
vitamin A.
Recommended Dietary Allowance (RDA)
The RDA for vitamin A is expressed in Retinol Activity Equivalents (RAE) to account for the
different bioactivities of retinol and provitamin A carotenoids.
• Men: 900 \mug RAE per day.
• Women: 700 \mug RAE per day.
• Pregnancy: 770 \mug RAE per day.
• Lactation: 1,300 \mug RAE per day.
Note: Excessive intake of preformed vitamin A can be toxic, with a Tolerable Upper Intake Level
(UL) for adults set at 3,000 \mug per day.
Biochemical Functions
Vitamin A is crucial for a wide range of physiological processes, including:
1. Vision
, • Visual Cycle: In the retina's rod cells, 11-cis-retinal, a derivative of vitamin A,
binds to the protein opsin to form the light-sensitive pigment rhodopsin. When light
strikes rhodopsin, the 11-cis-retinal isomerizes to all-trans-retinal, triggering a
cascade of events that sends an electrical signal to the brain, allowing for vision,
particularly in low light. The all-trans-retinal is then converted back to 11-cis-retinal
to regenerate rhodopsin.
2. Gene Regulation and Cell Differentiation
• Retinoic Acid: The form of vitamin A known as retinoic acid acts like a steroid
hormone. It binds to nuclear receptors (RARs) that regulate the expression of
hundreds of genes. This process is essential for the proper development and
differentiation of many cell types, especially epithelial cells in the skin, lungs, and
digestive tract.
3. Growth and Reproduction
• Vitamin A is necessary for normal growth and development in children. It also
plays a vital role in male and female reproductive systems.
4. Immune Function
• Vitamin A is critical for maintaining a healthy immune system. It supports the
function of various immune cells and the integrity of mucosal barriers, which are the
body's first line of defense against infection.
Deficiency Manifestations
Vitamin A deficiency is a major public health problem worldwide, particularly in developing
countries. The clinical manifestations primarily affect the eyes, skin, and immune system.
1. Ocular Manifestations (Xerophthalmia)
• Night blindness (Nyctalopia): This is one of the earliest signs of deficiency. It is
the inability to see well in dim light due to a lack of rhodopsin.
• Xerosis: The conjunctiva (the membrane covering the white of the eye) becomes
dry and wrinkled.
• Bitot's spots: Foamy, white or gray patches appear on the conjunctiva.
• Keratomalacia: In severe cases, the cornea softens, ulcerates, and can lead to
permanent blindness.
2. Other Manifestations
• Hyperkeratosis: The skin becomes dry, scaly, and rough due to the defective
, • Hyperkeratosis: The skin becomes dry, scaly, and rough due to the defective
differentiation of epithelial cells.
• Impaired immune function: Increased susceptibility to infections, especially
respiratory and diarrheal infections, which can lead to higher morbidity and
mortality, particularly in children.
• Growth retardation: Vitamin A deficiency can impair normal growth and
development.
• Reproductive issues: Infertility and impaired reproductive function can occur in
both males and females.
Wald’s Visual Cycle
Wald's Visual Cycle, also known as the retinoid cycle, is a series of biochemical reactions that
regenerate the visual pigments in the eye. This process is essential for continuous vision and for
adapting to different light conditions.
Here are the key points of the cycle:
• Light Absorption: The cycle begins when a photon of light is absorbed by
rhodopsin, the light-sensitive pigment found in rod cells.
• Isomerization: This light absorption causes the 11-cis-retinal component of
rhodopsin to change its shape, or isomerize, into all-trans-retinal.
• Dissociation: The all-trans-retinal then detaches from the opsin protein, a process
known as "bleaching" of the rhodopsin.
• Transport and Reduction: The free all-trans-retinal is reduced to all-trans-retinol,
which is then transported to the retinal pigment epithelium (RPE), a layer of cells
adjacent to the photoreceptors.
• Regeneration: In the RPE, the all-trans-retinol is converted back into 11-cis-retinal
through a series of enzymatic steps.
• Return and Recombination: The newly formed 11-cis-retinal is transported back
to the photoreceptor cells, where it recombines with opsin to regenerate rhodopsin.
This cycle allows the photoreceptor cells to be ready to absorb another photon of light, thus
enabling sustained vision. The regeneration of rhodopsin is a slower process, which explains why
it takes time to adapt to changes in light, like when moving from a brightly lit room to a dark one.
The cycle's reliance on vitamin A highlights the importance of this vitamin for healthy vision.
,The cycle's reliance on vitamin A highlights the importance of this vitamin for healthy vision.
VITAMIN D
Vitamin D is unique among vitamins because it can be obtained from both dietary sources and
from the body's own synthesis.
• Sunlight (Endogenous Synthesis): This is the most significant source for most
people. The skin contains a precursor molecule called 7-dehydrocholesterol. When
exposed to ultraviolet B (UVB) rays from sunlight, this molecule is converted into
cholecalciferol (Vitamin D_3).
• Dietary Sources:
• Fatty Fish: Oily fish like salmon, sardines, mackerel, and herring are
excellent natural sources of Vitamin D.
• Egg Yolk: The yolk of eggs contains Vitamin D.
• Fortified Foods: In many countries, foods are fortified with Vitamin D to
prevent deficiency. Common fortified foods include milk, some breakfast
, cereals, and margarine.
• Mushrooms: Some mushrooms, especially those exposed to UV light, can
be a source of Vitamin D_2 (ergocalciferol).
Recommended Dietary Allowance (RDA)
The RDA of Vitamin D is 400 International Units (IU), which is equivalent to 10 micrograms (mcg).
The countries with good sunlight such as India the RDA is 200 IU
Biochemical Functions
Vitamin D, specifically its active form calcitriol (1,25-dihydroxycholecalciferol), functions more
like a hormone than a typical vitamin. It plays a critical role in calcium and phosphate
homeostasis.
1. Calcium and Phosphate Homeostasis: This is the primary and most well-
known function. Calcitriol regulates the serum concentrations of calcium and
phosphate by acting on three target organs:
• Intestines: Calcitriol stimulates the synthesis of a calcium-binding protein in
the intestinal cells. This protein enhances the absorption of dietary calcium
and phosphate, increasing their levels in the blood.
• Kidneys: Calcitriol, along with parathyroid hormone (PTH), promotes the
reabsorption of calcium from the renal tubules, preventing its loss in urine. It
also helps in phosphate reabsorption.
• Bones: Calcitriol is essential for normal bone formation and mineralization.
When blood calcium levels are low, it can work with PTH to mobilize calcium
and phosphate from bone, helping to maintain blood levels.
, 2. Gene Regulation: Calcitriol binds to a specific receptor called the Vitamin
D receptor (VDR). This complex then binds to DNA, acting as a transcription
factor to regulate the expression of various genes, including those
responsible for calcium-binding proteins.
3. Other Functions: The VDR is found in most cells of the body, suggesting
that Vitamin D has non-skeletal functions. It is involved in cell differentiation
and growth, and has been linked to the regulation of immune, endocrine, and
cardiovascular systems.
Deficiency Manifestations
A deficiency in Vitamin D leads to impaired calcium and phosphate absorption, which in turn
affects bone health. The most prominent deficiency manifestations are:
• Rickets (in children): Rickets in children is characterized by bone deformities due to
incomplete mineralization, resulting in soft and pliable bones and delay in teeth formation. The
,weight-bearing bones are bent to form bow-legs. In rickets, the plasma level of Calcitriol is
decreased and alkaline phosphatase activity is elevated. Alkaline phosphatase is concerned
with the process of bone formation. There is overproduction of alkaline phosphatase related to
more cellular activity of the bone.
• Osteomalacia (in adults): In adults, Vitamin D deficiency leads to the demineralization of
existing bone. This causes the bones to become soft and weak, leading to bone pain, muscle
weakness, and an increased risk of fractures.
• Secondary Hyperparathyroidism: A chronic Vitamin D deficiency can lead to low blood
calcium levels (hypocalcemia). This triggers the parathyroid glands to produce more parathyroid
hormone (PTH) in an attempt to raise blood calcium. This condition, known as secondary
hyperparathyroidism, leads to increased bone breakdown to release calcium, further worsening
bone health.
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