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Vitamin A
Vitamin A is linked to a family of similarly shaped molecules, the
retinoids, which complete the remainder of the vitamin sequence. Its important part is the retinyl group, which can be found in several forms. In foods of animal origin, the major form of vitamin A is an
ester, primarily
retinyl palmitate, which is converted to an
alcohol (
retinol) in the small intestine. Vitamin A can also exist as an
aldehyde (
retinal), or as an acid (
retinoic acid). Precursors to the vitamin (
provitamins) are present in foods of plant origin as some of the members of the
carotenoid family of compounds.
[1]All forms of vitamin A have a
beta-ionone ring to which an
isoprenoid chain is attached. This structure is essential for vitamin activity.
[1] The orange pigment of
carrots -
beta-carotene - can be represented as two connected retinyl groups, which are used in the body to contribute to Vitamin A levels. The retinyl group, when attached to a specific protein, is the only primary light absorber in
visual perception, and the compound name is related to the
retina of the eye.
Vitamin A can be found in various forms:
retinol, the form of vitamin A absorbed when eating animal food sources, is a yellow, fat-soluble,
vitamin with importance in vision and bone growth. Since the alcohol form is unstable, the vitamin is usually produced and administered in a form of retinyl
acetate or
palmitate.
other
retinoids, a class of chemical compounds that are related chemically to vitamin A, are used in medicine.
[2]Recommended daily intake of vitamin A
Life Stage Group
RDA/AI*
μg/day
UL
μg/day
Infants
0–6 months7–12 months
400*500*
600600
Children
1–3 years4–8 years
300400
600900
Males
9–13 years14–18 years19 - >70 years
600900900
170028003000
Females
9–13 years14–18 years19 - >70 years
600700700
170028003000
Pregnancy
<19>50 years
750770
28003000
Lactation
<19>50 years
12001300
28003000
RDA = Recommended Dietary AllowancesAI* = Adequate IntakesUL = Upper Limit
Sources of vitamin A
Vitamin A is found naturally in many foods:
liver (beef, pork, chicken, turkey, fish)
sweet potatocarrot [
broccoli leaf
kalebutterspinachleafy vegetablespumpkincollard greenscantaloupe meloneggapricotpapayamangopeabroccoli
winter squashMetabolic functions of vitamin A
Vision
Gene transcription
Immune function
Embryonic development and reproduction
Bone metabolism
Haematopoiesis
Skin health
Reducing risk of heart disease
Antioxidant Activity
Vitamin B12
Vitamin B12 is a water soluble
vitamin with a key role in the normal functioning of the
brain and
nervous system, and for the formation of
blood. It is one of the eight
B vitamins. It is normally involved in the
metabolism of every
cell of the body, especially affecting
DNA synthesis and regulation, but also
fatty acid synthesis and energy production.
Vitamin B12 is the name for a class of chemically-related compounds, all of which have vitamin activity. It is structurally the most complicated vitamin and it contains the biochemically rare element
cobalt.
Biosynthesis of the basic structure of the vitamin can only be accomplished by
bacteria, but conversion between different forms of the vitamin can be accomplished in the human body. A common synthetic form of the vitamin,
cyanocobalamin, does not occur in nature, but is used in many pharmaceuticals and supplements, and as a food additive, due to its stability and lower cost. In the body it is converted to the physiological forms,
methylcobalamin and
adenosylcobalamin, leaving behind the
cyanide, albeit in minimal concentration. More recently,
hydroxocobalamin, methylcobalamin and, adenosylcobalamin can also be found in more expensive pharmacological products and food supplements. The utility of these is presently debated.
Historically, vitamin B12 was discovered from its relationship to the disease
pernicious anemia, which is an autoimmune disease that destroys parietal cells in the stomach that secrete
intrinsic factor. Intrinsic factor is crucial for the normal absorption of B12, therefore, a lack of intrinsic factor, as seen in pernicious anemia, causes a vitamin B12 deficiency. Many other subtler kinds of
vitamin B12 deficiency, and their biochemical effects, have since been elucidated.
Functions
Vitamin B12 is normally involved in the metabolism of every cell of the body, especially affecting the DNA synthesis and regulation but also fatty acid synthesis and energy production. However, many (though not all) of the effects of functions of B12 can be replaced by sufficient quantities of
folic acid (another B vitamin), since B12 is used to regenerate
folate in the body. Most "B12 deficient symptoms" are actually folate deficient symptoms, since they include all the effects of
pernicious anemia and
megaloblastosis, which are due to poor synthesis of DNA when the body does not have a proper supply of
folic acid for the production of
thymine. When sufficient folic acid is available, all known B12 related deficiency syndromes normalize, save those narrowly connected with the B12 dependent enzymes
Methylmalonyl Coenzyme A mutase (MUT), and
5-methyltetrahydrofolate-homocysteine methyltransferase (MTR), also known as methionine synthase; and the buildup of their respective substrates (
methylmalonic acid, MMA) and
homocysteine.
Symptoms and damage from deficiency
Vitamin B12 deficiency can potentially cause severe and irreversible damage, especially to the brain and nervous system. At levels only slightly lower than normal, a range of symptoms such as
fatigue,
depression, and poor
memory may be experienced. However, these symptoms by themselves are too nonspecific to diagnose deficiency of the vitamin.
Vitamin B12 deficiency can also cause symptoms of mania and psychosis.
Vitamin B12 deficiency has the following pathomorphology and symptoms
Pathomorphology includes: A
spongiform state of neural tissue along with
edema of fibers and deficiency of tissue. The
myelin decays, along with axial fiber. In later phases, fibric
sclerosis of nervous tissues occurs. Those changes apply to dorsal parts of the spinal cord, and to pyramidal tracts in lateral cords. The pathophysiologic state of the spinal cord is called
subacute combined degeneration of spinal cord.
In the brain itself, changes are less severe: they occur as small sources of nervous fibers decay and accumulation of
astrocytes, usually subcortically located, an also round hemorrhages with a torus of glial cells. Pathological changes can be noticed as well in the posterior roots of the cord and, to lesser extent, in peripheral nerves.
Clinical symptoms : The main syndrome of vitamin B12 deficiency is Biermer's disease (
pernicious anemia). It is characterized by a triad of symptoms:
Anemia with bone marrow promegaloblastosis (
megaloblastic anemia)
Gastrointestinal symptoms
Neurological symptoms
Each of those symptoms can occur either alone or along with others. The neurological complex, defined as
myelosis funicularis, consists of the following symptoms:
Impaired perception of deep touch, pressure and vibration, abolishment of sense of touch, very annoying and persistent
paresthesiasAtaxia of dorsal cord type
Decrease or abolishment of deep muscle-tendon reflexes
Pathological reflexes -
Babinski,
Rossolimo and others, also severe
paresisDuring the course of disease, mental disorders can occur which include: irritability, focus/concentration problems, depressive state with suicidal tendencies,
paraphrenia complex. These symptoms may not reverse after correction of hematological abnormalities, and the chance of complete reversal decreases with the length of time the neurological symptoms have been present.
Sources
Foods
Vitamin B12 is naturally found in meat (especially liver and
shellfish),
milk and eggs. Animals, in turn, must obtain it directly or indirectly from bacteria, and these bacteria may inhabit a section of the gut which is posterior to the section where B12 is absorbed. Thus,
herbivorous animals must either obtain B12 from bacteria in their
rumens, or (if fermenting plant material in the
hindgut) by reingestion of
cecotrope fæces.
Eggs are often mentioned as a good B12 source, but they also contain a factor that blocks absorption.
[31] Certain insects such as
termites contain B12 produced by their gut bacteria, in a manner analogous to ruminant animals.
[32] An NIH Fact Sheet lists a variety of food sources of vitamin B12.
According to the U.K.
Vegan Society, the present consensus is that any B12 present in plant foods is likely to be unavailable to humans and so these foods should not be relied upon as safe sources, as the B12 analogues can compete with B12 and inhibit
metabolism. Also,
vegan humans who eat only plant based foods must ordinarily take special care to supplement their diets accordingly. The only reliable
vegan sources of B12 are foods fortified with B12 (including some soy products and some breakfast cereals), and B12 supplements.
While
lacto-ovo vegetarians usually get enough B12 through consuming
dairy products, vitamin B12 may be found to be lacking in those practicing
vegan diets who do not use
multivitamin supplements or eat B12 fortified foods. Examples of fortified foods often consumed include fortified breakfast cereals, fortified
soy-based products, and fortified
energy bars.
Claimed sources of B12 that have been shown through direct studies of vegans to be inadequate or unreliable include
laver (a
seaweed),
barley grass, and human gut bacteria.
Natural food sources of B12
Vitamin B12 is found in foods that come from animals, including fish, meat, poultry, eggs, milk, and milk products. One half chicken breast provides some .3 µg per serving or 6.0% of one's daily value (DV), 3 ounces of beef, 2.4 µg, or 40% of one's DV, one slice of liver 47.9 µg or 780% of DV, and 3 ounces of
molluscs 84.1 µg, or 1,400 % of DV, while one egg provides .6 µg or 10% of DV.
Supplements
Vitamin B12 is provided as a supplement in many processed foods, and is also available in vitamin pill form, including multi-vitamins. Vitamin B12 can be supplemented in healthy subjects also by liquid, transdermal patch, nasal spray, or injection and is available singly or in combination with other supplements.
Cyanocobalamin is converted to its active forms, first
hydroxocobalamin and then
methylcobalamin and
adenosylcobalamin in the liver.
The sublingual route, in which B12 is presumably or supposedly absorbed more directly under the tongue, has not proven to be necessary or helpful. A 2003 study found no significant difference in absorption for serum levels from oral vs. sublingual delivery of 500 µg (micrograms) of cobalamin. . Sublingual methods of replacement are effectively only because of the typically high doses (500 micrograms), which are swallowed, not because of placement of the tablet. As noted below, such very high doses of oral B12 may be effective as treatments, even if GI tract absorption is impaired by gastric atrophy (pernicious anemia).
Injection and patches are sometimes used in cases where digestive absorption is impaired, but there is evidence that this course of action may not be necessary with modern high potency oral supplements (such as 500 to 1000 µg or more). Even pernicious anemia can be treated entirely by the oral route. These supplements carry such large doses of the vitamin that 1% to 5% of high oral doses of free crystalline B12 is absorbed along the entire intestine by passive diffusion.
However, if the patient has inborn errors in the methyltransfer pathway (cobalamin C disease, combined
methylmalonic aciduria and
homocystinuria), treatment with intravenous, intramuscular
hydroxocobalamin or transdermal B12 is needed.
[41][42][43][44][45]Cyanocobalamin is also sometimes added to beverages including
Diet Coke Plus and many
energy drinks (one example would be
Chaser's Five Hour Energy Drink, which contains 8333% (500 µg)of the
Recommended Daily Value of Vitamin B12).
[
edit] Recommendations
The
Dietary Reference Intake for an adult ranges from 2 to 3 µg (micrograms) per day.
Vitamin B12 is believed to be safe when used orally in amounts that do not exceed the recommended dietary allowance (RDA). The RDA for vitamin B12 in
pregnant women is 2.6 µg per day and 2.8 µg during
lactation periods. There is insufficient reliable information available about the safety of consuming greater amounts of Vitamin B12 during pregnancy.
The
Vegan Society, the Vegetarian Resource Group, and the
Physicians Committee for Responsible Medicine, among others, recommend that vegans either consistently eat foods fortified with B12 or take a daily or weekly B12 supplement. Fortified
breakfast cereals are a particularly valuable source of vitamin B12 for
vegetarians and
vegans. In addition, adults age 51 and older are recommended to consume B12 fortified food or supplements to meet the RDA, because they are a population at an increased risk of deficiency
Allergies
Vitamin B12 supplements in theory should be avoided in people sensitive or
allergic to cobalamin, cobalt, or any other product ingredients. However, direct allergy to a vitamin or nutrient is extremely rare, and if reported, other causes should be sought.
Vitamin C
