Vitamins Essential to Human Nutrition

The Science Behind Health
With Doctor Bones (Don R. Mueller, Ph.D.)
The Funny Man of Health
Educator
Entertainer
J
U
G
G
L
E
R
Scientist

Vitamins are essential organic compounds, needed in small
amounts for normal functioning of the human body's metabolism,
growth and reproduction. A vitamin is called "essential" because
either we cannot make it in the body (called synthesis) or that we
produce too little of it for good health.
For example, humans cannot synthesize vitamin C. Therefore, we
must obtain it via our diet. On the other hand, we can synthesize
enough vitamin D in our skin if we receive sufficient exposure to
sunlight. If we are sun deficient, then we may also find ourselves
vitamin D deficient and must obtain vitamin D through our diet.

Vitamins are essential organic compounds, needed in small
amounts for normal functioning of the human body's metabolism,
growth and reproduction.
The 13 or so vitamins that we require for good health include the
fat-soluble vitamins A, D, E and K and the water-soluble B-
Vitamins (B1, B2, B3, B5, B6, B7, B9, B12) and Vitamin C.

The 13 or so vitamins that we require all
play important roles in sustaining life.
The fat-soluble vitamins that we need
include vitamins A, D, E and K. Fat-
soluble vitamins dissolve in fat and
therefore can be stored in the fatty
tissues of the body. As a consequence,
we do not need to ingest fat-soluble
vitamins daily.
An assortment of B-vitamins and vitamin C are the water-soluble
vitamins that we require. Water-soluble vitamins dissolve in
water and not in fat and therefore cannot be stored in the fatty
tissues of the body. In order to avoid a deficiency, water-soluble
vitamins are needed on nearly a daily basis.

Did you know that the word vitamin comes from the word
"vitamine?" It's true! Vitamine, which means "vital amine," was
coined in 1911 by the Polish-American biochemist Casimir Funk
because it was thought at the time that these substances
contained amines, compounds that have nitrogen (N) as part of
their molecular structure.
The letter "e" in vitamine was later dropped when it was
determined that not all vitamins contained nitrogen. It was
Casimir Funk who discovered vitamin B1 (thiamine).

A Science-Based Examination of the
Fat-soluble Vitamins ( A, D, E and K)

Fat-soluble Vitamins ( A, D, E and K)
Vitamin A (also called retinol) is fat-soluble, and as such, can
be found in the body’s fatty tissues, although the majority is
stored in the liver as retinyl palmitate. Vitamin A can be obtained
either directly from foods that are substantial in vitamin A (e.g.,
beef liver, fish liver oils, egg yolks and butter) or by converting a
substance called beta-carotene into vitamin A.
In 1913, the biochemist Elmer V. McCollum and his
colleagues conducted nutritional studies that led
to the discovery of vitamin A in butterfat and cod
liver oil. McCollum originally called it "fat-soluble
factor A" and then later "vitamine A."

An early sign of vitamin A deficiency is night
blindness. You've probably heard the saying,
“Carrots for good eyesight!" In truth,
carrots contain a substance called "beta-
carotene" (carotene, coming from carrots)
which the body can convert to vitamin A
when needed.
"Eat your carrots!”
Beta-carotene originates from compounds called "carotenoids."
These substances exhibit characteristic yellow and orange colors.
In addition to its role as a provitamin for vitamin A, beta-carotene
also functions as an important antioxidant compound, protecting
cells from the harmful effects of free radicals.

Vitamin A has many Important Functions:
 Plays an important role in vision, especially night vision.
 Helps regulate cell development.
 Promotes the proper growth of bones and teeth.
 Boosts the body's immune system helping to increase our
resistance to infectious diseases.
 Vital in the formation and maintenance of healthy hair,
skin and mucous membranes.
 Vitamin A holds an important place in sexual reproduction.

Vitamin D - "The Sun Vitamin"
Vitamin D3 is synthesized in the body when
ultraviolet light (mostly UV-B radiation) strikes our
skin. Fair-skinned people need 20 to 30 minutes a
day in bright sunlight to meet their vitamin D
needs. The darker your skin the longer you need to
be in the sun in order to make enough vitamin D.
Overcast skies, smog, clothing and sunscreens all decrease your
exposure to UV-B radiation and hence decrease the amount
vitamin D you can produce from sunlight. Fortunately, we can also
meet our vitamin D needs with vitamin D fortified foods (milk, for
example is one such item) and vitamin D supplements.

For humans, the two most important forms of vitamin D are
vitamin D2 and vitamin D3. Vitamin D2 (ergocalciferol) is derived
from plants and irradiated yeast and fungi. Vitamin D3
(cholecalciferol) as discussed previously, is synthesized in the
body when we are exposed to sunlight and can be obtained from
foods such as milk, fortified cereals, tuna, salmon and fish oils.
Vitamin D2 and vitamin D3 have equal biological
activity. Both can be converted, first to
calcifediol (a transport form of vitamin D) in the
liver and then to the bioactive form of vitamin D,
calcitriol (1,25-dihydroxycholecalciferol) in the
kidneys. Calcitriol is then transported via a
carrier protein to the various sites in the body
where it is needed.

Some of the many useful functions of Vitamin D
 Vitamin D aids in the absorption of calcium and phosphorus
from the small intestine for the normal mineralization of bone.
 Vitamin D helps regulate calcium levels in the blood, ensuring
that nerves and muscles function properly, as calcium is vital for
nerve cell transmissions and muscle fiber contractions.
 There is evidence that vitamin D (specifically, vitamin D3) is
involved in regulation of the body's immune system.
 Vitamin D is essential for normal insulin secretion by the
pancreas and therefore control of blood sugar levels.
 The bioactive form of vitamin D3, (calcitriol) is believed to play
an important role in the regulation of the genes involved in cell
growth, differentiation and proliferation.

Vitamin E - The Antioxidant Vitamin
The fat-soluble Vitamin E, is actually a family of eight different
vitamin E molecules. Four of the eight vitamin E molecules are
called the tocopherols (alpha, beta, gamma and delta).
Foods containing significant amounts of tocopherols include a
number of oils (corn, safflower, soybean, cottonseed and canola),
nuts (almonds, hazelnuts, and walnuts), wheat germ and
vegetables like spinach, kale, sweet potatoes and yams.

The remaining four Vitamin E molecules are known as
tocotrienols (alpha, beta, gamma and delta). Sources of
tocotrienols include: oils from palm, rice bran and annatto beans.
In foods, alpha-tocopherol is in the form of d-alpha-tocopherol
(the vitamin E isomer preferred by the body), which is found in a
number of oils, including safflower and sunflower. It is also found
in wheat germ. “Trienol” refers to three C=C double bonds.
C=C double bonds

Some of the Important Functions of Vitamin E
 Vitamin E in the form of d-alpha-tocopherol is an important
fat-soluble antioxidant, scavenging oxygen free radicals: lipid
peroxy radicals and singlet oxygen molecules before these
various radicals can do further harm to cells.
 Vitamin E helps maintain the structural integrity of cell
membranes throughout the body.
 The isomer, d-alpha-tocopherol has been shown to inhibit the
aggregation or "clumping" of blood platelets, hence, helping to
reduce the risk of heart attack and stroke.
 Studies show that d-alpha-tocopherol protects the fat
component in low-density lipoproteins (LDLs) from oxidation and
exhibits moderate cholesterol-lowering capabilities.

Vitamin K - The Anti-Hemorrhagic Vitamin
Vitamin K, was first isolated in 1939 by the Danish biochemist
Henrik Carl Peter Dam, later winning the Nobel Prize in Medicine
in 1943, for his work involving vitamin K.
A decade earlier, it was shown that this fat-soluble substance
(present in green leafy vegetables) was required for normal
coagulation of the blood.
The "K" comes from the German word "koagulation.” Vitamin K
is essential for the functioning of several proteins involved in
normal blood clotting. Vitamin K is needed to make four blood
coagulation factors, including: prothrombin (factor II),
proconvertin (factor VII), Christmas factor (factor IX) and the
Stuart-Power factor (factor X).

Vitamin K1 is the major dietary source of vitamin K and is found
in green leafy vegetables like lettuce, kale, parsley, spinach and
various greens (turnip, beet and mustard). Broccoli is also a
good source of vitamin K1 as are certain vegetable oils (soybean,
cottonseed, canola, and olive).
Vitamin K1 (phylloquinone)
There are three basic forms of vitamin K, namely:

Vitamin K2, which is the most biologically active form of vitamin
K, is found in egg yolks, butter, liver, cheddar cheese and yogurt.
Vitamin K2 is also produced by certain "friendly" intestinal
bacteria in humans and it has been suggested that products
yogurt, kefir and acidophilus milk (fermented milk) may help to
increase the functioning of these useful bacteria.
Vitamin K2
(menaquinone)

Important Functions for Vitamin K
 The primary role for Vitamin K in human health is in the
regulation of normal blood clotting.
 Vitamin K has been shown to improve bone health, particularly
for those at risk for osteoporosis.
 Used to reduce the hazard of bleeding in liver disease.
Vitamin K3 or "menadione" is a fat-
soluble synthetic (man-made) vitamin
K compound, used mainly in animal
feed and pet foods. Although vitamin
K3 is converted to vitamin K2 in the
body, it is generally not recommended
for use in humans.
Vitamin K3

A Science-Based Survey of the B-Vitamins
Thiamin (B1) Riboflavin (B2) Niacin (B3)
Pantothenic Acid (B5) Biotin (B7)
Pyridoxine (B6)
Folic Acid (B9) Cobalamin (B12)
B7
B9

In a wide range of biological processes, B-vitamins function as
coenzymes, uniting with certain protein molecules to form active
enzymes. These enzymes are catalysts in important biochemical
reactions, such as turning carbohydrates into energy and
metabolizing proteins and fats. A catalyst changes the rate of a
chemical reaction without the catalyst itself being consumed.
The B-vitamins being water-soluble, tend not to be stored in the
body for long periods to time. Consequently, it is important that
we continually replenish B-vitamins via a well-balanced diet.
B-Complex vitamin formulas usually contain all of the B-vitamins
and several vitamin-like compounds: namely, Choline, Inositol,
and Para-aminobenzoic Acid (PABA).

The B vitamins are a group of water-soluble vitamins:
Thiamin (B1) Riboflavin (B2) Niacin (B3)
Pantothenic Acid (B5) Pyridoxine (B6)
Biotin (B7) Folic Acid (B9) Cobalamin (B12)
1 2 3
4 5
6 7 8
Eight

Vitamin B1, known as thiamine is also sometimes called aneurine.
Thiamine, which was first isolated in the 1930's, was one of the
first organic compounds to be recognized as a vitamin.
Vitamin B1 (Thiamine or Thiamin)
In humans, thiamine exists both
as free thiamine and in several
phosphorylated forms:
(1) Thiamine monophosphate
(2) Thiamine diphosphate
(3) Thiamine triphosphate.
Thiamine
1

Thiamine diphosphate (thiamine pyrophosphate) is the bioactive
form of thiamin and a cofactor for several very important enzymes,
which help convert foods into energy. Thiamine pyrophosphate
(TPP) comprises both pyrimidine and thiazole ring structures along
with a double-phosphate (pyrophosphate) group.
Thiazole ring
Pyrimidine ring
Pyrophosphate
The phosphates (PO4
3-) in
TPP, are taken from ATP.
Actually, to make TPP from
thiamine requires not only
ATP, but also magnesium
and the enzyme, thiamine
pyrophosphokinase.
Thiamine Pyrophosphate (TPP)
Thiamine + ATP AMP + TPP

Thiamine in the form of TPP is essential for the metabolism of
complex carbohydrates into simple sugars (glucose).
Thiamine is also important for the proper functioning of nerve and
muscle cells. Thiamine is stored in small amounts (25 to 30 mg) in
organs with high metabolic needs, such as the skeletal muscles,
heart, brain, liver and kidneys. As a coenzyme, thiamine plays an
important role in chemical reactions that stimulate the release of
acetylcholine, an important neurotransmitter.
Great sources of vitamin B1 include pork,
beef liver, egg yolks and salmon, whole grain cereals
and flours, wheat germ, brown rice, soy, navy,
kidney and garbanzo (chickpeas) beans, sunflower
seeds, peanuts and brewer's yeast.

2 Vitamin B2 (Riboflavin) was once called Vitamin G.
Riboflavin is constructed from an isoalloxazine ring system linked
to ribitol, which is a 5-carbon alcohol (C5H12O5) formed by the
reduction of ribose, an important 5-carbon sugar (pentose).
Riboflavin
Isoalloxazine
Ring
Ribitol

Vitamin B2 (Riboflavin) is involved with the metabolism of
carbohydrates, proteins and fats.
The isoalloxazine ring in vitamin B2 (riboflavin) is a component
of the flavin coenzymes, flavin mononucleotide (FMN) and
flavin adenine dinucleotide (FAD).
FMN
FAD

Both FMN and FAD are required for enzyme catalyzed oxidation-
reduction (redox) reactions connected with energy metabolism.
The tricyclic isoalloxazine ring system (in FMN and FAD) is the
center of attention in these redox reactions.
For example, flavin adenine dinucleotide (FAD) is the oxidized
form of the coenzyme required for several redox reactions
connected with carbohydrate and fat metabolism. When FAD
accepts two electrons (2 e-), it becomes FADH2 (the reduced form
of FAD). The FADH2 is an electron carrier in these reactions.
FAD FADH2
+ +2 H+ 2 e-

Milk is perhaps the best single source of vitamin B2
(riboflavin), containing nearly two milligrams of
riboflavin per quart, which is enough riboflavin per
day for both children and adults.
Excellent Sources of Vitamin B2:
Other good sources of riboflavin include various cheeses, yogurt,
eggs, liver, beef, chicken, pork, tuna, dark green vegetables such as
broccoli and spinach, cereals, breads, wheat germ, wild rice,
mushrooms, soybeans and brewer's yeast.
For the biochemist, Oxidation-Reduction reactions are electrochemical processes
that are generally concerned with the addition of either oxygen or hydrogen to a
biomolecule. In chemistry, the term “Oxidation” signifies a loss of electrons from
an atom or molecule, whereas “Reduction” suggests a gain of electrons.

Vitamin B3 (Niacin)3
Vitamin B3 (Niacin) is yet another of the B-vitamins needed for the
conversion of foods into energy. Vitamin B3 was originally called
nicotinic acid before being changed to niacin, which is actually a
name used to describe both nicotinic acid and nicotinamide.
Nicotinic acid Nicotinamide

It is the oxidized form of nicotinamide, which serves as a
component in two coenzymes required for energy metabolism:
1) NAD+ (nicotinamide adenine dinucleotide)
2) NADP+ (nicotinamide adenine dinucleotide phosphate).
NAD+ NADP+

Coenzymes NAD+ and NADP+ serve as electron acceptors (also
called electron carriers) in important redox reactions in cells. By
accepting a hydride ion H-, which means a gain of two electrons,
both NAD+ and NADP+ are converted to their respective reduced
forms, NADH and NADPH. (Reduction being a gain of electrons.)
Both NADH and NADPH can transfer (donate) the two electrons to
several types of electron carriers in the electron transport chain,
which takes place inside the cell's mitochondria (within the inner
membrane).
In aerobic respiration, oxygen (O2) is the final acceptor of these
electrons. By shuttling two electrons to the electron transport
chain, NAD+ and NADP+ are regenerated and can serve again as
electron acceptors. The electron donor-acceptor cycle continues.

Sources of Vitamin B3:
Good sources of niacin are protein-rich foods such as meats,
chicken, fish, eggs, dairy products, dried beans/peas and nuts.
One-half cup of dry soybeans provides nearly 12 milligrams of
niacin, while a 4-ounce slice of tofu contains nearly 16 milligrams
of niacin. Fortified cereals are niacin-rich as well.
Our body can make niacin
from tryptophan, but the
synthesis is rather slow. To
make 1 milligram (mg) of
niacin, it takes about 60 mg
of tryptophan. Tryptophan

4 Vitamin B5 (Pantothenic Acid)
The name pantothenic acid is derived from the Greek word
pantothen, meaning "from everywhere," and is so named
because pantothenic acid is available in so many different foods.
OH
HO-CH2-CH—CH—C—NH-CH2CH2-C—OH
CH3
CH3
OO
IIII
I
II
Pantothenic Acid
Vitamin B5 is required for the synthesis of coenzyme A. In fact,
pantothenic acid is largely the “arm” of the coenzyme A molecule.

Coenzyme-A is constructed from three different molecules:
1)Pantothenic acid 2) Cysteine and 3) ATP
Coenzyme-A
Pantothenic acid
Cysteine
ATP
( also called co-A )

The metabolism of fats requires acetyl-coA. From acetyl-coA a
number of important molecules are synthesized, including
triglycerides (fats), cholesterol, phospholipids, prostaglandins,
and of course, ATP.
Sources of Vitamin B5 (Pantothenic Acid):
Good sources of Vitamin B5 are numerous:
including various meats, chicken, fish, eggs,
milk, cheese, dried beans and peas, nuts of all
sorts, whole-grain breads and cereals,
avocados, bananas, carrots, corn, oranges,
mushrooms and so many more.
Pantothenic Acid is truly a ubiquitous vitamin!

5 Vitamin B6 (Pyridoxine)
Vitamin B6 is actually a set of closely related molecules, including:
pyridoxine, pyridoxal and pyridoxamine. All three are efficiently
converted to pyridoxal phosphate, the bioactive form of vitamin B6.
Pyridoxine Pyridoxal
Pyridoxal phosphate
Pyridoxamine

Conversion to pyridoxal phosphate requires the enzyme pyridoxal
kinase. Pyridoxine hydrochloride is a more stable form of vitamin
B6 and is the one found in commercial vitamins.
Pyridoxal phosphate participates as a coenzyme (cofactor) in
a wide variety of important biochemical reactions involving
carbohydrate, protein and fat metabolism:
 Amino acid metabolism
 Breakdown of glycogen
 Synthesis of globular proteins
 Conversion of certain fatty acids

Sources of Vitamin B6:
A wide range of foods that are good sources
of vitamin B6, include beef, pork, chicken,
fish, eggs, liver, dairy products, brown rice,
whole grain breads and cereals, soybeans
and lentils, various nuts and seeds, potatoes,
carrots, avocados, bananas and more.
Interestingly, vitamin B6 is needed in the production of red blood
cells (erythrocytes). Doctors, for example, prescribe vitamin B6 to
treat certain types of anemia. Vitamin B6 also plays a vital role in
maintaining the body's immune system.

6 Vitamin B7 (Biotin) sometimes called Vitamin H.
Vitamin B7 or "Biotin" is sometimes called vitamin H and also
coenzyme R. The name biotin derives from the Greek word bios,
meaning "life." As a coenzyme, biotin plays important roles in a
variety of biochemical processes including the metabolism of
carbohydrates, protein and fats.
Biotin is a bicyclic compound
formed from three molecules:
1) Thiophene ring
2) Ureido group
3) Valeric acid side chain1
2
3

Biotin serves as a cofactor in enzymes called carboxylases,
which transfer carboxyl (CO2) groups in the form of
bicarbonate (HCO3
-) in a number of important biochemical
processes, including the synthesis of glucose (known as
gluconeogenesis) and fatty acids and in the catabolism
(breakdown) of certain amino acids.
In humans, biotin participates in four different carboxylases:
1) Acetyl-CoA carboxylase (ACCase)
2) Propionyl-CoA carboxylase (PCCase)
3) Pyruvate carboxylase (PCase)
4) Beta-Methylcrotonyl CoA carboxylase (beta-MCCase)

How Biotin Works
Biotin present in foods is bound to specific proteins and must be
released before it can be absorbed through the small intestine.
Attaching biotin to another molecule is called "biotinylation."
Action by proteolytic enzymes on protein-bound biotin, results in
a complex called biocytin (also known as epsilon-N-biotinyl-L-
lysine) whereby biotin is attached to the amino acid lysine.
Biocytin
Lysine

Although biocytin is easily absorbed in the small intestine, the
body can only use biotin in its free form (i.e., without the lysine
residue). The enzyme biotinidase cleaves biocytin into biotin and
lysine. Biotin is then free to perform its duty as a coenzyme.
Sources of Vitamin B7 (Biotin):
Excellent sources of biotin include egg yolks,
liver, milk, wheat germ and Brewer's yeast.
For those who would prefer other sources,
biotin is found in fortified breads and cereals,
rice, soybeans, peanuts, fish (e.g., herring and
mackerel), mushrooms and bananas.

7 Vitamin B9 (Folic Acid) also called vitamin M.
Vitamin B9 or "folic acid" is also known as folate and folacin.
Actually, it is folate, when found in a variety of leafy green
vegetables and folic acid (the synthetic form of folate) when
added to foods and vitamin supplements. Not surprisingly, the
name "folate" is derived from the Latin word "folium" for leaf.
Folate
Folate in Focus:
Folate is recommended
for woman of
childbearing age to help
prevent neural tube
defects (NTDs) in the
developing fetus.

Folic acid (folate) is constructed from three molecules:
 6-Methylpterin
 Para-aminobenzoic acid (PABA)
 Glutamic acid (glutamate)
6-Methylpterin PABA Glutamate

Tetrahydrofolate (THF): A Vital Coenzyme
Inside cells, folate is converted to its biologically active forms:
7,8-dihydrofolate (DHF) and 5,6,7,8-tetrahydrofolate (THF).
Tetrahydrofolate (THF) is a vital coenzyme in reactions that
involve the transfer of single carbon functional groups such as
methyl (-CH3), methylene (-CH2) and formyl (-HC=O). THF serves
both as an acceptor and a donor of these one-carbon units.
THF
5,6,7,8-tetrahydrofolate

Folate is found in a variety of leafy green
vegetables, bell peppers, oranges, liver, egg
yolks, rice, barley and various legumes.
Folic acid is the synthetic form of folate and
is added to a number of processed foods.
Folic Acid Alert: An interplay between vitamin B12 and vitamin B9
Vitamin B12 and vitamin B9 deficiencies are both a common cause
of anemia. Treating anemia solely with folic acid supplements and
at dosages exceeding 1 milligram (1000 micrograms) per day can
mask the symptoms of a vitamin B12 deficiency.
Sources of Vitamin B9 (Folate):

8 Vitamin B12 (Cobalamin)
The “Red” Vitamin
Called the “Red" vitamin, because it
forms a dark red crystalline compound.
Vitamin B12 is the only vitamin to contain the cobalt (Co3+) metal
ion, which by the way, gives it the red color. Vitamin B12, is also
called cobalamin, cyanocobalamin and hydroxycobalamin. The
coenzyme forms of vitamin B12 serve in a number of capacities.
Dr. Dorothy Crowfoot Hodgkin (a pioneer in the field of X-ray
crystallography) elucidated the structure of vitamin B12 in 1955.

Although it looks quite complicated, vitamin B12 is really built
from some basic parts: namely a nucleotide and a corrin ring.
The nucleotide, consisting
of a nucleic acid, ribose
sugar and a (PO4
3-) group,
is attached to the corrin
ring as are several methyl
groups and amino acids.
The corrin ring (shown
in blue) is made from
four pyrrole groups. At the
center of the corrin ring is
the cobalt (Co) atom
(shown in red).Vitamin B12

The four basic components of Vitamin B12:
Corrin Ring
Cobalt (Co) Amino Acid
Structure
Nucleotide Structure
(1)
(2)
(3)
(4)

Attached to the cobalt Co atom is an R-group (shown in
purple) used to represent the groups associated with the two
cobalamins and the two coenzyme forms of vitamin B12.
If the R-group is cyanide
(CN), then vitamin B12 is
in the configuration of
cyanocobalamin.
In the hydroxycobalamin
form of vitamin B12, the
R-group is given by the
hydroxyl (OH) molecule.
Vitamin B12

With respect to the two coenzyme forms of vitamin B12, R equals
an adenosyl group in adenosylcobalamin and R equals a methyl
group (CH3) in methylcobalamin.
The coenzyme forms of vitamin B12 serve a number of important
functions. Adenosylcobalamin, for example, is the coenzyme for
the enzyme methylmalonyl-CoA mutase, which in mitochondria
converts methylmalonyl-CoA to succinyl-CoA.
Isomerization

This type of conversion or "intramolecular rearrangement" is a
process called isomerization. The reaction is associated with the
breakdown (catabolism) of fatty acids having an odd number of
carbons. The succinyl-CoA can then be metabolized to produce
energy via the Kreb's cycle or used in the synthesis of fatty acids.
People with elevated methylmalonic acid levels in their blood or
urine are not converting methylmalonyl-CoA to succinyl-CoA
efficiently. They may, in fact, have a vitamin B12 deficiency.
Vitamin B12 in foods is bound to proteins and must be released
before we can utilize it. Hydrochloric acid in the stomach does the
job of releasing vitamin B12. The free vitamin, then binds to
glycoproteins called R-binders, forming a vitamin B12/R-binder
complex, thus protecting it from destruction by hydrochloric acid.

A glycoprotein (i.e., sugar-containing protein) compound that has
come to be known as the "intrinsic factor" is required for the
proper absorption of vitamin B12. The intrinsic factor is secreted
by the same parietal cells, which secrete hydrochloric acid into
the stomach. The intrinsic factor binds to vitamin B12 (the
"extrinsic factor") thus enabling its absorption through the small
intestine and into the bloodstream.
The “Intrinsic Factor"
In the stomach, the pH is low (acidic) and the affinity of vitamin
B12 for the intrinsic factor is low, while its affinity for the R-
binders is high. This is ideal, as it keeps vitamin B12 from being
destroyed by the acid. Before vitamin B12 can bind to the
intrinsic factor, the vitamin B12/R-binder complex must enter the
small intestine where the pH is much closer to neutral (pH ≈ 7).

Sources of Vitamin B12 (The “Red” Vitamin):
The human body generally stores from 2 to 10 mg of
vitamin B12 distributed mostly amongst the liver,
kidneys and the nervous system. The liver can secure
enough of the vitamin to last for several years. As a
consequence, vitamin B12 is not needed daily as are
most of the water-soluble B-vitamins.
Vitamin B12 is found in organ and muscle meats, fish,
shellfish, dairy products, eggs and in fortified foods
such as breakfast cereals. Strict vegetarians (those who
avoid dairy products and eggs) are generally at risk of
developing a vitamin B12 deficiency if they are not
taking a vitamin B12 supplement.

Vitamins Essential to Human Nutrition

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