Vitamin B12
Function of B12 in the body
Metabolism
Vitamin B12
is essential for metabolism of fats and carbohydrates and the synthesis
of proteins. Vitamin B12 is also essential for the transport and storage
of folate in cells and for conversion to its active form. Rapidly dividing
cells, such as those in the epithelium and bone marrow, have the greatest
need for vitamin B12.
Brain and nervous system
Vitamin B12
is involved in the manufacture of the myelin sheath, a fatty layer which
insulates nerves. It is also essential in the formation of neurotransmitters.
Blood cells
The manufacture
and normal functioning of blood cells requires vitamin B12.
Genetic material
Vitamin B12
is necessary for the production of nucleic acids, which make up DNA, the
genetic material of the cell.
Absorption and metabolism
A compound
known as intrinsic factor which is secreted by the cells lining the stomach
is necessary for absorption of vitamin B12 from the small intestine. Those
with malabsorption problems; such as celiac disease, low stomach acid,
or who have had stomach or intestinal surgery; may have problems absorbing
vitamin B12. Calcium and iron assist with vitamin B12 absorption.
Vitamin B12 is bound to proteins known as transcobalamins in the blood. It is excreted in the bile and re-absorbed. Those on diets which are low in vitamin B12 may obtain more from re-absorption than from food. Because of this re-absorption, vitamin B12 deficiency can take many years to become apparent.
The Schilling test, which uses a small dose of radioactive vitamin B12 and then a larger dose of normal B12 to flush this out, is used to measure the ability of a person to absorb vitamin B12.
Deficiency
As the body
stores vitamin B12, symptoms of deficiency can take up to four to five
years of poor dietary intake or lack of intrinsic factor production to
appear. Deficiency is more commonly linked to the inability to absorb the
vitamin due to lack of intrinsic factor than to insufficient dietary intake.
Elderly people
Vitamin B12
deficiency is more common in the elderly than in younger people, with around
15 per cent of elderly men and women affected. This is usually because
of decreased absorption due to reduced production of intrinsic factor or
to a stomach disorder known as atrophic gastritis. Supplementation can
prevent irreversible neurological damage if started early. Elderly people
with vitamin B12 deficiency may show psychiatric or metabolic deficiency
symptoms even before anemia is diagnosed. Screening for low vitamin B12
levels is necessary in elderly people with mental impairment, although
it has also been found that deficiency states can still exist even when
blood levels are higher than the traditional lower reference limit for
vitamin B12. Patients who are most at risk of vitamin B12 deficiency include
those with gastrointestinal disorders, autoimmune disorders, Type I diabetes
mellitus and thyroid disorders, and those receiving long-term therapy with
gastric acid inhibitors.
Blood
Vitamin B12
deficiency causes pernicious anemia with symptoms of tiredness, pallor,
lightheadedness, breathlessness, headache and irritability. Red blood cells
become abnormally enlarged and reduced blood platelet formation causes
poor clotting and bruising. A high intake of folic acid can prevent the
red blood cell changes caused by vitamin B12 deficiency. It does not, however,
prevent the nerve damage which may only become apparent in later stages
and which may not be reversible. Strict vegetarians, whose folic acid intakes
are high while their vitamin B12 intakes are low, may be at particular
risk of nerve damage.
Immune system
Vitamin B12
deficiency leads to reduced numbers of white blood cells which causes increased
susceptibility to infection. Recent research has shown that elderly patients
with low vitamin B12 levels have impaired antibody response to bacterial
vaccine, even when there are no clinical signs of deficiency.
Brain and nervous system
Vitamin B12
deficiency eventually leads to a deterioration in mental functioning, to
neurological damage and to a number of psychological disturbances including
memory loss, disorientation, dementia, moodiness, confusion and delusions.
Alzheimer's disease sufferers are often found to have low vitamin B12 levels,
although it is unclear whether these are a contributing factor or a result
of the disease.
Vitamin B12 deficiency leads to a loss of nerve-insulating myelin which begins at the peripheral nerves and eventually moves up to the spine causing decreased reflexes, abnormal gait, weakness, fatigue, poor vision and impaired touch or pain sensation. Other signs include tingling or loss of sensation and weakness in hands and feet, and diminished sensitivity to vibration and position sense.
Gastrointestinal system
Vitamin B12
deficiency causes poor cell formation in the digestive tract and leads
to nausea, vomiting, loss of appetite, poor absorption of food, soreness
of the mouth and tongue, and diarrhea.
Heart disease
Vitamin B12
deficiency may lead to increased levels of an amino acid called homocysteine,
which has been linked to an increased risk of heart disease.
Other symptoms
Vitamin B12
is involved in production of the genetic material of the cell and deficiency
may cause defective production which could lead to cancer. A 1997 Australian
study found that low levels of vitamin B12 could contribute to chromosome
damage in white blood cells. Low levels of Vitamin B12 may also contribute
to diabetic neuropathy, poor vision, recurrent yeast infections and infertility.
Vitamin B12 affects bone cells, and deficiency may be risk factor for osteoporosis.
Sources
Good sources
of vitamin B12 include liver and organ meats, muscle meats, fish, eggs,
shellfish, milk and most dairy products. Sea vegetables and fermented soybean
products such as miso also contain forms of vitamin B12, although some
research suggests that the human body may not be able to absorb these forms
and they may even block true vitamin B12 absorption. Many vegetarian and
vegan products are fortified with vitamin B12, including yeast extract,
vegetable stock and soya milk. Cooking has little effect on vitamin
B12 although some may be lost when food is cooked to temperatures above
212 degrees F.
RDAs for vitamin B12 have
recently been raised in the US.
Men 2.4 mcg
Women 2.4 mcg
Pregnancy 2.6 mcg
Lactation 2.8 mcg
Supplements
Vitamin B12
is available in several supplemental forms, both oral and injectable. Cyanocobalamin
is the main synthetic form and has a cyanide molecule attached. Methylcobalamin
is one of two active forms of vitamin B12 and may be a more effective supplement.
Vegans are at particular risk of vitamin B12 deficiency and may need supplements. Vitamin B12 tablets should be taken one hour before food for optimal absorption.
Toxic effects of excess
intake
There have
been no reports of toxic effects even at high doses.
Therapeutic uses of supplements
Pernicious anemia
Both oral
and injectable vitamin B12 supplements are used to treat pernicious anemia.
In those who lack sufficient intrinsic factor and cannot absorb vitamin
B12, it is usually given by injection, although there is evidence that
oral administration in high enough doses is effective. An intranasal gel
is also available.
HIV/AIDS
Lower than
normal serum vitamin B12 levels are common in those infected with HIV and
may help predict those patients in whom the disease will progress most
rapidly. AIDS patients often show signs of nerve damage including numbness
and tingling in the hands and toes, and vitamin B12 may be useful in treating
these symptoms. Recent studies have found that deficiency of vitamin B12
is associated with lower measures of immune system effectiveness in HIV-positive
people and that increasing vitamin B12 levels increases these counts.
Sleep
Some research
suggests that vitamin B12 might affect sleep quality and performance. In
a 1996 study, researchers explored the effects of 3 mg of vitamin B12 on
the quality of sleep and work performance of ten healthy, male staff members
of an Austrian industrial plant. The results showed better sleep quality
and shorter total sleep time in those taking supplements.
Interactions with other
nutrients
Vitamin B12
works closely with folic acid and vitamin B6 in a number of body functions.
Vitamin B6 deficiency reduces vitamin B12 absorption.
Interactions with drugs
Acids and
alkalis, water, sunlight, alcohol, estrogen and sleeping pills can destroy
vitamin B12. Antacids, anti-epileptic drugs, cholestyramine and colchicine
may decrease vitamin B12 absorption. Chloramphenicol and other bone marrow
suppressant drugs may interfere with the red blood cell functions of vitamin
B12. Smoking affects vitamin B12 metabolism.
Cautions
The clinically
available cyanocobalamin form of vitamin B12 should not be used in patients
with hereditary optic nerve atrophy or suspected cobalt hypersensitivity.
Large doses of vitamin B12 should be used with caution in those with low
blood levels of potassium (due to diuretic drugs or other causes).