An abnormally low level of vitamin B12 (cobalamin) is a factor in many disorders.
The absorption of dietary vitamin B12 occurs in the small intestine and requires a secretion from the stomach known as intrinsic
factor. If intrinsic factor is deficient, absorption of vitamin B12 is severely diminished. Vitamin B12 deficiency impairs
the body's ability to make blood, accelerates blood cell destruction, and damages the nervous system. The result is pernicious
anemia (PA). In the classical definition, PA refers only to B12 deficiency anemia caused by a lack of intrinsic factor.
True pernicious anemia is probably an autoimmune disease. The immune system destroys cells in the stomach that secrete
intrinsic factor. Many people with PA have both chronic inflammation of the stomach lining, called atrophic gastritis, and
antibodies that fight their intrinsic factor-secreting cells.
The term pernicious anemia is sometimes used colloquially
to refer to any anemia caused by vitamin B12 deficiency. Vitamin B12 deficiency can be due to malabsorption of dietary B12
despite normal levels of intrinsic factor. For example, celiac disease and Crohn's disease may cause B12 malabsorption,
which can lead to anemia. Less common causes of B12 deficiency include gastrointestinal surgery, pancreatic disease, intestinal
parasites, and certain drugs. Pregnancy, hyperthyroidism, and advanced stages of cancer may increase the body's requirement
for B12, sometimes leading to a deficiency state.
Low stomach acid, known as hypochlorhydria, interferes with the
absorption of B12 from food but not from supplements. Aging is associated with a decrease in the normal secretion of stomach
acid. As a result, some older people with normal levels of intrinsic factor and with no clear cause for malabsorption will
become vitamin B12-deficient unless they take at least a few micrograms per day of vitamin B12 from supplements.
Caution: pernicious anemia is a serious medical condition. When fatigue, often the first symptom of PA, is present, a qualified
healthcare practitioner should be consulted. Symptoms of PA can be caused by other conditions, none of which would respond
to vitamin B12 supplementation. Moreover, if true vitamin B12 deficiency exists, the cause-lack of intrinsic factor, general
malabsorption conditions, lack of stomach acidity, or dietary deficiency-must also be properly diagnosed by examination and
blood tests before the appropriate treatment can be determined.
Symptoms of severe vitamin B12 deficiency (regardless of the cause)
may include burning of the tongue, fatigue, weakness, loss of appetite, intermittent constipation and diarrhea, abdominal
pain, weight loss, menstrual symptoms, psychological symptoms, and nervous system problems, such as numbness and tingling
in the feet and hands. Most symptoms can occur before the deficiency is severe enough to cause anemia. Healthcare professionals
have a series of laboratory tests that can determine B12 deficiency at earlier stages that are not accompanied by anemia.
Vitamin B12 is
found in significant amounts only in animal protein foods-meat and poultry, fish, eggs, and dairy products. Even small amounts
of these foods supply sufficient amounts of vitamin B12 to provide enough for healthy people.
Except for vegans
(vegetarians who also abstain from eggs, dairy, and other animal products), virtually no one in North America has a diet deficient
in vitamin B12. Those who avoid animal protein foods can easily take vitamin B12 supplements instead. Strict vegans generally
develop a dietary deficiency of vitamin B12, but it is often many years before a deficiency becomes severe enough to cause
symptoms or to be diagnosed. Doctors recommend that all vegans supplement with vitamin B12.
People who lack intrinsic
factor or have a malabsorption condition need to depend on high amounts of vitamin B12 from supplements and not the lower
amounts found in food. Similarly, older people with a vitamin B12 deficiency due to a lack of stomach acid, but not a lack
of intrinsic factor, cannot depend on food-based vitamin B12.
Alcohol abuse can lead to gastritis and damage to the lining of the intestines,
both of which can interfere with vitamin B12 absorption. If B12 deficiency is due to alcoholism, abstinence may prevent further
impairment of B12 absorption.
Normally,
only 3 to 4 mcg per day of vitamin B12 is required to prevent dietary deficiency. If gastrointestinal function is normal,
even these small amounts of vitamin B12 from oral supplementation can prevent deficiency in vegans. If a deficiency already
exists, most doctors will recommend an initial vitamin B12 injection, then oral amounts ranging from 500 mcg to 1,000 mcg
per day until symptoms subside; this is followed by a maintenance level of approximately 10 mcg per day to prevent future
deficiencies.
In a person with true PA, initial B12 supplementation should begin with an injection given by a qualified
healthcare professional. After blood abnormalities are reversed, maintenance supplementation can be successfully accomplished
with oral vitamin B12 at 1,000 to 2,000 mcg (1 to 2 mg) per day and does not require further injections. In a person lacking
intrinsic factor, only about 1% of this oral amount (10-20 mcg) will be absorbed, but that amount is more than sufficient
to prevent future vitamin B12 deficiency.
People with a vitamin B12 deficiency due to a malabsorption condition
must have an appropriate treatment tailored to their individual needs by a healthcare professional. In older people who have
inadequate absorption of vitamin B12 from food due to low stomach acid, prevention of deficiency can be achieved with small
amounts of supplemental vitamin B12 found in B-complex and multivitamins. However, if a deficiency already exists in such
people, a vitamin B12 injection is typically the initial treatment, followed by varying amounts of oral supplemental vitamin
B12 depending on the extent of the deficiency.
Caution: Individuals with vitamin B12 deficiency
must not take large amounts (greater than 800 mcg per day) of folic acid without the supervision of a doctor. At high levels,
folic acid can mask the signs of vitamin B12 deficiency, potentially resulting in serious and irreversible nerve damage.
