What Causes Vitamin B12 Deficiency?

I am a 48-year-old female and recently found out I have vitamin B12 deficiency with peripheral neuropathy. Why does this happen?

Vitamin B12 is a vitamin necessary for the proper functioning of many tissues, including the bone marrow, nervous system and gastrointestinal tract. A deficiency of B12 is most commonly seen in the anemia called pernicious anemia. This is an autoimmune disease in which antibodies against certain cells in the the stomach prevent a person from making a substance know as intrinsic factor, which is necessary for the absorbtion of B12. Without intrinsic factor, a person slowly uses up their supply of B12, and eventually becomes deficient, with the development of pernicious anemia.

Vitamin B12 is available only from animal sources. Therefore, strict vegans who eat nothing from an animal source are at risk of deficiency. Because the body’s stores of B12 may last 20-30 years after one stops absorbing, most vegans don’t get into difficulty, but their newborn children may have early problems due to a lack of B12, because they do not get a large supply from their mother — as most of us do.

So why am I going on about pernicious anemia when today’s writer has only mentioned a peripheral neuropathy due to a lack of B12? As I mentioned above, B12 deficiency affects many different tissues, including the nerve cells in the peripheral nerves, those that run from the brain or spinal cord out to our muscles and other tissues. The cells of the spinal cord and brain can also be affected, and if the deficency goes on long enough, those nerve cells die, and cannot be regenerated. The anemia of B12 deficiency is easily treated and the blood count will return to normal, but the damage to the nervous system may unfortunately be permanent.

It has been known for many years that taking folic acid will prevent or correct the anemia due to B12 deficiency, but will do nothing to prevent or correct the damage to nerve cells. Therefore, someone who is B12-deficient for any reason and takes folic acid will not become anemic. This is a problem, since checking for anemia with a routine blood count is a simple, cheap screening test that most doctors do regularly, and if pernicious anemia is found, treatment with B12 can be started before damage to the nervous system has taken place. Taking folic acid sets the stage for the development of the nervous system changes without the accompanying anemia, which may be the situation in which our writer finds herself. Folic acid has been added to foods as a supplement since 1994, after much controversy, since many neurologists feared that this scenario of nervous system disease without anemia could become much more common. Because folic acid is now added to foods, and many people take it as a vitamin on their own, the only way to screen for B12 deficiency will be to measure the B12 level in the blood, a much more expensive test than a blood count, and not one usually done as a screening test. The alternative may be to recommend a B12 supplement for everyone over the age of 40, since even people who lack the intrinsic factor can absorb some of the vitamin when large doses are taken.

The earliest symptoms of the nervous system involvement are usually paraesthesias, numbness and tingling of the feet, and ataxia, a type of uncoordination that comes from loss of the nerves that tell us what position our feet and legs are in. Weakness may develop later. If the condition is caught before there has been much loss of nerve cells, then B12 supplementation may reverse some of the symptoms.

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