DEAR DOCTOR K:
I was recently diagnosed with a vitamin B12 deficiency. Can I take B12 supplements by mouth? Or do I need the shots?
Tissues throughout the body need vitamin B12, especially in the brain, spinal cord and bone marrow, where blood cells are made. Vitamin B12 in the diet gets absorbed in the part of the small intestine called the ileum. There, it enters the blood. Vitamin B12 deficiency can cause anemia, and problems with the bones, brain and spinal cord.
Low vitamin B12 levels in the blood basically have two causes: Either there is not enough B12 in the diet, or the B12 in the diet has trouble getting absorbed by the ileum. B12 is found naturally only in animal products like meat, fish, poultry, eggs and milk. Many cereals are fortified with it.
In the United States, Canada and other developed nations, there is only one relatively common situation where there is not enough B12 in the diet. Vegans, who avoid all animal-based foods, are at increased risk. I order blood tests for vitamin B12 for all my patients who are vegans.
While there is only one common cause of B12 deficiency due to inadequate amounts of the vitamin in the diet, there are many causes of poor absorption of the vitamin by the gut. The most serious is a disease called pernicious anemia. This autoimmune disease is caused by an attack of the immune system on the ability of the ileum to absorb vitamin B12 in food.
One of the hallmarks of pernicious anemia is that the red blood cells are both larger and less numerous than normal. (I’ve put an illustration of this, below.)
Pernicious anemia was a fatal disease until the mid-20th century. Scientists here at Harvard Medical School discovered that vitamin B12 (which was extracted from the livers of animals) could be given as treatment. It was a life-saving discovery that was honored with the Nobel Prize.
In pernicious anemia, it won’t do any good to take vitamin B12 pills — not even megadoses. The disease won’t let it get absorbed by the gut. As a result, it won’t get into the blood and get carried to all the cells of the body that need it. So, for pernicious anemia you need the vitamin B12 shots. The vitamin B12 in the shots goes directly into the blood: It doesn’t need to be absorbed by the gut.
Besides pernicious anemia, there are other conditions that interfere with the ability of the gut to absorb the vitamin. Any diseases that affect the ileum interfere with the absorption of vitamin B12. An example is Crohn’s disease. Often, B12 shots are necessary. And if Crohn’s disease has injured the ileum badly enough that it needs to be surgically removed, then a B12 pill definitely won’t work.
In tomorrow’s column, I’ll describe the other conditions that can make your vitamin B12 level low, and how to treat them.
B12 deficiency and anemia
In rare cases, low vitamin B12 levels can cause pernicious anemia, a condition in which the bone marrow produces red blood cells that are both larger and less numerous than normal. Symptoms can include yellowish skin, fatigue, shortness of breath, and headaches. Numbness or tingling in the hands and feet and trouble keeping balance are also common.