In 1849, Dr. Thomas Addison described the first documented cases of what is now called “pernicious anemia.” The patients were pale and weak and got sicker and sicker until they inevitably died. People with pernicious anemia don’t have enough red blood cells, and many of the ones they do have are abnormally large. Unlike other forms of anemia, the condition didn’t respond to any available treatment, and it seemed to affect the nervous system as well as the blood. Eventually, doctors realized that pernicious anemia was a major public health problem, affecting about 1% to 2% of the adults over age 50 in the United States. It occurred mainly in people of European ancestry.
By 1880, it was clear that pernicious anemia resulted from damage to the lining of the stomach. The stomach was failing to secrete something (later called “intrinsic factor”) that the body needed in order to get a particular nutrient from food. The importance of a healthy stomach was later proved through a clever, though disgusting experiment. Healthy people were fed a big serving of ordinary beef, which was allowed to digest for a while in their stomach. Then the partially digested beef was pumped from their stomach and fed to someone with pernicious anemia, who was then able to make normal new red blood cells!
In the 1920s, researchers at Harvard Medical School figured out that they could keep people with pernicious anemia alive if they fed them huge amounts of liver. Liver contains an enormous dose of whatever it is that cures pernicious anemia, enough to compensate for poor absorption. Researchers then tested various kinds of liver extracts, to see if they could isolate and identify that nutrient. They found a water-soluble organic compound that contains the metallic element cobalt and named it vitamin B12.Three of the researchers who contributed to solving this puzzle won the 1934 Nobel Prize in Medicine or Physiology.
Although pernicious anemia is due to a deficiency of vitamin B12, most of the people who were getting the disease were eating food that contained more than enough vitamin B12. They simply weren’t absorbing the vitamin. The vitamin B12 that occurs naturally in animal-based food tends to be bound to protein. The acid in the stomach helps to break the vitamin B12 free from those proteins. Then the vitamin B12 binds to another protein produced by the stomach itself. This protein is called intrinsic factor. When vitamin B12 is bound to intrinsic factor, it is very easily absorbed from the intestine. If the stomach doesn’t produce intrinsic factor, only about 1% of a very large dose of vitamin B12 would get absorbed into the bloodstream.
In other words, people with a normal digestive system can efficiently absorb vitamin B12 from their food or from a supplement they take by mouth. People whose stomach doesn’t make intrinsic factor can’t absorb vitamin B12 efficiently from their food or from supplements. They can meet their needs for vitamin B12 either by getting vitamin B12 shots (thus bypassing the need for intrinsic factor) or by taking such huge doses of vitamin B12 by mouth that they end up absorbing enough to meet their needs.
Theoretically, you could also get a vitamin B12 deficiency by eating a diet that doesn’t contain enough vitamin B12. Dietary deficiency of vitamin B12 wouldn’t be a problem for carnivores or omnivores, because animal-based foods contain vitamin B12. Even though plant foods don’t contain vitamin B12, most plant-eating species don’t have a problem with vitamin B12 deficiency. That’s because they can absorb the vitamin B12 that is made by bacteria in their digestive tract.
Cattle and sheep are particularly good at getting vitamin B12 from their gut bacteria. That’s because plenty of vitamin B12 is being produced in the stomach and is then absorbed in the small intestine. So “foregut fermenters” like cattle and sheep never have a problem with vitamin B12 deficiency as long as there’s some source of cobalt in their food.
The problem gets a bit tricky for “hindgut fermenters” like rabbits and gorillas and human beings. In a hindgut fermenter, the bacterial action that produces vitamin B12 is produced in the large intestine, too late for the vitamin to be absorbed in the small intestine. Rabbits solve this problem by eating some of their own droppings. Gorillas might do the same thing, but they probably get enough vitamin B12 from the termites and other creepy crawlies in their diet. Although plant foods don’t contain any vitamin B12, people who don’t eat animal products can easily get vitamin B12 from a nice clean supplement.
Most cases of vitamin B12 deficiency in human beings result from a disease of the stomach. Dietary deficiency of vitamin B12 is extremely rare, even among alcoholics, vegetarians, and malnourished people and people with liver or kidney disease. That’s probably because the liver can store over a 3-year supply of the vitamin. There have been a few reports of vitamin B12 deficiency in people who eat nothing but plant-based foods or in babies breast-fed by women with a vitamin B12 deficiency.
People who eat no animal products whatsoever are strongly encouraged to take a vitamin B12 supplement.
It’s possible to get “pernicious anemia” without having anemia. In other words, if you have plenty of folic acid in your system, you can end up with the nerve damage from vitamin B12 deficiency without the low red blood cell count and abnormal red blood cells that would help your doctor figure out what’s wrong. This could allow the nerve damage to progress undetected. This was one of the drawbacks to using folic acid supplements instead of eating foods that contain natural folate.
Starting in the late 1990s, the U.S. Food and Drug Administration started requiring refined cereal products, such as white bread and pasta, to be fortified with folic acid. The purpose of this fortification was to prevent birth defects by making sure that women had enough folate in their system when they became pregnant. This policy has reduced the number of babies born with severe birth defects, such as paralysis from an open spine. Unfortunately, the folic acid fortification could end up causing cases of pernicious anemia in elderly people to be overlooked until severe nerve damage occurs.