Studies have found that 20% of Americans ages 65 and over have low levels of the vitamin in their blood... More serious shortages can result in dementia or anemia, because B12 is essential for the production of red blood cells in the bone marrow. Breast-fed infants of mothers with a B12 deficiency are at risk for severe developmental abnormalities and irreversible neurological damage. Some experts say that it’s the most common nutritional deficiency in the developing world and possibly in the United States as well.
...the crystalline form of the vitamin contained in pills and breakfast cereals is more readily absorbed than the “natural” form found in food... Plants neither use nor contain vitamin B12, so the only dietary sources are animal-derived: meat, fish, shellfish, poultry, eggs, milk, and milk products. Beef liver and several varieties of fish contain high amounts, dairy products and eggs not so much. Vegetarians can eat tempeh, which is made from fermented soybeans (the bacteria produce B12).
In normal digestion, stomach juices pry B12 loose from animal protein. But the small intestine can’t absorb the vitamin in this liberated, solo state. To be absorbed, it must be combined with a protein called intrinsic factor that is produced by cells in the lining of the stomach.
Four ways deficiencies develop
1. Diets without food derived from animals. (Explained above.)
2. Lack of stomach acid. Up to 30% of people ages 50 and over suffer from atrophic gastritis, a thinning of the stomach lining. This condition reduces the acid secretions that free B12 from animal protein, so much less is absorbed by the small intestine.
3. Lack of intrinsic factor. Some people’s stomachs don’t make enough intrinsic factor, so they can’t absorb enough B12.
4. Gastrointestinal disorders and surgeries. Crohn’s disease might cause a B12 shortage because it interferes with the functioning of the small intestine. Some people develop a B12 shortage after a surgical procedure that shrinks the stomach, such as gastric bypass, because a smaller stomach means far fewer of the cells secreting stomach acid and intrinsic factor.
I sure am glad I take my multi-vitamins!
Source: Harvard Health Letter, Volume 30, No. 10 - "Are You Getting Enough of This Vitamin?"