Vitamin B12 plays an important role in folate metabolism and DNA synthesis. B12 deficiency can lead to folate deficiency, it reduces homocysteine levels and it helps maintain the myelin sheath. Tapeworms, overgrown stomach bacteria, absence of ileum or stomach, lack of enzymes, r-protein or intrinsic factor, reduction of gastric acid production all affect the absorption of Vitamin B12.
Recommended Daily Allowance (RDA)
The recommended daily allowance for men and women aged 19 – 50 is 2.4 micrograms per day. People with atrophic gastritis (inflammation of the stomach lining) should focus on fortified food and dietary supplements for most of their B12 intake because they are well absorbed.
With the exception of fortified foods and some soy milks, animal derived foods are the only good source of B12. Cooked clams, cooked beef liver, cooked oysters, cooked chicken liver, cooked herring, cooked Alaskan king crab, cooked blue crab, canned sardines with bones, cooked salmon, cooked lobster and cooked extra lean ground beef have a high daily value of B12, providing 20% or more of the daily Vitamin B12 requirement. Whole milk, 2% milk, cooked squid, 1% milk, beef frankfurters, cooked cod, beef bologna, cooked pork chops are all good sources of B12, providing 10 – 19% of the daily Vitamin B12 requirement.
Vitamin B12 deficiency is almost always caused by impaired absorption. Injections are used to relieve deficiency because they go directly into the bloodstream. The liver stores substantial amounts of B12 so monthly shots are sufficient. Mega doses of 300 times RDA and B12 nasal gel are also used to ease deficiency.
Aside from people with impaired absorption, vegetarians are at risk for B12 deficiency unless they take supplements or eat fortified cereal regularly. Adult livers store 3 – 5 milligrams of B12 so deficiencies develop slowly over the course of 3 – 12 years. These deficiencies may cause B12 deficiency anemia and pernicious anemia.
No upper intake limit has been established for B12. High levels have not been shown to cause harmful side effects in healthy people. Doses of 1 milligram are routinely used to treat people with deficiencies without any adverse side effects.
USDA.gov. United States Department of Agriculture, 27 Jan. 2012. Web. 27 Jan. 2012. <http://www.fns.usda.gov/tn/Resources/appendd.pdf>.
Insel, Paul M., R. Elaine. Turner, and Don Ross. Nutrition. 3rd ed. Sudbury, MA: Jones and Bartlett, 2007. Print.