According to the National Institutes of Health Office of Dietary Supplements, vitamin B12 is responsible for keeping blood cells and nerve tissue healthy, preventing a specific kind of anemia called megaloblastic anemia, and forming DNA. Insufficient B12 in a person's diet leads to pernicious anemia, neuropathy, depression, fatigue, constipation and lack of appetite, among other problems.
The Office of Dietary Supplements states that adults should consume 2.4 micrograms of B12 daily to maintain proper health. Vitamin B12 can only be found naturally in animal foods and products. The best food sources for B12 are beef liver and clams, followed by fish, meat, poultry, eggs and milk. B12 supplements are also available as dietary supplements or injections. People who do not eat meat must supplement their diet with B12 as it cannot be provided by plant products.
Some people cannot metabolize B12 properly, possibly due to advanced age or an inability to produce the intrinsic factor protein necessary to absorb B12. Such people need to supplement their B12 intake with shots or nasal sprays if oral B12 supplements are not effective. Some over-the-counter and prescription drugs can inhibit B12 absorption, including the active ingredients in many antacid medications and Metformin, reports the Office of Dietary Supplements. A person should consult a physician if he is concerned about possible B12 deficiency, especially if he is over 50 or is a vegetarian or vegan.