According to the National Institutes of Health (NIH), the best food sources for B12 are beef liver and clams, followed by fish, meat, poultry, eggs and milk. Vitamin B12 can only be found naturally in animal foods and products.
The NIH states that vitamin B12 is responsible for the formation of DNA, keeping blood cells and nerve tissue healthy. It also prevents a specific kind of anemia called megaloblastic anemia. Insufficient B12 in a person's diet leads to pernicious anemia, neuropathy, depression, fatigue, constipation and lack of appetite, among other problems.
People who do not eat meat must supplement their diets with B12, as it cannot be provided by plant products. B12 supplements are commonly added to fortified foods, such as breakfast cereals, and are also available as dietary supplements or injections.
Some people cannot metabolize B12 properly, possibly due to advanced age or an inability to produce the intrinsic factor protein necessary to absorb the vitamin. These people must 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. Individuals should consult a physician if they are concerned about possible B12 deficiency, especially people over 50, vegetarians, and vegans.