Vitamin B-12 is an essential nutrient that does not cause harm in high amounts. Your body needs vitamin B-12 to produce healthy red blood cells, metabolize protein and fat, synthesize DNA and maintain proper neurological function. Inadequate intake or absorption of vitamin B-12 can lead to vitamin deficiency anemia, a condition that can lead to neurological disorders if left untreated. The body absorbs only a small percentage of vitamin B-12 from food and supplements, and no evidence suggests that excessive intake of vitamin B-12 causes adverse effects, according to the Institute of Medicine.
Upper Intake Level
Because of vitamin B-12's low potential for toxicity and adverse effects, the Institute of Medicine has not established a UL level, or tolerable upper intake level, for this vitamin. UL values represent the highest daily intake of a nutrient that most of the general population can tolerate without adverse effects. No data suggests that high doses of vitamin B-12 can cause health problems.
Absorption
Vitamin B-12 must attach to intrinsic factor, a glycoprotein in the stomach, before the body can absorb it in the gastrointestinal tract. Intrinsic factor exists in finite amounts, and once intake of vitamin B-12 exceeds the capacity of intrinsic factor, absorption decreases. The body absorbs only 56 percent of a 1 mcg oral dose of vitamin B-12, according to the National Institutes of Health, and an even lower percentage of higher doses. For example, the body absorbs only about 10 mcg of a 500 mcg oral dose, or about 2 percent. Because of the difficulty of absorbing this vitamin, vitamin B-12 deficiency represents a serious health problem and excessive B-12 intake poses little threat.
Vitamin B-12 Deficiency
Many conditions can limit the body's ability to absorb vitamin B-12, leading to vitamin B-12 deficiency. People with atrophic gastritis, Crohn's disease, celiac disease or pernicious anemia and those who have had gastrointestinal surgeries may not be able to absorb adequate amounts of vitamin B-12 from food or supplements. Vegans and vegetarians who do not consume animal products may not get enough vitamin B-12 without taking supplements or eating fortified foods. Symptoms of vitamin B-12 deficiency include weakness, fatigue, dizziness, shortness of breath, pale skin, sore tongue and difficulty concentrating. Severe or long-term vitamin B-12 deficiency can cause serious neurological problems, including difficulty balancing, confusion, depression, memory loss and dementia.
Food Sources
Vitamin B-12 exists naturally in animal products, including meat, poultry, seafood, eggs and dairy products. Fortified breakfast cereals provide vegetarian and vegan-friendly sources of vitamin B-12. Foods with high amounts of vitamin B-12 include liver, with 48 mcg per serving and clams, with 34.2 mcg per serving.
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