B12 is a vitamin commonly found in dietary sources such as poultry, shellfish, milk and milk products, meat and eggs, as well as in various types of supplements. Although upper limits for the vitamin have not been established, maintaining adequate daily levels of B12 will help keep you from developing potentially harmful vitamin deficiencies.

Normal Levels

Your doctor can measure your vitamin B12 levels through a blood test. Because of the general safety of the vitamin, upper limits for normal intake do not exist. If your blood values for the vitamin fall below a range of 170 to 250 picograms per milliliter, you may develop a B12 deficiency. However, blood tests do not necessarily accurately reveal the presence of a deficiency -- testing levels of a substance called methylmalonic acid may give a better picture of potential B12-related problems.

Deficiency Symptoms

If you have abnormally low levels of vitamin B12, you may experience symptoms that include lack of adequate red blood cells, also known as anemia, as well as weakness, constipation, weight loss and lack of appetite. You also may experience nerve-related symptoms, such as tingling or numbness in your feet or hands, as well as confusion, depression, memory deficits and balance problems.

Dietary Factors

Since vitamin B12 sources include mainly animal-based foods, you may be at risk for B12 deficiency if you follow a vegan or vegetarian diet. If this is the case, you can add some amount of B12 to your diet through fortified cereals, as well as through some forms of nutritional yeast. Be sure to examine the vitamin content of any products you buy to make sure they provide adequate amounts of B12 1.

Absorption Difficulties

Normally, your body absorbs vitamin B12 in food and stores it in your liver 1. However, you may not gain sufficient benefit from foods rich in the vitamin if you have a disorder that decreases your ability to properly absorb it. Examples include Crohn’s disease, pernicious anemia and atrophic gastritis. If you have these types of disorder, your doctor may recommend injectable forms of B12, which do not require gastrointestinal absorption.

Potential Complications

If your vitamin B12 levels fall below normal, you may experience an increase in a blood-borne amino acid called homocysteine. In turn, elevations in homocysteine may increase your risks for the development of coronary heart disease. Vitamin B12 deficiency, combined with heightened levels of homocysteine, also may increase your risk for Alzheimer’s disease and other forms of dementia.

Intake Recommendations

As often as possible, your daily B12 intake should be through a well-rounded diet of B12-containing foods. Recommended B12 amounts for adults are 2.4 micrograms per day and recommended amounts for children range from .9 to 1.8 micrograms per day. Recommended amounts for infants range from .4 to .5 micrograms per day.