The big picture: Vitamin B12 is a nutrient that helps convert food into energy for your body, produce DNA and RNA and maintain healthy nerve cells. Deficient levels increase your risk for anemia, cardiovascular disease and neurological impairment.

Vitamin B12, also called cobalamin, is one of eight B vitamins. All B vitamins help the body convert food (carbohydrates) into fuel (glucose), which is used to produce energy.

Vitamin B12 is a nutrient primarily sourced from your diet. It is present in many animal products, including meats, dairy and eggs. The highest concentrations are in clams and liver.

Excess vitamin B12 is stored in the liver, decreasing the likelihood of deficiency.
However, when vitamin B12 cannot be absorbed, for example, due to dietary insufficiency (vegetarian/vegan diet), or malabsorption (celiac disease), liver stores are depleted and deficiency ensues.
Levels may be artificially elevated in individuals with alcoholism, liver disease or cancer because it means there’s decreased clearance of proteins in the liver which result in higher circulating levels of B12.

Your body needs Vitamin B12 to make healthy blood cells. Low vitamin B12 levels may result in anemia.

Vitamin B12 works closely with vitamin B9 to help make red blood cells and to help iron work better in the body. Together they produce S-adenosylmethionine (SAMe), a compound involved in immune function and mood.
Anemia is a condition that develops when you can’t make enough healthy red blood cells, which are the blood cells that carry oxygen to all parts of the body.

Low levels of Vitamin B12 increases your risk of heart disease via its important role in the metabolism of homocysteine.

Vitamins B12, B6, and B9 work together to control blood levels of homocysteine, an amino acid. So a low B12 status may result in elevated homocysteine.
Meta-analyses of 30 prospective and retrospective studies (n=16,786) show the risk of any coronary heart disease event increased approximately 20% for each increase of 5 μmol/L of homocysteine. Furthermore, lowering homocysteine by 25% or 3 μmol/L was associated with a 11% lower risk of ischemic heart disease and 19% lower risk of stroke.

Vitamin B12 deficiency can also lead to severe neurological impairment. Symptoms such as numbness and tingling in the hands and feet, muscle cramps, dizziness, cognitive disturbances, difficulty walking, erectile dysfunction, as well as fatigue and psychiatric symptoms like depression and even psychosis.