Much is now known about the biochemistry and metabolism of vitamin B12, however, the diagnosis of its deficiency has become more complicated with the classification of a "sub-clinical" deficiency category, characterized by serum vitamin B12 concentrations that were once considered to be adequate. Vitamin B12 deficiency was previously thought to take many years to develop, and only in strict vegetarians or those with pernicious anaemia. More recent research has suggested that there are disease implications associated with sub-clinical B12 deficiency, which develop most commonly due to malabsorption or dietary inadequacy. The rates of sub-clinical deficiency of vitamin B12 are high in developing countries, in the elderly, and in vegetarian populations. The long term consequences are not fully known but may include adverse effects on pregnancy outcomes and aspects of ageing. Vitamin B12 deficiency is most commonly caused by low intakes, but can also result from malabsorption, certain intestinal disorders, low presence of binding proteins, and use of certain medications. Vitamin B12 is rare from plant sources, so vegetarians are more likely to suffer from vitamin B12 deficiency. Infants are at a higher risk of vitamin B12 deficiency if they were born to vegetarian mothers. The elderly who have diets with limited meat or animal products are vulnerable populations as well. Vitamin B12 deficiency may occur in between 40% to 80% of the vegetarian population who are not also consuming a vitamin B12 supplement.