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Deficiency of B12 due to low amounts of intake alone, is quite rare, and generally limited to vegan children, who are at risk due to higher needs of the vitamin, no intake of animal products that contain it, and low body stores (which in adults may last for years; see below). Vegans are generally encouraged to ingest B12 supplements, which are vegan since all commercial B12 is produced directly by bacteria. The total amount of vitamin B12 stored in the body is about 2 � 5 mg in adults. Around 50% of this is stored in the liver. Approximately 0.1% of this is lost per day by secretions into the gut, as not all these secretions are reabsorbed. Bile is the main form of B12 excretion; however, most of the B12 secreted in the bile is recycled via enterohepatic circulation. Due to the extremely efficient enterohepatic circulation of B12, the liver can store several years’ worth of vitamin B12. How quickly B12 levels may change when dietary intake is low, depends on the balance between how much B12 is obtained from the diet, how much is secreted, and how much is absorbed. B12 deficiency may arise in a year if initial stores are low and genetic factors unfavourable, or may not appear for decades. In infants and children, B12 deficiency appears much more quickly when the diet becomes poor in the vitamin. |
