Vitamin B12’s primary functions are in the formation of red blood cells, DNA synthesis and the maintenance of the nervous system. If B12 deficiency occurs, DNA production is disrupted and abnormal cells called megaloblasts manifest, which results in anemia. B12 protect the nerves which are surrounded by an insulating fatty sheath called the myelin. B12 plays a vital role in the metabolism of fatty acids essential for the maintenance of myelin. Prolonged B12 deficiency can lead to nerve degeneration and irreversible neurological damage. When deficiency occurs, it is more commonly linked to a failure to effectively absorb B12 from the intestine rather than the lack of B12-rich foods.
Vitamin B12 can be stored in small amounts by the body. Total body store is 2-5mg in adults. Around 80% of this is stored in the liver. Vitamin B12 is excreted in the bile and is then effectively reabsorbed – this is known as enterohepatic circulation. People on diets low in B12 may be obtaining more B12 from re-absorption than from the food they ingest. Re-absorption of the B12 from the bile is the reason it can take over 20 years for deficiency disease to develop. In comparison, if B12 deficiency is due to a failure in the absorption mechanisms it can take only 3 years for deficiency disease to occur.
The only reliable dietary sources of vitamin B12 are meat, dairy and eggs. The considerable research into possible plant food sources of B12 turned up few positive results. Fermented soya products, seaweeds and algae have all been proposed as possible sources of B12 because they are products which have been “pre-digested” into various amino acids, making them easier to absorb. However, analysis of fermented soya products, including tempeh, miso, shoyu and tamari, found no significant B12.
Spirulina, algae available as a dietary supplement in tablet form, and nori, a seaweed, have both appeared to contain significant amounts of B12 after analysis. However, it is thought that this is due to the presence of compounds structurally similar to B12, known as B12 analogues. These cannot be used satisfy dietary needs. The body is unable to differentiate between B12 and its analogues. Researchers have suggested that supposed B12 supplements such as spirulina may in fact increase the risk of B12 deficiency disease, as the B12 analogues can compete with B12 and inhibit metabolism. The current nutritional consensus is that no plant foods can be relied on as a safe source of vitamin B12.
Bacteria present in the large intestine are able to synthesize B12. In the past, it has been thought that the B12 produced by these colonic bacteria could be absorbed and utilized. However, the bacteria produce B12 too far down in the intestine, and as B12 is not absorbed through the colon lining, this B12 just gets flushed out of the system. Supplementations such as the injections should be used if you begin to experience the symptoms of B12 deficiency and you know that you are at risk.