Vitamin B12

Vitamin B12 is a water soluble vitamin with a key role in the normal functioning of the brain and nervous system, and for the formation of blood. It is one of the eight B vitamins.

It is normally involved in the metabolism of every cell of the body, especially affecting DNA synthesis and regulation, but also fatty acid synthesis and energy production.

The human physiology of vitamin B12 is complex, and therefore is prone to mishaps leading to vitamin B12 deficiency. Unlike most nutrients, absorption of vitamin B12 actually begins in the mouth where small amounts of unbound crystalline B12 can be absorbed through the mucosa membrane.

The total amount of vitamin B12 stored in body is about 2,000-5,000 mcg 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.

Vitamin B12 deficiency can potentially cause severe and irreversible damage, especially to the brain and nervous system. At levels only slightly lower than normal, a range of symptoms such as fatigue, depression, and poor memory may be experienced. However, these symptoms by themselves are too nonspecific to diagnose deficiency of the vitamin.

Supplements

Vitamin B12 is naturally found in meat (especially liver and shellfish), milk and eggs. Animals, in turn, must obtain it directly or indirectly from bacteria, and these bacteria may inhabit a section of the gut which is posterior to the section where B12 is absorbed.

Thus, herbivorous animals must either obtain B12 from bacteria in their rumens, or (if fermenting plant material in the hindgut) by reingestion of cecotrope fæces. Eggs are often mentioned as a good B12 source, but they also contain a factor that blocks absorption.

Certain insects such as termites contain B12 produced by their gut bacteria, in a manner analogous to ruminant animals.

According to the U.K. Vegan Society, the present consensus is that any B12 present in plant foods is likely to be unavailable to humans and so these foods should not be relied upon as safe sources, as the B12 analogues can compete with B12 and inhibit metabolism.

While lacto-ovo vegetarians usually get enough B12 through consuming dairy products, vitamin B12 may be found to be lacking in those practicing vegan diets who do not use multivitamin supplements or eat B12 fortified foods. Examples of fortified foods often consumed include fortified breakfast cereals, fortified soy-based products, and fortified energy bars.

Claimed sources of B12 that have been shown through direct studies of vegans to be inadequate or unreliable include laver (a seaweed), barley grass, and human gut bacteria.

Recommendations

The Dietary Reference Intake for an adult ranges from 2 to 3 µg (micrograms) per day.

Vitamin B12 is believed to be safe when used orally in amounts that do not exceed the recommended dietary allowance (RDA). The RDA for vitamin B12 in pregnant women is 2.6 µg per day and 2.8 µg during lactation periods. There is insufficient reliable information available about the safety of consuming greater amounts of Vitamin B12 during pregnancy.