Vitamin B-12 plays a role in the synthesis of protein and DNA in our cells. Deficiency of this vitamin results in damage to the central nervous system (brain and spinal cord), and this damage is usually at least partly irreversible. Cognitive dysfunction, that is, problems with memory and thinking, is sometimes the earliest sign. Pernicious anemia, a decline in red blood cells (sometimes accompanied by a decline in white blood cells) is also a characteristic feature of vitamin B-12 deficiency, but it often does not present until well after the neurologic damage has begun. Unfortunately, many doctors are unaware of the fact that brain and spinal cord damage often precede the anemia and fail to test for vitamin B-12 deficiency in patients who may have signs and symptoms of neurologic damage but are not yet anemic.
Vitamin B-12 occurs almost exclusively in animal products. Tiny amounts, not enough to meet our needs, can also be found in plants grown with manure fertilizers. The Recommended Dietary Allowance is 2.4 micrograms a day in adults with normal digestive systems (slightly more for pregnant or lactating women). The vast majority of people who do not consume any animal products will ultimately develop vitamin B-12 deficiency, although, because of the efficiency with which the body preserves and stores this vitamin, it usually takes two to five years or more for deficiency to occur. The vast majority of cases of vitamin B-12 deficiency, however, occur not because of lack of vitamin B-12 in the diet but rather because of gastrointestinal absorption problems. It has been estimated that 1 out of 8 persons over the age of 65 is vitamin B-12 deficient because of changes in the stomach.
People who eliminate animal products from the diet (or greatly reduce their consumption) must, therefore, include a source of vitamin B-12 to avoid deficiency. Some individuals choose to consume fortified foods, such as soy milk, cereals, and vegetarian formula nutritional yeast. Otherwise, a supplement is required. For most people, 1,000 micrograms weekly should suffice. Why such a large amount if the daily requirement is only 2.4 micrograms? When someone consumes a single dose of vitamin B-12, about 60-80% of the first 2 micrograms is absorbed. At most, 1% of the remainder is absorbed. The explanation is that there are two ways in which vitamin B-12 is absorbed in the small intestine. The first 2 micrograms can be absorbed by a facilitated route, which involves a glycoprotein called intrinsic factor that is produced by the stomach, as well as certain receptor sites (in the last part of the small intestine) to which the vitamin B-12 intrinsic factor complex binds. Anything over 2 micrograms exceeds the capacity of this system and must be absorbed passively, a very inefficient process. Please note that blue-green algae, sea vegetables, tempeh, or other foods are not considered reliable sources of vitamin B-12 despite what you may read elsewhere.