Vegan for Life - Jack Norris [14]
The scientific name for vitamin B12 is cobalamin because the B12 molecule contains the mineral cobalt at the center of its structure. Commercial preparations of vitamin B12 used in supplements and fortified foods are called cyanocobalamin. This supplemental form is converted in the body to vitamin B12 coenzymes, which are the compounds needed for B12 activity. Some people prefer to take supplements of vitamin B12 that are already in the form of the coenzyme methylcobalamin, which doesn’t require any conversion for some of its uses. But because there are questions about the stability of methylcobalamin, supplements must contain much higher amounts and there is less available research on their effects on B12 status. The recommendations we make in this chapter are based on supplements and fortified foods that utilize cyanocobalamin.
VEGAN SOURCES OF VITAMIN B12
All of the vitamin B12 in the world is made by bacteria, and that includes bacteria living in the digestive tracts of animals and humans. It seems like we could just use what these bacteria produce, but they are too far down in the intestines to be of any use to us. We absorb vitamin B12 in our small intestine; the bacteria producing it live in our large intestine.
There are also molecules that are very similar to vitamin B12 but that have no true vitamin activity for humans. These are inactive B12 analogues. Most methods for measuring vitamin B12 in foods don’t differentiate between true vitamin B12 and the inactive analogues. That’s been a source of confusion for a long time. Foods like fermented soy products, tofu, sourdough bread, and some sea vegetables have all been credited at one time or another as good sources of vitamin B12. But studies show that what they really contain are primarily inactive analogues.1 There is a double risk associated with depending on these foods for vitamin B12, because the inactive analogues can actually block the activity of true vitamin B12.2
Some companies may claim that a food contains active vitamin B12 even though the testing methods they use can’t discern between active B12 and inactive analogues. Currently, the only way to know if a food contains active vitamin B12 is to feed it to humans and look for vitamin B12 activity. The standard way to do this is to see how different foods affect levels of a compound called methylmalonic acid (MMA). MMA levels increase in B12 deficiency, and consuming foods that contain active vitamin B12 causes those levels to drop. Many foods that are commonly believed to be good sources of vitamin B12 actually have no effect on MMA levels, which means that they contain primarily inactive analogues.
Plants have no need for B12, which is why they usually don’t contain any. Occasionally, a plant food might be “contaminated” with an inactive B12 analogue. That is, it contains vitamin B12 by accident. For example, the “starter” used to make tempeh, which is a fermented soyfood, might accidentally contain B12-producing bacteria. Seaweed might pick up bacteria that produce B12-analogues. There is some evidence that sea vegetables such as chlorella, dulse, and nori contain vitamin B12, but again, these haven’t been shown to be reliable and significant sources of the active vitamin.3
Most humans get vitamin B12 by eating animal products. Animals such as cows and other true herbivores are able to absorb the vitamin B12 produced in their intestines by bacteria. Others, including many primate species, eat at least small amounts of animal products (including insects) or feces, which can be a good source of B12.
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