Vitamin B12 (aka cobalamin) is not just the odd duck of the B-vitamin family, but one of the most unusual vitamins on earth. It’s the only one to contain metal (a cobalt atom) and is, molecularly speaking, the largest and most structurally complex vitamin there is. B12 is essential for many of the key cellular processes keeping us alive—including DNA replication and neurotransmission—though we only require it in the most minuscule quantities: about two to three micrograms per day, which is equal to about 0.003 percent of the recommended daily intake for vitamin C.
While there are usually many ways to get a vitamin into our diets—vitamin C, for example, is in fruits like limes and lemons, vegetables such as broccoli and spinach, and animal milks—no animal or plant can produce B12 on its own. It can only be made by bacteria or archaea (another type of single-celled microorganism). This means that, unless you are directly consuming large amounts of bacterial cells that produce the vitamin (and we’ll get to that in a minute!), getting B12 into your diet generally means a certain amount of obligatory carnivorousness, or at least consumption of animal products, since meat, milk, and fish are the richest sources of it.
But let’s say you don’t want to rely on animal products or pills. It has been posited that certain fermented products—like tempeh, or algae like spirulina—may be good alternative sources, though some research has suggested that they really contain compounds that are only structurally similar to B12, and that those can actually interfere with regular B12 uptake.
When we look to other members of the animal kingdom, we can see that herbivores use a different strategy. Rabbits, hares, and pikas, for example, have a decent population of B12-fermenting bacteria in their colons. They perform a complicated digestive process called “hindgut fermentation,” in which chewed food passes to their stomachs, through the small intestine, and into the large intestine, where it picks up a healthy dose of bacteria. Then it’s forced backwards (via reverse peristalsis, the same process that cows use to regurgitate their cud), out of the top of the colon and into the cecum, a pouch between the small and large intestines. There, it ferments, producing sugars and B12.
Then comes the exciting part. The rabbit eliminates this fermented mixture in the usual manner, then proceeds to gobble up this “cecotrope” (yep, its own poop, but it gets a special name because of the double-digestion process). Why let all that B12 go to waste?
Vegetarians and vegans—whose diets are somewhat like rabbits’—are particularly at risk for B12 deficiencies. Humans lack the digestive architecture necessary to produce cecotropes—even though we do have B12-producing bacteria in our colons, we can’t absorb it there.
However, one gross study (described in the scientific literature as “one of the less appetizing but more brilliant experiments in the field of vitamin B12 metabolism”) demonstrated a possible colon bacteria-B12 link in humans by collecting stool samples from B12-deficient vegans, turning them into a water extract-cum-tea, and then feeding that back to the same study subjects. This may have proved to be a rich enough source of B12 to cure the subjects’ anemia.
Should vegans, and dairy-hating vegetarians, literally eat shit? Perhaps not, but this is one place where the fascinating, diverse, and delicious plant kingdom has limitations regarding its ability to keep us healthy and alive. While the world of fermented foods can be a magical wonderland, the evidence for fermented sources of B12 is dicey enough that most respectable vegetarian organizations recommend supplements. So eat your vegetables—but take your vitamins, too.