Vitamin B12: Can Gut Bacteria Synthesize It?


Subtitle: Yes, but science says you shouldn’t rely on it — as they make it for their own needs.
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A Primer on B12

Vitamin B12 — also called cobalamin — is a priority for vegans and vegetarians to address. Because the human body cannot ssynthesiseB12 and plant foods don’t contain it unless they’re fortified with B12. That’s why vitamin B12 is usually sourced from animal foods such as liver, fish, chicken and eggs.

Vitamin B12 helps the body to synthesize new DNA and maintain healthy red blood cells and neurons. Vitamin B12 deficiency, therefore, leads to a plethora of side effects including tiredness, anaemia, constipation, weight loss, memory problems, depression, tingling in hands and feet and other manifestations of neurological issues.

B12-Synthesizing Gut Bacteria

“Vitamin B12, otherwise known as cobalamin, is the biggest and most intricate vitamin,” write Chinese researchers in a peer-reviewed book chapter titled Biosynthesis of Vitamins by Probiotic Bacteria. Why? Vitamin B12 has the most complex structure out of all vitamins synthesizable by gut bacteria. Vitamin B12 synthesis requires 30 different genes in a bacterial genome to be activated in an intricate order.

In a 2019 review written by Japanese researchers in the Frontiers of Nutrition, they provided a list of gut bacteria that can make vitamin B12:

  • Bacteroidetes: Bacteroides fragilis and Prevotella copri.
  • Firmicutes: Clostridium difficile, Faecalibacterium prausnitzii, Ruminococcus lactaris, Lactobacillus plantarum, L. coryniformis and L. reuteri.
  • Actinobacteria: Bifidobacterium animalis, B. infantis and B.longum.
  • Fusobacteria: Fusobacterium varium.

But these bacteria all reside in the large intestine or the colon. And receptors that uptake vitamin B12 is only present in the small intestine wherein they absorb dietary B12 after protein digestion in the stomach. The gut microbial B12 is, therefore, not bioavailable to the host.
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What About Coprophagy?

Some mammals such as rabbits, hippos, pandas, elephants, and non-human primates do practice coprophagy — the act of ingesting feces.

“These animals receive the benefit of microbial cobalamin [vitamin B12] production in the large intestine by consuming their feces which localizes microbial cobalamin (and other vitamins) to the upper part of the digestive tract where it can be absorbed,” Andrew Goodman, assistant professor in the Department of Microbial Pathogenesis at Yale University and colleagues wrote in Cell Metabolism.

However, even animals prefer B12 from foods if given a choice. Experiments with rodents and dogs showed that they tend to engage in coprophagy if their diet were deficient in B12. Provide them meals with B12, and the coprophagy tendency stops.

No credible studies to date have tried administering something like a ‘stool capsule’ to people deficient in vitamin B12. I mean, people would just settle for other nutritional supplements or, for vegans and vegetarians, other non-animal sources of B12 such as algae or B12-fortified plant foods.
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What About B12-Synthesizing Small Intestinal Bacteria?

“The human small intestine also often harbors a considerable microflora,” stated an old 1980 study that examined healthy Southern Indians. “We now show that at least two groups of organisms in the small bowel, Pseudomonas and Klebsiella species may synthesize significant amounts of the vitamin.”

Yet vitamin B12 deficiency is common in India, probably of the lack of animal foods in their diet. Indians also typically have a more enriched small intestinal microbiota than other geographical regions for unclear reasons.

And increased numbers of bacteria in the small intestine — such as during small intestinal bacterial overgrowth (SIBO) — is known to induce vitamin B12 deficiency. Because small intestinal microbes outcompete the host for the valuable vitamin B12 required for DNA synthesis, explain researchers from the University of Virginia School of Medicine.

The same applies to microbes in the large intestines, especially the butyrate-producing bacteria and Bacteroides thetaiotaomicron that have multiple B12 transport genes. Butyrate has systems-wide anti-inflammatory effects crucial for overall health. Whereas B. thetaiotaomicron helps the host to break down antinutrients — phytates and saponins — present in plant foods: 5 Antinutrients That Can Be Degraded by Gut Bacteria.

In a sense, B12 also nourishes the gut microbiota. And this is also the primary reason why large and small intestinal bacteria synthesize B12 — to support the growth of other microbes, serving their roles as members of the microbial ecosystem. If the host’s diet has sufficient B12, however, the microbes would instead conserve the energy needed to activate 30 genes to produce B12.

“Although cobalamin [vitamin B12] is synthesized by some human gut microbes, it is a precious resource in the gut and is likely not provisioned to the host in significant quantities,” Goodman and colleagues concluded.

Put it simply, gut bacteria can produce vitamin B12. But we shouldn’t rely on them to provide our daily requirement of B12. Because, most of the time, they make B12 for their own needs.

This article was originally published in Microbial Instincts with minor modifications.

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