Vitamin B12 Deficiency

A genuine paradox

Many gut bacteria can produce vitamin B12, also called cobalamin, and studies estimate gut microbes could theoretically supply a meaningful share of the body's daily requirement. There's a catch. B12 can only be absorbed in the terminal ileum, near the end of the small intestine, using a specific carrier protein called intrinsic factor. Most B12-producing bacteria live further along, in the large intestine, past the point where absorption can happen. This means the vitamin B12 gut bacteria make is often nutritionally out of reach, a genuine biological paradox that explains why B12 deficiency remains common worldwide despite how widespread the bacteria that produce it actually are.

Source: The Vitamin B12 Paradox: Microbial Abundance, Human Deficiency, and Food-Based Nutritional Solutions

When bacteria make things worse, not better

Gut bacteria's relationship with B12 isn't purely a case of missed opportunity. In small intestinal bacterial overgrowth (SIBO), where bacteria multiply in the wrong part of the gut, certain bacteria can actively compete with the body for the B12 that is available, using their own proteins to intercept it before intrinsic factor can bind it and carry it through for absorption. This is one of several recognised causes of B12 malabsorption, alongside conditions like coeliac disease, Crohn's disease, and pernicious anaemia, an autoimmune condition that destroys the cells that make intrinsic factor in the first place.

Source: Competition Between Bacteria and Intrinsic Factor for Vitamin B12

What this means for you

B12 deficiency causes fatigue, pale skin, a sore tongue, and, if untreated for long enough, nerve damage that can become permanent, so persistent unexplained fatigue or tingling in the hands or feet is worth a blood test rather than guessing. People following a vegan diet, older adults, those on long-term acid-reducing medication, and anyone with an underlying gut condition such as coeliac disease or SIBO are at higher risk and may need testing more proactively. Since dietary B12 mainly comes from animal products rather than gut bacteria, food sources or a supplement, not a general probiotic, are the reliable way to correct a deficiency, though treating any underlying gut condition remains important for keeping absorption working properly. A varied, plant-rich diet supports the wider gut ecosystem alongside that, which is the same idea behind GutLab, everyday gut health starts with regularly feeding the microbiome a broad range of plant-rich ingredients.