A friend eats a strict plant-based diet full of vegetables and fruit, but takes no supplements or fortified foods. Why is she genuinely at risk of B12 deficiency?

Plants don't make B12, so her diet supplies almost none

Cooking vegetables destroys most of their B12

Plants don't make B12, so her diet supplies almost none

🦠 The one vitamin that neither plants nor animals can make

Vitamin B12 (cobalamin) is unique among nutrients: no plant, animal, or fungus can synthesize it. It is made almost exclusively by bacteria and archaea. Grazing animals get it because microbes in their gut (or soil) produce it, and it accumulates in their tissues — which is why meat, fish, eggs, and dairy are its reliable dietary sources. Plants have essentially no consistent B12, which is why strict plant-based diets need fortified foods or a supplement.

B12 deficiency often hides for a long time. A healthy liver can stockpile several years' worth, so symptoms may not appear until stores are badly depleted. Absorption is also surprisingly fussy: it requires intrinsic factor, a protein secreted by the stomach, so older adults and people on certain acid-reducing medications can become deficient even while eating meat.

The stakes are real. Shortfalls cause megaloblastic anemia (oversized, poorly functioning red blood cells) and progressive nerve damage — tingling, balance problems, and memory issues — some of which can become irreversible if left untreated. That combination of a microbe-only origin and a slow, silent onset is what makes B12 easy to overlook.