Vitamin B12 isn't naturally present in any plant food.

Vitamin B12 is produced only by certain bacteria and archaea, not by plants or animals themselves. Animals accumulate it by eating microbe-contaminated feed or, in the case of ruminants like cows, hosting B12-producing bacteria in their own gut, which is why meat, fish, eggs, and dairy are reliable sources for humans while plant foods naturally contain essentially none.

This makes B12 the one nutrient that vegan diets can’t obtain from whole plant foods alone, regardless of how varied or well-planned the diet is. Products sometimes marketed as plant-based B12 sources, such as certain algae or fermented foods, generally contain inactive B12 analogues that don’t function in the human body and can even interfere with tests for genuine deficiency. As a result, major health organizations recommend that people on vegan diets get B12 from fortified foods or supplements rather than relying on food combinations, since no amount of plant variety substitutes for a food source that simply doesn’t contain it.