[alexmingoia]

Yeah great points. B12 is produced by a bacteria that lives in animal (and human) small intestines. It's not clear why it lives in some humans and not others, so the only way to ensure you're getting it is to take a supplement (or eat animal products since animals are supplemented). Nutritional yeast does not contain B12 unless they're fortified with it.

Contrary to Wikipedia and popular belief, B12 producing bacteria has been found in human small intestines (https://www.ncbi.nlm.nih.gov/pubmed/7354869). It's just not clear why it lives in some people and not in others. Prior to modern hygiene practices vegetarian humans and animals probably got it from ingesting feces (in contaminated water or food fertilized with feces).

https://veganhealth.org/intestinal-bacteria-as-b12-source/#s...