Thread by @ck_eternity_

Veganism can still cause B12 deficiency even if an individual is taking B12 supplements

This is because in healthy individuals only about 60% of the RDI for B12 can be absorbed per serving, so taking a single supplemental dose per day isn't sufficient

In individuals with gut inflammation issues like IBS, B12 absorption is impaired and can be even lower

Part of the reason most only last 5-10 years max on a vegan diet is because that's how long it takes the average person's liver B12 stores to be completely depleted

If you supplement B12, bioavailability can be increased through sublingual, topical, or injection administration

Methyl-B12 is the most optimal form for most, as it's the activated form used most by the body, and the form found in animal products

For a well-rounded B12 profile supplemental adenosyl-B12 and methylfolate can also be included

Adenosyl-B12 is another activated form used specifically in mitochondrial energy metabolism, and methylfolate is a direct cofactor for many methylation reactions involving B12

Source on B12 bioavailability in healthy adults:
pubmed.ncbi.nlm.nih.gov/18606874/

About 1.5 mcg per serving is absorbed, while the RDI is 2.4 mcg for most adults and 2.8 mcg for pregnant women