This.
I had a very low B12, even eating meat. I had to get monthly B12 shots AND take B12 and folic acid supplements.
I’m almost 2 weeks almost meat-free (just some chicken and fish) but I still take a mega-B12 dose. It will be 3 weeks mammal meat-free when I go for blood work next week. I’m curious to see my level. I don’t get shots currently but I take the supplements.
Latest stuff I read was insinuation that there is a genetic factor that affects assimilation of it.