I just learned that I will have to take in vitamin B12 if I go vegan. (I already take vitamin pills, so that's no problem.) However, I'm in need of good reasons to shut people up about that it's apparently natural to eat meat, because otherwise you can't obtain enough vitamin B12 through food.
I like to tell people that for all we know, the plant source of B12 has gone extinct or is extremely rare because of the havoc we have wreaked upon the environment. You can also get your B12 from certain brands of nutritional yeast, which is very tasty too. ;o)
Just tell them that the only reason there is B12 in meat is because animals eat plants that naturally contain bacteria that contains B12. If we picked our plants directly from the ground and ate without washing we'd also get B12. It's not really about being vegan, it's about us being so clean. This is the way I understand it. Here is a link that explains it a bit. The animals are not producing their own B12.
There is a good post called Vitamin B12 in the Superhero forum, under Getting Started. I recommend looking over this--especially the last few posts. "Bacteria are responsible for producing vitamin B12. Animals get their vitamin B12 from eating foods contaminated with vitamin B12 & then the animal becomes a source of vitamin B12." Theoretically, you should be able to eat unwashed, organic produce, & still naturally absorb enough B12; however, due to humans, there is probably not much of this bacteria still alive in most of our soil, food, etc today. I would also keep in mind & remind people the daily recommendation for B12; point out, people consume way more meat than "naturally" required to reach this. Also, have you read TKD? I love Alicia's description & information on eating meat, how it's digested in the body--or not digested--& humans as carnivores. It's simple, basic information supported by facts, & it provides a good foundation concerning opinions about whether eating meat is "natural", especially when we should be able to get our own B12 through helpful bacteria.