Milk products. Probably the single biggest reason that vegans need B12 supplementation is they don't consume milk products. It's only animal products that B12 is available from... unless you are an ungulant, wherein you get B12 from the fermentation of vegetation in your rumen.
It's all (not just B12) debated by various nutritionists all the time. One study shows this, another shows that. Red wine is good, then it's bad. This oil is good oil, and then it's bad. Good cholesterol, bad cholesterol, don't eat this, don't eat that. Eat more kale. No don't.
Frankly, I'm quite sick of it, and am more like what Wirey said ... stop concerning yourself much with it, all that stress over nutrition is worse than poor nutrition.
We don't really even think about nutrition much here, although in mu cardio-rehab I have to go see a nutritionist this Friday as part of the program. Maybe I'll find out some more new-fangled stuff I can ignore.
It reminds me of the 40s when cigarettes were good for you.