Despite what some people believe, it’s possible to eat a completely vegan diet without supplements.
Most of us vegans need a good vegan vitamin B12 supplement, but that’s because of how our food is cleaned.
Vitamin B12 is made by bacteria or archaea (a single-celled organism).
This bacteria usually hangs out in the dirt that plants (vegetables) grow in.
So in the past, when people ate plants, they would also eat a bit of dirt that contained this bacteria, and some B12 that it made.
But for most people today, our plants are washed so well that we don’t get any (or very little) B12 from them. That’s why B12 is so hard to get without animal products.
We do get some of the vitamin B12-producing bacteria sometimes, but that resides in our colons. So even though it still produces B12 inside your body, it’s too far down in the digestive tract to actually absorb (this is not true for certain animals like cows).
That’s all there is to it. Humans can get vitamin B12 naturally while eating plants that aren’t cleaned too well. For non-vegans, they can also get B12 that is stored inside animals who have consumed or produced B12.
I don’t recommend fully abandoning supplements, even if you grow your own vegetables, unless you’re getting regular blood tests. If you’re planning on doing that, check with your doctor first.