B12 is kind of tricky. Dr. Gregar has a good video on it. If you can't find it yourself then ask me and I'll find it and include a link.
But the gist of it is that our bodies can only absorb around 30% of the RDA at one time (like in one meal). Then the "receptors get clogged". After the receptors get clogged the body can only absorb B12 at a 1% efficiency. so you need to take 10000% * of the RDA for good /full absorption. That does sound like a lot but there are affordable pills exactly that size.
(forum members - please correct me if I made any mistakes. I didn't cross check these facts)
Or you can just take at least 30% three times a day. This is what I have chosen to do. Since my plant milk contains about 50% I take 2 glasses a day. that leaves me a little shy so I take a multi too.
Iron is also a little tricky. I have always tested borderline anemic so I think there is something to the idea that some of us just aren't that good at absorbing iron. And even if your food contains 100% of the RDA, non-heme iron just isn't as absorbable. And to make it even worse, some of the foods we eat interfere with iron absorption. I'm pretty sure caffeine interferes with iron absorption. And vitamin C promotes absorption. On the other hand, a lot of vegan food has iron.
My multi contains 100% of the RDA of B12. And 30% of iron. I tried a multi with 50% iron but my poops became rock hard - which indicates that my body is excreting iron. So I went back to 30%. Iron in large quantities is toxic. but our bodies can excrete excess iron. but the brick-like poops are uncomfortable. ?
* not sure that is the right number of zeros.
