I never understood the part when it says that the metal is ingested?
Wikipedia and Mistborn wiki describe it exactly like that "burning" and "ingestion", with no further explanation what that means, what am I supposed to make out of that?
The Wikipedia article says it's a "genetic ability that allows a person to metabolize ("burn") metals." So "burn" is in quotes. I don't remember precisely how it was described in the books; it's been too long since I read them. I think it's more of a metabolization, the way your body metabolizes things like food or whatever, except this is a world with magic so....magic. However, yes, metals can burn: The Burning of Metals
From what I remember, the metal is in the form of flakes when swallowed. So it's not like trying to metabolize large ingots.