Another vote here for heme iron. I like beef spleen supplements, which have more iron than liver. And another vote for making sure your iron is actually low as there are many nutrients needed for iron metabolism - copper, vitamin A, biotin, B2, B5, B6, B12, B9, lysine, antioxidants to reduce oxidized iron so that the iron can be reused... Also, hepcidin levels must be low enough.