Jini Says: Iron cannot be absorbed by the body without the presence of Vitamin B12, folic acid and Vitamin C. So, the best iron supplements provide these co-factors along with the iron itself. If iron tends to constipate you, then use the Angstrom Iron Plus Vitamin C. This nanoparticle-sized iron is revolutionary since it does not require digestion and therefore does not irritate the intestinal tract, trigger bleeding, or constipate.