Hi! Like Nicole said, meds we take (not just antibiotics) deplete our mg levels. I take a supplement every day. The best form to get is chelated magnesium glycinate. The brand I take is Solgar. As for iron, the best supplement to take if you are low is Floradix. It is the most bio-available and doesn't have the side effects of the conventional pills. When I was iron deficient during my last pregnancy, Floradix is what my midwife recommended and it did help! It didn't taste too bad either.
I'm a huge proponent of supplemental magnesium. Most of the US (don't know where you are from..) is deficient and then you add CF and our meds and we have a double whammy. There are many subtle symptoms of mg deficiency and once corrected you realize that you feel better. I don't think you could get enough from magnesium rich foods to counteract our situation. If our soils aren't rich in minerals anymore (and they aren't), the plants will be lacking somewhat as well. Foods are where you should start, but I don' think they're enough. Some CF centers are finally recognizing that mg deficiency is real and a problem for CFers. Sounds like yours is headed in that direction since they even bothered to check! The dietitian at my clinic once told me that almost nobody is deficient in mg - that is flat out misinformation.