Pretending for a moment that his comment has some validity (there could be some truth to it...), the only reason any minerals might be hard to replace is because the typical American diet is so deficient in them thanks to agribusiness's farming technique, but this is just a ploy by your doctor to convince you not to try this "crackpot" technique.
You can easily replace any minerals which are lost by simply taking colloidal (trace) mineral supplements. Common one's are T. J. Clark's Colloidal minerals and marine grade Coral Calcium, but there are several sources available. Find one you like.
There is also the possibility of growing your own food using live soil techniques, if you have some unpolluted soil to use... By live soil, I mean soil containing microbes and larger organisms such as worms, all of which work together to provide minerals to the plantlife grown there which are then usable by humans. This is a far better idea than taking supplements -- especially non-colloidal supplements -- as this natural method of mineral acquisition is what we've lived with and grown accustomed to over however long we've been on Earth. It's only the recent use of chemical farming which has killed the soil and robbed us of these trace minerals.