The starches are too complex to break down easily, fibre, phytic acids etc etc. I couldn't tell you exactly without a bit of reading right now and I can't be arsed. Maybe I could start a thread some time, but I am sure it is not in most peoples interest here.
I was however just looking for an alternative. I remembered reading about how corn was used as a staple in the Americas, they would beat it, mix it with lime or ash and ferment it somehow. The process is called 'nixtamalization': when the process was foregone in later years, on a similar diet, natives suffered from niacin, protein, vit A, zinc deficiencies. Nixtamalisation made the proteins much more bioavailable (though lower in number in scientific terms since it washes away damaging proteins), minerals and vitamins available (the process removes most of the mineral leeching phytic acid),higher in calcium (from the lime), and much easier to digest. I wonder how available such a product is now. It is a very laborious product to make traditionally, the old women spend much of their time bashing corn and preparing this stuff.
Regarding the calcium level: I know dentists who volunteered in Guatemala who noted the natives' teeth were very difficult to extract and their bones were VERY strong.