Phosphorus in the bran of whole grains is tied up in a substance called phytic acid(or phytate). This combines with minerals like magnesium zinc etc. in the intestine, blocking their absorption. Traditional societies usually soak or ferment their grains before eating them, processes which neutralize phytates, and in effect pre-digest the grain so more nutrients are available. If you eat muesli for breakfast, soaking it overnight in acidulated water (with a teaspoon of something acid like lemon juice or yoghurt in the soaking water) will improve it in this way.

If you haven't found it already, you might like the Westonaprice.org website, which teaches how to treat our food for best nutritional results.