Thanks to pyridoxine glucoside, B6 from cereal grain products is absorbed with far less efficiency than that from animal foods
Comparative vitamin B-6 bioavailability from tuna, whole wheat bread and peanut butter.
http://www.ncbi.nlm.nih.gov/pubmed/6655507
the high phytate content of whole grains binds to minerals such as iron, calcium, magnesium, and zinc in the gastrointestinal tract, significantly reducing their absorption by the body
http://www.sciencedirect.com/science/article/pii/S0140673673...
EFFECTS OF PURIFIED PHYTATE AND PHYTATE-RICH BREAD UPON METABOLISM OF ZINC, CALCIUM, PHOSPHORUS, AND NITROGEN IN MAN
http://www.ncbi.nlm.nih.gov/pubmed/1657026
Effects of dietary fiber and phytic acid on mineral availability
It has long been recognized that high cereal grain consumption induces vitamin D deficiency in various animal species, including primates, our closest animal relatives. By studying the fate of radio-labelled vitamin D, researchers observed significantly increased excretion of vitamin D in healthy human volunteers fed sixty grams of wheat fiber daily.
http://www.ncbi.nlm.nih.gov/pubmed/6299329
Reduced plasma half-life of radio-labelled 25-hydroxyvitamin D3 in subjects receiving a high fiber diet.
Full analysis:
http://anthonycolpo.com/the-whole-grain-scam/