Is it true that the high amount of phytic acid in collard greens can prevent the body from absorbing calcium?

Collard greens are among a small number of foods that contain measurable amounts of oxalates, naturally occurring substances found in plants, animals, and human beings. When oxalates become too concentrated in body fluids, they can crystallize and ...

from The World's Healthiest Foods http://ift.tt/Ynch5z