Actually I have disagree with the statement that the body has a very hard time digesting grains and meats... to some extent that is true. For instance me being of blood type O thrive on high protein diets and have the plenty of stomach acids and enzymes in my body to do that, but when it comes to grains my body doesn't respond so well to high doses of it so I keep my grain in take limited to rice, spelt, kamut and quinoa.
On the other hand, a type A whose stomach isn't well geared for meat actually thrives quite well on a high grain intake.
The blood type factor plays a big role in all this.