That is one of the problems with the diet. The theory is that type Os have a lot more stomach acid than other types. O is the oldest blood type and goes back to the time when our ancestors were nomadic hunter-gathers. Apparently high protein meats are a more natural and nutritive food and grains are not handled well at all.
I am a type O and do eat a little meat maybe three times a week, but not as much as the diet recommends. Use high protein beans as an important staple in your diet and be sure and eat plenty of leafy green vegetables for the iron. Eat grains only if they do not bother you. I find I cannot eat a whole lot of wheat without some distress, but handle brown rice and oatmeal quite well.
Now, I just use the Blood Type Diet as a starting point, but continue to experiment to discover what does and does not work for me.