"The Blood Type - Diet Connection", as the introduction to Dr. D'Adamo's book is entitled, is about as technical as I would care to get but it makes sense to me.
Either there's something to the Blood Type Diet or there isn't.
The only way to tell is to try it for a while.
Most important, in my case, is avoiding the foods in the "avoid" column. I did a Colon Cleanse which may also have helped to get off on the right foot with the Diet.
All I know is, it has improved my life.
Pay close attention to the "Supplement Advisory". A little vitamin B can do wonders (for type O).