I believe the book entitled "Eat Right for Your Type" by Dr. D'Adamo may explain this. He states it's basically a blood type phenomenon. For example, Type O people will lose weight better on Atkins, whereas, Type A people lose best eating veggies and pineapple.

He says the significance of blood type can be found in the story of human evolution, i.e, Type O is the oldest with the caveman-type diet; Type A evolved with the change to an agricultural society; Type B emerged as humans migrated north into colder, harsher climates; and Type AB is a modern adaptation which is a result of the intermingling of the 3 groups.