Certain foods are not compatible with type O receptors, and this causes the body to form lectins which can accumulate over a period of time. The problems caused by lectins are mostly inflammatory and auto-immune difficulties.
Three weeks after I started the type O diet, all of my physical problems disappeared. My arthritis disappeared when I followed D'Adamo's advice for type O's. I guess you could say I'm a believer.
Research has been done. You should read D'Adamo's books. They're actually very scientific and well informed.