Peter D’Adamo is a naturopathic physician who is also an author, researcher-educator, Ivesian, amateur horologist, budding software developer and air-cooled enthusiast. He is considered a world expert in glycobiology, principally the ABO (ABH) blood groups and the secretor (FUT2) polymorphisms. In 1996 Dr. D’Adamo wrote the NY Times Bestseller Eat Right For Your Type.
Each Blood Type has unique levels of important intestinal enzymes.
No single diet theory can address all aspects of our individuality, and only a fool would claim that soy, red meat, grains, coconut oil or anything else is universally good or universally bad for everyone.
For example, people who are blood type O appear to derive significant benefit from a diet including hormone and antibiotic free meats and poultry. There is a very basic physiologic reason for this: those with type O blood have almost three times the levels of an enzyme in their intestines called intestinal alkaline phosphatase (IAP) . This enzyme performs two very important functions in the body. First, IAP splits dietary cholesterol into smaller fragments, allowing for their proper breakdown. Second, IAP enhances the absorption of calcium from the diet. Now you’d think this was cutting-edge, late-breaking news since it is obviously of tremendous interest in these nutrigenomic times. However, the first observations were made over four decades ago.
In addition to these two critical functions IAP is an important influence on the ability of the digestive tract to heal. Thus in most of our type O patients (44% of the population) we see a marked improvement in their IBS, colitis and Crohns disease when they increase their protein and cut back on their carbohydrates.  Amazingly, for a very brief period in our fetal life this enzyme is the most abundant enzyme in our body: exactly at the moment when the fetus is busy manufacturing the delicate microvilli lining that powers the absorption mechanisms of our small intestines.
Blood type B makes considerable amounts of IAP as well, but type A’s make very little. This probably explains why most studies that have looked at heart disease and blood type show a significantly higher rate of problems with blood type A individuals. These folks really should follow a Mediterranean-type diet.
Later studies showed that type A not only secreted almost no alkaline phosphatase in their intestines, but whatever little they did secrete was in and of itself inactivated by the presence of their own A antigen. 
Thus, we have here one of the strongest indications for the long term benefit of a low-fat diet in type A, both with regard to the susceptibility to cardiovascular disease, and (although not mentioned here) their additional susceptibility to cancer. Following the type A eating plan, with its emphasis on a healthy fats and moderate to lower animal protein, is the best method to maximize digestive efficiency in type As, lower their level of intestinal dysfunction, and to influence their susceptibility to cardiovascular disease.