The Blood Type Diet starts with your blood type, and helps you determine which types of food are best suited for you based. In essence it is helping you determine which group of humans - and therefore metabolism - you fall into.
Dr. Amato is the main proponent of the Blood Type Diet and has many books on the topic. He feels strongly that your blood type should determine what you eat.
If you don't understands the basics of blood types or how this affects your body's health and metabolism, I have articles at the bottom of this page that help you understand those basics. It is definitely true that the blood type you have affects your health. It does this directly, by affecting what types of diseases tend to affect you. It also does this indirectly, by indicating what group of humans your traits came from and therefore affect your metabolism.
So once you know your blood type, what does the Blood Type Diet say you should eat?
Blood Type B
We'll start with the rarest and newest blood type - B. This only came into existence fairly recently (in the evolutionary sense) and has the smallest number of people out of A, B, and O. Only about 20% of people in the world are type B. B seems to have sprung into being in Asia. India has one of the highest percentages of B people. The diet recommends high volumes of vegetables (India is known for its vegetarianism, is this coincidence?) and low numbers of grains.
Dr. Amato says that B is one of the only blood types that handles dairy well as an adult. However, interestingly, we know that Asians almost universally are lactose intolerant - they CANNOT handle dairy well as an adult. So this would seem to be a contradiction.
Blood Type A
Interestingly scientists are still arguing over whether A or O "came first" which causes a problem for this diet. The diet claims that O was the original blood type and therefore A, coming later, is about farming and grains. It suggestions Type A people avoid meat and to eat natural farm-grown foods. However, what if A turns out to have been the original blood type? In any case, A is found in "cold areas" - the northern areas of Canada, in northern Europe, and southern Australia. So you *can* say that A groups seem to be the hearty groups. So if you compare the diets of say native Alaskans and Scandinavians and so on, you'll find they eat large quantities of fish and monounsaturated fats. These people tend to eat few grains, which seems to go against the Blood Type Diet's recommendations that these "historic farmers" should therefore eat a high grain diet with little meat.
Blood Type AB
The smallest blood group is when you mix the A and B people together! Only 5% of the world has blood type AB. You mix the Indians with the Inuit and come up with vegetarians who eat fish :)
Blood Type O
Again scientists are still arguing about the A vs O primacy in the blood world. Some feel O came first and was the typical meat-eating-hunter. Others say genetic testing proves A came first and that O has only become more prevalent because people with A are more susceptible to dying from diseases like smallpox. If you go with the O=caveman theory, type O people should eat a lot of meat. Almost all of South America and western US are type O.
Is the Blood Type Diet Valid?
Many scientists are quite skeptical about the Blood Type Diet. Part of this stems from Dr. Amato insisting that type O people were the original people, the caveman hunters. He therefore says O people should eat a lot of red meat. However, what if O people were, as many scientists feel, a "later development"? Shouldn't that mean that according to Dr. Amato that those A people should be eating lots of meat, and the O people should be the grain-eating farmers?
I tend to look at this not from a caveman / farmer point of view, but from a "what culture were my ancestors from" point of view. Let's say you are a B blood type which we can clearly see on distribution maps is high in India and China. It is high in areas where people tend to be vegetarians. It makes a lot of sense that if gigantic groups of people are vegetarian and thrive with that diet that someone from that background should at least consider why.
For example, most Asians are lactose intolerant. Their ethnic background determines how their body processes food. This isn't saying Asians are better or worse or anything else. It's simply a biological fact of metabolism. There are many other similar situations for other ethnic groups. By understanding more fully where your ancestry comes from - and investigating what diet and metabolism repercussions that might have - you can do the best job of feeding your body what it needs and taking good care of yourself!