The basic premise of the blood group diet, or the genotype diet, is the concept of nutrigenomics, a growing area of research. Taken straight from the blood group diet's website, nutrigenomics and this diet recognize "the biochemical uniqueness of each individual and the need to tailor treatments and prescriptions to match that individual variability. While a person's genetic code, ultimately, is the basis of this individuality, basing treatments on genetic factors is too broad an approach and not consistent with alternative medicine". Basically, the plan suggests that your blood type indicates the prehistoric group you're descended from, and it reveals what your digestive system can tolerate. Interesting, but is there any truth behind this?
There is no published research to date that shows that such diets are particularly effective, but one thing is for sure; they can put people at risk of nutrient deficiencies. For example, Group A's are recommended to consume a vegetarian-style diet with more vegetables and greens, cereals, nuts, seeds and carbohydrates such as pasta. It is proposed that meat should be consumed in lesser quantity. Perhaps this isn't a huge deal, but what about Type O's, for whom whole grains and dairy are off limits? It is recommended that they follow a 'prehistoric' style of eating, consuming lots of red meat and fish. You don't have to be a rocket scientist to realize that excluding dairy and grains, while eating lots of red meat raises many red flags.
Also, I might remind you that our food has changed in an astronomical number of ways since the time of hunters and gatherers, and humans have changed too, so trying to copy their diets expecting the same results just doesn't follow. Most respected medical experts simply don't back this diet because they don't want to put their patients or clients at risk for deficiencies and best practices are always backed by sound research. Realistically, the best road to good health is ensuring that you get all the nutrients you need from a balanced diet and avoid excluding entire food groups. Balance is the name of the game, not blood type!