Type A types
should basically stick to fruits and vegetables (high carbs / low fat). They have thicker blood than other blood types, a sensitive immune system, and should not consume dairy products, animal fats and meats. They are at a heightened risk for cardiovascular disease, diabetes and cancer.
Type B types
should consume a balanced diet (fruits and vegetables, grains, fish, dairy, meat, but avoid chicken). They have the best chance of bypassing or overcoming everyday types of diseases, including heart disease and cancer.
Type AB types
should consume a mostly vegetarian diet, and only on rare occasions some fish, meat (no chicken), and dairy.
Type O types
should basically stick to a high protein diet (including red meat), low carbs, and enriched with fruits and vegetables. They should limit the intake of wheat germ, whole wheat products, corn, and avoid dairy products and most nuts. Type O types are commonly affected with hypothyroidism, high stomach acid (leading to ulcers), and thinner blood with greater resistance to blood clotting.
Peter D'Adamo proposes that lectins cause agglutination (clotting) of blood cells in an individual with the wrong blood type, and which in turn may create serious liver or kidney problems as visible under a microscope (lectins are sugar-containing proteins found on the surface of some foods that may cause various molecules and some types of cells to stick together).
He theorizes further that elevated urine indican levels - prevalent in many gastrointestinal diseases such as celiac disease, diverticulitis, pancreatic insufficiency, inflammatory bowel diseases and others - can also be attributed to specific blood types affecting the interactions of foods with intestinal bacteria, and creating polyamine abnormalities.
In addition, different blood types - according to P. D'Adamo - affect the body's secretory performance in respect to digestive juices, whereby a blood Type O for instance is capable of producing higher than average stomach acid levels, which could lead to a greater incidence of gastric ulcers.