Much study has been done on blood types, includeing their effects not only on nutritional needs, but personality.

Blood type O is the oldest and most common on the planet, deriving from the original hunter-gatherer population. These people survived best by eating a lot of animal protein with some vegetables, fruits and wild grains supplementing the meat. Thus, O blood types need adequate strong proteins in their diets, especially wild game, vegetables, some fruit and small amounts of whole grains and dairy (both of which were introduced to the world about 10,000 years ago and are very new to the O blood type metabolism). This blood type should especially avoid pok prodducts and refined grains and flour products.

Blood type A, the next blood type appearing after type O, generally generally does well on a vegetarian diet. They need protein that is leaner or from carbohydrates (legume) sources. They tolerate more grains, legumes and vegetables. A blood types should limit red meat and dairy.

Blood type B appeared less than 10,000 years ago, after the introduction of grains into the diet. Thus, B blood types can handle a wide range of animal products, dairy and complex carbohydrates. This blood type should avoid chicken and sesame seeds/oil as they cause clumping in B type blood.

Blood type AB does very well on a vegetarian diet. They can handle dairy, grains and domesticated meat. This is the most recent blood type on the planet, and thus the rarest.