From Harvard Health Publishing: Each of these theories has been challenged. For example, there is evidence that type A was actually the first blood group to evolve in humans, not type O. In addition, there is no proven connection between blood type and digestion. So, in addition to a lack of evidence that the diet works, serious questions remain about why it should work in the first place. https://www.health.harvard.edu/blog/diet-not-working-maybe-its-not-your-type-2017051211678