I think the ‘theory’ behind it is some thing like this:
Originally, there was just one blood type, back in the distant past, when we were hunter/gatherers. Obvoiusly we would have eaten plenty of maet, fat and some roots, nuts, seeds and berries. What we would not have eaten is carbohydrates in anything like the amounts (or types) we do today.
So if you have this original ‘ols school’ blood type, you should eat an old school’, paleo type diet (according to the theory).
Then some of our ancestors became subsistence farmers and began cultivating crops and husbanding livestock. At around this point a new blood type emerged and was found mainly within this proto-agrarian culture. These folks were better adapted to eat corn, bread, flax, etc, etc.
Later still, as more intense farming and more freely available refined carbohydrates appeared on the scene, another bloodtype emerged.
So the theory says that if you are one of the older blood types, then you shaould avoid carbs, since you weren’t getting any ‘back in the day’. The newer types can eat carbs, but not necessarily the refined ones. If you have the new blood types, you can eat what you want.
As you can see, I only skimmed the basic facts (from a freind who believes in this) and as you can see, whilst on some superficial level there may be a form of logic to the theory, I don’t personally agree with it.
Did they actually verify by any means – perhaps time travel and directly testing these ancient peoples, or perhaps DNA testing of thousands-of-years-old bodies which are in such plentiful supply from most cultures – that these different sorts of cultures had different blood types?
Or did they pull it out of their ass like the rest of the theory?