As Todd says, Dr. D'Adamo doesn't really recommend wheat for any blood
group, but my understanding is that he only considers gluten to be a serious
problem for Type O's.  He tells Type A's to restrict wheat because it causes
excess acidity.  Since Dr. D believes that gluten isn't a problem for Type
A's, and autistics are often sensitive to gluten, he was surprised when Todd
mentioned that most autistics are Type A.

Not all traditional hunter-gatherer groups were exclusively Type O.  For
example, about three-quarters of the Blackfoot and Blood Indians were Type
A;  ditto for about 30% of the Navajo.  I'm not sure about the Blood, but
the Blackfoot relied on buffalo meat as a dietary staple.  When I mentioned
this to Dr. D'Adamo, he replied that this may be because Type A blood
increased the resistance of these tribes to certain infectious illnesses.  I
suspect that resistance to disease may be the main selective pressure
affecting blood type distribution.  Since certain illnesses would have been
most common in agricultural societies, this may have resulted in a spurious
correlation between blood type and metabolic type.

Incidentally, does anyone have any solid stats on the ABO type distribution
among the Inuit?  Some people have said that they were virtually all Type O,
but Mourant's "Blood Relations" book stated that a significant proportion
were Type A or B.  The book also says that populations which were completely
isolated became entirely Type O over a period of several millenia (partly
because Type O mothers will abort Type A or B fetuses more often than Type O
ones); however, it also mentions h-g populations which were not completely
isolated, where the A and B types persisted.

I'm another Type A who fared very poorly on a "Type A" vegetarian diet, so I
share the skepticism of others on this list.  As Todd suggested, the blood
type diet confounds two issues: (1) sensitivity to particular foods, and (2)
metabolic needs.  Blood type may in fact be correlated with metabolic needs,
but there are many exceptions.  From an evolutionary perspective, it makes
little sense that within a population, in which everyone ate the same foods
for thousands of years, there would be sub-groups with completely different
dietary needs; hunting tofu was not an option for the Blackfoot.

Cheers,

-- Aaron Wieland