Indole/ Indican/ Polyamine/ Putrescene/ Skatole (use these terms in searches) do go up in most type A's on a heavy meat/ high fat diet. This data I have been collecting since 1987 on rather large numbers of patients. But to say that this are exclusively related to low stomach acid may not be correct. One colleague of mine did Heidelberg Gastric Analysis on a variety of patients in his office a few years back. he did note a trend toward low stomach acid in type A's versus other types.
1. Intestinal alkaline phosphatate (a fat busting enzyme) may be the cause of the malabsorption as its occurence in highest in the order O-B-AB-A. This has been postulated to account for the rates of hypercholesterolemia and ischemic heart disease as per ABO blood group (observed to be in the same order) since the early 1960's [editorial, Lancet 1966] Another enzyme which known to be polymorphic in the same distribution, pepsinogen, is important in breaking down proteins to amino acids.
2. Polyamines are known to be stimulated by lectins.
3. ABO blood group has a profound effect on the type of bacteria found in the intestinal tract. Since polyamine synthesis usually involves colonization of the upper intestine, one could argue that gut permiability could also play a role.
4. What is interesting to me about this idea is that polyamine synthesis is usually the result of bacterial fermentation of protein macromolecules or the individual amino acid tryptophane. So it might be that in type A's who are even capable of at least partially breaking down protein by sufficient acid hydolysis may still have more problems with the residues than the other types.