A comment below about gluten intolerance (which is associated with problems digesting products with wheat) made me curious. In much of Eurasia this would be a serious problem since wheat is the staff of life. Hard numbers are difficult to come by. This is as good as anything else I’ve seen:
Celiac disease affects as many as 1 in 300 people in Italy and southwestern Ireland, but is extremely rare in Africa, Japan, and China…According to a multicenter study in 2003, there is a 1 in 133 chance that people with no risk factors or family history in the U.S. have celiac disease. Additionally, a person’s risk increases to a 1 in 22 chance if they have a first-degree relative with celiac disease and a 1 in 39 chance if they have a second-degree relative…Around 60,000 Americans are diagnosed with celiac disease annually and a total of over 2 million have the disease, making it perhaps the most common genetic disorder in the United States…Celiac disease can occur at any age, and females are more commonly affected than males. Of females presenting during their fertile years, the male to female ratio is almost 3 to 1….
The vast majority of coeliac patients have one of two types of HLA DQ, a gene that is part of the MHC class II antigen-presenting receptor (also called the human leukocyte antigen) system and distinguishes cells between self and non-self for the purposes of the immune system. There are 7 HLA DQ variants (DQ2 and D4 through 9). Two of these variants-DQ2 and DQ8-are associated with coeliac disease. Every person inherits two copies, one from each parent. The gene is located on the short arm of the sixth chromosome, and as a result of the linkage this locus has been labeled CELIAC1.
Coeliac disease shows incomplete penetrance, as the gene alleles associated with the disease appear in most patients, but are neither present in all cases nor sufficient by themselves cause the disease. Over 95% of coeliac patients have an isoform of DQ2 (DQA1*0501:DQB1*0201 haplotype or more simply DQ2.5) and DQ8 (DQA1*0301:DQB1*0302), which is inherited in families.
Incomplete penetrance might be due to the fact that there are other genetic actors which haven’t been elucidated that are necessary for the emergence of this syndrome. Or, there might be environmental or pathogenic triggers which only affect a minority with the necessary genetic predisposition. But in any case, my first thought was gluten intolerance might be the result of an incomplete selection sweep as populations shifted from hunter-gatherer lifestyles to agricultural ones. I’m skeptical of this since populations in Africa and Australia which don’t have a history of wheat agriculture don’t exhibit this syndrome. Additionally, though wheat agriculture is practiced in north China this was originally a region of millet production. Finally, all the reports suggest massive underestimates of the extent of this condition within the population. Like lactose intolerance this isn’t a disease with a clean set of symptoms which are easy to assay quantitatively (is there a way a metric for stool firmness?). The implication of MHC loci as necessary preconditions makes me wonder if gluten intolerance is simply a low frequency condition which is a byproduct of a disease adaptation on the genes in question which was operant in western Eurasia.