I have heard that in the study of ying and yang, a ancient Chinese philosophy, that people adapt to locally produced food that are indiginous to the areas they live in. For example, a bananna will have no effect on someone in central America, but may clash with someone in New York.
Pineapple or coconut fine in Hawaii, but not so healthy for someone in Iowa.
American Indians lived off of the local food fare like venison and fish, berrys and nuts. When europeans introduced other foods into the hemisphere, the Indian people started suffering from illness they had never experienced. Their immune systems were adversely affected by food items not indiginous to their local habitat.
I don’t know for certain if this is a valid arguement for health problems. But it certainly in theory could have a bearing on many food allergies based on ethnic heritage.
For example, there are studies of Europeans that point to 500 relatively recent (w/in last 5,000 years) gene mutations that enable them to consume milk as adults. The Chinese, derived from the same Ice Age population, do not have those mutations but they have their own large number of mutations that appear to enhance their ability to consume rice.
With wheat, the argument is that the genes for Celiac (gluten intolerance) arose in a population (the Sa'ami) with no access to grain. On the other hand, there's a plant, Birthwort (Aristolochia clematis), that likes to grow in wheat fields. It contaminates wheat with a poison that can wipe out your liver and kidneys in a couple of days.
Almost certainly a population with Celiac would survive such contamination. The recent finding of a number of alleles that can cause Celiac implies that humanity has faced a crisis regarding Birthwort many times before.
Even the Pentatuch focuses on the need to clear wheatfields before sowng the grain ~ take a look at: http://www.medicalnewstoday.com/articles/90839.php