Yes Smed, I agree. Tolerated diet has much to do with our genes and how they are regulated as our neuroimmune system LEARNS to interact with our microbiome. There is ample evidence that diet can also feedback through our nuroimmune system to alter gene expression and that that gene expression carries over to the next generation mostly through the mother. What your great grandmother ate, thought and experienced is passed on to you.
It is said that the mother spends her whole life preparing her body to nurture the baby. That isn't sexists and science shows that epigenitics and the microbiome are strongly intertwined and inherited. Genes are a minority of the story. Cultural and social capital in the sense of resources, values and habits are passed on too; it all matters.
I like the way these two contrasting rationales for what is the right diet exemplify how important empirical evidence is in comparison to just a good theory that convincingly weaves facts together.
The empirical diet (diets are individualistic):https://www.youtube.com/watch?v=0z03xkwFbw4
The theoretical diet (there is one diet for all humans):https://www.youtube.com/watch?v=2wG3b3ql34A
Again, diet can alter gene expression. We are not locked into a diet based on our genes but I do suspect that optimum health is subtly dictated by genes.
Having that dreaded HLA-B27 gene seems to be a detriment if you eat grains: there is anthropological evidence that rates of vertebra fusion was abnormally high among migrants that moved from the northern latitudes into Egypt to work and ended up eating more grains (note: the story that King Tut had AS has changed). But perhaps having the HLA-B27 gene also reduces the rates of cancer and suseptibialty to pathogens -- it's a double edged sword!
But having emphasized the importance of "evidence" I still think there are times that doctors need to treat the patient and symptoms instead of the blood tests and biopsies for humanitarian purposes.
Sometimes you just have to have faith. For example, I had faith that the prophecies of the Standard Model of Particle Physics was correctly prophecizing the existence of the Higgs Boson -- it just had to exist to make the model work and there was way too much evidence already supporting the model. And so it was observed 40 years after its prediction! Related to that, the evidence is growing that gravity is not a fundamental force field but actually an emergent property of more fundamental principles in physics. Lastly, I take a leap of faith and hypothesize that nothing is real but consciousness. So maybe I can just think my diseases away!
May the Force be with you, always!