My working hypothesis is that from 3 million years ago to 100,000 years ago, our ancestors primarily ate megafauna. The optimal diet that we evolved for is to nose-to-tail eating of megafauna, so basically cows and sheep, since we killed all the mammoths and deinotherium.

In more recent times, human diets have been under rapid selection pressure, with most human groups being able to tolerate grains, and some being able to thrive off of milk, and other human groups . These populations that underwent selection for certain diets then get mixed around a lot with various migrations, conquests, and intermarriage, and so it is a bit of crapshoot what kind of diet any given modern caucasian American can tolerate. Pretty much any American can thrive on a diet that is very heavy in cow meat, but what kind of dairy or plants can be tolerated is more random.

Heck a good friend of mine, if he follows exactly the same keto diet I do, felt physically sick and lethargic for 2 months before he gave up. Meanwhile I was out there doing 3 hour a day intense workouts.

Out of curiosity, what are the rough outlines of this diet that did not work for your friend?