Clearly, humans can adapt to many types of diets involving multiple macronutrient combinations with varying amounts of fat, protein and carbohydrate. However, our genetic constitutions, including our nutritional requirements were established in the remote past over eons of evolutionary experience. Human health and well being can be optimized when we use the evolutionary paradigm as the starting point for present day nutrition. Obviously, humans have had little evolutionary experience with the modern high carbohydrate, high fat, cereal based diet which is omnipresent in western, industrialized countries, and there is considerable evidence to suggest that these types of diets have the potential for creating health problems in some, but not all people. That varies by the person.
Some people can handle more cereal grains than others.
For a celiac patient a single teaspoonful of gluten containing grains is too much. The human dietary staple for more than 2 million years was lean game meat supplemented by fresh fruits and vegetables. I suspect that for most people, a simple subjective test can be conducted in which they reduce the amount of cereal grains in their diet and replace the grains with more fresh fruits, vegetables and lean meats and seafood. They have poorly absorbable vitamin B6, and the phytate levels in grains impairs the absorption of most of the divalent minerals. Excessive consumption of cereal grains are associated with a wide variety of health problems.