This is not scientific precision. This is not peer reviewed. This might not resist the rigor of an editor. These are simply observations intended to spur thought and look laterally.
Fat is probably the most desirable and quintessential food for humans. It is the supreme appetiser. It carries essential vitamins. Not only has fat the highest calorific gain of all foodstuffs but it has a low-calorie demand for ingestion and digestion. And yet it has powerful feedback mechanisms. Therefore, although fat stimulates appetite it also produces satiation relatively rapidly. Rapid satiation allows food to be spread to the entire pack, in keeping with expectations of le milliere exterior which demands survival of the group, not the greedy individual.
Protein is probably neutral tasting without the fat and salt, not particularly palatable and not having the “addictive” quality of carbohydrate. (Refined and manufacturer altered carbohydrate once ingested, prompts the desire to keep on eating it.)
Carbohydrate was probably never particularly attractive to most early humans. Yes, I know well enough that some carbohydrates, the sugary carbohydrates, are exceedingly attractive. But in primitive societies, all carbohydrates were not attractive. Sugar is a relatively new evolution and the current sweet fruits and even potatoes are the product of intentional selective breeding to make carbohydrates more palatable.
The metabolisms of carbohydrate, the sugars, are again very different from fat, in that the same metabolic pathways are used for both the anabolism and the catabolism of carbohydrate. This is distinct from fat where the anabolic and catabolic pathways are very different, and so allowing feed-back curb. Satiety occurs rapidly when fat is eaten.
Carbohydrate’s prime quality is that it is cheap. As a consequence commerce has “wrapped” carbohydrate in both fats and sugar to make it more attractive, if not compulsive, eating at a cheap price. Amongst the most tempting ingestants are those that have both sugar and fat, as in chocolate.
Therefore it might not be what you eat, but the combination of foods that is eaten, which influences the health or disease of individuals. There is some evidence that individuals like to eat the same food and can become habituated to certain foods.
“Humans like variety, humans need variety, and humans need a balanced diet”. This may not have been the case with evolutionary man and it is certainly not the case with many animals. Animals can adapt to a particular foodstuff (clearly one that is available for them) and then continue eating that foodstuff by choice, even where alternatives are available.
The legend has arisen that individuals need a “mixed and balanced” diet. As far as I am aware there is no evidence that this needs be in the same meal. True enough, one needs the vitamins and one needs the different proteins, fat and carbohydrate. But does one need them simultaneously, wrapped around each other and made into tempting compotes?
Satiation by fat is strongly influenced by salt. Therefore, salted fat and perhaps salted protein can oppose satiation causing fat to become a “compulsive” foods, inducing eating until gorged. So we have yet another factor in “additions” to food which induce compulsive feeding, prompted by those intent on making money out of food. Salt is impregnated into carbohydrate (in the form of chips and French fries). Sugar is impregnated into cakes or spread on the top as icing. Fat is used as a layer to make bland carbohydrates or even carbohydrates and protein more palatable, as in deep fried foods – where salt is also added detrimentally.