There are reasons to believe that the diet ideal for the long-term health of an average human should consist of: 20 to 35 percent carbohydrates, 50 to 65 percent fats, and about 15 percent proteins (the percentages are by calorie).
Reason #1: That’s what all mammals consume.
Not exactly. Different mammals consume different proportions of macronutrients. However, the differences arise from differences in their digestive systems. Some have a bigger colon and some have a different set of gut bacteria. The difference results in different capabilities to convert one nutrient into another. However what remains constant is the ratio of macronutrients left once the food has gone through all the interconversion. So if some species seems to eat a lot of carbohydrates, it’s because its digestive tract has a more developed carbs-to-fats conversion mechanism. And what’s the ratio of macronutrients left after this preprocessing? 20 to 35 percent carbohydrates, 50 to 65 percent fats, and about 15 percent proteins.
But why should we assume that just because all mammals consume this ratio, this is the right ratio for us? Why should we believe that they have figured out the optimal diet for them when humans, a much more intelligent species, have not?
We do not need to attribute excellent optimization skills to the non-human mammals to conclude that they are doing things right. We only need to attribute the optimization skills to evolution. Any non-domesticated species has consumed pretty much the same kind of diet for the last several millions of years. This means evolution has optimized their body to respond well to the diet. That is, the mammals have not optimized their diet to suit their bodies; evolution has optimized the body to suit the diet! Once we transfer the burden of optimization to natural selection, the argument starts to make sense.
Humans also lived on a similar diet for millions of years in the paleolithic time, but things changed drastically once they invented agriculture.
Reason #2: That’s what the human body is composed of.
The human body (and the bodies of all mammals) consists of 20 to 35 percent carbohydrates, 50 to 65 percent fats, and about 15 percent proteins (see Perfect Health Diet) for details. Why does that mean this is the correct ratio for our diets as well?
Because throughout most of evolution, our diet mostly consisted of our own cells. We spent a lot of time fasting because food was not so readily available. And the only food our body consumed during the fasts was our own cells.
Reason #3: Human breast milk has a similar composition.
Again, not exactly. Breast milk is supposed to be for babies and babies have slightly different nutritional requirements than adults because of the accelerated growth of the brain during the early years of one’s life.
The human brain feeds on less fat than the rest of the body. Fats can be used by pathogens and viruses as carriers and thus if the brain fuelled itself with fat, it would be more susceptible to infection. This has led the brain to evolve means of fuelling itself with glucose and ketones. If we adjust for this difference, the breast milk composition is almost 20 to 35 percent carbohydrates, 50 to 65 percent fats, and about 15 percent proteins.
But why does the composition of breast milk have anything meaningful to say about the ideal composition of an adult diet? Because breast milk has been optimized by evolution to be the ideal diet, at least for babies. Adjust for the differences in the nutritional requirements of babies and adults, and you get a reasonable formula for an ideal adult diet.