A thought experiment (and ONLY a thought experiment):
If you think about it, when do we typically encounter something in this universe (in nature, or in the modern world) that requires a storage mechanism? Typically when the thing we encounter, the input, is rare, or when we anticipate that in the future, it will be rare or it will most likely be needed (an analogy is saving money in a bank)…
Mother nature gave us storage mechanisms for things that were rarely encountered or for the anticipation that in the future, these stored things would most likely be needed. We have storage mechanisms for sugar (glycogen) and fat (triacylglycerol). The storage mechanism for fat is much more efficient; one gram of fat containing more energy than one gram of carbohydrate. This is due to their respective chemical structures; glycogen is hydrophilic and can carry three times it’s weight in water, whereas triacylglycerol is hydrophobic and able to package itself efficiently into small droplets that exclude water. Indeed, much more ATP is produced from a gram of fat compared to a gram of carbohydrate (more ATP produced via fatty acid oxidation vs. glucose oxidation).
However, we don’t have a storage mechanisms for protein. Almost all protein has a function other than storage… thus when protein is utilized or is needed, it is taken from muscle or anywhere else in the body where it can be found (usually muscle is the first to go). From an evolutionary perspective, why would this happen? One can hypothesize that this could happen only if protein and amino acids were readily incorporated in the diet, thus minimizing the need for a storage mechanism.
A storage mechanism for protein would come at an unnecessary cost if it was the case that it was frequently consumed in the diet. Think about it this way… ruminants vs. omnivores and carnivores: omnivores (like us) and carnivores can trade stomach mass for brain mass because we eat nutrient dense foods (like meat), and thus don’t need large digestive tracts to digest mass quantities of plants (cellulose and carbohydrates) like cows. It is an evolutionary advantage to be able to do this, and one could argue it led to the survival of our species.
What was rare back then, when we evolved? Well everything pretty much so it’s hard to really say… but a case can be made that protein was probably consumed more frequently. Sugar is seasonal (fruits), and wild game contains not much fat (hence the term “gamey” to describe grass fed and wild meat because they contain less fat and a different fat profile).
Perhaps this suggests humans may be favorable to a diet with a macronutrient profile of low carb, low fat, and high protein?
I myself am an advocate and practitioner of “Paleo” (low carb, “low” fat, and “high” protein) and Ketogenic (high fat, low protein, and low carb) diets, although I usually tip the balance in favor of “Paleo”. The one thing I almost always limit however is sugar and carbs, with the exception of the rare occasional bout of carb consumption for hormesis effects.
Of course this could all be bogus. There are things I’m definitely overlooking and not discussing here. Even if this were true, nutritional requirements and metabolism are not the same for every person, and may be different for people in a healthy state versus in a diseased state.
This was just meant to be a thought exercise :)