[throwaway_kufu]

Keto is the primary energy system.

Consider when you don’t eat (fasting) your body goes into Ketosis. In other words it’s the default energy system.

You literally have to introduce Specific foods that knock you out of Ketosis...it just so happens now a days those are the primary foods people eat. Foods that spike insulin and knock you out of Ketosis really wouldn’t have been available in a constant supply to keep people running on glucose pre-agriculture.

Also consider the blood brain barrier, ketones are much more efficient at crossing the blood brain barrier to fuel the brain compared to glucose.

[mrob]

Even in the environment of evolutionary adaptedness, prolonged fasting is not the default. If you're going to argue that metabolism under abnormal conditions defines the primary energy system, alcohol must be the primary energy system. It's used at higher priority than either carbs or fats.

[throwaway_kufu]

Alcohol is an energy source just as much as carbs, fat and protein for sure...but the brain can’t run on alcohol, only ketones and glucose.

Your natural state is ketosis (that makes it the primary fuel source). And we are not talking about “prolonged fasting” or “abnormal conditions”, we are talking about natural states. So if you happen to kick yourself out of ketosis and start running primarily on glucose most people will burn through their glucose stores in just a few hours and their body will crash/shutdown to preserve the remaining glucose for the brain (think of marathon runners “hitting the wall” which doesn’t happen in ketosis, you basically have unlimited fat energy stores and your body will internally produce the limited glucose the brain needs)