It’s interesting. Human baby is in ketosis when born, breast feeding maintains it. There is no essential carb.. Switching to ketogenic diet is cure for many health issues.

Then again, I know fruitarian who runs ultramarathons. He eats only sugar, so clearly human body can run on two different fuels very well.

Maybe we shouldn’t mix them but choose just the other? Carbs or fat. It would make sense that if there are two paths to get energy from eating, the system may not run optimally if you keep feeding both at the same time?

Humans seem to do alright with most diets except for the eat too much one

Why would that make sense? People are always coming up with diets that look something like “stop eating bananas because they have been secretly making you sick and only eat blueberries which cleanse all your toxins” (and whatever variants thereof). It makes sense in a “doctors hate this one simple trick” sense but not biologically.