[PudgePacket]

Humans can produce all the glucose they need when in nutritional ketosis or starvation ketosis.

https://en.wikipedia.org/wiki/Gluconeogenesis

[shlant]

but does that make it the "default energy System"?

[shock]

From an evolutionary standpoint it would make sense to evolve the ability to produce all the secondary "fuels" from the most abundant(primary) "fuel". I don't know if things are so in reality, alas.