This is mostly hunch/n=1, but I think a significant factor is not just sugar, but the sugar/fiber ratio. This is one reason why whole fruits (and berries especially) are preferable to fruit juice: the fiber slows down the blood sugar spike (some of the energy is used up breaking apart the fibers for digestion).

I’ve started eating a pinch/handful of leafy greens twice a day, as a sort of medicine-for-the-gut; and while the effect is subtle, I’ve definitely noticed a stability boost to mood and energy, especially while I’m not in ketosis.