This is a bit of a thought experiment. I've been mulling over a lot of the competing theories wandering the paleosphere over the last year - especially in light of the Taubes-Guyenet cage match that took place in the months immediately after AHS11 - and have some ideas of my own regarding what we should eat and why we react as we do to some foods. Not based on any scientific studies or anything, just distilled out of a lot of reading other people's thinking on nutrition.
Let's start by removing man's ability to store food prior to eating it. If we don't eat something we have hunted or gathered, it will spoil and make us ill. That means that we eat what's available when it's available, naturally, rather than what has miraculously been shipped around the world and kept fresh for us year-round. If we ate that way, what would it look like? I think it would look a bit like this (I'm looking at this from the high plains of North America - regional variances would obviously apply).
Start with January - plants would be scarce; we might find some leaner animals or fish if the weather permits. But we'd have to live to some degree off our own fat stores. Early February might look a lot the same, maybe we'd start seeing a few leaves or shoots toward the end of the month if the winter had been warmer, but still a lot of living off those fat stores. As Spring gets rolling, the plant matter increases, and the animals would be competing with us for it - but that might also make it easier to hunt them - and, even if it would mean "eating Bambi", I suspect that mothers and young might be easier targets.
Summer would bring a big increase in available plant food and fatter animals to eat; and as it progresses, our plant-based diet would move into fruit, with its relatively high sugar content. The taste would provide high reward, and we would gorge as much as we could. This would increase our fat stores as the season continues. In addition, meat and fish would be easier to get than in the depths of winter, since we wouldn't be fighting the weather.
In the Fall, our plant food availability would start declining, and would shift to tubers and cool-weather greens, maybe some winter squashes, which don't spoil so quickly. The animals would be at their fattiest, but as the weather grew colder, would start getting harder to find, and as Winter returns, we'd be back at scarcity, again relying on the fat stores built up in our fruit gorge over the summer.
So, maybe from November to May, we'd be operating on a ketone-based fuel, June-October on glucose, with the result that we are fatter ourselves as Fall deepens, and burn our own fat during the colder months as well as any animals we can find and eat.
Note that I mention the word "gorge" in late summer. Fruit sweetens, the reward increases, we eat until we can't, based on the reward. That fuels a glucose/storage cycle that fattens us up - as should happen. But, because it's not available all the time, this isn't a problem; we wouldn't choose fruit over meat in January, because there isn't any.
And if ketones fuel our brain in ways that seem to sharpen our intellect, does that not also make sense in terms of needing to be a bit sharper during ketone-fueled months to find scarce resources and manage to kill them to eat?
This thought experiment leads to these implications, for me, at least.
- Glucose is not toxic, per se, but it has predictable effects on human health. Ditto fructose, which generally should be accompanied by the rest of the fruit of origin to mediate the effects.
- The fat-storage mechanism that correlates with high insulin levels has a beneficial purpose that makes sense when seen in context.
- There's a reason we are a dual-fuel creature
- Plants are in symbiosis with animals and we should act in accordance with that symbiosis. Our role in the circle of plant life is to spread whole seeds around in excrement; modern technology has allowed us to blow the symbiosis out of the water (refining sugar and seed products to eat them in isolation, etc).
- It might be worth exploring if seasonality itself is important, or if we can use the information above in isolation. Might there be the benefits to eschewing food storage and eating as if some foods were less or not available during part of the year and in crazed abundance at other times?