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[Tangential musing] I haven't given this much thought or really studied it in any detail but it strikes me that a high blood glucose is metabolically harmful and could quite easily occur during (eg) a fruit glut.
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I agree about the metabolic harm but what did ancient mankind most need to do in the summer? Store fat for the winter.
So anything that raised your blood sugar needed to be shunted into fat reserves ASAP. For most of the rest of the year you lived on a complex mix of low GI carbs, animal protein and fibre.
The only time of year you needed to worry about your glucose level was in midsummer.