Depends on whom you ask;
some arguing from an incorrect understanding of evolution*, will tell you that we've evolved to low-carb and that any HbA1c higher than 5.5% is bad (not what you asked for, but you get the picture)
* Think about what evolution does - it selects the candidate best able to pass on their genes; since diabetes only kills us well after we would have had kids and grandkids in the past, our evolution is completely irrelevant to the condition.
Further, any diet humans have evolved to eat would be advantageous for evolution - producing lots of offspring and dying young - which is not something usually considered desirable today.
Finally, there is evidence tht humans haven't stopped evolving after agriculture was developed; humans have largest brains and smaller digestive tracts compared to apes of human size I.e. We need easily digested calorie dense food to meet our energy requirements (the brain, located outside the well-insulated torso, has to be kept at body temperature)