This! carb load is dictated by activity level. Nuff said really. We could go back and forth posting things like this Glucose Hysteresis as a Mechanism in Dietary Restriction, Aging and Disease but there's a tit for every tat in the debate so its fairly useless. The concept that either glucose or fat is a preferred fuel source is an oversimplification. Your body will preferentially burn fat at rest and up to 70% of you max heart. Over that you'll preferentially burn glucose. Some body parts burn fat predominantly or respond very well to ketones as a fuel while others prefer glucose and only partial displacement by ketones.
The fact is that you spend most your time in a state of low to mid level activity that fat can fuel quite well. The problem with going into all this is that people want an either/or solution, or have latched on to some new ideas and won't accept the wide variety of diets humans can readily adapt to and thrive on. Since hundreds of healthy human societies for thousands of years have thrived on low carb it is quite obvious we are well adapted to it (far more societies than just the Inuit as some would have you believe). Likewise, you can find healthy human societies that thrive on a higher carbohydrate load. So the only thing you can say for certain is that humans are perhaps born with the capacity to thrive in either environment.
So now enters our diseases of civilization. That is where it gets hairy. Those with autoimmune disorders, neurological, metabolic syndrome, diabetes, lipid dysfunction, thyroid problems, athletes and other extremes fall here too....and on and on. For each of these snowflakes it behooves them to do the N=1 necessary to find their optimum. The history of humans shows what we are capable of living well on.....picking what works best for you current condition and recognizing that it has evolutionary precedent is liberating! Low carb is perfectly safe and healthy and has precedent as a diet our ancestors thrived on. So do certain carb sources obviously so athletes or certain conditions or relatively healthy individuals obviously have that option available to them.
Damn, I went and let myself post this crap.....oh well