alt Hacker NewsI think common sense here can be a guide though. You don't need sugar at all, excluding high levels of anaerobic exercise. Your liver can produce the glucose your body actually needs from other sources (gluconeogenesis) and a lot of your tissues that use glucose also can use fatty acids or ketones. Fructose isn't needed at all. ("low blood sugar" isn't a symptom of not consuming enough sugar, it's a symptom of a disregulated metabolism -- ie insulin resistance or other conditions)
Saturated fats have all sorts of uses biologically.
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That has nothing to do with whether excesses of those nutrients cause cardiovascular disease, though. The general consensus is that the healthiest diet is one with 5-10% of total calories from saturated fat. For most people, it's necessary to restrict saturated fat to land in that range. We also need to distinguish between sugar and carbohydrates. Again, the general consensus is that intake of sugar and refined carbohydrates should be minimized, while 50-75% of total calories should come from sources of complex carbohydrates like vegetables, beans, and whole grains.
Looks like it's true that low-carb adapted athletes rely more on fat oxidation during exercise but performance suffers nonetheless because of increased oxygen demands that basically cannot be met.
Your entire argument here applies in the other direction as well. You do not need dietary saturated fats, and sugar has all sorts of uses biologically.
I would caution that just because your body can make something doesn't mean it will have optimal performance when doing so. People in ketosis do have worse peak performance in sports than those that eat more carbs/sugar.