> Brain needs glucose.

That is actually incorrect, just a common misconception. You might want to read up on ketogenic diets, and specifically the state of ketosis itself.

If you have an ultra-low carb diet (<30 g/day) with only moderate protein consumption (<30% of daily required calories), then the body can’t produce enough glucose to power the brain. Instead, it starts to convert fats to ketones in the liver, and the brain actually runs fine on ketones as well. Alternatively, if you’re not underweight, you can fast for 24 hours with a moderate activity level, and should enter ketosis regardless of diet (as you start converting stored body fat into ketones to power the brain).

Interestingly, you then notice that the “low blood sugar” mental haze disappears as your brain switches over to ketones, and you kinda avoid the rollercoaster between mental highs and lows throughout the day that you usually get with a carb-based diet – instead, mental energy is kinda just at a constant “medium” throughout the day. It’s also easy to measure more objectively, if you pick up a glucose monitoring device + “keto sticks” from a pharmacy.