I do multi day fasts (water Ott broth fasts, so no carbs) and once I am in ketosis I actually feel like I can concentrate as well as when I eat. It's key to no

I do multi day fasts (water Ott broth fasts, so no carbs) and once I am in ketosis I actually feel like I can concentrate as well as when I eat. It's key to not refeed carbs at all during fasting (no juices, honey, etc). I also occasionally do keto between the fasts which might have helped my metabolism switching to ketosis more quickly. Ketosis is - in brief - a metabolic state where the body primarily used fats to burn for energy. Our bodies are perfectly suited to operate on ketosis. But this is only reached if glycogens are completely burned which means to u need to avoid refeeding carbs entirely. Basically it is simpler to eat nothing at all, than to eat at a caloric deficit. Think of it this way: our bodies have evolutionarily adapted to be able to live off our fat reserves (when overweight). I recommend reading material or watching videos of Dr. Jason Fung, a Canadian nephrologist who makes compelling well-sourced arguments and explanations.