I've done HIIT in a fasted state just fine, and intellectually I actually feel sharper when fasting, which makes some sense. If we couldn't focus our senses wh

I've done HIIT in a fasted state just fine, and intellectually I actually feel sharper when fasting, which makes some sense. If we couldn't focus our senses while running on stored fat, the human race would have died out a long time ago. There's some data corroborating this, albeit small studies: https://www.ncbi.nlm.nih.gov/pubmed/18779282 https://www.ncbi.nlm.nih.gov/pubmed/18779282 https://www.ncbi.nlm.nih.gov/pubmed/7473300 https://www.ncbi.nlm.nih.gov/pubmed/7473300