" a ketone (also called a ketone body). Ketones are safe, desirable, energy-efficient forms of fuel. They are quite literally the fourth fuel. In fact, when you become well adapted to burning fats on the Primal Blueprint, one of the side benefits is that you will also become keto-adapted. "

― Mark Sisson , The Primal Blueprint: Reprogram Your Genes for Effortless Weight Loss, Vibrant Health, and Boundless Energy