" The good news is that you can quickly reclaim your genetic fat-burning abilities. Metabolic flexibility allows you to feel great all day long, with stable mood, energy, cognitive functioning, and appetite, whether or not you eat regular meals. I believe that reigniting your metabolic flexibility is the holy grail of all health pursuits. With it, you can naturally derive energy from a range of sources: the fat on your plate of food or the fat on your butt, belly, or thighs; the carbohydrates in your meal, the glucose in your bloodstream, or the glycogen in your muscles; and even from ketones, the superfuel that your liver makes when you’re fasting or restricting dietary carbs. The best part is that your body doesn’t care where those calories came from, because your source of calories to burn will move seamlessly from one substrate (fuel source) to another depending on your immediate energy needs. "

― Mark Sisson , Two Meals a Day: The Simple, Sustainable Strategy to Lose Fat, Reverse Aging, and Break Free from Diet Frustration Forever