When you eat three square meals a day (and fill your plate with representatives of each of the four food groups) your brain burns sugar. Glucose, to be precise, and boatloads of it. That’s all well and good, because your brain runs on heavy fuel. It vacuums up two out of every ten calories you consume. If you’re swaying in the hammock under a shady tree and contemplating life’s mysteries (or having a vivid daydream) that percentage may rise to an even fifty. When your blood sugar dips, your ever-voracious brain commands your body to produce an alternative energy source (ketones).