I’ve always been told that I have a fast metabolism. I stay thin no matter what I eat; it’s only in the past few years, as I’ve entered my mid-thirties, that I’ve experienced growing horizontally. I play squash a few times a week, run with a friend on Thursdays, and walk the dog. Otherwise I spend whole days at the computer, then sedentary on the couch, then asleep. And yet I stay lanky and get “hangry” easily; in the afternoons, after a hearty breakfast and two helpings at lunch, I go looking for another meal. I sometimes wake up hungry in the middle of the night. Where’s all the food going?
Our bodies require a lot of calories, and most of them are spent just keeping the machine running. You don’t particularly feel your liver, but sure enough it’s always there, liver-ing; likewise your kidneys, skin, gut, lungs, and bones. Our brains are major energy hogs, consuming around a fifth of our calorie intake despite accounting for just a fiftieth of our body weight on average. Possibly mine is less efficient than yours: I have an anxious cast of mind—I ruminate—and maybe this is like running in place. I sometimes feel sluggish while writing, after working a paragraph over in my head, and I used to assume that this meant I needed caffeine. Eventually, I discovered that a sandwich worked better. The effort of thinking had run my calories low, and it was time to throw another log on the fire.
Fire isn’t merely a metaphor for metabolism. In the eighteenth century, the French chemist Antoine-Laurent de Lavoisier conducted a series of ingenious experiments to prove that our life force was fire. First, he figured out what air was made of; he then, through precise measurements, showed that fire removed oxygen from the air and deposited it in the form of rust. Later, he made a device in which packed ice surrounded a compartment that could be filled with either a lighted flame or a small animal; by measuring how much ice melted, he could relate the energy burned by the flame to that “burned” by the creature. He even created a “respirometer,” an apparatus of tubes and gauges that measured a person’s precise oxygen consumption as they took on various tasks. He concluded that “respiration is nothing but a slow combustion of carbon and hydrogen, similar in all respects to that of a lamp or a lighted candle.” Both flames and living beings exchange energy and gases in what’s known as a combustion reaction. In fire, this reaction runs fast and out of control: energy is ripped from fuel with violent abandon, and nearly all of it is released immediately, as light and heat. But life is more methodical. Cells pluck energy from their fuel with exquisite control, directing every last drop toward their own minute purposes. Almost nothing is wasted.
