Exercised: Why Something We Never Evolved to Do Is Healthy and Rewarding

by Daniel E. Lieberman

You’ll be relieved to know that to measure your DEE, we no longer follow you around all day with a gas mask. Instead, we measure your pee. To be more precise, we’d ask you to drink some very expensive, harmless water that contains a known quantity of rare (heavy) hydrogen and oxygen atoms and then collect samples of your urine over the next few days. This sounds like a creepy magic trick, but by measuring the rate at which these heavy atoms become less abundant in urine, we can calculate the rates at which both the hydrogen and the oxygen atoms leave the body from sweating, urinating, and breathing. Because hydrogen exits the body only in water but oxygen leaves in both water and carbon dioxide, the difference in the concentration of these two atoms in urine allows us to compute exactly how much carbon dioxide someone produced from breathing, hence how much energy he or she used.9 Thousands of people’s metabolisms have been measured this way, providing a remarkable database on human energy expenditure. If you weigh 180 pounds, your DEE is probably about twenty-seven hundred calories a day. Because we already learned your RMR is about seventeen hundred calories a day, that means nearly two-thirds (63 percent) of the energy you expend each day is spent just on your resting metabolism. Who would have thought that being a couch potato was so expensive? Is