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

by Daniel E. Lieberman

basic function is to cause the body to store energy. Among its many actions, insulin directs special molecules on the surfaces of fat and muscle cells to transport sugar from the bloodstream into those cells to be stored or burned. Type 2 diabetes arises when the effects of metabolic syndrome prevent insulin receptors on these cells from binding with insulin (a phenomenon termed resistance). A vicious cycle ensues. When insulin binding doesn’t happen, the glucose transporters don’t take up sugar from the bloodstream. Then, as blood sugar levels rise, the brain desperately commands the pancreas to produce yet more insulin, but with diminishing effect, causing blood sugar levels to stay dangerously high. Symptoms include frequent thirst and peeing, nausea, tingly skin, and swollen feet. Eventually, the overworked pancreas fails, requiring injections of insulin to avoid death.