Getting old can be a drag, for both people and nations. As people age, they tend to become less physically active. This leads to loss of muscle mass and the gain of fat, which causes the body’s metabolism — the process of converting nutrients into energy — to decrease. When the population of a nation ages, a similar effect plays out. The labor pool dwindles, fatty debts build up, and the nation’s economic muscle, or labor productivity, atrophies, leading to a decrease in the nation’s metabolic rate and slower growth overall.