Think of your younger body as a thriving business in its best quarter. Flush with revenue, strong fundamentals, healthy reserves. In that context, cutting discretionary spending works beautifully. You trim the excess, the business barely notices, and you come out leaner and more efficient. The caloric deficit model was essentially that. Your hormones were the profitable quarter. The calorie cut just took credit for it.

When we think about health, we often focus on separate systems—the heart, the gut, the brain—but what if there was a single nerve that...