This story was originally published by Knowable Magazine.

From the earliest days of their evolution, guts and brains have been the best of friends.

It’s a mutually beneficial relationship. Guts prepare nourishment for delivery to the brain. And brains guide the behaviors needed to fill the gut with raw materials.

Even today, the primitive need to serve the gut’s hunger remains implanted in the human brain’s blueprint for directing behavior. But nowadays, food sometimes drives the brain to behave in ways that aren’t as useful for survival as the original evolutionary programming. In recent decades a mismatch has evolved between the food available to hungry humans and the brain circuitry designed to acquire it. Instead of scrounging for scarce sources of high-quality calories as in the era of hunting and gathering, modern humans are flooded with a glut of ultraprocessed foods, designed to appeal to the brain’s ancient evolutionary imperatives—whether the body needs the food or not.