If you’ve ever “gone with your gut” to make a decision or felt “butterflies in your stomach” when nervous, you’re likely getting signals from an unexpected source: your second brain. Hidden in the walls of the digestive system, this “brain in your gut” is revolutionizing medicine’s understanding of the links between digestion, mood, health, and even the way you think.⁠ ⁠ Scientists call this little brain the enteric nervous system (ENS). And it’s not so little. The ENS is two thin layers of more than 100 million nerve cells lining your gastrointestinal tract from the esophagus to the rectum.⁠ ⁠ Unlike the big brain in your skull, the ENS can’t balance your checkbook or compose a love note. Its main role is controlling digestion, from swallowing to the release of enzymes that break down food, to the control of blood flow that helps with nutrient absorption, to elimination. The enteric nervous system doesn’t seem capable of thought as we know it, but it communicates back and forth with o...