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YOUR SECOND BRAIN

My students tell me after they learn about the vastness of their brain, they have a whole new sense of worth, that their self-esteem grows overnight. Here’s more good news: You are not limited to just one brain, you have a second—your gut. Have you ever had a “gut feeling”? That moment when you just knew? If you’ve ever “gone with your gut” to make a decision or felt “butterflies in your stomach,” did you ever wonder why that was? 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 esophagus to rectum. Science is only beginning to understand the brain-gut axis and how it affects our brains, our moods, and our behavior. You may hear it referred to as the “brain-gut connection.” In the last decade, we’ve discovered that the gut has an outsize effect on the way our brains function. One can liken it to the way a tree functions. The roots in the ground are drawing up vital nutrients and water from the soil as well as communicating with other plants. Those nutrients are then brought up into the body of the tree, fortifying and building the trunk, and giving the tree what it needs to sprout new leaves each spring, which in turn gather light, another energy source.
In the same way, the nutrients we take in are absorbed through our intestines. We rely on those nutrients to fuel our brains. While our brains take up very little of our total body weight, they use 20 percent of the energy we take in, so nutrients make a huge difference in the way our brains function on a day-to-day basis.

The gut is lined with more than a hundred million nerve cells, and it makes up part of the ENS. When a baby grows in the womb, the ENS and the CNS develop from the same tissue and remain connected via the vagus nerve. In many ways, the two systems mirror each other in structure. They also both use many of the same neurotransmitters to function, including serotonin, dopamine, and acetylcholine. As with CNS, we used to believe that we are each born with a certain amount of cells—and that’s it. But like the brain, we now know the ENS makes new neurons throughout adulthood and can be repaired when damaged.⁵ The gut is made up of these neurons as well as a network of bacteria that form the microbiome, and as with the brain, each of us has our own unique microbiome.

What’s more, these nerve cells operate through startlingly similar pathways as the brain. In 2010, neuroscientist Diego Bohórquez of Duke University discovered that the enteroendocrine cells of the gut had “footlike protrusions” that resembled the synapses that neurons use to communicate. This caused Bohórquez to wonder if these cells could “talk” to the brain using signals similar to the way neurons do. He hypothesized that if this were happening, they would have to be using the vagus nerve, which connects the gut and the brain stem.⁶ After further testing, they discovered
that the cells do in fact use the vagus nerve to take up messages and send them to the brain, faster than could be done via the bloodstream.