Probably three brains if you consider the neural activity going on in the gut and how it interacts with our brain.

In a study published in 2023, the scientists found that long COVID is associated with a reduction in the body’s natural levels of serotonin, a signaling molecule that is produced the brain and the gut. In the brain, serotonin influences mood, sleep and memory. In the rest of the body, it plays a role in physiology, but Thaiss and Levy found that gut-produced serotonin can affect the brain as well.

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Before these findings, scientists debated how SARS-CoV-2 could cause cognitive problems, as it is unclear if the virus breaches the blood-brain barrier. But the study by Thaiss and Levy suggests that the disease could have direct influence on the brain via the vagus nerve, regardless of whether the virus enters the brain.

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The gut also contains the largest number of neurons outside the brain of any structure in the body — more than 100 million neurons line the human digestive tract, from the esophagus to the anus. These cells make up what is known as the enteric nervous system, which some scientists refer to as a “second brain.”

It never ceases to amaze me how complicated our bodies and our minds are.