Big caveats about mouse studies here: but this scientist is doing very important work on how the microbiome could affect emotions via the vagus nerve. Also he may have found a physical linkage between irritable bowel and depression. He's not prescribing yogurt and kimchi yet, but I don't think it could hurt to add some natural probiotics to your diet :)

I really like this concept of "second brain":

Microbes are in the news these days. Specifically, the microbes that live in and on the human body, making up our “microbiome.” Michael Pollan made a splash with a column titled “Some of My Best Friends are Germs” about a year ago, and now Martin Blaser, director of the Human Microbiome Project at NYU, has published a book called Missing Microbes: How the Overuse of Antibiotics Is Fueling Our Modern Plagues.

In a short period of time, bacteria, fungi and other microbes have gone from enemy to friend in the public consciousness.

But in addition to the many studies finding out about the numbers and diversity of the microbes with whom we share our bodies and their roles in our nutrition and immune function, some researchers have made some surprising findings: the bugs in your gut might actually impact your emotions.

The bidirectional connection between our brains and our guts is not news. When we are hungry, full, queasy, or suffering from gas or constipation, our guts let our brains know. And our emotions can easily impact how we feel in our guts, like when one has “butterflies in the stomach.” The link between emotions and the gut is so strong that we talk about “gut instinct” or gut feelings.

What’s more, the human gut is connected to the brain by the vagus nerve. Within our guts, we have what is called the “enteric nervous system” (ENS), which is so significant it is often referred to as a “second brain.”

Premysl Bercik, an associate professor of gastroenterology at McMaster University, is one scientist on the forefront of researching the link between gut and emotions. He began by studying low-grade gut inflammation in patients with bowel disorders like irritable bowel syndrome (IBS) and functional dyspepsia. “These are basically patients who do not have any structural abnormalities but their gut misbehaves,” he explains. Over the years, his research drifted to studying how microbiota impact gut function and “from there it was only a step” to look at how they impact emotions.