It's no secret that the brain communicates with the gastrointestinal (GI) tract. When you feel upset, your brain may send signals that trigger a stomachache or diarrhea. People with chronic conditions like inflammatory bowel disease (IBD) often have flare-ups when they're stressed.

It goes the other way too. An irritated GI tract sends signals to the brain that can trigger mood changes and sleep problems.

How does this two-way communication happen? Many experts believe that the bacteria living in your gut are responsible. Here's what scientists know — and the questions that still remain.

The incredible microbiome

The lining of your gut and all the surfaces of your body are covered in trillions of microscopic organisms — mostly bacteria. Together, these organisms are called the microbiome.

While you probably don't notice its presence, your microbiome plays a big role in fighting disease and keeping you well. There are 3 ways that gut bacteria send signals to the brain:

• Affecting the vagus nerve, which runs from the base of the brain to the digestive system. The vagus nerve helps oversee immune response, digestion, heart rate and mood.
• Producing neurotransmitters like serotonin and dopamine. More than 90% of the body's serotonin, which plays an important role in mood and depression, is in the gut.
• Playing a role in inflammation and the immune system. Both of these can affect the brain and mental health.

It's clear that imbalances in gut bacteria are linked to conditions like depression, IBD and cancer. But what experts don't know is which way this works. Does a problem in gut bacteria cause a health condition, or does the health condition lead to the gut imbalance?

More questions than answers

Despite the growing interest in this area, experts still have questions. They're the same ones you're probably asking yourself right now.

• How exactly does gut bacteria affect anxiety and depression?
• Are there any digestive treatments that can improve mental health?
• Do genes play a role in how the microbiome impacts mental health?

Finding answers will be tricky. Research in this area is fairly new. And most of the studies use mice, so it's not clear if the results apply to humans as well. For example, probiotics can reduce anxious behavior in rodents, but studies haven't yet shown whether that works for people too.

Plus, mental health conditions like depression and anxiety have many causes, including genetics and environment. Unraveling how they're related to gut bacteria is complex.

In the future, providers may be able to help people improve mental health by prescribing dietary changes or nutritional supplements. That's an exciting idea — but medicine isn't quite there yet. For now, talk to your health care provider about the best available treatments for anxiety, depression and mental health conditions.