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Can bacteria cause anxiety?

This question originally appeared on Quora. Answer by Keck Medicine of USC.

Your gut feeling could be causing your anxiety.

For some time, doctors have observed that patients who have gastrointestinal diseases such as GERD and irritable bowel syndrome also suffer from depression and anxiety. This link was confirmed in a recent study. Scientists found that strains of two bacteria in the gut, Lactobacillus helveticus and Bifidobacterium longum, reduce anxiety-like behavior in rats. A complementary study noted that these “good” bacteria, when taken as a probiotic formula, decrease stress-induced gastrointestinal discomfort in humans.

There is a connection between anxiety and an upset gut.

In a normal digestive tract, trillions of bacteria are constantly at work to keep your body healthy. They protect against infection, provide nutrition to cells in the digestive system and convert food to energy. However, when the bacteria’s normal functions are disrupted, it can also disrupt your mood and behavior, both of which can lead to anxiety. In the study, when the bacteria’s healthy functions were restored, mood and behavior leveled out, reducing or eliminating anxious feelings.

“Leaky gut” leads to depression.

The name itself is unsettling. Leaky gut is tough to diagnose because its common symptoms, bloating, gas, food sensitivities and cramps can also be symptoms for other digestive issues. The condition occurs when the walls in your stomach and intestines allow toxic bacteria to leak into your bloodstream. These displaced bacteria cause inflammation and a host of other issues, the most prominent being depression. Doctors don’t know why; they only know that patients who are depressed also have a leaky gut.

Stress can be triggered by a glut in gut bacteria.

Too much bacteria in your gut releases abnormal levels of trace amines and toxins into the blood, tricking your brain into thinking it’s tired, depressed, anxious and stressed. A poor diet causes the bacterial imbalance; when treated, stress levels subside. Eliminating refined carbohydrates and high-sugar foods can get your digestive tract back on track.

Common medicines and foods can upset your gut.

Everyday foods and medicines are known to knock the digestive system out of whack. Junk food, high-sugar diets and refined flours fuel harmful bacteria, upsetting the natural balance in your stomach. Antibiotics kill all bacteria, even the good kind, leaving a void that is all too-attractive to toxin-producing bacteria. Birth control pills have the same effect.

Irritable bowel syndrome causes more than physical discomfort.

People with irritable bowel syndrome (IBS) are familiar with the bloating, gas, diarrhea and abdominal pain that accompany it. But there is also a connection to anxiety. When overgrowth of harmful bacteria occurs in the small intestine, it leads to IBS; a gut off-balance triggers anxiety and stress. Doctors theorize that a weak immune system is to blame.

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