The average adult carries up to five pounds of bacteria in their digestive tract, which is made up of trillions of living microbes known as probiotics.
These probiotics occur naturally but they can be supplemented by eating foods such as yogurt and soy yogurt, which both have added live cultures.
The link between gut bacteria and mental health first emerged when scientists discovered that autistic children have higher HPHPA levels, which is a chemical byproduct of the clostridia bacteria.
After those children were given antibiotics to fight the bacteria, they soon began to display fewer symptoms of autism.
Another case reported on ABC News recently found that when a teenager with OCD, ADHD and an array of digestive disorders was given powerful probiotics to boost the good bacteria in her gut, her mental health problems slowly faded away. Within a year, the girl claimed that both her OCD and ADHD had disappeared.
The journal Science recently reported that thin and large people have different bacteria in their guts, a discovery that scientists think could lead to more effective weight-loss programmes.
Not every strain of bacteria found in the gut has been identified by scientists, but they can test for them by analysing the chemical byproducts released in the urine.
Boston-based psychiatrist Dr James Greenblatt tests patients in this way. He said: “Eight out of 10 people are fine. But in the two patients where [the bacterial levels are] elevated, it can have profound effects on the nervous system.”
He believes every psychiatric patient should be tested in this way.
Other studies have found that anxious behaviour could also be determined by gut bacteria. In one study by McMaster University in Canada, scientists found a link between intestinal microbiota and anxiety-like behaviour.
Normal baby mice were compared to baby mice stripped of microbes. The researchers found that those with no bacteria had higher levels of cortisol, the hormone related to stress and risk-taking. They also had altered levels of the brain chemical BDNF, which has been linked to anxiety and depression in humans.
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