Bugs in your belly affect your brain

When things go amiss in your gut, they can also go amiss in your brain. Emerging research shows a link between intestinal bacteria and brains that can influence mood and behaviour — a link scientists are only beginning to understand.

Roughly 100 trillion microorganisms live here. Research suggests they "talk" to our brains. (Joey Yee, flickr.com)

Roughly 100 trillion microorganisms live here. Research suggests they “talk” to our brains. (Joey Yee, flickr.com)

For instance, up to 80 per cent of people with irritable bowel syndrome (IBS) have increased anxiety and depression. And some people with autism — a syndrome characterized by problems interacting with others —have abnormal mixes of gut bacteria.

“Many people with chronic intestinal conditions also have psychological disturbances and we never fully understood why,” says McMaster University gastroenterologist Stephen Collins.

Now, researchers such as Collins are starting to come up with answers, most of which point to the teeming community of microorganisms in our guts.

Our lower gastrointestinal tract is home to almost 100 trillion microorganisms, most of which are bacteria. They are largely “good” bacteria that help us digest food and release the energy and nutrients we need. They also crowd out “bad” bacteria that can make us sick.

But bad bacteria can sometimes take over, affecting our command and control centre for mood and behaviour: the brain.

Collins and fellow McMaster University gastroenterologist Premysl Bercik have done some of the seminal research into the bacteria-brain-behaviour connection. In one study, they changed the behaviour of mice by giving them fecal transplants of intestinal bacteria through a feeding tube.

They gave adventurous mice bacteria from timid ones, thereby inducing timid behaviour. Before the transplant, adventurous mice placed in a dark, protected enclosure spent much of their time exploring an attached bright, wide-open area. After the transplant, they rarely ventured beyond their enclosure.

The researchers also did the reverse — transplanting bacteria from adventurous mice into timid mice, which then became adventurous.

The mice’s brain chemistry gives some insight into what might be going on, says Collins. The newly adventurous mice had decreased levels of a naturally occurring chemical called brain-derived neurotrophic factor (BDNF), which is linked to anxiety.

“We can’t do experiments like this in humans. But it’s highly likely this is applicable in humans too,” says Collins.

Probing probiotics

He and Bercik have just completed a study in which they transferred gut microbes from three different groups of people into mice. The groups were divided into those who were healthy, those with irritable IBS, and those with IBS plus anxiety. The researchers found that when they transferred microbes from people with IBS plus anxiety, the mice also became anxious.

In another recently completed pilot study, Collins and Bercik tested a probiotic on people with IBS and depression. The researchers had already established that this probiotic — a mixture of beneficial microorganisms — alleviates symptoms of depression in mice.

Half of the 44 IBS study participants drank the probiotic, while the other half drank a placebo. Those who had the real thing reported reduced depression. Collins and Bercik hope to repeat their findings in a larger study.

But how exactly do bacteria alter mood and behaviour? Research shows there are three mechanisms. The first has to do with bacterial waste called metabolites. Bacteria feed on nutrients we ingest with our food, and they produce chemicals as they digest those nutrients. These chemicals may get into the blood and possibly into the brain, changing its chemistry and function. And chemical activities in the brain underlie mood and behaviour.

The second way bacteria affect the way we feel and act is by activating the body’s immune system, resulting in the release of signals that can influence the brain. The third way is through the vagus nerve, a long nerve that runs between the gut lining and brain. Gut bacteria, or their metabolites, somehow communicate with the brain over this nerve.

Regardless of how the signalling takes place, scientists hope to eventually find what constitutes bad bacteria and good bacteria, and how many good ones it takes to make a difference. “We are in the process of trying to narrow that down,” says Collins.

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