Written by Nicholas Talley for The Conversation. 

If you’ve eaten a fabulous meal recently, the experience was pleasant, comfortable and pain-free because your stomach and intestinal system worked seamlessly to move the food along and eventually absorb it.

Our gastrointestinal tract, or gut, is sometimes described as our “second brain”. This is because it is controlled by its own complex nervous system comprising hundreds of millions of neurons – more than all the nerves in your spinal cord.

The gut and brain talk to each other through nerve signals, the release of gut or stress hormones, and other pathways. We have long known that emotions can directly alter gut function.

But lately we’ve been discovering that it works the other way too: our gut actually has an effect on our brain. And because it’s easier (and generally safer) to manipulate the gut than the brain, this knowledge provides the possibility that doing so could treat some chronic psychological and brain diseases.

How your brain affects you gut

Think of a time you had to do an exam and had “the runs” (diarrhoea) or felt anxious and developed butterflies in your stomach. This is your brain driving your gut. If you are stressed or anxious, you even change the production of stomach acid through nerve connections.

Traditionally it was thought gut symptoms came about from an underlying psychological disorder, such as anxiety. Anxiety changes gut function. Over time, this can lead to unpleasant symptoms such as pain, diarrhoea, bloating or excessive fullness.

Many people who suffer from irritable bowel syndrome (IBS) or severe indigestion are anxious, for instance. And doctors have investigated antidepressants and psychological treatments in these disorders with variable success.

But actually many signals go up to the brain from the gut as well as in the downward direction. So could it be that in some cases, changes in the gut are actually driving anxiety experiences rather than the other way around? Accumulating evidence suggests this is likely to be the case.

How the gut changes your brain

We followed 1,002 people over a 12-year period in Sydney and found about 50% of the participants with chronic gut issues had been anxious first and then developed their gut problems.

But the other 50% developed the gut disorder before the psychological problems arose. In other words, their gut appeared to get sick first and this led to brain dysfunction manifesting as anxiety, not the other way around.

We later observed similar findings – that psychological distress can predict later onset of gut disorders and vice versa – in a large study in the United Kingdom.

We know that some people with IBS have mild gut inflammation. We have also identified that some people with IBS have elevated levels of cytokines in their blood. These are byproducts of inflammation; part of the immune response.

One study showed a clear increase of certain cytokines in people with both anxiety and IBS. Higher anxiety levels strongly correlated with higher cytokine levels. Based on this new information, we concluded that gut inflammation releases cytokines that may cause anxiety in IBS.

How the bugs in your gut alter your brain

Everyone’s gut is chock a block full of bugs (trillions of them) that can be good, bad or indifferent. They hang out all the way from the mouth through to the end of the bowel.

The bugs talk to the nervous system through pathways, including the immune system, that keeps them in check. Experimental work suggests an imbalance in these bugs can affect the brain and, in some cases, may lead to anxiety or depression.

Altering gut bacteria is a new way to treat many diseases of the gut and possibly the brain, including through diets (changing your diet rapidly changes your gut bugs), or by providing “good” bacteria and suppressing “bad” bacteria, that can be done with probiotics. Other methods include transplanting stool from healthy people to those in need.

Intriguing observations could also unlock new ways to manage currently incurable degenerative nervous diseases. For instance, altered gut function manifesting as constipation is often the first symptom of Parkinson’s disease.

And studies are currently exploring the role of the gut in neurological diseases such as multiple sclerosis.

But for the moment, new evidence suggests when the gut is inflamed, it may affect the brain and lead to psychological dysfunction.

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