The primary information connection between the brain and gut is the vagus nerve, the longest nerve in the body.

Probiotics can do more than improve gut health. They may also indirectly enhance your cognition too.

The gut has been called a ‘second brain’ because it produces many of the same neurotransmitters as the brain does, like serotonin, dopamine and gamma-aminobutyric acid, all of which play a key role in regulating mood. In fact, it is estimated that 90% of serotonin is made in the digestive tract.

In an article published by Harvard Health Publishing, of Harvard Medical School, the authors state that what affects the gut often affects the brain and vice versa. “When your brain senses trouble – the fight-or-flight response – it sends warning signals to the gut, which is why stressful events can cause digestive problems like a nervous or upset stomach. On the flip side, flares of gastrointestinal issues like irritable bowel syndrome (IBS), Crohn's disease, or chronic constipation may trigger anxiety or depression.”

The brain-gut axis works in other ways, for example, in regulating appetite by telling the brain when it's full. About 20 minutes after eating, gut microbes produce proteins that can suppress appetite, which coincides with the time it often takes to begin feeling full.

The gut-brain axis

Some research has found that probiotics may help boost mood and cognitive function and lower stress and anxiety. A study (2016) by Frontiers in Aging Neuroscience found that Alzheimer's patients who drank milk made with four probiotic bacteria species for 12 weeks scored better on a test measuring cognitive impairment compared with those who had regular milk.

A small 2013 study reported in the journal Gastroenterology found that women who ate yoghurt with a mix of probiotics, twice a day for four weeks, were calmer when exposed to images of angry and frightened faces compared with a control group. MRIs also found that the yoghurt group had lower activity in the insula, the brain area that processes internal body sensations such as those emanating from the gut.

“It's too early to determine the exact role probiotics play in the gut-brain axis since this research is still ongoing. Probiotics may not only support a healthier gut, but a healthier brain, too,” the authors concluded.

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Source: Harvard Medical School