Having probiotics can not only regulate gut bacteria -- trillions of microorganisms in the gut which perform key functions in the immune system and metabolism -- but also improve brain function and, thus, reduce anxiety, find researchers.Probiotics are living organisms found naturally in some foods that are also known as "good" or "friendly" bacteria because they fight against harmful bacteria and prevent them from settling in the gut.Recent research also suggests that mental disorders could be treated by regulating the intestinal microbiota, but there was no specific evidence to support this."Gut microbiota can help regulate brain function through something called the 'gut-brain axis,'" found the researchers from the Shanghai Jiao Tong University School of Medicine.To reach this conclusion, the researchers reviewed 21 studies that had looked at 1,503 people collectively.The team found that probiotic supplements in seven studies within their analysis contained only one kind of probiotic, two studies used a product that contained two kinds of probiotics, and the supplements used in the other five studies included at least three kinds.Overall, 11 of the 21 studies showed a positive effect on anxiety symptoms by regulating intestinal microbiota, meaning that more than half (52 per cent) of the studies showed this approach to be effective.Of the 14 studies that had used probiotics as the intervention, more than a third (36 per cent) found them to be effective in reducing anxiety symptoms, while six of the remaining seven studies that had used non-probiotics as interventions found those to be effective - a 86 per cent rate of effectiveness.Non-probiotic interventions were also more effective.Most of the studies did not report serious adverse events, and only four studies reported mild adverse effects such as dry mouth and diarrhoea."People who experience anxiety symptoms might be helped by taking steps to regulate the microorganisms in their gut using probiotic and non-probiotic food and supplements," suggested the review of studies published in the journal General Psychiatry.In addition to the use of psychiatric drugs for treatment, "we can also consider regulating intestinal flora to alleviate anxiety symptoms," said researchers.