Some bacteria found in the intestines of humans could fuel depression, according to a new study that adds evidence to the theory.

The researchers found that among more than 2,100 adults, those with depression had differences in specific groups of intestinal bacteria. And people with higher concentrations of some other intestinal insects have generally reported better mental well-being.

The research, published online Feb. 4 in Nature Microbiology, is the latest in revealing links between human health and the gut microbiome. The term refers to the billions of bacteria and other microorganisms living in the intestines.

These microbes are thought to do much more than facilitate digestion. Research suggests that they are involved in everything from immune system defenses to the production of vitamins, anti-inflammatory compounds and even chemicals that act on the brain.

But most research on this "gut-brain" communication has been done on animals, said Jeroen Raes, principal investigator of the new study.

His team therefore looked for links between intestinal microbes and depression in more than 2100 adults in two health studies. Investigators found that levels of two specific groups of intestinal bacteria – Coprococcus and Dialister – were "systematically depleted" in people with depression.

Meanwhile, people with higher levels of Coprococcus and another group of bacteria called Faecalibacterium generally gave a better assessment of their quality of life.

Both types of bacteria break down dietary fiber to produce an anti-inflammatory compound called butyrate.

None of this proves that these bacteria contribute in one way or another to depression or the protection of depression, according to Raes, professor at KU Leuven-University of Leuven, Belgium.

But, he said, new studies should target the bugs.

"After all the studies on mice," said Raes, "we finally have reliable human data indicating interesting target organisms that could lead to new drugs and probiotics in the future."

Dr. Emeran Mayer is a professor in the Faculty of Medicine at the University of California at Los Angeles and author of The Mind-Gut Connection.

Mayer said the new findings add to the evidence of an association between the intestines and mental well-being.

But like Raes, he said that they do not prove that microbes in particular cause depression.

"It's about the chicken and the egg," said Mayer. "Depressed people certainly have different diets and habits than people without depression, and this would affect the intestinal microbiome."

Mayer suspects the existence of a "circular process", in which depressed people can alter the composition of their intestinal microbes – which, in turn, "strengthens" the symptoms of depression.

Raes said additional research was needed to determine if there was such a "vicious circle". For the moment, he says, "we can not say that."

Studies conducted in recent years, mainly in the animal, have highlighted links between the composition of the intestinal microbiome and the risks of various diseases, ranging from other brain-related disorders, such as dementia, to Obesity, going through autoimmune diseases such as type 1 diabetes, and rheumatoid arthritis.

However, the extent to which intestinal bacteria contribute to human diseases is still unknown.

And even though the intestinal microbiome had an influence on the symptoms of depression, Mayer doubted that something as simple as a probiotic supplement would offer a quick fix. The microbiome and depression are too complex.

However, he said, intestinal bacteria release metabolic byproducts, or "postbiotics". And research on these compounds should give some idea of ​​how the microbiome might benefit or feed human illness.

Mayer said that he thought any effective intestinal therapy should affect his microbial balance "globally" – as opposed to simply adding a bacterial strain or two.

Diet changes can do that, he stressed.

In this study, there were allusions that butyrate-producing bacteria were beneficial. And, says Mayer, these bacteria produce butyrate when they break down various plant food fibers.

"I would say to eat a diet that is largely herbal and highly variable in types of plant foods," Mayer said. "If you're just eating tomatoes, it's not enough."

This is not just for producing butyrate. From what we know until now, says Mayer, it's the diversity of our intestinal bacteria that matters: the more our microbes are diversified, the better. "

And a diet low in processed foods and rich in plant foods – vegetables, fruits, legumes, nuts and grains – promotes a more diverse intestinal microbiome, Mayer said.

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