If there’s one thing the last decade of research has made clear, it’s that the microbial makeup of the digestive system can have a monumental impact on health . The list of possible links between diseases and imbalances in the intestinal flora is impressive; It is difficult to find a condition whose relationship to the microbiome has not been explored. Psychological illnesses have arrived late to the party, and the study of their connection with the microfauna that inhabits our guts has only just begun.
In the wake of the barrage of data published on this topic over the last few years, products rich in beneficial bacteria flooded the shelves of supermarkets; fermented foods loaded with supposed probiotics , such as Bulgarian kefir or yogurt and kombucha , were a trend; and lurid topics like fecal transplants made their way onto the media pages.
The idea that manipulating the gut microbiota can improve health is not new. More than a century ago, the Nobel Prize in Medicine Ilya Mechnikov (1845-1916) was the first to defend it. Convinced that aging was related to the colon, the Russian scientist discovered that life expectancy was higher in certain regions of Bulgaria, and attributed it to the consumption of yoghurts. His hypothesis gave rise to the concept of a probiotic, which remained a curiosity until the modern explosion of studies on the microbiome. Today, several beneficial bacterial strains have been described in laboratory animals, but the challenge remains of transferring these results to the human species.
Recent experiments have revealed that, against all expectations, fermented dairy products do have the ability to alter behavior. For example, scientists from the University of California, Los Angeles, showed in 2013 that daily yogurt consumption alters brain regions associated with controlling emotions.
No two intestinal flora are the same
Through the use of neuroimaging techniques, the experts verified the action of the probiotics. “The contrast between before and after was quite clear,” says gastroenterologist Emeran Mayer, director of this study. Although the participants had only consumed the daily dose of yogurt for two weeks, the bacteria took effect.
While, as a species, we share 99.99% of the genome, the microbiota is so different from one individual to another that it could be used as a method of identification . 70% of the species belong to two great phyla, Firmicutes and Bacteroidetes , but the variability is astonishing, with some geographic and social trends.
Several studies indicate, for example, that in developed countries – where the consumption of foods with fats and proteins predominates – the genus Bacteroides stands out , while in areas with diets rich in fiber, Prevotella predominates. The meaning of the differences, beyond their link to food, has not yet been established.
Comparative studies have tried to find relationships between these patterns and the development of certain diseases. Thus, it has been shown that obese people have 20% more Firmicutes and almost 90% less Bacteroidetes than slim people. Rob Knight, an expert at the University of California, San Diego, advises caution against such results: “It is very difficult to say whether the differences associated with diseases are causes or consequences.”
In 2011, another work, published in the journal Brain, Behavior, and Immunity, indicated that stress causes a decrease in microbial diversity in the intestines of mice, with the disappearance of beneficial species and a growth of pathogens. The same phenomenon has been detected in wild squirrels.
Although analyzing these interactions in humans is technically more difficult, Simon Knowles of Swinburne University of Technology (Australia) analyzed the microbiome of university students before and during exam time. In the tense evaluation stage, their intestinal flora lost lactobacilli, a group of lactic acid-producing bacteria to which some of the species most studied as probiotics belong.
While stress is an adaptive response to dangerous situations, it can develop into a chronic reaction. In the long term, it is associated with a predisposition to fall ill, especially mental ailments.
As in all ecosystems, the richness of the intestinal microbiome is synonymous with health . A biodiversity forest is capable of overcoming external aggressions because, although some species become extinct, it has others capable of fulfilling the same functions.
“We do not have a complete picture of what has happened, but our research indicates that human behavior has had a very harmful impact on its microbiome over the last two thousand years,” explains anthropologist Christina Warinner, from the University of Oklahoma. Indeed, modern man would live in a permanent state of dysbiosis – bacterial alteration.
And how to improve the set of microbes that we carry inside? Well, with prebiotics. Unlike probiotics, living organisms are compounds that provide sustenance to the microorganisms already present in our intestinal community. Essentially we are talking about fiber, an indigestible part of food of plant origin that reaches the colon almost intact, where it serves as a substrate for bacteria.
Legume diet to release tension
In mice, it has been shown that the ingestion of a specific type of fiber, known as GOS, promotes the presence of lactobacilli and bifidobacteria, an increase associated with a higher concentration of certain neurotransmitters that reduce anxiety. Although in humans there is still no such concrete data, a study carried out by scientists at the University of Oxford confirmed that those who consume GOS – very abundant in legumes, for example – have lower levels of cortisol, the stress hormone.
Unfortunately, the Western diet is deficient in fiber , which implies a reduction in the diversity of the microbiome and its by-products, such as the important short-chain fatty acids. Three of them, propionate, acetate and butyrate, are only available if produced by the intestinal flora . The first two are essential for liver function, while butyrate is the main source of energy for colon cells.
These compounds are not by themselves neuroactive, although they can affect the brain indirectly. Being involved in the stimulation of intestinal transit, for example, they promote the secretion of serotonin, the hormone of happiness. In addition, an incorrect metabolism of short-chain fatty acids has been linked to gastrointestinal problems suffered by more than 70% of patients with autism spectrum syndrome.
Taking into account all this information, the psychiatrist Timothy Dinan, the nutritionist Catherine Stanton and the neuroscientist John Cryan coined the term psychobiotics in 2013 to define a new class of probiotics “capable of producing and supplying neuroactive substances that act on the brain-axis intestine “ .
Now that the prescription of antidepressants and anxiolytics has reached levels never seen before, these experts advocate less aggressive alternatives and without side effects. At the APC Institute of Microbiome at the University of Cork (Ireland) they have already tested the effects on the brain of various bacteria. For example, the species Lactobacillus rhamnosus , known for its ability to modulate the immune system, reduces anxiety levels in adult mice through alterations at the level of the neurotransmitter GABA receptors. It is the same mechanism of action as diazepam, the most commonly prescribed drug for treating anxiety.
For now, these results have not been confirmed in people, although another psychobiotic, Bifidobacterium longum , managed to influence the same factors in a study conducted last year. Dinan and Cryan showed that it not only reduces the subjective sensation of stress felt by the subjects of the experiment, but also decreases cortisol levels, modifies brain activity and improves cognitive function, particularly memory.
We must trust our microbes
Other work indicates that various species of lactobacilli may be helpful in fighting the symptoms of depression, minimizing anxiety, and even treating chronic fatigue. However, a review published late last year looked at all the data available to date and concluded that “there is very little evidence for the efficacy of probiotic interventions in the prognosis of mental illness.”
“This is a very recent field of research, and more clinical trials are necessary to be able to draw conclusions,” says the article. Still, the safest bet is to trust the microbes in us; learn to adjust our diets, our lifestyle and our environment to the needs of the intestinal flora. A vast and diverse ecosystem that will take decades to understand, but that holds revolutionary promise.
Images: CC0 / A Doubt via Wikimedia / CC
