With more research being conducted on the gut-brain axis, studies have reported that the gut microbiota plays an important part in regulating brain function.
Taking into account the prevalence of anxiety symptoms in mental diseases and certain physical disorders (particularly those linked to stress), researchers have associated intestinal microbiota dysbiosis with anxiety.
While probiotics have been recommended to help alleviate anxiety via the gut-brain axis, there is no specific evidence to support the efficacy of this method, or the general treatment of anxiety through intestinal microbiota regulation.
Probiotics: Pro or con?
Based on this, researchers at the Shanghai Jiao Tong University School of Medicine conducted a systematic review to find evidence supporting improvement of anxiety symptoms through the regulation of intestinal microbiota.
Focusing on RCTs, they searched databases such as PubMed, EMBASE, the Cochrane Library, OVID, Web of Knowledge, China National Knowledge Infrastructure (CNKI), Wanfang Data, VIP databases and SinoMed.
They then settled on 21 studies involving 1,503 subjects; 14 of the studies used probiotics as interventions to regulate the subjects' intestinal microbiota, while the rest used non-probiotic methods, including dietary adjustments.
Among the probiotic studies, seven used a single type of probiotic each, two used a product containing two kinds of probiotics, and the rest used at least three types of probiotics each.
Of the 16 studies that used IRIF interventions alone, 11 used probiotics and five used other methods.
Nine of these studies (56%) reported improved anxiety symptoms, with four of the five non-probiotic studies (80%) having positive results.
On the other hand, five of the 11 probiotic studies (45%) reported improved anxiety symptoms in their subjects.
The researchers added: "Although we can regulate the intestinal flora in two ways, the non-probiotic intervention is significantly better than the probiotic intervention."
Dissecting the results
One of the reasons for this result is that the main energy source of gut microbiota growth is food. This means adjusting the gut microbiota dietary structure modulation can directly change the energy-supplying structure of the gut microbiota, which plays a "decisive role" in gut microbiota growth.
Secondly, the species of probiotics used in the studies were diverse, with survival competitions in implanted flora and primitive flora — this meant the possibility that not all the imported probiotics had been effectively implanted.
Lastly, the four- to eight-week intervention period of most of the reviewed studies could have been too short to significantly raise the abundance of the imported microbiota, so the subjects' original intestinal flora could not be effectively adjusted.
The researchers added: "67% of the studies used probiotic intervention to regulate intestinal flora, while only 33% of the studies used non-probiotic ways, such as low FODMAPs, scFOS and supplementary resistance dextrin.
"On the one hand, this indicates that more and more researchers have realised that microflora plays an increasingly important role in human health, but on the other hand, the function of diets in daily life has been neglected by people.
"The effect of dietary structure adjustment is better than that of probiotic supplements. In the future, more attention can be paid to the regulation of intestinal flora through non-probiotic ways, or the combination of probiotic and non-probiotic means, which may have unexpected effects."
They added that for patients suffering from somatic diseases and who therefore could not take psychiatric drugs for anxiety, probiotic and / or non-probiotic methods such as FODMAPs could be “applied flexibly according to clinical conditions”.
However, they also acknowledged that some studies had found that regulating intestinal flora to improve anxiety symptoms had limited effects.
As such, they recommended more relevant clinical intervention studies, alongside unified anxiety assessment scales and statistical methods used to determine the link between intestinal flora adjustment and alleviation of anxiety symptoms.
In conclusion, they wrote: "We find that more than half of the studies included showed it was positive to treat anxiety symptoms by the regulation of intestinal microbiota.
"There are two kinds of interventions (probiotic and non-probiotic interventions) to regulate intestinal microbiota, and it should be highlighted that the non-probiotic interventions were more effective than the probiotic interventions.
"More studies are needed to clarify this conclusion since we still cannot run meta-analysis so far."
Source: General Psychiatry
"Effects of regulating intestinal microbiota on anxiety symptoms: A systematic review"
Authors: Beibei Yang, et al.