To answer this question, a group of scientists from the Laboratory of Theoretical and Applied Functional Genomics at Novosibirsk State University and ITMO University conducted a study in which 248 volunteers followed a nutritionist’s recommendations for two weeks aimed at increasing the amount of beneficial plant fibres in their diet and reducing ‘empty’ calories. The scientists were interested in the effect of diet on the body in terms of changes in the composition of the bacterial community (microbiota) in the volunteers’ intestines. To do this, two samples of microbiota were collected from each volunteer — before and after following the recommendations. These samples were analysed using metagenomic DNA sequencing. The results of the study were published in the scientific journal Nutrients (Klimenko et al., Microbiome Responses to an Uncontrolled Short-Term Diet Intervention in the Frame of the Citizen Science Project). Most intestinal bacteria are human symbionts: they protect us from infections, produce vitamins and oleic acid necessary for intestinal cells, break down complex polysaccharides and participate in the formation of immunity. When the balance of the microbiota and the human body is disturbed, the number of opportunistic bacteria increases, causing inflammatory processes in the intestine and other health problems. In the community, each microbe performs specific functions, being a link in a complex metabolic network, and the composition of nutrients coming from outside with food largely determines which microbes will gain a competitive advantage. On the other hand, health status often affects the composition of the community: the human microbiota can be used to predict the presence of diseases such as type 2 diabetes, ischaemic heart disease and inflammatory bowel disease with relatively high accuracy. The composition of the microbiota varies considerably among individuals and provides insight into both their dietary preferences and health status. The presence of fermenting bacteria, which convert complex polysaccharides into short-chain fatty acids that are beneficial to humans, is one of the important signs of a healthy microbiota. The study showed that following the recommendations led to a significant increase in the presence of these bacteria. This suggests that even a two-week shift towards a healthier diet can have a beneficial effect on the microbiota. However, against the backdrop of an increase in the representation of microbes associated with a healthy state of the body, some destabilisation of the intestinal community was also observed: this was primarily expressed in a slight decrease in its diversity. The more diverse the community, the more resistant it is to external influences such as stress or antibiotic use. The effect of reduced diversity observed in the study was quite small. Apparently, it is related to the fact that the microbiota did not have time to ‘recover’ from the fairly rapid change in diet. ‘The research materials formed the basis for a microbiota test, and the conclusion of the study is very practical: you can improve the condition of the intestine by increasing the amount of dietary fibre. And this works not only in rigorous clinical studies, but also at home,’ said Dmitry Alekseev, senior researcher at the Laboratory of Theoretical and Applied Functional Genomics at NSU. Another interesting observation was that different people’s microbiota reacted differently to the diet: in some people, the composition of the community changed more significantly. The groups did not differ in terms of their diet prior to the study or the recommendations of a nutritionist, but the composition of their microbiota prior to the diet could be predicted with a fairly high degree of accuracy. This observation suggests that in order to increase the effectiveness of dietary recommendations, it is important to consider not only a person’s current diet, but also the composition of their intestinal community.