The composition of the microbiome of the intestinal flora is influenced by several external and internal environmental factors: genetics, age, gender, and so on. Of all the external factors examined so far, dietary habits have the greatest effect on the composition of the intestinal flora.
Recent studies have suggested that the intestinal microbiome plays an important role in modulating the risk of several chronic diseases, including inflammatory bowel disease, obesity, type 2 diabetes, cardiovascular disease, and cancer. At the same time, it is now understood that diet plays a significant role in shaping the microbiome, with experiments showing that dietary alterations can induce large, temporary microbial shifts within 24 h. Given this association, there may be significant therapeutic utility in altering microbial composition through diet. (1)
Stool microbiome studies show that the composition of the intestinal flora responds dynamically to the introduction of a new diet, however, if this new diet lasts only a short time, it is not sufficient to permanently restore the dysbiosis seen in diabetes, for example.
Differences in the microbiota are influenced by the housing location, hospitalization, rehabilitation, long-term home care, and diet. Microbiotas of people under long-term care are significantly less diverse than others and the elderly living separately are weaker, more fallible. Deteriorating health is indicated by the change in the structure of the gut microbiota, caused by the diet (2).
The composition of the gut microbiota is also influenced by fiber intake, even during adulthood. The effect of the fibers is that the Bacteroidetes/Firmicutes ratio increases, because of an increase in the number of the former, however, the body mass index is not related to this. The total of the bacterial genes of fiber consumers was similar but different from the microbiome of those ignoring fiber (3).
Food choice is manipulated by the microbiota, but at the same time, the microbiota can be manipulated also by food choice. Gut microbes can send signals to the brain through neural connections (nervus vagus), and thus urge people to consume foods that are necessary for optimal living conditions of the microbiota and the suppression of their competitors, regardless of whether this is beneficial for the „host” or not. The usual diet is satisfactory for certain members of the microbiota, but maybe it is not for others, and these can modify desires for food. For example, Prevotella requires carbohydrates for propagation, while dietary fibers mean a competitive advantage for Bifidobacteria, and it is exactly this competition that determines which group stays alive and becomes dominant. In overweight / obese individuals, an imbalance of the intestinal flora is also observed.
A microbiome study of people with a Western lifestyle found that a diet high in fiber and carbohydrates increased the proportion of Prevotella bacteria, while a diet rich in fat and protein favored the Bacteroides group. In overweight individuals who have been on a low-calorie, low-calorie, low-calorie diet for at least a year, the rate of Bacteroidetes: Firmicutes has increased. This change was greater the more weight the subjects in the experiment managed to get rid of. Also in the study of obese/overweight individuals, it was found that after a 6 + 6-week low-calorie, protein-rich diet, their metabolism was improved and the species richness of the intestinal flora was increased. However, the same change was negligible for individuals who had a more species-rich intestinal flora from the beginning.
To assert their interests, microbes quasi capture the nervous system of the host organism by exploiting the microbiome-gut-brain axis. In addition to the neural pathway, microbes can modify the secretion and operation of hormones influencing mood and behavior (dopamine, serotonin), and of certain receptors (e.g. taste), and can modify dietary preferences. Prebiotics, probiotics, fecal transplants, dietary changes can modify the microbiome even within a single day.
Now we know that gut microorganisms have been shown to play a role in a wide range of human diseases, including obesity, psoriasis, autism, and mood disorders. The close relationship between diet, the gut microbiome, and health suggests that we may improve our health by modulating our diet.
Further investigations have revealed more specific roles for some bacterial species in mediating host immunity and immunologic diseases. For example, the segmented filamentous bacteria have been found to promote autoimmune arthritis. On the other hand, lactic acid bacteria and Bifidobacteria are known to secrete factors that dampen inflammation
Intestinal SCFAs have also been shown to protect against allergic airway inflammation and decrease the secretion of several pro-inflammatory cytokines. Besides immunity, gut microorganisms have also been shown to impact host metabolic health. Individuals with metabolic disorders such as obesity and diabetes have been shown to have intestinal dysbiosis in relation to healthy individuals. Further characterization of the link between the gut microbiome and obesity has revealed several bacterial groups that may specifically contribute to the disease. In particular, obese individuals have a high baseline Firmicutes to Bacteroidetes ratio. In these subjects, reduction of caloric intake was noted to lower the Firmicutes to Bacteroidetes ratio. Intriguingly, hosts with a gut microbiome dominated by Firmicutes have altered methylation in the promoters of genes that are linked to cardiovascular disease and obesity. Additionally, Lactobacillus spp. has been shown to alleviate obesity-associated metabolic complications. The beneficial effects of Lactobacillus may be attributed to interactions with obesity-promoting bacteria in the gut and direct modulation of host immunity and gut barrier function. (4)
Although microbiome testing has undergone tremendous development over the past decade, the exact role of the intestinal microbiome in the development and maintenance of obesity, diabetes, or other disease remains unanswered. Despite, the next generation microbiome testing can help to detect all species of bacteria and along with the use of the existing knowledge it is now possible to develop some kind of therapies, recommend targeted diets that can help to balance the microbiome and prevent the development of the above diseases by correcting dysbiosis.