**3.1 Associations between physical activity and changes in gut microbiota in animal studies**

Many animal experiments have been performed on mice and rodents, which are good models for mimicking human physiology. In animals, different forms of exercise, especially voluntary and forced, resulted in different effects on the composition of the microbiome. Many germ-free animal studies have indicated the relationship between gut microbiota and host function [49, 50]. Alteration in gut microbiota and its metabolites can affect the structure of the mucus layer and immune system after gut microbiota colonization in germ-free animals. It was demonstrated [50] that exercise training triggered changes in gut microbiota community structure in donor mice and in gut physiology in recipient mice after 5 weeks of gut microbiota transplantation and colonization. Thus, the composition of the gut microbiota of recipient mice is dependent on the physical activity of their respective donors. This suggests that physical training directly alters the host response through cytokines and the production of intestinal metabolites.

SCFAs upregulated after exercise contribute to improved energy production and reduce inflammation in the gut of physically active individuals [51]. In addition, voluntary exercise training increases host butyrate concentration and its bacterial genera, which is associated with an increase in fat-free mass in early life [52].

It is believed that an increase in butyrate levels after exercise protects against intestinal inflammation and colon cancer [53, 54]. The mechanism of these changes is not yet fully understood. However, voluntary and/or forced exercise certainly influences the composition of the gut microbiota in animals.

Maternal gut microbiota during pregnancy and lactation influences the gut microbiota of rat offspring. Physical activity during pregnancy affects maternal obesity in offspring and plasma insulin and glucose concentrations [55]. Exercises started in youth can influence the bacteria ratio. In some studies, a decrease in Firmicutes and/ or an increase in Bacteroidetes was observed [56–58], while other studies showed the opposite effect [49, 59–61] or no effect [62].

Early childhood exercise can influence the composition of the gut microbiota in rats and improve the development of brain function [52]. The authors confirmed the anti-inflammatory effect of regular exercise, which protects from chronic inflammatory diseases [63].

In addition, recent studies have linked the microbiota to muscle function after antibiotic use. Depletion of the microbiota by antibiotic use resulted in decreased running performance and contractile muscle function [34, 46]. A similar effect was observed with low-carbohydrate diets, which decreased SCFA production.
