**3.2 Associations between physical activity and changes in gut microbiota in human studies**

A positive effect of physical activity on the composition of the gut microbiota has been found in human studies and confirmed animal findings. A positive effect of moderate exercise on the gut was the shortening of stool time and contact time between pathogens and the gastrointestinal mucosa [64], so exercise prevents the risk of many inflammatory diseases and various cancers. Other possible beneficial effects of moderate exercise include reduction of LPS production, increased production of SCFAs and immunoglobulins, and increase of butyrate concentration with anticarcinogenic and anti-inflammatory properties [64].

### *Regular Physical Activity Influences Gut Microbiota with Positive Health Effects DOI: http://dx.doi.org/10.5772/intechopen.110725*

Similar to animal studies, exercise-induced changes in microbiota diversity may reduce obesity-related complications in humans. The effect on the microbiota can be assessed by measuring diversity or functions. α-diversity represents the overall diversity of samples, while β-diversity compares how different bacterial species are distributed across different samples [42].

Based on the available studies, intense exercise, compared to moderate exercise, seems to cause more significant disturbances than moderate exercise on the human body's homeostasis [34]. High levels of inflammation (higher inflammatory interleukin-6 (IL-6)) and tumor necrosis factor-alpha (TNF-α), as well as gastrointestinal symptoms with increased intestinal permeability, were found in elite athletes [65, 66]. This may be related to diet as macronutrient intake before, during, and after exercise may influence performance and inflammatory responses in athletes [67]. Adequate carbohydrate intake after acute exercise lowers inflammatory cytokines. In addition, differences in fiber consumption impact the type and amount of SCFAs produced by microbiota [68]. Several studies have reported that fiber intake in athletes is low compared with dietary guidelines. In addition, special attention should be paid to protein supplementation in athletes. It has been demonstrated that long-term protein supplementation can have negative effects on the gut microbiota (abundance of *Bacteroidetes*) [69].

Importantly, the fitness status of participants also affects gut microbiota; individuals with good physical condition have more butyrate-producing bacterial taxa from the *Firmicutes* phylum, and 6-week intervention study in lean adults increased fecal SCFAs [34].

The World Health Organization (WHO) has published recommendations for physical activity in adults (150 minutes of moderate physical activity or at least 75 minutes of vigorous physical activity per week). The composition of the gut microbiota in women who exercised according to the recommendation of WHO was modified [70]. Similarly, in male participants with insulin resistance, both high-intensity and moderate-intensity continuous exercise resulted in an increase in *Bacteroidetes* and a decrease in inflammation [71]. A 6-month intervention with progressive exercise training leads to an increase in α-diversity as well as in the concentration of some physiologically relevant metabolites [72].

A large study conducted on 86 elite rugby athletes showed a greater gut microbiota richness/diversity compared to controls [73]. This study among elite rugby players provided evidence of the beneficial effect of exercise on gut microbiota diversity. However, the results indicated that these differences between the elite and control groups were associated with dietary extremes that could represent confounding factors.

Another study on international rugby players showed differences in the composition and functional capacity of gut microbiota as well as in microbial and humanderived metabolites [74].

In addition, a positive correlation was found between cardiorespiratory fitness (CRF), an indicator of physical fitness, and microbial diversity in 39 healthy individuals, especially in taxa that augmented the production of butyrate [75]. The authors concluded that exercise can be prescribed in patients with dysbiosis-associated diseases.

The microbiota of professional and amateur cyclists was studied by Petersen [64]. They found that the gut microbiota of professional cyclists differed from that of amateurs. In addition, a correlation between certain microorganisms in professional cyclists and high training intensities was confirmed. This study suggests that training intensity influences bacterial community structure.

Higher exercise intensity leads to changes in the gut microbiota (**Figure 1**). Exercise leads to a positive change in the bacterial composition of the gut microbiota. Higher exercise intensities require dietary intervention to prevent gastrointestinal dysfunction and inflammatory responses. Longitudinal studies monitoring exercise intensity, diet and other characteristics, and gut microbiota are still lacking. To express the intensity level of physical activity, the rate of energy expenditure expressed as metabolic equivalents (METs) is used: 1 MET is the rate of energy expenditure at rest, which for most people approximates an oxygen uptake of 3.5 milliliters per kilogram of body weight per minute (**Figure 1**).

Murtaza and coworkers [76] investigated the effects of different nutritional protocols on the fecal microbiota of elite endurance race walkers during an intense training program. This study showed that an intense training load with different dietary patterns had effects on the diversity of the gut microbiota. Specifically, it was found that a ketogenic, low-carbohydrate, and high-fat diet resulted in changes in the richness of some bacterial species [76].

Furthermore, the health benefits of physical activity in older adults have been established in several scientific studies. A relationship between physical activity and the diversity of the intestinal microbiota has been found in elderly people [77]. The abundance of *Bacteroides* significantly increased after aerobic exercise training in elderly women [78]. Results of many studies reported that gut microbiota composition does not change in some conditions, such as hypertension, obesity-associated inflammation, and gastrointestinal diseases [53, 79]. Exercise can modulate the gut microbiota diversity and could have positive effects on the pathogenesis of mentioned conditions. Since lower inflammation has been demonstrated, it is possible that exercise could decrease inflammatory markers in older adults. Exercise-induced changes in microbial composition are related to exercise duration. Recently, it has been confirmed that short-term endurance exercise in elderly men has little effect on the composition and diversity of the gut microbiota.

### **Figure 1.**

*Exercise can increase the number of beneficial microbial species and enrich the diversity of microflora.*

### *Regular Physical Activity Influences Gut Microbiota with Positive Health Effects DOI: http://dx.doi.org/10.5772/intechopen.110725*

However, small changes in the microbiota have been associated with lower cardiometabolic risk factors [79]. The study suggests that the gut microbiota is influenced by high-intensity exercise and diet and might play a crucial role in modulating cardiovascular disease development [80].

Overall, physical activity could be a strong modulator of gut microbiota composition. Experimental data showed that physical activity between 60 and 70% VO2max affected β diversity; interestingly, exercise at 70% VO2max resulted in an increase of α diversity or a decrease in *Clostridium difficile* [46].

Further studies need to clarify the effects of different types, intensities, and frequencies of physical activity on microbiota diversity and function. High intensity of physical activity decreases producers of SCFAs and increases pathogenic bacteria. This condition requires dietary supplementation [59] or a nutritional strategy [1] to maintain the structure and richness of the gut microbiota.
