4.3.2. Gut Microbiota and Probiotics

The concept of a healthy resilient gu<sup>t</sup> microbiome relies on its high richness and biodiversity [268]. The intestinal microbiota plays an important role in many metabolic processes that are beneficial to the host such as synthesis of vitamins and production of short chain fatty acids (SCFA), but also in the development of the mucosal immune system [269]. On the other hand, it has also been associated with chronic inflammation resulting from impairment of mucosal barrier, thereby contributing to the development of inflammatory, autoimmune and metabolic diseases [270]. Lower bacterial diversity has been observed with advanced age, and accumulating evidence suggests that the gu<sup>t</sup> microbiota is linked to health status in aging [271]. Diet, particularly protein intake, and exercise can modify the composition, diversity and metabolic capacity of the gu<sup>t</sup> microbiota [272–274] and may, thus, provide a practical means of enhancing gu<sup>t</sup> and systemic immune function. It has been reported that exercise-induced changes in the microbiota (e.g., increased butyrate-regulating bacterial taxa and microbiome SCFA-producing capacity) are more substantial in lean versus obese adults and are largely reversed once exercise training discontinued [274], suggesting an association between microbial diversity, body composition, and physical function. Cardiorespiratory fitness (VO2max) has been observed to correlate with gu<sup>t</sup> microbial diversity and fecal butyrate, a SCFA associated with gu<sup>t</sup> health, in healthy young adults [275], and with intestinal *Bacteroides* in healthy elderly women [276]. Thus, VO2max seems to be a good predictor of gu<sup>t</sup> microbial diversity and metabolic function in healthy humans. Recent findings from the American Gut Project revealed that chronic exercise training benefits older adults by maintaining the stability of the gu<sup>t</sup> microbiota (microbial composition and function) induced by aging [277]. Recent data demonstrated that masters athletes can be seen as a model of healthy aging also from the perspective of the microbiota [278]. Compared to community-dwelling older adults, senior orienteering athletes displayed a more homogeneous composition of gu<sup>t</sup> microbiota, with higher levels of *F. prausnitzii* that is associated with positive health benefits, such as good gastrointestinal health, as well as psychological well-being that is most likely due to changes in the gut–brain axis [279]. Conversely, a substantial proportion of endurance athletes report gastrointestinal problems (e.g., abdominal discomfort, diarrhoea) during long-distance runs or competitive events and these symptoms may be related to gu<sup>t</sup> ischaemia-associated leakage of bacterial endotoxins into the circulation [280]. Excessive exercise, but also other factors, such as drugs or illness, may be linked with dysbiosis of the gu<sup>t</sup> microbiome, promoting inflammation and a catabolic state that can negatively influence muscle mass and function, particularly during aging [281].

Probiotics are live microorganisms that are thought to confer immunomodulation properties on both local and systemic immunity. Probiotics have been found to modify the population of the gu<sup>t</sup> microflora and have been shown to increase some aspects of mucosal and systemic immunity in healthy humans such as altered cytokine production, increased natural killer cell cytotoxic activity, increased secretory immunoglobulin A (IgA) levels, and enhanced resistance to infections [282]. On the other hand, probiotics exert important anti-inflammatory 'tolerogenic' effects that may reduce the infection risk of athletes at a harmless level [258] and there is now some evidence from a number of studies in support of this [283–287].The proposed mechanism behind the protective effects of probiotics against infections in athletes may be attributable to modulation of the gu<sup>t</sup> microbiota (enhanced intestinal barrier function and protection from pathogens) the mucosal immune system (enhanced bioactive metabolite production, such as SCFA and neurotransmitters) and lung macrophage and T lymphocyte functions [288,289]. Data

from our own study indicate that some of these effects appeared to be connected with alterations in tryptophan metabolism [287]. Daily supplementation with a multi-species probiotics (1 × 10<sup>10</sup> colony-forming units (CFUs) during 12 weeks of winter training limited exercise-induced drops in tryptophan levels and reduced the incidence of URI. Trained young athletes who developed URI demonstrated higher degradation rates of tryptophan compared to those without URI. A previous study undertaken by well-trained cyclists reported sex differences with probiotic supplementation (1 × 10<sup>9</sup> CFUs of *L. fermentum* PCC®) for 11 weeks with a significant reduction in lower respiratory illness symptoms (duration and severity) in males, but with some evidence of an increase in symptoms in females, while the effects of probiotic supplementation on URI were unclear in males and females [290]. Although anti-inflammatory effects of probiotics exist in young athletes, these results cannot be directly extrapolated to masters athletes due to changes in the immune system that occur with ageing. The influence of probiotics on immune function and infection risk in the elderly has been studied only sporadically. The most comprehensive study so far in free-living elderly aged >65 years demonstrated a clear association between probiotic (3 × 10<sup>7</sup> CFUs of *L. delbrueckii* ssp. *bulgaricus* 8481) consumption for 6 months and enhanced systemic immunity in older people [291]. Clinical benefits by probiotics are reported from respiratory infection studies where in one study the rate of common cold infections was decreased by dietary intake of yoghurt fermented with *L. delbrueckii* ssp. *bulgaricus* OLL1073R-1 [292] and in another study the duration but not the incidence of episodes decreased with *L. casei* DN-114001 [293]. Theoretically, combining probiotics with omega-3 fatty acids may offer a promising nutritional strategy to counteract metabolic challenges associated with aging via the gu<sup>t</sup> microbiota, especially relevant for older people that suffer from anabolic resistance, systemic inflammation, and mood disorders (through the gut–brain axis) [279,294,295]. However, more research is required to understand the connection between exercise, nutrition and the gu<sup>t</sup> microbiota in the elderly population in general and in master athletes in particular.

## **5. Conclusions and Future Perspectives**

In summary, physiological and diet-related challenges that occur with the aging process can be positively influenced by chronic exercise training and appropriate nutrition. Even though limited evidence exists on nutritional requirements for masters athletes, based on the analysis of current nutrition guidelines for young athletes and older non-athletic populations, recommendations can be suggested for older athletes (Figure 6). Particular attention should be given to proper energy and protein intake for preventing low bone and muscle mass and optimizing post-exercise recovery. Furthermore, a higher demand for specific nutrients (e.g., vitamin D, probiotics omega-3 fatty acids) should be considered in older athletes to improve both mucosal and systemic immunity, enhance resistance towards inflammatory and metabolic diseases (including infections), and, last but not least, to preserve psychological well-being, which itself can help boost the masters athletes' immune system.

Although we have proposed nutritional recommendations for older athletes, there are still many open questions concerning the nutritional requirements of masters athletes. For example, how do older athletes train and eat and, what is the nutrition knowledge and practice of masters athletes? What role does the diet play, for both decreasing fracture risk and enhancing the healing process after fracture? In addition, further research is required demonstrating the benefits of tolerogenic nutrients for their immunological effects in older athletes. There is limited research available on how adaptations to exercise impact the gu<sup>t</sup> microbiota and how nutritional factors, such as restricted energy or higher protein consumption, influence the gu<sup>t</sup> microbiota in older athletes. As such, future longitudinal studies should address what kind of diet and specific nutrients are most suitable for the athletic lifespan to better assist masters athletes.

**Figure 6.** Overview of physiological and diet-related challenges associated with aging and nutritional strategies to assist masters athletes to stay healthy, optimize adaptation and post-exercise recovery. Body mass, BM; colony-forming units, CFU; carbohydrates, CHO; energy availability, EA; gastrointestinal, GI; international units, IU; lean body mass, LBM; polyunsaturated fatty acids, PUFA; upper respiratory illness, URI.

> **Author Contributions:** B.S., D.P., J.R., J.B. and M.B. wrote and contributed to specific sections of the manuscript. All authors have read and agreed to the published version of the manuscript.

> **Funding:** This work was funded in part by grants from the Austrian Federal Ministry of Education, Science and Research represented by the Austrian Research Promotion Agency (FFG) as part of the ERA-Net Cofund HDHL-INTIMIC (JPI HDHL).

**Data Availability Statement:** Data sharing is not applicable to this article.

**Conflicts of Interest:** The authors declare no conflict of interest.
