Table 3 Future areas of gut microbiota research in relation to athletes and exercise

1. Athletic-focused studies have assessed the taxonomic composition of the gut microbiota, while placing less emphasis on functional capacity.

• The functional capacity of gut microbiota, as assessed by metagenomics and other omic-driven techniques (e.g., metabolomics), can be significantly altered without major shifts in community structure. Effects on the host may be more dependent on the metabolic capacity and metabolites of the microbiota, instead of the composition per se. Therefore, investigators should incorporate functional assessment of the gut microbiota.

2. At present it is unclear how exercise affects the gut microbiota over long durations (i.e., > 6 months).

• Assess the effect of exercise on the gut microbiota, as well as the gut microbiota in athletes over longer periods of time.

3. Little research is available directly comparing the potential differences in the gut microbiota between athletic disciplines, exercise routines, training loads, and physical fitness status.

• Examine the potential differences in the gut microbiota between these factors in both cross-sectional and longitudinal investigations.

4. There is limited research on how environmental factors affect the gut microbiota in athletes.

• Explore the effect of temperature, humidity, pollen/allergens, and other relevant environmental factors in relation to the gut microbiota in athletes.

5. There is limited research available on how intrinsic adaptations to exercise impact the gut microbiota.

• Investigate how such factors as decreased blood flow, tissue hypoxia, and increased/decreased transit and absorptive capacity in the GI tract can lead to changes in the gut microbiota.

6. Some research is available showing prolonged excessive exercise has a detrimental influence on intestinal function.

• Investigate how prolonged, excessive exercise impacts intestinal function (i.e., increased intestinal permeability) and the gut microbiota.

7. Currently the gut-brain axis remains generally unexplored in athletes.

• Explore the gut-brain axis in athletes, as well as the impact of exercise in sedentary individuals.

8. Currently the role of Akkermansia muciniphilia has not been fully elucidated in humans, although appears to be more present in athletes compared to non-athletes.

• Continue to investigate the role Akkermansia muciniphilia plays in the gut microbiota and its functional impact on metabolism.

9. In relation to obesity, some athletes who may be defined as physically active using common criterion may not necessarily be healthier based on BMI. There is limited research comparing these athletes to sedentary individuals matched for BMI.

• Investigate this comparison at the obese classification. Findings from this research could provide important data in connection with the pathogenesis of obesity and the gut microbiota.

10. The influence of energy stores (obese or lean state) and energy intake (positive or negative energy balance) on ability to alter the gut microbiota remain unclear.

• Gut microbiota research in athletes with high, as well as low energy consumption requires further investigation.

• Research in those implementing caloric reduction with exercise for weight loss is also needed.

11. The functional capacity of the athletic gut microbiota is not fully understood, particularly the relationship to the significant energy demands and tissue adaptation that occurs during intense exercise and elite sport.

• Intervention-based studies should be conducted to further delineate this relationship.

• Results from such research will be important and may provide further insights into optimal therapies to influence the gut microbiota, and its relationship with health and disease as well as athletic performance.

12. Few studies have focused on the impact that voluntary exercise has on gut microbiota.

• More animal research should be conducted looking at the difference between forced vs. voluntary exercise in relation to exercise volume.

13. Some animal research reported exercise induces more effective changes in the microbiota in juvenile rats compared to adult rats.

• Research in animal models, as well as humans, should examine the effect of exercise on the gut microbiota comparing different age classifications.

14. Currently a few preclinical experiments have shown that beneficial microbes from athletes can be effectively transferred to enhance performance.

• Research in animal models should continue to test the functions of these microbes expressed in athletes (e.g, Veillonella) and explore their implications for broader human health and performance applications.

15. Limited research has shown that exercise-induced gut microbiota changes may be temporary and require continual stimulus.

• More research is needed investigating the temporal effect of exercise stimuli on the gut microbiota and its functional capacity, both at the acute and chronic level.

16. Limited research has reported a segregation in compositional and functional changes between exercise responders and non-responders, accompanied by distinct alterations in microbial metabolites.

• Replication of these findings and further investigation of the possibility that the makeup of the gut microbiota may be a determiner for the efficacy of exercise (i.e., exercise responders vs non-responders)

• Explore the possibility of targeting the gut microbiota of ‘non-responders’ to increase the benefit of exercise.

17. To date, no study in athletes has addressed RED-S^b syndrome in relation to the gut microbiota. Moreover, little is known on the effects of energy reduced diets in athletes looking to healthfully reduce bodyweight and/or improve body composition.

• Dissect the impact of restricted energy consumption and/or increased energy expenditure on the gut microbiota in athletes.

18. The effects of high-protein (without concurrent high-fat) consumption on gut bacteria are not well studied. This also includes the effects of high-protein and high-fiber intake.

• Investigate the effect of higher protein consumption on the gut microbiota, particularly in the context of lower fat and higher fiber intakes.

• The types and amounts of fats consumed in conjunction with protein should be investigated in the overall effect on the gut microbiota.

• Examination of the effect of prebiotics and probiotics in conjunction with increased protein intake.

19. Protein intake appears to be a strong modulator of gut microbiota diversity, however there is little research examining the effect of protein supplementation.

• More research needs to be conducted investigating the effect of protein supplementation, such as whey and vegetable-based proteins, on the gut microbiota.

20. Early research has reported changes in the gut virome from protein supplementation, suggesting virus particles from whey protein transmit to the gut from consumption.

• The effect of the gut virome on the gut microbiota and host requires further investigation, particularly in relation to food and supplement consumption.

21. The genus Prevotella has been reported to be positively associated with both health and disease states.

• More studies in humans are needed to better understand Prevotella’s role in athletes, as well as its role in disease. For this, more in-depth metagenomic studies will be required to reveal the health- or disease-modulating properties of Prevotella, particularly at species and functional level.

22. Little is known about exercise-nutrient interactions that underpin adaptation and performance.

• Further research is needed to determine the synthesis kinetics and clinical consequence of microbial by-products during increased nutritional status and metabolic demands during exercise.

• Determine if specific nutrient recommendations aimed at improving performance can be made by enhancing certain metabolites during exercise and recovery. This includes limiting those that produce toxic metabolites that may worsen the consequences of exercise stress.