A recent update established that there is a one-to-one ratio between bacterial cells and human body cells. The microbiota refers to the micro-organisms whereas the microbiome is a term which refers to these organisms, and the activities which result from them are specific ecological niches and the surrounding environmental conditions.
The gastrointestinal tract is an organ that has many functions and not only takes in food and nutrients that it digests to extract and absorb energy and expel waste, but also consists of the upper tract and the large intestine which includes microvilli and the epithelium layer; this is where the microbiota hosts this bio-directonal ‘cross-talk’ with the rest of the body.
At the level of bacterial strains as seen in classical microbiology, the gut microbiota demonstrates tremendous diversity between individuals. Certain microbiome fermentations lead to the generation of metabolites that are very relevant to athletes; these include the production of short-chain fatty acids, lactate and branched-chain fatty acids. Short-chain fatty acids are volatile products of the microbial fermentation of mainly fibres and some peptides and amino acids. Short-chain fatty acids have distinct physiological effects as they can not only be an energy source by the host cell and the intestinal microbiota, but they can also contribute to shaping the gut environment, thereby influencing the physiology of the colon and participating in host signaling. Included in this ecosystem is also, to a large extent, the immune system of which 70% resides in the gut.
Gastrointestinal disorders for ultra runners and ultra marathon runners who do not finish the race are common due to the detrimental effects of the microbiome balance on their endurance and exercise, and so it is worth looking at the mechanisms of actions, both long- and short-term, for exercise and the gut microbiome.
Recent studies have shown that moderate exercise has positive effects on the health of athletes because it may reduce inflammation and leaky gut as well as improving body composition. It may also have positive impacts on the gut microbiome and the metabolites produced within the gastrointestinal tract. Intense exercise in the short-term on the other hand may increase leaky gut, diminish the mucous lining and enable potential pathogens to enter the bloodstream, and this may cause inflammation. Elite athletes, however, who have trained for many years appear to have a higher gut microbial diversity and produce higher numbers of metabolites such as short-chain fatty acids, showing that in the long-term endurance training is beneficial for the gut microbiome. These studies have even indicated that there is a bio-directional relationship between exercise and gut microbiota meaning that the gut microbiome may influence performance.
To summarise, physical inactivity leads to a decrease in bacterial diversity, whereas light, moderate and intense activity leads to an increase in bacterial diversity. An increase in bacterial diversity leads to an increase in short-chain fatty acids producing species which are beneficial for our health in many ways. In fact, elite athletes, after several years of intense training, may have special features in terms of athletic performance but also in terms of metabolic adaptations. A human study among elite rugby players found that, in terms of diversity, they scored well, and interestingly some studies have even shown an effect between the gut microbiome and performance indicating that there may be a positive impact on performance based on the gut microbiome diversity.
Physical adaptations that may occur due to endurance training include an upregulation of mitochondrial biogenesis and glucose metabolism as well as an increase in energy sources from the diet, an increase in species diversity, intestinal barrier function and a decrease in inflammatory status.
Exercise and diet together may have the largest benefit for the human gut microbiota; for example, a study investigating half-marathon runners observed interactions between exercise and diet composition on the intestinal microbiome. Fibre consumption should be taken into account before drawing any conclusions about these studies because dietary fibres are digested in the large intestine and have an effect on transit time. Stool frequency, and overall consumption of fibre also impacts the amount of short-chain fatty acid producing microbiota; for example, a diet containing large amounts of fibre significantly increases short-chain fatty acid production in many cases.
Fibre intake is often low in the diets of athletes. Several studies involving female gymnasts reported that fibre consumption was often below the nutritional guidelines of 25g per day in the US. Athletes may be reluctant to adopt such dietary habits because of digestive issues, and gastric intestinal discomfort issues that may come from fibre, and to avoid gastrointestinal symptoms with exercise some athletes also turn to a low FODMAP diet which removes some particular fibres from the diet that may cause bloating and discomfort. However this diet should not be adhered to in the long term.
Dietary protein intake also differs largely among elite athletes and more sedentary. In a recent study which looked at the effects of protein supplementation on the gut microbiota, protein supplementation may increase the presence of some beneficial bacteria and decrease the presence of some un-beneficial bacteria. The authors of this study concluded that long-term protein supplementation may have a negative impact on the gut microbiota, but another study comparing the microbiota characteristics in healthy sedentary men, bodybuilders and distance runners found that high daily protein intake long-term negatively impacted the gut microbiota, which indicates that high quality protein in the diet may negatively impact the gut microbiota. However, more research is needed in this area.
The consumption of carbohydrates is a well-known strategy for increasing performance because carbohydrates are absorbed into the blood flow and can improve endurance performance as well as provide a better experience for athletes, and so moderate carbohydrate intake may be beneficial for performance in addition to protein consumption for post-workout gains.
Probiotics have many beneficial health effects and have been used for many centuries to mitigate intestinal issues linked to antibiotic travel or illness. There are many different strains and species of probiotics and each will have different potential health effects for athletes. The effects of probiotics on gastrointestinal symptoms and inflammation have been studied in elite and competitive athletes, but the effects differed between males and females and so not much evidence has been proven as yet.
Probiotic supplementation has been shown to increase running time to fatigue, reduce intestinal permeability in gastrointestinal discomfort as well as lower the severity of GI symptoms. Probiotic supplementation may also decrease muscle tension and muscle damage after exercise and may correlate with maintained performance. Body fat percentages were significantly lower in a probiotic group that was studied using female soccer and volleyball players, and correlated with higher diversity in distance runners.
In summary, moderate, light and intense exercise may be beneficial for the gut microbiome; in fact, for long-term elite athletes it may also help to increase the diversity of the gut microbiome in their endurance training. The gut microbiome may then in turn have a bio-directional positive impact on the system as a whole. It could also be said that pairing this with good nutrition, i.e. a diet that is high in fibre and probiotics, may also benefit anyone who is training at a moderate to professional level.