probioticThe topic of gut health seems to have hit in the mainstream in the last couple of years, with media articles urging you to support the bacterial population that live in your digestive tract by consuming yogurt, sauerkraut and sourdough bread or drinking bone broth to prevent ‘leaky gut’. You might have wondered if this has any relevance to you as an endurance athlete. The answer is yes.

To train and race to your best potential, you need to be healthy. Days lost to sickness compromise your ability to be race ready. High volume and high intensity training also acts as a considerable stress on the body and raises the risk of illness, making adequate recovery an essential part of the training mix. Endurance athletes are more prone to Upper Respiratory Tract Infections (URTIs – essentially the common cold) than the general population, for example. They may also experience digestive health problems such as bloating, diarrhoea, constipation and abdominal discomfort, particularly during longer duration races. This is where maintaining a healthy gut may play an indirect role in improved performance, by helping to prevent issues which impact on training time or races.

One of the keys to a healthy gut is to keep the bacteria that live there in a state of balance. You have probably heard the term ‘probiotics’: this refers to the beneficial species of bacteria, which support human health in a number of ways. Examples are the digestion and absorption of certain nutrients including carbohydrate and short chain fatty acids (both sources of energy), synthesis of B vitamins and vitamin K to supplement the amount obtained through food, absorption of minerals such as calcium and magnesium (both play important roles in exercise performance), keeping your gut moving smoothly, aiding the detoxification process, resisting infection by pathogens such as parasites and harmful species of bacteria, and boosting levels of the main antibody in the gut, Secretory IgA, which plays a key role in your immune system.

But probiotics are not the only inhabitants of your gut. You might have heard the term ‘microbiome’ or ‘microbiota’; this refers to the bacterial colonies of your gut in its entirety. The human microbiome consists of around 100 trillion microbial cells, outnumbering cells by ten to one. The gut microbiome is the subject of huge interest to researchers, who are gradually uncovering the role that the bacteria play in human health and disease. I’ve already mentioned the beneficial contributions made by probiotic bacteria; it is when the balance of the microbiota is disturbed that health issues arise. Two main imbalances occur: (1) dysbiosis, which refers to the colonisation of pathogenic (harmful) bacteria, compromising the levels of probiotic (beneficial) bacteria and adversely affecting immunity in the gut; (2) increased permeability of the gut wall – also known as ‘leaky gut’ – due to an increase in the circulation of inflammatory chemicals known as cytokines which may then allow partially digested food and bacteria to enter the blood stream, in turn leading to an immune response. Both compromise optimal health and may have a negative impact on performance as a result.

One way to prevent or reduce disturbance of the microbiota is to increase colonisation of the beneficial bacteria, either by taking probiotic supplements containing bacterial species known to support health or by increasing the consumption of fermented foods that contain these bacteria such as yogurt, sauerkraut and sourdough bread. You should also aim to eat more foods that are rich in pre-biotics such as inulin or fructo-oligosaccharides (FOS). This is the type of fibre that probiotic bacteria feed on, causing the colonies to multiply. Examples of pre-biotic foods are garlic, onions, leeks, beans, lentils and chickpeas.

But what’s the evidence for these strategies being helpful to endurance athletes specifically? There are actually a number of studies which support the case for taking steps to support your gut bacteria if you participate in endurance sports. Here are some examples:

  • In a double-blind crossover study 1 20 healthy elite male distance runners were given the probiotic Lactobacillus fermentum or placebo over a four month period of winter training. Subjects reported less than half the number of days of respiratory symptoms when taking the probiotic compared with taking the placebo. Severity of symptoms was also lower. There were no changes in markers of immunity (salivary IgA) or inflammation (interleukin 4 & 12).
  • In a double-blind, placebo controlled study 2 involving 84 highly active individuals engaged in endurance-based activities, those receiving a daily probiotic (Lactobacillus casei Shirota) during four months of winter training experienced a lower average number of weeks with URTI symptoms than those receiving the placebo. Severity and duration of URTI symptoms showed no difference in this study, but salivary IgA concentration was higher in those taking the probiotic, suggesting that this may have been a contributing factor.
  • 99 competitive cyclists were randomised to receive either Lactobacillus fermentum probiotic or placebo treatment for 11 weeks 3. Male subjects taking the probiotic saw reduced severity in gastrointestinal symptoms compared with those taking placebo, particularly as the training load increased. Males also saw reduced load (duration x severity) of lower respiratory illness symptoms. The same benefits were not seen in female subjects.
  • 23 trained men received a multi-species probiotic or placebo for 14 weeks and performed an intense cycling exercise over 90 minutes at the start and end of the trial period 4. Zonulin, a marker for intestinal permeability (‘leaky gut’) decreased with the probiotic supplementation and was significantly lower in the probiotic group than the placebo group after 14 weeks. Levels of TNF-α, an inflammatory cytokine, were also reduced.

However, not all studies have shown benefits. The species of probiotic bacteria tested is likely to be of significance. For example, the same group who conducted the second trial above also studied the effect of Lactobacillus salivarius on infection, cold symptom duration and markers of immunity in 66 endurance athlete subjects 5. The proportion of subjects taking the probiotic who experienced a week or more of URTI symptoms during four months of spring training was not different from the proportion of those taking a placebo. The number of URTI episodes and the severity and duration of symptoms were also similar. The amount of salivary IgA antibodies did no change over the course of the study in either group. There is still much research to do in this area.

In summary, although not conclusive, there is evidence that supporting the probiotic bacteria population in your gut may have indirect benefits for performance in endurance sport by reducing training days lost to upper respiratory tract infections, particularly during the winter. If you experience gastrointestinal health issues, probiotic supplementation may also help to reduce intestinal permeability which may be a contributing factor. Other benefits may come from overall improved immunity, better absorption of minerals, increased synthesis of B vitamins and vitamin K, and enhanced gut motility. So my recommendations for you as an endurance athlete are as follows:

  • Try to include at least one fermented food source in your diet every day to boost your probiotic bacteria. Try sourdough bread, yogurt, kefir (similar to yogurt), sauerkraut, kimchi (Korean fermented vegetables), tempeh and miso (fermented soya products) and kombucha (fermented teas). These products can be found in health food shops and are becoming more widely available in some supermarkets and lunch places.
  • Regularly consume pre-biotics like garlic, onion, beans and lentils to help feed your probiotic bacteria.
  • Take a good quality, multi-species probiotic supplement during winter, heavy training blocks and when travelling abroad, especially for races. Two examples I can recommend are also both Informed Sport certified, making them safe for athletes subject to drug testing under WADA: BioAcidophilus Forte from Biocare and High Strength Probiotic from Healthspan Elite

I hope this information helps you to stay healthy and therefore contributes to your performance goals.

Jo Scott-Dalgleish BSc (Hons) is a BANT Registered Nutritionist, writing and giving talks about nutrition for endurance sportBased in London, she also works as a Registered Nutritional Therapist, conducting onetoone consultations with triathletes, distance runners and cyclists to help them eat well, be healthy and perform better through the creation of an individual nutritional plan. To learn more about these consultations, please visit


1 Cox et al. Oral administration of the probiotic Lactobacillus fermentum VRI-003 and mucosal immunity in endurance athletes. Br J Sports Med. 2010; 44(4): 222-6.

2 Gleeson et al. Daily probiotic’s (Lactobacillus casei Shirota) reduction of infection incidence in athletes. Int J Sport Nutr Exerc Metab. 2011; 21(1):55-64.

3 West et al. Lactobacillus fermentum (PCC®) supplementation and gastrointestinal and respiratory tract illness symptoms: a randomised controlled trial in athletes. Nutr J. 2011; Apr 11. 10:30

4 Lamprecht et al.Probiotic supplementation affects markers of intestinal barrier, oxidation and inflammation in trained men; a randomized, double-blinded, placebo-controlled trial. J Int Soc Sports Nutr. 2012; 9(1): 45

5 Gleeson et al. Effect of a Lactobacillus salivarius probiotic intervention on infection, cold symptom duration and severity, and mucosal immunity in endurance athletes. Int J Sport Nutr Exerc Metab. 2012; 22(4): 235-42