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Probiotics: can they help to prevent illness?

What are probiotics?

Probiotic-rich foods and supplements contain non-pathogenic bacteria that colonise the gut and can potentially yield a variety of health benefits that include reduced incidence of respiratory and gastrointestinal illness. There are several possible ways in which probiotics can act to produce these effects. By their growth and metabolism, probiotics help inhibit the growth of other bacteria, antigens, toxins and carcinogens in the gut, and reduce potentially harmful effects. Probiotics can also influence immune function via interaction with immune cells associated with the gut. Probiotics are found in several foods, particularly dairy products such as milk, yoghurt and cheese, although concentrations are relatively low. Consequently, there is widespread interest in use of probiotic supplements in both the general and sporting communities.

Potential health benefits of probiotics

Probiotics have been used for over a century to manage common gastrointestinal conditions including stomach cramps, irregular bowel movements, excessive flatulence, diarrhoea, and irritable bowel syndrome. In research settings, the focus has been on verifying the clinical benefits of regular probiotic supplementation, and underlying mechanisms of action. Many studies have been conducted on the effects of probiotic use on gastrointestinal problems and upper respiratory tract infection (URTI) in the general population. A recent systematic review (King et al 2014) of 20 placebo-controlled trials in both children and adults concluded that probiotic use resulted in lower numbers of illness days, shorter illness episodes and fewer days of absence from school or work. The most recent Cochrane systematic review of probiotic benefits for URTI using data from randomised controlled trials involving 3,720 non-athletes from 12 studies concluded that probiotics were better than placebo in reducing URTI incidence by 47%, and the average duration of an acute URTI episode by 2 days (Hao et al 2015).

Probiotics were better than placebo in reducing URTI incidence by 47%, and the average duration of an acute URTI episode by 2 days

The most important mechanisms of probiotic action are thought to be via influences on local immunity (by interaction with gut-associated lymphoid tissue and maintenance of gut barrier function) and systemic immunity (by enhancing some aspects of both innate and acquired immune responses). Certain probiotics, particularly those containing Lactobacillus or Bifidobacterium species, have been shown to enhance several aspects of immune cell functions including natural killer cell activity, microbicidal capacity of neutrophils and monocytes, modify the production of cytokines and elevate levels of antibodies (Hao et al. 2015), with effects that can extend beyond the gut to other mucosal sites, including the respiratory tract.

Evidence of probiotic benefits for athletes

A recent comprehensive review (Pyne et al 2015) identified 15 relevant experimental studies that investigated immunomodulatory and/or clinical outcomes of regular probiotic use in athletes. Of the 8 studies that recorded URTI incidence, 5 found reduced URTI frequency or fewer days of illness and 3 reported trivial or no effects. A randomised, placebo-controlled trial involving physically active individuals (West et al 2014) reported that 27% fewer URTI episodes were experienced in those who ingested daily a Bifidobacterium probiotic compared with placebo over a 150-day intervention period. The studies that have shown reduced URTI incidence in athletes have been mostly limited to Lactobacillus and Bifidobacterium species and used daily doses of ~1010 live bacteria. Although most studies have examined probiotic effects in relatively small numbers of recreationally active individuals over periods lasting less than 6 months, there is now sufficient understanding of the mechanism of action of certain probiotic strains, and enough evidence from trials with athletes and highly physically active people (in addition to 12 studies cited in The Cochrane review (Hao et al. 2015) on children and adults) to signify that there are mostly positive effects.

Probiotics advice

Other benefits of probiotics

Other potential benefits of probiotics could be reduced risk of gastrointestinal discomfort symptoms and diarrhoea (e.g. so-called runner’s trots) during prolonged exercise, reduced endotoxaemia during exercise in the heat, and reduced incidence of gastrointestinal infections – a particular concern when travelling abroad. Future research is likely to establish the long-term tolerance of probiotic supplementation in highly-trained athletes over several months to years, the possible benefits, if any, of cycling on and off probiotics, and the effectiveness of multi-component formulations combining several different probiotics species, or probotics and prebiotics (non-digestible food ingredients that promote the growth of beneficial microorganisms in the intestines).

Other benefits of probiotics could be reduced risk of GI discomfort symptoms during prolonged exercise, reduced endotoxaemia and reduced incidence of GI infections

Practical application and advice

Take a daily dose of probiotic containing Lactobacillus and/or Bifidobacterium species containing at least ~1010 live bacteria (referred to as colony forming units, CFU). This is probably better than multi-strain probiotics as different strains can produce different effects which may oppose each other. Take the probiotic in the morning with breakfast. Probiotics need to be taken for several weeks before positive health effects can be expected.

Probiotics advice


  1. Hao Q, Dong BR and Wu T (2015) Probiotics for preventing acute upper respiratory tract infections. Cochrane Database Systemic Review 2:CD006895.

  2. King S, Glanville J, Sanders M, Fitzgerald A and Varley D (2014) Effectiveness of probiotics on the duration of illness in healthy children and adults who develop common acute respiratory infectious conditions: a systematic review and meta-analysis. British Journal of Nutrition 112:41-54.

  3. Pyne DB, West NP, Cox AJ and Cripps AW (2015) Probiotic supplementation in athletes: clinical and physiological effects. European Journal of Sports Science 15:62-72.

  4. West NP, Horn PL, Pyne DB, Cripps AW, Gebksi V, Lahtinen S and Fricker PA (2014) Probiotic supplementation for respiratory and gastrointestinal illness symptoms in healthy physically active individuals. Clinical Nutrition 33:581-587.


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