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What is a probiotic?

Gut Health 101

Yoghurt? Kimchi? Powders? Pills? We explore what probiotics are - and what they're not.

Put very simply, a probiotic is a type of beneficial bacteria or yeast that naturally lives in your body. You can take a probiotic supplement or eat fermented foods such as kimchi, kefir or kombucha if you want to add beneficial bacteria to your gut.

However, in order for a supplement or food to be classed as a "probiotic", it should meet the World Health Organisation definition of "Live microorganisms, that when administered in adequate amounts, confer a health benefit on the host."


To meet the scientific definition, a probiotic should adhere to the WHO guidelines, which are as follows:

  1. An adequate number of microbes in your supplement or food must be able to survive the digestive process to reach your colon. This is very difficult given how sensitive live bacteria are. A true probiotic should be able to prove this survivability through human clinical trials.

  2. Strains must be identified genetically, and classified using the latest terminology. This means it's not enough to just say "probiotic" or "good for the gut," you should have access to the strain name - for example:  Lactobacillus plantarum DSM 9843.

  3. Human studies must be performed using the strain(s). If the strains have only undergone animal studies, it should be clearly marked as "under experimental testing."

  4. Strains shown to confer a benefit for one condition, may not be a probiotic for another application, so check that the research targets the specific issue you want to address.


The effect that the gut has on our health and wellbeing is phenomenal. Science continues to unearth new ways in which the gut influences the body as a whole; from the immune system to hormones, to mental health.

A healthy, balanced microbiome (the collective name for the organisms in our gut) is essential for overall health. A probiotic may support this ongoing process of balancing the bacteria inside us.

In addition, our diet and lifestyle factors that include rest, movement and our environments all play a key role in keeping the bacteria within us in-check.


Some, although not all, probiotics are able to perform the following actions in the body:

  • They may promote diversity by adding beneficial bacteria to the gut. This diversity of species helps to create balance and, as current research suggests, leads to a stronger and healthier gut. When the overgrowth of a few organisms occurs (particularly undesirable organisms) this balance may be disturbed, which can be referred to as ‘dysbiosis’.

    Dysbiosis may manifest with direct digestive symptoms.

  • They can strengthen the mucosal barrier function. Having a healthy and balanced gut microbiome enables sufficient mucus secretion, which strengthens the gut epithelium (or gut barrier). Maintaining a strong gut barrier is key to help prevent unwanted substances and potential pathogens moving out of the gut into the bloodstream.

  • Some produce short chain fatty acids. Certain gut microbes produce positive substances called short-chain fatty acids that help to nourish the gut barrier as well as having an important role in managing inflammation in the body more generally.

  • They can support Immune Function: Up to 70-80% of your immune system is located in your gut, and the gut supports your immune system in several ways:


    Gut microbes help to support the integrity of the intestinal barrier to prevent potentially harmful pathogens and foreign substances passing into the bloodstream.


    Gut microbes have an important role in training the immune system to recognise friend from foe and maintain a healthy immune response.


    A well-nourished gut microbiome and a strong gut epithelium plays a crucial role in the regulation of your immune system.

  • They can support nutrient absorption.Having a healthy and balanced gut microbiome helps to support digestion and absorption processes.

Kechagia, M., Basoulis, D., Konstantopoulou, S., Dimitriadi, D., Gyftopoulou, K., Skarmoutsou, N. and Fakiri, E., 2013. Health Benefits of Probiotics: A Review. ISRN Nutrition, 2013, pp.1-7. [Online] Available from: https://www.ncbi.nlm.nih.gov/pmc/articles/PMC4045285/  [Accessed 26 February 2021].

Ohland, C. and MacNaughton, W., 2010. Probiotic bacteria and intestinal epithelial barrier function. American Journal of Physiology-Gastrointestinal and Liver Physiology, 298(6), pp.G807-G819. [Online] Available from: https://journals.physiology.org/doi/full/10.1152/ajpgi.00243.2009 [Accessed 26 February 2021].

Reid, G., Gadir, A. and Dhir, R., 2019. Probiotics: Reiterating What They Are and What They Are Not. Frontiers in Microbiology, 10. [Online] Available from: https://www.ncbi.nlm.nih.gov/pmc/articles/PMC6425910/  [Accessed 26 February 2021].

Sheridan, P., Bindels, L., Saulnier, D., Reid, G., Nova, E., Holmgren, K., O'Toole, P., Bunn, J., Delzenne, N. and Scott, K., 2013. Can prebiotics and probiotics improve therapeutic outcomes for undernourished individuals?. Gut Microbes, 5(1), pp.74-82. [Online] Available from: https://www.ncbi.nlm.nih.gov/pmc/articles/PMC4049942/  [Accessed 26 February 2021].

Silva, Y., Bernardi, A. and Frozza, R., 2020. The Role of Short-Chain Fatty Acids From Gut Microbiota in Gut-Brain Communication. Frontiers in Endocrinology, 11. [Online] Available from: https://www.frontiersin.org/articles/10.3389/fendo.2020.00025/full  [Accessed 26 February 2021].