What are Prebiotics?

Kerry Beeson BSc (Nut. Med.) Nutritional Therapist

Prebiotics are a source of food for probiotics to grow, multiply and survive in the gut. Think of them as your friendly bacteria’s favourite food! 

Definition of prebiotics

The prebiotic definition was recently updated in 2017 to 'a substrate that is selectively utilised by host microorganisms conferring a health benefit1. Generally speaking, most prebiotics are indigestible fibres that the human body can’t break down, instead they specifically feed beneficial microbes in the gut, especially Bifidobacteria

What are prebiotic foods

You can find prebiotics in food and in a supplement format. Prebiotics occur naturally in our diet, and prebiotic fibres can be found in jerusalem artichokes, garlic, chicory, and onions amongst other foods. Unfortunately, you generally need to eat a variety of these foods in large quantities to attain the prebiotic benefits. Plus, harsh cooking methods can impact the prebiotic structures.

This is why some individuals may prefer to take a supplement to ensure they are getting a set amount of prebiotic. This can also be an easier option for fussier eaters! You may find prebiotics on their own or in combination with a probiotics  (known as synbiotic). For higher doses of prebiotics, they will likely be in powder format. Some prebiotics usually have a natural sweet taste so can be very pleasant to take. A benefit of prebiotics is that they are very shelf stable and do not easily degrade over time. They can also be added to a variety of food and drinks as part of your daily routine. A good example of this type of supplement is OptiBac 'For every day', which contains a combination of well researched probiotic strains and FOS fibres. 

for every day probiotics

What are the best prebiotics

The three most recognised and researched prebiotics are:

  1. Fructo oligosaccharides (FOS)
  2. Galacto oligosaccharides (GOS)
  3. Inulin

They all meet the prebiotic criteria and are well known to increase levels of Bifidobacteria in the gut, thus supporting a healthy microbiome and overall health. From our digestive, immune, hormonal, metabolic and even mental health.

Another type of prebiotics are human milk oligosaccharides (HMO’s). They are found in breast milk and because of their extensive benefits, some formula milks are starting to include one or two types. HMO’s are the most selective prebiotic out there. You can find more information on HMO’s in Baby Probiotics

Candidate prebiotics (i.e. those who have yet to be given the official prebiotic status) include xylo oligosaccharides, polydextrose, isomalto oligosaccharides, gluco oligosaccharides, malto oligosaccharide, mannan oligosaccharides and many more!

For more detail on FOS and Inulin, take a look at this blog; A closer look at FOS and Inulin

prebiotic capsules
Prebiotic capsules

What are prebiotics made of?

Prebiotics are fibres i.e. carbohydrates. As our body can’t break them down, they provide little/no calorific value. Our microbes use them as a food source for energy, enabling them to survive and grow.

The biochemistry of prebiotics is truly fascinating, as their structure can affect their function. This is currently being explored in research, with the potential to develop unique structures for specific conditions or functions. FOS, GOS and inulin are all different prebiotics, and so are structurally and chemically different.

FOS and Inulin are the most similar in their make up, however FOS is much shorter than inulin. Inulin chains are much longer (over 10 units). Whereas, GOS is made up of different units to FOS and inulin but has a similar chain size (10 units and below) to FOS. This can affect where in the gut they are broken down.

What’s more, there are different types of FOS, GOS and inulin’s. It depends how the prebiotic was made or where they were isolated from (i.e. what type of vegetable). Their unique structure can affect what species or strains of friendly bacteria use them ultimately affecting their overall function. 

A common myth is that prebiotics feed harmful bacteria in the gut. It’s important to understand that the gut is a huge complex ecosystem, with thousands of microbes competing for space and nutrients and interacting with one another.  For a microbe to break down a prebiotic, they must contain specific enzymes. Many harmful microbes do not harbour these enzymes and thus cannot break down the prebiotic. It is these enzymes, that are commonly found in friendly bacteria that dictate if and how well a strain of bacteria can use a prebiotic.

Now, there could be an element of ‘cross feeding’, this is where the substances produced by one microbe feeds another. Essentially- one bug’s trash becomes another one’s treasure! So it could be that prebiotics feed good bacteria and then these bacteria produce metabolites which then feed neutral gut microbes. So, we might see very small increases in some other types of bacteria, but there the key thing is to look at the bigger picture and the overall benefits the prebiotic provides.

You can read more on does FOS feed bad bacteria too?  

What are prebiotics good for?

Prebiotics can benefit health in many ways. When a prebiotic is fermented (i.e. broken down), bacteria produce short chain fatty acids (SCFA’s). The most common SCFA’s are acetate, butyrate and propionate. They support a healthy gut environment and have systemic benefits as well. Butyrate typically attracts the most attention due its extensive benefits on gut health. It can stimulate our gut cells helping to promote regular bowel movements thus reducing the risk of constipation. Plus, butyrate supports a healthy gut lining ensuring proper digestion and absorption of nutrients.

In fact, prebiotic supplemented have been positively associated with:

  • Gut health2
  • Constipation3
  • Immunity4
  • Sleep5
  • Autism6
  • Metabolic health7
  • Mineral absorption8
  • Skin health9,10
  • Satiety11

Healthcare professionals can find out more about the mechanisms of actions and clinical trials on each health area in mine and Dr Aisling’s blog; The uses of prebiotics.

What are prebiotics summary

Hopefully its now really clear what are prebiotics, the best prebiotics and what prebiotics are good for. They can make a really nice addition to your daily routine and support the composition of your microbiome and your overall health. 

  • Prebiotics can be defined as ‘a substrate that is selectively utilised by host microorganisms conferring a health benefit’
  • You can find prebiotics naturally in some foods
  • You can also take a prebiotic supplement to ensure you receive a specific amount
  • FOS, GOS and Inulin are the most recognised and researched prebiotics
  • Prebiotic fibres can support our gut and overall health

Prebiotic research is booming, with new studies coming out regularly on more benefits and modes of actions of prebiotics. The potential of prebiotics is exciting and one to keep an eye on!

If you enjoyed reading about this subject, then take a look at these related articles:

Should you take prebiotics alone?

Gut health and sleep on the BBC

Health professionals can visit the Professionals site to read these articles:

Could prebiotics help with anxiety?

Could prebiotics help to tackle obesity?


  1. Gibson G et al., "Expert consensus document: The International Scientific Association for Probiotics and Prebiotics (ISAPP) consensus statement on the definition and scope of prebiotics," Nature Reviews Gastroenterology & Hepatology , vol. 14, p. 491–502, 2017.
  2. Sanders M et al., "Probiotics and prebiotics in intestinal health and disease: from biology to the clinic," Nature Reviews Gastroenterology & Hepatology volume , vol. 16, pp. 605-616, 2019.
  3. Collado Yurrita L et al., "Effectiveness of inulin intake on indicators of chronic constipation; a meta-analysis of controlled randomized clinical trials," Nutr. Hosp, vol. 30, pp. 244-252, 2014.
  4. Vulevic J et al., "Influence of galacto-oligosaccharide mixture (B-GOS) on gut microbiota, immune parameters and metabonomics in elderly persons," British Journal of Nutrition, vol. 114, pp. 586-595, 2015.
  5. Thomson R et al., "Dietary prebiotics and bioactive milk fractions improve NREM sleep, enhance REM sleep rebound and attenuate the stress-induced decrease in diurnal temperature and gut microbial alpha diversity.," Front Behav Neurosci, vol. 10, p. https://doi.org/10.3389/fnbeh.2016.00240, 2017 .
  6. Grimaldi R et al., "A prebiotic intervention study in children with autism spectrum disorders (ASDs)," Microbiome, vol. 6, p. 133, 2018 .
  7. Beserra B et al., " systematic review and meta-analysis of the prebiotics and synbiotics effects on glycaemia, insulin concentrations and lipid parameters in adult patients with overweight or obesity.," Clinical Nutrition, vol. 34, pp. 845-858, 2015.
  8. Abrams S et al., "Young adolescents who respond to an inulin-type fructan substantially increase total absorbed calcium and daily calcium accretion to the skeleton," Journal of Nutrition, vol. 137, pp. 2524-2526, 2007.
  9. Lolou et al., "Functional Role of Probiotics and Prebiotics on Skin Health and Disease," Fermentation, vol. 5, no. 2, p. https://doi.org/10.3390/fermentation5020041, 2019.
  10. Salem I et al., "The Gut Microbiome as a Major Regulator of the Gut-Skin Axis," Front Microl, p. 9: 1459, 2018.
  11. Kellow N et al., "Metabolic benefits of dietary prebiotics in human subjects: a systematic review of randomised controlled trials," British Journal of Nutrition, vol. 111, pp. 1147-1161, 2014.