What are Prebiotics?

Dr Kate Stephens PhD Food and Microbial Sciences; Gut Microbiology (University of Reading), BSc Medical Microbiology

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! 

Prebiotics definition

The prebiotic definition was recently updated in 2017 to 'a substrate that is selectively utilised by host microorganisms conferring a health benefit1. In simple terms, prebiotics are indigestible fibres - mostly carbs - which the human body can’t break down, but which our beneficial microbes in the gut microbiome love to feast on, especially Bifidobacteria. Read more about The microbiome here.

What are prebiotic foods?

You can consume prebiotics in certain foods but many people also like to add prebiotic supplements to their daily health regime. Prebiotic fibres are found naturally in many different fruits and vegetables, including Jerusalem artichokes, garlic, bananas, chicory, onions, and dandelion leaves! Another example of a prebiotic food is raw, unpasteurised apple cider vinegar, a fermented food containing prebiotics from the apples that it is made from. Unfortunately, today’s typical ‘Western-style’ diet is quite lacking in the types of healthy foods that contain prebiotics, so people are not getting the levels that they need to maintain good gut health. Plus, harsh cooking methods can damage the prebiotic structures and reduce our intake still further.

This is why some individuals may prefer to take a prebiotic supplement to ensure they are getting a good intake of these healthy fibres each day. This can also be an easier option for fussier eaters, or those who do not enjoy vegetables, though supplements should not be used to replace a healthy diet. Prebiotics may be found in a supplement on their own, or in combination with probiotics  (known as a synbiotic). Higher doses of prebiotics are usually found in powder format, as they are too bulky to fit into a small capsule. They have a naturally sweet taste so can be pleasant to take, and they mix easily in to a variety of different foods and drinks. Another benefit of prebiotics is that they are very shelf stable and do not easily degrade over time, making them an excellent addition to a daily health routine.

prebiotic capsules
Prebiotic capsules

Which are the best prebiotics?

The three most recognised and researched prebiotics are:

  1. Fructooligosaccharides (FOS)
  2. Galactooligosaccharides (GOS)
  3. Inulin

These all meet the prebiotic criteria and are well known to increase levels of Bifidobacteria in the gut, thus supporting a healthy microbiome. Having a healthy balance of microbes in the gut is important for many aspects of health, including: our digestive, immune, hormonal, metabolic and even mental health.

Another type of prebiotic is found in human breast milk. Human Milk oligosaccharides (HMO’s) have been shown to have extensive health benefits for babies, so much so that some formula milks are now adding them as an ingredient. HMO’s are a very selective prebiotic, nurturing the proliferation of only certain Bifidobacteria strains in an infant gut. For babies and infants breast milk is undoubtedly one of the best prebiotics foods with many health benefits. You can find more information on HMO’s in Baby Probiotics

Beyond the prebiotics already mentioned there are several other types of non-digestible fibre that are yet to be officially classified as ‘prebiotics’. These include: xylooligosaccharides, polydextrose, isomalto-oligosaccharides, gluco-oligosaccharides, malto-oligosaccharides, mannan-oligosaccharides and many more!

For more details about FOS and inulin, take a look at this blog; A closer look at FOS and Inulin

What are prebiotics made of?

Prebiotics are typically indigestible carbohydrates, made up of linked chains of sugars. The human body can’t break them down, so they provide little/no calorific value. Our microbes however, use them as a food source for energy, enabling them to survive and grow in numbers.

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 health 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 has a much shorter chain length 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, and by which gut microbes.

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 can also 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.

prebiotic sachets
Prebiotics may also come in sachet form

You can read more about this subject in this article does FOS feed bad bacteria too?  

What is the difference between prebiotics and probiotics?

Many people get confused about the difference between pre- and pro-biotics. However, once we understand that prebiotics are non-digestible fibres that the human body cannot digest, but that our friendly gut bacteria do, the distinction becomes clear. In its simplest terms, prebiotics are non-digestible carbohydrates that friendly strains of gut bacteria use as a ‘fuel’ source. Whereas probiotics are the friendly bacteria themselves. Prebiotics are essentially a food source for probiotics.

What does a prebiotic do?

Prebiotics can benefit health in many ways. One of their best known benefits is that when they are fermented (i.e. broken down) by our gut bacteria, the bacteria are able to produce short chain fatty acids (SCFA’s) as a by-product. The most common SCFA’s produced are: acetate, butyrate and propionate which 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. Additionally, butyrate supports a healthy gut lining ensuring proper digestion and absorption of nutrients. By ensuring that we provide our ‘friendly bacteria’ colonies with prebiotics to use as their fuel source, they are in return able to support our health through the provision of these beneficial SCFA’s.

When looking at prebiotics benefits however, there are many more than just SCFA production. In fact, prebiotic supplements have been positively associated with:

  • Gut health2
  • Constipation3
  • Immunity4
  • Sleep5
  • Autism6
  • Metabolic health7
  • Mineral absorption8
  • Skin health9,10
  • Satiety11
  • Weight loss12

There has been much interest in prebiotics for weight loss, and several scientific studies have yielded promising results. One such study12 involved 42 overweight and obese children. They were given either prebiotic fibre (inulin) or placebo once a day for 16 weeks. The prebiotic group of children gained less weight in the 4 month intervention period, than those in the placebo group.

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

Should I take prebiotics?

Once you have an understanding of what prebiotics do, it makes it easier to decide whether to take a prebiotic supplement. When you increase your intake of prebiotics you are aiming to increase the levels of friendly bacteria in the gut and add fibre to your diet. If you eat a wide selection of different fruits, vegetables, and fermented foods and/or a probiotic supplement, then an additional prebiotic supplement may not be necessary. Taking probiotics also increases the levels of friendly bacteria in the gut, so the question of ‘prebiotics vs probiotics’ arises. The thing to note is that taking probiotic supplements allows you to choose the strains of bacteria which are most suitable for your needs, but prebiotics will have a more global effect.

Individuals following a low FODMAP diet may in fact need to avoid prebiotics either in their diet or in supplement form, particularly whilst they are in the initial  strictest phase of the diet. The ‘O’ in FODMAP stands for oligosaccharides, which need to be avoided on this diet, and most commercially available prebiotic supplements contain either fructo- or galacto-oligosaccharides (FOS or GOS). Prebiotics can usually be phased back in to the diet once the ‘maintenance’ and ‘reintroduction’ phases are reached.


This article hopefully answers the questions of: what prebiotics are, which prebiotics are the best ones to use, and what prebiotics are good for. They can make a really nice addition to a daily health routine, and support the composition of the microbiome and overall health of many people. 

  • The approved definition of a prebiotic is: ‘a substrate that is selectively utilised by host microorganisms conferring a health benefit’
  • Prebiotics encourage the growth of good bacteria in the gut
  • Prebiotic fibres can support our gut and overall health
  • Prebiotics are found naturally in some foods
  • Prebiotic supplements can be taken to optimise daily intake
  • FOS, GOS and Inulin are the most recognised and researched prebiotics

Optibac Bifido & Fibre contains 4g of FOS along with the highly researched probiotic strain, Bifidobacterium lactis BB-12® 

Prebiotic research is booming, with new studies coming out on a regular basis highlighting their many benefits and modes of actions. 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 the following articles:

Could prebiotics help with anxiety?

Could prebiotics help to tackle obesity?


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  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.
  12.  Reimer A et al., Prebiotics reduce body fat in overweight children. American Gastroenterological Association, 2017.