Guide to Prebiotics

The gut microbiome is a sensitive ecosystem of bacteria, yeast, viruses and more that live in our digestive tract. Together they are responsible for keeping us digesting and absorbing important nutrients and vitamins, keeping our body as healthy as possible.

What are prebiotics?

Prebiotics are types of soluble fibres that promote the growth of beneficial bacteria in the gut (probiotics), an essential part of our digestive health. They are found in certain kinds of carbohydrates naturally in a variety of foods, as well as supplements, and are often consumed with probiotics as they work so closely together. Both are essential to keeping the gut microbiome healthy.

What do prebiotics do?

Prebiotics are soluble fibres which aren’t digested in our stomach. Instead they pass into the colon undigested, where they are broken down by prebiotic bacteria, allowing them to provide a host of benefits.1,2 For example, extensive research has shown that the prebiotic, inulin, can boost immune function and reduce gastrointestinal discomfort.3

Improve digestion and relieve constipation

The obvious health benefit of taking prebiotics is to aid digestion. It’s thought that at any given time, 1 in 7 UK adults will have constipation. This is clearly a very real and very common problem.4

Some prebiotics, such as galactooligosaccharides and fructooligosaccharides are often used as an alternative therapy to laxatives or an increase in dietary fibre for the treatment of constipation.5 Numerous studies have found evidence to support the use of prebiotics to improve stool frequency and consistency.6

Psychological wellbeing and lower stress levels

There is convincing evidence to suggest a link between psychological wellbeing and our gut microbiome, for example, some bacteria found in our digestive tract could even tell us whether you have a predisposition to post traumatic stress disorder. 7

Prebiotics in particular have been found to reduce blood levels of stress hormone, cortisol. Taking a prebiotic containing galactooligosaccharides, participants produced a lowered stress response compared to the placebo group.8

The effect prebiotics have on stress may also be linked to their ability to improve sleep.9

Insulin resistance

Not only can prebiotics improve overall digestion and wellbeing, but there’s also evidence to suggest that it could benefit those with type 2 diabetes. A 2015 study looked at the use of prebiotic supplements in women with type 2 diabetes and found that it not only reduced inflammation but can also improve insulin resistance.10 Data from Public Health England revealed in 2016 that around 9% of the UK adult population now have diabetes and is an increasing health concern for the nation.11 The research into prebiotics and insulin resistance is a positive step forward in the management of such a common disease.

Getting prebiotics from your diet

There are lots of ways to get prebiotics into your diet, as they occur naturally in a number of foods. These include raw garlic, onion, chicory root, green bananas, Jerusalem artichokes, etc. However, many of these require eating a large amount to give provide a noticeable difference, and not everyone is too keen on smelling like raw garlic all the time.


As prebiotics can provide a laxative effect, high doses can potentially lead to diarrhoea and abdominal pain. There is some evidence to suggest that large amounts can also increase gastrointestinal reflux and bloating. If you experience irritable bowel syndrome or pre-existing gastrointestinal reflux, it’s important to discuss the use of probiotics with your GP before taking them.12

Correct dosage

As with any supplement, it’s essential you follow the usage guidelines provided by the manufacturer.

Studies have found positive effects from probiotics across a variety of dosages.

UK Government guidelines suggest that our dietary fibre intake should be 30mg a day, but the average adult only gets 18g.13 However, this will vary depending on the individual and their lifestyle.

Prebiotics such as inulin, fructo-oligosaccharides (FOS), and galacto-oligosaccharides (GOS) are a type of dietary fibre which survive digestion to reach the large bowel. Here, they act as food for beneficial probiotic bacteria and selectively promote their multiplication and growth. Less beneficial bacteria are unable to use prebiotic fibre as fuel, so ensuring good intakes of prebiotics helps to promote a healthy balance of bacteria within the gut.

Dr Sarah Brewer

1 G.G, H.P, J.L, R.R. (2007). Dietary modulation of the human colonic microbiota: updating the concept of prebiotics. Nutrition Research Reviews.
2 J.S. (2013). Fiber and Prebiotics: Mechanisms and Health Benefits. Nutrients.
3 D.M, M.S-W. (2009). The bifidogenic effect of inulin and oligofructose and its consequences for gut health. European Journal of Clinical Nutrition
4 Constipation. NHS Inform. [online].
5 L.M, K.K, R.K. (2007). Galacto-oligosaccharides and bowel function. Scandinavian Journal of Food and Nutrition
6 S.M.J.H, S.M-M, L.L. et al. (2017). The Microbiome in Posttraumatic Stress Disorder and Trauma-Exposed Controls: An Exploratory Study. Psychosomatic Medicine
7 K.S, P.C, C.H, G.T, S.E, P.B. (2015). Prebiotic intake reduces the waking cortisol response and alters emotional bias in healthy volunteers. Psychoharmacology
8 R.T, R.R, A.M, B., R.K, M.C, B.B, M.F. (2017). 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. Frontiers in Behavioral Neuroscience.
9 R.T, R.R, A.M, B., R.K, M.C, B.B, M.F. (2017). 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. Frontiers in Behavioral Neuroscience.
10 A.A, P.D, B.P.G, M.A-J. (2015). Resistant dextrin, as a prebiotic, improves insulin resistance and inflammation in women with type 2 diabetes: a randomised controlled clinical trial. British Journal of Nutrition
11 3.8 million people in England now have diabetes. (2016). [online]. Public Health England.
12 M.P, S.P. (2004). Tolerance of probiotics and prebiotics. Journal of Clinical Gastroenterology.
13 How to get more fibre into your diet. NHS Choices. [online].




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