What are probiotics and what do they do?

Check out the top 10 probiotic foods to support your gut health.

What are prebiotics and how do they work?

Prebiotics are considered by some to be non-digestible carbohydrates, that are not digested by the body but nourish the micro-organisms in the colon. They occur naturally in the diet and are found in foods such as garlic, bananas, oats, onions and leeks. This idea has been criticised by some due to its poor definition and some scientists prefer to use the term 'microbiota accessible carbohydrates', as they are fermentable dietary fibre that the microbes can use. However, foods containing prebiotics are also the components of a healthy diet and should therefore be consumed regularly.

What can negatively affect gut bacteria?

There are a number of components that negatively affect gut bacteria including lifestyle factors such as smoking and high stress levels, as well as the use of antibiotics.

Designed to fight infections, antibiotics reduce and deplete the natural bacteria living in the gut. Resistance to antibiotics is becoming a serious problem worldwide and it is for this reason that we should only take these when absolutely necessary.

Stress can change the number and diversity of our gut bacteria, which in turn affects the immune system and may explain why certain conditions, such as eczema or acne, flare up when we are more stressed.

Excessive alcohol consumption can be inflammatory and may affect the lining of our digestive system, where a lot of our microbiome live, reducing them in numbers and therefore affecting our health.

A long-term reduced intake of fermentable carbohydrates for the treatment of IBS (as in a low FODMAP diet) can also negatively affect the bacteria in the gut. It is therefore important that these foods are only omitted for a specified time under the guidance of a specialist dietitian as the implications of long-term avoidance needs further research.

Smoking also has a negative impact on the bacterial composition of the gut in addition to the well documented health risks it carries.

What are the different types of probiotics?

There are numerous types of probiotics and each has different characteristics. They may be combined with others or appear on their own in powder, tablet or liquid dietary supplements. At the moment, foods that naturally contain probiotics are not eaten regularly in the UK and supplements are becoming more popular. The most common probiotics include lactobacillus acidophilus and bifidobacterium. These differ as they are made up of different types or strains of bacteria, and are recommended for different clinical conditions. Lactobacillus acidophilus have been clinically shown to lower the incidence of antibiotic-associated diarrhoea and can also result in a shorter length of stay in hospital for some. In order to experience this benefit, a vast quantity of food containing probiotics would need to be consumed. It is therefore easier and more effective to take a recommended probiotic supplement.

For those with a diagnosis of IBS, supplements containing bifidobacterium have been shown to reduce symptoms, including bloating, cramping and stool frequency, and can be taken for up to eight weeks.

What are the health benefits of probiotics?

Probiotics have been shown to improve symptoms in IBS, traveller’s diarrhoea and the duration of antibiotic associated diarrhoea. There is emerging evidence that probiotics may improve cholesterol levels in people with type II diabetes, and could play a role in benefitting cold or flu outcomes during stressful periods. However, these are very small studies and more research is needed before robust recommendations can be made.

What are the risks of probiotics?

Generally, probiotics are safe for healthy individuals, however those with a compromised immune system may be at risk and should seek advice from a specialist dietitian or GP before starting.

What should we look for when choosing a probiotic supplement?

There are two key issues when choosing a probiotic: the first is to ensure that the product contains enough bacteria to have an effect (107 to 1010 probiotic cells per gram); the second is to ensure that it survives the acidic environment of the stomach in order to reach the large intestine. Examples of probiotics that meet this criteria include Alforex, Yakult, Symprove and VSL #3. Each of these is designed to treat different symptoms, so if you are thinking about taking a probiotic, seek advice from a specialist dietitian or GP to ensure you take the correct one.

Unless stated differently by manufactures, probiotics should be taken for a minimum of four weeks before effects may be seen. If no improvement is noted, it is recommended that you try another brand or stop completely.

Is it best to take probiotics and prebiotics as foods, supplements or drinks?

If you are thinking about taking a probiotic or prebiotic and are unsure where to start or what to take, ask for help from a specialist. There are so many products available on the market it can be difficult to decide which to choose. The evidence is very much linked to the supplement so whether it’s a powder, tablet or drink you’re looking for, choosing the right one should depend on the symptoms you’re experiencing.

Speak to your GP if you're experiencing symptoms or if you're considering starting a supplement.

Microbes & me

Our new series, in collaboration with BBC Future, looks at all the factors that affect our unique microbiomes – from dietary choices to lifestyle factors.

We’ve worked with dietitian Emer Delaney to bring you expert information and specially selected recipes that will help you to understand how to eat for better digestive health.

Find out more...

How does diet affect gut health?
Gut-friendly vegan recipes
Visit our Microbes & Me series hub page

This article was reviewed on 2nd January 2019 by dietitian Emer Delaney.

Emer Delaney BSc (Hons), RD has an honours degree in Human Nutrition and Dietetics from the University of Ulster. She has worked as a dietitian in some of London's top teaching hospitals and is currently based in Chelsea.

Kerry Torrens is a qualified Nutritionist (MBANT) with a post graduate diploma in Personalised Nutrition & Nutritional Therapy. She is a member of the British Association for Nutrition and Lifestyle Medicine (BANT) and a member of the Guild of Food Writers. Over the last 15 years she has been a contributing author to a number of nutritional and cookery publications including BBC Good Food.

All health content on bbcgoodfood.com is provided for general information only, and should not be treated as a substitute for the medical advice of your own doctor or any other health care professional. If you have any concerns about your general health, you should contact your local health care provider. See our website terms and conditions for more information.

Have you tried probiotics or prebiotics, or do you have any questions about them? Ask us in the comments below.