What are probiotics?

The body contains 100 trillion microorganisms (also called microbiota or microbes), outnumbering human cells by 10 to one. The microbiome is a community, or ecosystem, of different microorganisms, including bacteria, fungi, yeasts and viruses, of which some are good, and some are bad. In a healthy person, they should co-exist in harmony, but imbalance can cause disease. For example, Staphylococcus aureus is naturally present in the nose in some people without any impact, but if its numbers grow too high, it can cause pneumonia and other life-threatening diseases.

Every individual has a different balance of microorganisms, which are found mainly in the gut, but also skin and elsewhere. The unique balance is influenced by a variety of factors such as genetics and early environment, such as whether a person is born via caesarean or via birth canal, or whether breastfed or bottle-fed in infancy.

Probiotics are beneficial live bacteria (and some yeasts). They exist in the body, but levels of beneficial bacteria can be increased by taking them via diet or they can be taken as supplements, in the form of pills, powder sachets, or drinks or applied topically. Taking probiotics adjusts the balance of microbes e.g. in the gut, vagina or skin.

The health benefits of probiotics are due to the variety of their functions including (1):

  • creating vitamins,
  • extracting energy from food,
  • immunity,
  • mood regulation,
  • improving digestion,
  • improving cholesterol,
  • Improving blood sugar levels,
  • Improving blood pressure and
  • reducing risk of bowel cancers.

Probiotics and digestion

As probiotics improve digestion, they can be of great benefit to individuals with digestive disorders. For example, individuals with Irritable Bowel Syndrome (IBS) symptoms such as abdominal pain, bloating, wind, constipation and diarrhoea, can see significant improvement in these symptoms and in quality of life with certain probiotics. Other probiotics are shown to prevent diarrhoea associated with antibiotic use and traveller’s diarrhoea.

In people with Ulcerative Colitis (UC), probiotics are shown to help improve symptoms (2) and can maintain remission or prevent relapse in UC and in pouchitis (3). Probiotics can also reduce the side effects of H Pylori and help treat it more quickly (4). Speak to a Dietitian for specific recommendations.

In Crohn’s disease, the evidence is not strong enough to recommend probiotic use.

Probiotics and weight loss

It’s known that an individual’s balance of gut microbiota can make them susceptible to diseases including type 2 diabetes, obesity, and heart disease. (5, 6) This balance of microbiota begins in infancy, but can also be affected by diet. And as the gut microbiota can influence the immune system, the neurological system and hormones, there is ongoing research in to how these processes can lead to obesity.

Select probiotics (Lactobacillus gasseri) have been studied in trials on people with obesity and type 2 diabetes mellitus, and are shown to reduce BMI and abdominal fat, improve metabolism of carbohydrates and insulin sensitivity, and improve blood lipid levels and liver function (7). Further studies (8) suggest supplementation of specific probiotics prevents fat gain.

Probiotics and immunity

The bacteria and other microorganisms in the gut help to protect immunity by acting as a barrier against harmful bacteria which could cause infection.

Taking specific probiotics may help to reduce the likelihood of catching a cold (9), increasing resilience towards infections, and can shorten the length of time of being unwell. They may also boost the effectiveness of certain vaccines, including flu, when taken everyday both before and after the day the vaccine is received.

For some infections, antibiotics are used as treatment. Antibiotics are not selective, and as well as eradicating harmful bacteria, they can reduce the numbers and diversity of good bacteria. Probiotics taken during and shortly after an antibiotic treatment can reduce the chance of getting diarrhoea, and help to replenish the gut bacteria.

Probiotics and mood

The gut-brain axis is a system that moves in both directions, where the brain influences the gut and vice versa. As well as digesting food, your gut microbiota produces hormones and other neurochemicals, such as serotonin, which affect how the immune system and brain functions. The microbiota has been shown to contribute to the development of neurodegenerative diseases including Autism Spectrum Disorders, Alzheimer’s Disease and Parkinson’s Disease as well as anxiety and depressive disorders (10).

Taking probiotics can help to influence emotions, but also pain perception and stress response. Pyschobiotics (11) are specific probiotics which produce neuroactive substances which can have benefit on individuals with psychiatric illnesses. A study in mice showed psychobiotic (L. rhamnosus) had anti-anxiety and anti-depressant effects in order to control the response to a stressful situation.

Conversely, being stressed can tip the balance of microbes in the gut, increasing vulnerability to infection, so as well as taking beneficial bacteria supplements, it’s important to partake in general self-care as both will improve gut health.

Probiotics and women’s health Graver

As well as the intestine, the vagina is colonised by microorganisms, mainly lactobacilli which produce lactic acid, preventing bacterial and yeast infections (12).

Specific strains of lactobacillus probiotics can be used to treat vaginal infections such as bacterial vaginosis or vulvovaginal candidiasis.

Probiotics and skin health

One thousand different species of bacteria live on the skin. They protect the body partly by activating the immune functions of the skin. When imbalanced, they can play a role in skin diseases, including acne vulgaris, rosacea, psoriasis, atopic dermatitis, and childhood eczema (13). Oral and/or topical probiotics can be used to treat these conditions. Studies also show benefits of probiotics in wound healing.

Common Probiotics

Example Probiotics Strains Format and dose Evidence
Bifidobacterium animalis DN-173 010
2 pots a day (yoghurt)

Improves discomfort, QoL score and bloating and stool frequency (if <3 stools/week)

*Multicenter, randomized, double-blind, controlled trial (14) (15)


Lactobacillus casei Shirota

1 bottle per day (liquid)


Reduces risk of antibiotic-associated diarrhoea (16)

Decreased likelihood of bowel injury from taking NSAIDS (17)


Lactobacillus casei

2 bottles per day (liquid)

Reduces risk of antibiotic-associated diarrhoea (18)



Twice daily

Contributes to eradication of H pylori (19)


- Lactobacillus casei
- L. acidophil,
- L. plantarum
- Enterococcus farcium

Once daily on empty stomach
12 weeks
Liquid format

IBS symptom severity (20)

VSL#3 (Vivvomixx)

L. paracasei
L. plantarum
L. acidophilus
L. delbrueckii
B longum
B. infantis
B. breve
S. thermophilus

twice a day: Morning and evening

Ulcerative colitis and pouchitis – maintains remission, (3)

Optibac / Biokult

S. boulardii

1-2 Daily
4 weeks

Quality of life in IBS with diarrhoea (21)

Prevents travellers diarrhoea and

Protects against antibiotic associated diarrhoea (22)

BioGaia Protectis

L. Reuterii DSM 17938

1 daily

Constipation – improved frequency (23)

Contributes to eradication of H pylori (24)

Choosing the right probiotic

  1. Understanding the purpose

As like medicines, you would choose a medicine according to the condition you want to treat, specific probiotics should be chosen according to the benefit they provide.

  1. Knowing the name

Probiotics are named according to their:

  • genus e.g. Lactobacillus or Bifidobacterium; describing a broad group of organisms with similar qualities
  • species e.g. plantarum, or acidophilus – sharing common features, but may be different in function or structure
  • an alphanumeric strain eg 299v or AD031 – particularly important for identifying the type of probiotic and how it will effect you
  1. Read the product label

Probiotics are not labelled like medicines, but are labelled as a functional food, and should provide the 3–part name as detailed above. The label must also inform you of the dose, eg 1 capsule, and the number of colony-forming units (CFUs), plus how to store it to preserve life of these live microorganisms, eg refrigerated or in a cool dry place, and expiry date as the probiotic will not be effective if it is not ‘live’.

  1. Take as directed

Probiotics are very specific, and if you have been recommended a specific strain, these should be taken and not substituted for another as they may not have the same benefit. Additionally, guidance recommends to take a probiotic as directed eg 1 capsule with a meal, daily for 4 weeks (some probiotics are recommended to take for longer eg 8 weeks or 12 weeks). If you don’t notice a difference in your symptoms, try an alternative probiotic, again, for 4 weeks as directed. Taking for less time or not at the recommended dose may reduce the likelihood of seeing a difference in symptoms.

Caution on probiotics

Though more studies are needed to confirm which specific probiotics are effective for which condition, they’re generally considered safe to take for healthy adults, as they are naturally occurring in the body already, and in commonly consumed foods. You can speak to your doctor or dietitian for more specific guidance on which probiotics have been studied and shown to be effective in your condition.

However, probiotics are not considered safe to take if immunosuppressed. You should also discus with your doctor before taking any supplement if you are pregnant or breastfeeding.

Probiotics are not regulated so reading the label is important (see above).

Probiotics for children

Children may benefit from probiotics for digestive health issues, colic, or atopic eczema.

Introducing probiotics into diet via foods is a safe way to introduce children to probiotics.

Some probiotic supplements are designed for use in infants and children, but it is best to speak with a paediatrician first.

Probiotics in foods

Check the label for ‘live’ or ‘active’ cultures. The ingredients label may also list the specific probiotic.

Examples of probiotic-rich foods include:

  • yoghurt, and cottage cheese
  • kefir
  • kimchi
  • kombucha
  • tempeh
  • miso
  • fermented sauerkraut

What about prebiotics and synbiotics?

Prebiotics are the food for the bacteria in your gut, which help to improve levels of the good bacteria which feed off those foods in particular. Prebiotics are complex carbohydrates, or high-fibre foods, or resistant starches, which aren’t readily digested, and so are available for bacteria to ferment in the large bowel.

Synbiotics are products which contain both probiotics and prebiotics.

By Naomi Leppitt, Registered dietitian, Gut health specialist


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