Health Benefits of Probiotic Bacteria

gut-bacteriaThere are thousands of different species of bacteria and other organisms in the gut, collectively known as the gut microbiome. Some are classed as “bad” as they can cause acute diarrhoea and, in the long-term, promote chronic inflammation and leaky gut syndrome, increasing the risk of premature ageing and degenerative and inflammatory disease. Healthy “good” gut bacteria, on the other hand, tend to reduce inflammation and work with the body to improve gut health and general immunity by helping prevent bacterial and viral infections as well as lowering the risk of food intolerances and food allergies.

This page describes the dietary and lifestyle factors which affect gut health and discusses the evidence for extra probiotic supplementation.


Screenshot 2019-12-01 at 16.51.20Lifestyle measures which affect our gut probiotic  bacteria

Non-nutritional factors: Regular exercise can improve gut health, while there are links between obesity and depression and poorer gut health.

Bacteria-rich foods: Many foods contain healthy bacteria, including fresh uncooked fruit and vegetables. Fermented and pickled foods are especially good, and these include live yoghurt, kefir, aged cheeses, miso soup, tempeh, kimchi, natto, sauerkraut and pickled onions. A good quality probiotic supplement can certainly help. Be careful of milk shots, however, which often contain sugar and include just one strain of bacteria.

probioticsPre-biotic-rich foods: These are foods which help promote the growth of healthy bacteria and impede the formation of unhealthy bacteria. Some, such as mushroom, have anti-biotic properties which directly kill unhealthy bacteria. These foods also protect bacteria from the enzymes in the saliva and stomach, enabling the formation of healthy colonies in the small bowel and colon. Pre-biotic-rich foods consist of two main categories – soluble dietary fibres (from linseeds, fruit grains etc) and polyphenol prebiotics (read more).


Potential benefits of probiotic supplements inside the gut

Travellers diarrhoea: Certain strains of probiotics have been shown to be effective at treating diarrhoea and gastroenteritis commonly caught whilst travelling, usually from drinking water. A study published in the Journal of Pediatrics concluded that Lactobacillus species are a safe and effective form of treatment for children with infectious diarrhoea. Other studies have shown that prophylactic use of probiotics containing Lactobacillus significantly reduce the risk of nosocomial diarrhoea in infants, especially nosocomial rotavirus gastroenteritis.

constipation-comAntibiotic-induced diarrhoea: Antibiotics play a vital role in treating bacterial infections but have the unfortunate side effect of also reducing healthy bacteria in our gut, altering the natural balance of the microflora, causing bloating and diarrhoea. A report published in the Journal of Pediatric Gastroenterology and Nutrition indicated that probiotics were helpful in the prevention or treatment of antibiotic-induced diarrhoea, especially in children.

Bacterial food poisoning: Most acute diarrhoea episodes are caused by viruses but some, especially those caught by ingesting infected food, can be bacterial. A study published in ‘Proceedings of the National Academy of Sciences’, found that probiotic bacteria can help protect against bacterial infections such as Listeria and the superbug Clostridium difficile infection. Some hospitals are giving people probiotics before going into hospital in order to reduce their risk of infection.

Chemotherapy-induced diarrhoea: Several chemotherapy agents commonly cause diarrhoea due to damage these agents inflict on the cells lining the gut. If severe, as well as being very distressing and uncomfortable for the patient, this can lead to dehydration, concentration of the chemotherapy in the bloodstream and, eventually, renal damage. A placebo-controlled randomised trial from Helsinki involving patients given 5FU chemotherapy to treat bowel cancer showed a highly significant reduction in the risk of diarrhoea among those given probiotics.

After chemotherapy: Even without diarrhoea, a course of intense chemotherapy can cause significant damage to the gut microbiome, leading to multiple problems including malabsorption, fatigue and inflammation. Many oncologists and nutritionists agree that restoring the gut flora with a high-quality probiotic is a good idea, and some research even recommends an autofaecal transplant! (see paper)

Radiotherapy bowel problems: Several well-conducted trials have demonstrated that probiotics can help prevent radiation bowel damage and aid recovery after radiotherapy – click here for an example of a published trial

inflammationIrritable bowel syndrome (IBS) is a condition associated with abdominal bloating and flatulence. A 2010 Cochrane review summarised the evidence from several clinical studies which showed that probiotics can help some sufferers of IBS. The National Institute for Health and Care Excellence (NICE) recommends taking them for at least four weeks at a dose recommended by the manufacturer, and if they work to take them continually.

Wheat and Lactose intolerance: As you get older, it’s common to develop an intolerance to gluten found in wheat and other grains. This causes IBS like symptoms with bloating and colicky pains. Reducing the intake of bread, pasta and cereals should always be the first measure taken, but a number of studies have demonstrated how probiotic intake can also be helpful. Intolerance to milk also is becoming more common, usually the result of a lactase deficiency. Lactase is an enzyme normally produced in your small intestine. It is needed to break down lactose and to enable you to digest milk. The probiotic Lactobacillus acidophilus helps with the digestion and absorption of lactose by producing lactase.

Potential benefits of probiotic supplements outside the gut

Hot flushes

Viral respiratory tract infections (colds and flu): A study from Australia gave healthy athletes either probiotics or a placebo. After one year, the probiotic group had fewer colds and, as a consequence, experienced less disruption to their training. Several studies have shown that regular intake of live lactobacilli and bifidobacteria, shortened the duration and severity of upper respiratory tract infections. In 2011, a summary of all international studies concluded that probiotics also reduce the incidence of upper respiratory tract infections.

Vitamin D levels:

Probiotics and gut health: As a fat soluble vitamin, the absorption of vitamin D is strongly linked to a healthy gut. A number of studies have linked the intake of a good quality probiotic supplement together with vitamin D supplements with higher blood levels of vitamin D3 [Jones]. Adequate vitamin D is important for a wide number of chronic disease including bone health and immunity – low levels have recently been linked to higher problems related to covid-19 [Jones]

Chronic inflammation: An overactive immune system leads to excess inflammatory cytokines which can cause numerous chronic diseases including arthritis, dementia, cancer and heart disease. The best anti-inflammatory diet is the macrobiotic diet, which features lots of fermented foods such as miso soup, kefir and kimchi. UK diets are traditionally low in these bacteria-rich foods.

Arthritis: People with arthritis often have suboptimal gut health. One study gave a probiotic capsule to people with arthritis for 8 weeks and found that several markers of joint oxidative stress decreased significantly (ref). In another study, patients who took two probiotic capsules a day for 12 months experienced a reduction in the number of tender and swollen joints, while also displaying reduced inflammatory markers such as  IL-1β (ref).

gut health rugbySports performance: Many athletes now take regular probiotic supplements. Alongside reducing the frequency and severity of “colds” avoids, mental alertness associated with a healthy gut can improve motivation to train. Reducing chronic inflammation can also help reduce joint pains and even damage associated with exercise. A study from Cork University has shown that elite rugby players have a much better gut profile than men in the general population at the same age.

Brain function: A study involving mice conducted by the Alimentary Pharmabiotic Centre at the University of Cork demonstrated how mice fed with Lactobacillus had significantly fewer stress, anxiety and depression-related behaviours than those fed with just broth. Moreover, bacteria-fed mice had lower levels of the stress-induced hormone corticosterone and had altered expression of receptors for the neurotransmitter GABA in the brain. In humans, a study from the University of California showed that healthy women consuming yoghurt containing probiotics were reported to have improved brain function and less environmentally induced markers of stress.

Bone health: Probiotic bacteria Screenshot 2019-03-24 at 22.32.18have been shown to reduce gut inflammatory markers, and there is a known link between chronic gut inflammation and bone health. A number of laboratory studies have indicated that just 4-6 weeks of probiotics significantly increase bone density in animals with established osteoporosis or prevented its development altogether [Chiang, Yacon, Ohlsson].

Blood Pressure: There is emerging evidence that probiotics may have a role in cardiovascular health. A meta-analysis of nine studies, published in the journal Hypertension, tracked over 543 adults with both normal and high blood pressure and found that, on average, taking probiotics lowered systolic blood pressure by 3.56 mmHg and diastolic blood pressure by 2.38 mmHg. The greatest effects were seen in subjects with BP above 130/85 and those who took probiotics with multiple strains were more likely to experience a lowering of BP than those taking single-strain sources.

Fatty liver and metabolic syndrome: Too much fat accumulating in the liver affects its function, eventually leading to damage. Several factors can cause a fatty liver, defined as fat stores >10% of its volume, including obesity, diabetes, alcohol abuse, poor diet and genetic susceptibility. Although there are no significant studies evaluating humans, researchers recently discovered that Lactobacillus and Bifidobacterium probiotic strains prevented the accumulation of fat in the liver of obese rats. Markers of inflammation (tumour necrosis factor and interleukin 6) and triglyceride counts also improved. In another animal study, mice were fed a high-fat diet for eight weeks and given either probiotics or a placebo. Mice who received the bacteria ate significantly less than the placebo group, had lower insulin levels and body fat, and reduced their fatty liver deposits. Trials in humans are planned.

Obesity: Obese patients usually have a poor microflora, which in turn promotes inflammation and storage of fats, leading to further weight gain. A number of studies have demonstrated that some probiotics taken before a meal reduce overall calorie intake [Hume].

Cholesterol levels: Research presented at the American Heart Association’s Scientific Sessions in 2012 reported that a formulation of Lactobacillus may be able to reduce blood levels of LDL (bad cholesterol).

Chronic Fatigue Syndrome: Scientists at the University of Cork reported in the journal Gut Microbes that Bifidobacterium may have benefits for patients with chronic fatigue syndrome.

Iron absorption: A trial from Gothenburg, Sweden published in 2015 showed that women taking a lactobacillus probiotic supplement were able to increase natural iron absorption from food [Hoppe]

Probiotics, gut health and cancer

While it is clear that more research is needed, the evidence so far strongly suggests that a diverse bacterial microbiome can protect us from cancer by:

  • Outcompeting (displacing) cancer-causing pathogens
  • Reducing inflammation
  • Priming immune responses

There is growing evidence that the immune responses to gut bacteria seem to help stimulate immune responses against cancer. In addition, through molecular cues such as cytokine IL-10, beneficial gut bacteria can counteract inflammation and strengthen anticancer T-cell responses not only in the gut but throughout the body.

Preliminary experiments in animals have found that stimulated growth of bifidobacteria in the colon leads to the inhibition of colon carcinogenesis. It has been suggested that this is due to the pH-lowering effect of bifidobacteria in the colon, which subsequently inhibits the growth of E. coli and clostridia. A decrease in the growth of pathogenic microorganisms also modulates bacterial enzymes that convert pro-carcinogens to carcinogens. In humans, taking regular probiotics has been linked to a lower rate of new polyp formation and bowel cancer relapse.

Healthy gut bacteria also produce butyrate, an acid which exerts a direct effect on cancer cells by turning off genes involved in cell growth and turning on genes that trigger cell suicide (apoptosis). Early research has shown that butyrate can block the growth of colon cancer cells in vitro, and experiments in mice suggest that dietary intake of probiotics and/or prebiotics with soluble fibre may increase butyrate production and slow tumour development. Besides fibre, gut bacteria can use other dietary sources, particularly polyphenols, to produce a range of cancer-preventive chemicals. These include:

  • anti-inflammatory urolithins, produced from ellagic acid found in pomegranates.
  • antioxidant equol, produced from daidzein found in soy-based foods.
  • anti-inflammatory and cancer-blocking isothiocyanate, found in broccoli.
  • anti-inflammatory conjugated linoleic acid, produced from linoleum acid, a component of vegetable oils.

A fibre and polyphenolrich diet can increase the abundance of cancer-preventive, butyrate-producing bacteria in the gut. High-protein, low-carbohydrate diets, on the other hand, may change fermentation in the intestine, leading to increased levels of harmful nitrosamines, and decreased levels of protective molecules like butyrate and phenols. Other research suggests that a high-fat diet can change the microbiome in a way that increases bacterial production of the bile acid DCA, which promotes colon and oesophageal cancers.

The microbiome and response to cancer treatments

white-label-60 front


Laboratory and clinical studies are revealing a fascinating role for gut bacteria in relation to cancer treatments. Scientists have recently discovered that particular species of gut bacteria increase the therapeutic benefit of CTLA-4 blockade (Bacteroides) in melanoma, while others enhance the effect of PD-1 blockade (Clostridiales) and PD-L1 blockade (Bifidobacterium). These findings suggest that individual differences in microbiome composition may be one of the reasons these immunotherapies work better for some people than others.

Scientists have a theory to explain the beneficial effects of gut bacteria during cancer treatment. Chemotherapy (cyclophosphamide and platinum salts) and immunotherapy can damage the intestinal lining, allowing certain types of gut bacteria and bacterial byproducts to leak into the blood circulation. Once they reach lymph nodes and lymphoid organs, these bacteria prime Th1 and Th17 T cells, which help recruit other immune cells to the tumour.

Microbiome research is a young, but fast-growing field. It is clear from animal and early clinical studies that the effect of the microbiome on cancer should not be discounted. While there are still many unanswered questions, in the not so distant future, cancer care will likely include an analysis of the patient’s microbiome at diagnosis to inform personalised treatment planning. It may also be possible to manipulate the microbiome to optimise treatment outcomes.


  • Bultman SJ: The microbiome and its potential as a cancer preventive intervention. Semin Oncol 43:97-106, 2016
  • Schwabe RF: The microbiome and cancer. Nat Rev Cancer 13:800-12, 2013
  • Viaud S. Gut microbiome and anticancer immune response: really hot Sh*t! Cell Death Differ 22:199-214, 2015
  • Shahanavaj K: Cancer and the microbiome: . Expert Rev Anticancer Ther 15:317-30, 2015.
  • Russo E: The interplay between the microbiome and immune response in cancer development. Therapy Adv Gastroenterol 9:594-605, 2016.
  • Pevsner-Fischer M: Role of the microbiome in non-gastrointestinal cancers. World J Clin Oncol 7:200-13, 2016
  • Poutahidis T: Gut microbiota and the paradox of cancer immunotherapy. Front Immunol 5:157, 2014
  • Zitvogel L et al: Microbiome and Anticancer Immunosurveillance. Cell 165:276-87, 2016
  • Gopalakrishnan V:  Diversity and composition of the gut microbiome with differential responses to PD-1 therapy in patients with metastatic melanoma. J Clin Oncol 35, 2017
  • Vetizou M : Anticancer immunotherapy by CTLA-4 blockade relies on the gut microbiota. Science 350:1079-84, 2015
  • Sivan A: Commensal Bifidobacterium promotes antitumor immunity and facilitates anti-PD-L1. Science 350:1084-9, 2015
  • Pitt JM: Fine-Tuning Cancer Immunotherapy: Optimizing the Gut Microbiome. Cancer Res 76:4602-7, 2016
  • Prevention of irinotecan-induced diarrhoea by probiotics: RCT  study. J Clin Oncology. Mego M et al 32:5s, 2014 (s 9611).
  • Hoppe M et al Probiotics increases iron absoption from fruit: a doubleblind cross over trial. British Journal of Nutrition 114,1195-1202
  • Hume M et al Prebiotic supplementation improves appetite control in children with obesity: a RCT. Am J Clin Nutrition 2017. 22, 2017, doi: 10.3945/ajcn.116.140947
  • Abrams SA et al Effect of prebiotic supplementation and calcium intake on body mass index. J Pediatr. 2007;151(3):293-8.
  • Health claims substantiation for probiotic and prebiotic products. Sanders M et al. 2011, Gut Microbes, 2 (3), 127-133.
  • Safety assessment of probiotics for human use. Sanders M, Akkermans L et al 2010 Gut Microbes, 1 (3) pp 164-185.
  • Probiotics: From Bench to Market. Annals of the New York Academy of Sciences (2010) Klein M, Sanders M, Duong T DOI: 10.1111/j.1749-6632.2010.05839.x
  • Guide to designing, conducting, studies involving probiotics. (2010) Gut Microbes Shane A, Michael D. Cabana M et al, Vol 1 (4) pp 243-253.
  • Food Formats for Effective Delivery of Probiotics. Annual Review of Food Science and Technology. 2009 Saunders M and Marco M. Vol. 1: 65-85.
  • How Do We Know When Something Called “Probiotic” Is Really a Probiotic? A Guideline for Consumers and Health Care Professionals (2009).
  • Saunders M: Probiotics: Their potential benefit to human health. J of the council for ag science and tech 2011  Oct 2011.
  • Probiotics in the management of chemo-induced diarrhoea. JPEN J Parenter Enteral Nutr 2009. 33 (5) 569-70.
  • Chiang S et al. Antiosteoporotic effects of Lactobacillus -fermented soy skim milk on bone mineral density. J Agric Food Chem. 2011, 27; 59(14):7734-42
  • Yacon F et al Bifidobacterium longum modulate bone health in rats. J Med Food. 2012; 15(7):664-70.
  • Ohlsson C et al Probiotics protect mice from ovariectomy-induced cortical bone loss. PLoS One. 2014; 17; 9(3):e92368.
  • Reid G et al. New scientific paradigms for probiotics and prebiotics. J Clin Gastroenterol. 2003; 37(2):105-18.
  • Delia et al Probiotics and radiotherapy bowel damage. World Journal of Gastroenterology. 2007; 13(6), 912
  • Jones ML, Martoni CJ, Prakash S. Oral supplementation with lactobacillus probiotic increases mean circulating 25-hydroxyvitamin D: a randomized controlled trial. J Clin Endocrinol Metab. 2013;98:2944–2951. doi: 10.1210/jc.2012-4262. [PubMed] [CrossRef] [Google Scholar]