Probiotics and health: a review of the evidence
Authors: Weichselbaum E. 
Published: November 2009

Probiotics are live microorganisms – mainly bacteria – which when administered in adequate amounts confer a health benefit on the host. There is rising interest in this area, but reports in the media are often conflicting. The aim of this review is to consider the current evidence on the effects of probiotics on health, focusing on gut‐related health issues and the immune system, with the objective to provide a clearer picture of whether and how probiotics can be beneficial for health. The outcomes of this review are based on more than 100 original studies, meta‐analyses and systematic reviews. A variety of different strains have been used in studies on probiotics, and it is important to remember that the effectiveness of probiotics is strain‐specific, which means that each single probiotic strain has to be tested to assess its potential health benefits.

Overall, despite the diversity of strains used in the studies included in this review, there is evidence that probiotics have the potential to be beneficial for our health. Studies in patients with inflammatory bowel disease show probiotic strains to be able to decrease the recurrence of ulcerative colitis and occurrence and recurrence of pouchitis, however, current evidence suggests that probiotics are ineffective in treating patients with Crohn's disease. Patients with irritable bowel syndrome show a reduction in symptoms when treated with selected probiotic strains, but high placebo effects have been reported as well. The evidence of the efficacy of probiotics in patients suffering from constipation is limited, but the evidence seems promising for some strains to bring relief to patients suffering from constipation.

There is good evidence that a number of probiotic strains are effective in preventing antibiotic‐associated diarrhoea. The most commonly studied strains are Lactobacillus rhamnosus GG (LGG) and Saccharomyces boulardii, but other strains and mixtures of strains seem to be effective as well. There is also promising evidence of a preventive effect of probiotics in Clostridium difficile‐associated diarrhoea, although some studies have been too small to obtain statistically significant findings. The effect of probiotics in acute diarrhoea, particularly in children, is well studied. Selected probiotic strains seem to be effective in reducing the duration of acute diarrhoea. LGG and S. boulardii are again the most commonly used strains and a number of studies have shown them to be effective, although one meta‐analysis showed that the effect of LGG was only significant in children in Western countries, not in children in developing countries, which may be due to different causes of diarrhoea in these regions. Studies investigating the preventive effect of probiotics in the context of common cold and flu infections show that the studied strains failed to lower the incidence of episodes but that they have the potential to decrease the duration of episodes, which suggests that the immune system may be more efficient in fighting off common cold and flu infections after consuming these strains. The evidence so far does not suggest that probiotics are effective in preventing or treating allergies or in treating eczema. However, some probiotic strains seem to lower the risk of developing eczema if taken by pregnant women and their infants in early life.