Probiotics are healthy bacteria reputed for the positive effect they have on the digestive system. In addition to occurring naturally within our bodies, these microorganisms are constantly marketed to us in everything from yogurts and milk to vitamin supplements and breakfast biscuits. While the positive effects probiotics can have on individuals with metabolic and gastrointestinal diseases are quite clear, according to a recent study, it’s less apparent if they are of any benefit to healthy adults.

For the study, now published online in the journal Genome Medicine, researchers from the Novo Nordisk Foundation Center for Basic Metabolic Research at the University of Copenhagen conducted a systematic review of seven randomized control trials investigating the effect of probiotic products on the digestive health of healthy adults.

Inside our gut and digestive tract are microbes that aid digestion by breaking down food and synthesizing certain vitamins and amino acids. Previous research has shown that certain conditions, such as diabetes and obesity, are marked by an uneven composition of healthy gut bacteria. Purposely adding healthy bacteria to one's diet, in the form of probiotics, has a measurable effect on these individuals as seen by either body mass index or insulin resistance. However, it is much more difficult to measure the effect that probiotics have on those without any pre-existing health conditions. This study attempted to measure this effect for the first time.

The researchers reviewed the seven previous studies, which consisted of healthy volunteers, between the ages of 19 and 88 years old, who consumed probiotics in the form of biscuits, milk-based drinks, sachets, or capsules for a period between 21 and 42 days. For the studies, the volunteers then had their feces analyzed to determine the overall structure of the fecal microbiota. This reveals the number of bacteria species present, the evenness of the bacteria populations, and whether the probiotic groups, as a result of their intervention, had different changes in the bacteria living in their gut when compared to the placebo groups.

"A systematic review of experimental evidence allows us to pull together evidence and look at the relationship between probiotic products and the composition of the fecal microbiota in healthy people using explicit, systematic methods, ensuring the highest level of evidence," junior study author Nadja Buus Kristensen said in a recent statement.

Results showed that out of the seven randomized controlled trials, only one showed greater changes in the microbiota composition of the group who had taken the probiotics when compared against individuals in  the control group who had not. In an email to Medical Daily, senior author Dr. Oluf Perdersen said, “In other words, the scientific evidence for an effect of probiotics in healthy adults is non-existent.”

While past research has suggested that probiotics can have a positive effect on healthy adults, the researchers suggest that factors, such as small sample sizes and variations in different probiotic strains, may have masked the true impact of probiotic intake.

While the results alone should not be enough to discourage you from enjoying your favorite probiotic yogurt, they do suggest that questioning the health claims made by manufacturers of products including probiotics may be in order.

“This lack of health benefits are in sharp contrast to the rapidly expanding global market for probiotics, which in 2015 was estimated to [be $33 billion] and in 2020 is projected to amount [to $46 billion],” Perdersen told Medical Daily.

According to Perderson, the results suggest that the probiotics industry might want to consider investing in further long-term trials to improve accuracy and support its current claims.

Source: Kristensen NB, Bryup T, Allin KH, Nielson T, Hansen TH, Perderson O. Alterations in fecal microbiota composition by probiotic supplementation in healthy adults: a systematic review of randomized controlled trials. Genome Medicine. 2016