Writing in Nature journal, the research team found evidence for probiotics’ role in improving stool consistency, bowel movement, and vaginal lactobacilli concentration in healthy adults.
However, the short-term improvement shown implies that supplementation with probiotics may need to be an ongoing process in order to maintain the gut microbiota changes.
“Gut microbiota is sensitive to multiple factors, such as lifestyle, aging, and disease,” the study authors said
“Even in apparently healthy individuals, changes in diet quality and alcohol intake can significantly affect gut symbiosis.
“A diet poor in fruit and vegetable intake (as a good source of prebiotics) may not provide the food required for probiotic survival and maintenance. This may explain the constant need for probiotic food and supplements to maintain gut symbiosis and health.”
The review comes as demand for probiotic food and supplements has increased over the past few decades. Globally, the probiotic market is expected to see up to 8% compound annual growth rates up to 2020.
While research has demonstrated positive effects of probiotic consumption on several health outcomes, the majority of the published literature is in populations with underlying conditions.
Evidence supporting the health-promoting effects of probiotics in healthy adults was considered limited and less consistent.
Despite this, probiotic manufacturers promote the use of their product to a broader consumer market than those with specific health conditions.
The review questions whether probiotic supplementation conveys benefit in healthy individuals as the team, led by Dr Saman Khalesi from Central Queensland University in Australia, included 45 studies to reach their conclusions.
Studies were included if they featured experimental trials, included healthy adults, aged 18+ years and used probiotics.
Healthy adults were defined as individuals with no reported status of the chronic or acute diseases, including cardiovascular disease (CVD), obesity (body mass index (BMI) over 30 kg/m2), liver disease, diabetes, chronic GI problems, autoimmune disease, cancer, psychological disorder, etc.
The team found that probiotic strains of the Bifidobacterium and Lactobacillus species such as B. animalis lactic, L. acidophilus johnsonii and L. casei Shirota were resistant to low pH environments, appearing to have good survival rates.
In addition, L. acidophilus, B. longum, and B. infantis demonstrated good resistance to bile salts.
While encapsulation and microencapsulation manufacturing techniques delivered probiotics by better preserving their viability, the research team said that they did not guarantee colonisation of the probiotic bacteria in the intestine.
Despite scepticism about the influence of probiotics on the gut microbiota of healthy adults, probiotics have proven beneficial effects when dysbiosis exists.
Aging is also associated with a relative dysbiosis in gut microbiota. Reductions in bifidobacteria count, diversity, and an increase in pathogenic bacteria are observed in the elderly.
“Overall, it seems that probiotic supplementation in healthy adults can lead to an increase in the colonization of specific probiotic strains,” the review said.
“However, this increase may be transient and return to baseline after supplementation stops.”
Mechanisms of action
Regarding immune function, the team found probiotic supplementation in healthy adults could improve immune function and the immune response to common cold infections.
However, the immune response to influenza infection and the effective duration, dose, and type of probiotics supplementation required further investigation.
The team also thought fermentation of non-digestible carbohydrates and production of SCFA could be potential mechanisms that probiotic supplementation follow in improving stool consistency, bowel movement, and reducing irritation caused by abdominal bloating.
“Although this review suggests that healthy adults (and in particular, older adults) may achieve some health benefits from the consistent use of probiotics, probiotic supplementation may have a similar fate to multivitamins. That is, they may be effective in specific cases or conditions,” the review concluded.
“Until further research is conducted, it would be prudent to advise consumers that supplementation with probiotics may be more effective in specific population groups and those with underlying pathologies.”
Source: Nature (European journal of clinical nutrition)
Published online ahead of print: doi:10.1038/s41430-018-0135-9
“A review of probiotic supplementation in healthy adults: helpful or hype?”
Authors: Saman Khalesi, Nick Bellissimo, Corneel Vandelanotte, Susan Williams, Dragana Stanley & Christopher Irwin