Probiotics

Probiotics are live microorganisms that, when administered in adequate amounts, confer a health benefit on the host. ^{[1] [2]} Lactobacillus and Bifidobacterium species are the most commonly used probiotics, with other genera including Bacillus, Streptococcus, Enterococcus, and Saccharomyces also used. ^[3] The word probiotic derives from Latin and means 'for life', and fermented products such as kefir and yoghurt were used for nutritional and therapeutic purposes long before probiotic microorganisms were understood. ^[4]

Probiotic administration has been linked to a multitude of health benefits, with gut and immune health being the most researched applications, and roughly 70% of the immune system is located in the gut. ^[1] A systematic review and meta-analysis of 26 randomized controlled trials in 1891 participants found that probiotics significantly improved gut barrier function and reduced inflammatory factors including CRP, TNF-alpha, and IL-6, while boosting enrichment of Bifidobacterium and Lactobacillus. ^[5] Despite the existence of shared core mechanisms for probiotic function, the health benefits of probiotics are strain- and dose-dependent. ^[1] Through the microbiota-gut-brain axis, probiotic bacteria have been implicated in the regulation of anxiety, mood, cognition, and pain. ^[6]

A meta-analysis found significant therapeutic efficacy for the yeast probiotic Saccharomyces boulardii in the prevention of antibiotic-associated diarrhea (relative risk 0.47), and it can be strongly recommended in adults for preventing antibiotic-associated and traveler's diarrhea. ^[7] A European pediatric working group recommends Lactobacillus rhamnosus GG or Saccharomyces boulardii for preventing antibiotic-associated diarrhea in children when risk factors are present. ^[8] A systematic review and network meta-analysis of irritable bowel syndrome found probiotics improved symptoms with a favorable odds ratio and no serious adverse events, with Bifidobacterium and Lactobacillus being the dominant effective strains. ^[9] A meta-analysis of 23 randomized controlled trials in 1401 patients found that probiotics produced a significant reduction in depression symptoms and a moderate reduction in anxiety symptoms in clinically diagnosed populations. ^[10] Not all indications are supported: the Infectious Diseases Society of America does not recommend the use of probiotics for prevention of Clostridioides difficile infection. ^[11]

Probiotics can be used in the prevention and treatment of conditions spanning digestive disorders, diabetes, obesity, inflammatory, cardiovascular, respiratory, and central nervous system diseases. ^[3] Products that contain probiotics must include the genus, species, and strain of each live microorganism on the label, as well as the total estimated quantity of each strain at the end of shelf life measured in colony forming units. ^[1] Targeting the gut microbiome, including through probiotics, is emerging as a promising avenue to prevent, alleviate, and ameliorate aging-related disorders, though this remains an emerging framework rather than established anti-aging therapy. ^[12]