Probiotics: Benefits, Dosage, and Safety

What probiotics is

Probiotics are live microorganisms that, when administered in adequate amounts, confer a health benefit on the host (definition of the International Scientific Association for Probiotics and Prebiotics). They consist mainly of bacteria but also include yeasts, and are found in some fermented foods, added to certain food products, and sold as dietary supplements. Not all products labeled as probiotics have proven health benefits. Probiotic products commonly contain organisms from the genera Bifidobacterium, Saccharomyces, Streptococcus, Enterococcus, Escherichia, and Bacillus, and from the family Lactobacillaceae (which includes many species formerly in the Lactobacillus genus before its 2020 restructuring). Each probiotic is identified by genus, species, subspecies where applicable, and an alphanumeric strain designation, and effects can be specific to a particular strain. Probiotics differ from prebiotics (complex carbohydrates that gut microbes use as fuel), synbiotics (products with both), and postbiotics (preparations of dead or fragmented microbes).

What it does

• May inhibit the growth of pathogenic microorganisms in the gastrointestinal tract by fostering colonization resistance, improving intestinal transit, producing antimicrobial substances, or helping normalize a disturbed microbiota
• May produce bioactive metabolites such as short-chain fatty acids and reduce luminal pH in the colon
• Species-specific roles can include vitamin synthesis, gut barrier reinforcement, bile salt metabolism, enzymatic activity, and toxin neutralization
• Strain-specific roles can include cytokine production and immunomodulatory activity
• May transiently colonize the gut mucosa in highly individualized patterns depending on the person's baseline microbiota, the strain, and the region of the gastrointestinal tract

Intake and dosing

Probiotics is not an essential nutrient, so no Recommended Dietary Allowance or Tolerable Upper Intake Level has been set. Doses in studies vary widely. Because supplement doses are not standardised, discuss an appropriate amount with a clinician or pharmacist.

Deficiency and who is at risk

Not applicable. Probiotics are not an essential nutrient, so there is no deficiency state or deficiency disease. The body does not require dietary probiotics to function, and ODS does not describe a probiotic deficiency.

Who may need more

• No formal recommendations exist for or against probiotic use in healthy people
• People taking antibiotics who are at risk of antibiotic-associated diarrhea (evidence supports LGG or Saccharomyces boulardii started within 2 days of the first antibiotic dose in children and adults age 18 to 64)
• Pediatric patients with acute infectious gastroenteritis, per some expert groups (ESPGHAN), though the American Gastroenterological Association recommends against use in the US and Canada
• People with mild-to-moderate ulcerative colitis, where some evidence suggests modest benefit as an adjunct to conventional therapy
• Adults with irritable bowel syndrome, where probiotics may modestly reduce some symptoms
• Preterm, low-birth-weight infants in hospital settings to reduce necrotizing enterocolitis risk, under specialist care only, given FDA safety concerns

Food sources

Fermented foods such as yogurt (made with Lactobacillus bulgaricus and Streptococcus thermophilus, and sometimes containing Bifidobacterium or Lactobacillaceae). Some fermented foods contain live cultures but not proven probiotic microorganisms, including many cheeses, kimchi, kombucha, sauerkraut, miso, pickles, and raw unfiltered apple cider vinegar. Certain unfermented foods such as milks, juices, smoothies, cereals, nutrition bars, and infant and toddler formulas may have probiotics added and labeled with strain and viable-cell count. Some fermented foods (sourdough bread, most commercial pickles) are processed after fermentation and contain no live cultures when eaten.

Forms and absorption

Available as dietary supplements in capsules, powders, liquids, and other forms, containing single or multiple strains at a wide range of doses. Potency is expressed in colony-forming units (CFU), for example 1 x 10^9 for 1 billion CFU. Because probiotics must be alive to have benefits and can die during shelf life, look for products labeled with the CFU count at the end of shelf life (expiration or use-by date), not at time of manufacture. Current US labeling regulations only require the total weight of microorganisms (live plus dead) on the Supplement Facts label; manufacturers may voluntarily list CFU. Higher CFU counts are not necessarily more effective. Storage varies by product, with some requiring refrigeration and others stable at room temperature.

Safety, excess and interactions

Too much

There is no established Tolerable Upper Intake Level. In healthy people probiotics are unlikely to cause harm and side effects are usually minor, consisting of self-limited gastrointestinal symptoms such as gas. The main serious safety concern is not classic overdose toxicity but infection: probiotic use has been linked to bacteremia, fungemia (fungi in the blood), and infections resulting in severe illness, mostly in people who were severely ill or immunocompromised. At least 60 reports since 1966 describe fungemia associated with Saccharomyces cerevisiae-containing probiotics, often in ICU patients with central venous catheters, enteral or parenteral nutrition, or broad-spectrum antibiotics. A retrospective analysis of ICU patients found those given LGG had a higher risk of Lactobacillus bacteremia.

Interactions to know

• Antibiotics: antibiotics disrupt the gut microbiome and can also kill probiotic organisms; when probiotics are used to prevent antibiotic-associated diarrhea, starting them within 2 days of the first antibiotic dose is more effective than starting later (ODS)
• Immunosuppressive therapy and serious illness: in severely ill or immunocompromised people, probiotics have been linked to bacteremia and fungemia; the World Gastroenterology Organisation advises restricting use to strains and indications with proven efficacy in these patients (ODS)
• 5-ASA (mesalamine) in ulcerative colitis: some evidence suggests combining certain probiotics with 5-ASA may be superior to 5-ASA alone for inducing remission, though the evidence is limited and of low certainty (ODS)

How status is measured

Not routinely measured. There is no standard blood or laboratory marker of probiotic status. Products are quantified by colony-forming units (CFU) on the label rather than by a clinical test of the person.

Regulated by the FDA as dietary supplements (or as food ingredients when added to foods) under DSHEA, not as drugs, so they do not undergo FDA premarket review for safety, effectiveness, or quality for medical uses. FDA labeling guidance (2018) requires only the total weight of microorganisms on the Supplement Facts label; CFU listing is voluntary. In 2023 the FDA issued a news release warning that administering probiotics to preterm infants may cause infection or invasive, potentially fatal disease, noting probiotics used this way have not been approved by the agency.

What the evidence supports

We grade the strength of evidence behind each common reason people take probiotics, mirroring how the NIH describes it.