What should you do to support your gut with probiotics and prebiotics?
Eat a variety of fermented foods daily (yogurt, kefir, kimchi, sauerkraut) and prebiotic-rich foods (garlic, onions, asparagus, bananas, oats). Use specific probiotic supplements only for targeted conditions with evidence-matched strains.
For general gut health, the Stanford-led 2021 study in Cell found that a high-fermented-food diet (6+ servings daily) increased microbiome diversity and decreased inflammatory markers more effectively than a high-fiber diet alone over 10 weeks. Even 1-2 servings of fermented foods daily provides beneficial bacteria in a natural food matrix.
For prebiotic support, aim for 25-35g of total dietary fiber daily from varied sources. Specifically prebiotic fibers include inulin (garlic, onions, leeks, asparagus), fructo-oligosaccharides (bananas, artichokes), galacto-oligosaccharides (legumes), and beta-glucan (oats, barley). These fibers selectively feed beneficial bacteria like Bifidobacteria and Lactobacilli, promoting a healthier gut ecosystem.
What are probiotics and how do they work?
Probiotics are live microorganisms that, when consumed in adequate amounts, confer a health benefit. They work by competing with harmful bacteria, producing antimicrobial substances, strengthening the intestinal barrier, and modulating immune responses.
The term 'probiotic' covers a wide range of bacteria and yeasts, each with different properties. The most common genera are Lactobacillus, Bifidobacterium, and Saccharomyces. Within these genera, specific species and strains have vastly different effects — Lactobacillus rhamnosus GG has robust evidence for diarrhea prevention, while other Lactobacillus strains may have no effect (Source: World Gastroenterology Organisation Guidelines).
Most ingested probiotics are transient — they pass through the gut without permanently colonizing. Their benefits come from temporary activity during transit: competing with pathogens for binding sites, producing short-chain fatty acids and antimicrobial compounds, stimulating mucus production that strengthens the gut barrier, and interacting with immune cells in the gut-associated lymphoid tissue. Effects typically cease within 1-3 weeks of stopping supplementation.
Which probiotic strains have the best evidence?
The strongest evidence exists for Lactobacillus rhamnosus GG (antibiotic-associated diarrhea), Saccharomyces boulardii (C. difficile prevention), Bifidobacterium infantis 35624 (IBS), and VSL#3 (ulcerative colitis maintenance).
For antibiotic-associated diarrhea (AAD), a Cochrane review of 33 RCTs found that Lactobacillus rhamnosus GG and Saccharomyces boulardii both significantly reduced diarrhea incidence — by approximately 50%. The NNT (number needed to treat) was 13, meaning 1 in 13 people taking these probiotics during antibiotics avoided diarrhea. Start the probiotic on the first day of antibiotics and continue for 1-2 weeks after completion.
For IBS, Bifidobacterium infantis 35624 (Align) has the most consistent evidence, showing significant improvement in global IBS symptoms, bloating, and abdominal pain in multiple RCTs. For inflammatory bowel disease, VSL#3 (a multi-strain high-dose formulation) is evidence-based for maintaining remission in ulcerative colitis. For traveler's diarrhea prevention, Saccharomyces boulardii shows modest benefit.
- L. rhamnosus GG — antibiotic-associated diarrhea prevention (Cochrane-reviewed)
- S. boulardii — C. difficile prevention and traveler's diarrhea
- B. infantis 35624 — IBS symptom relief (bloating, pain)
- VSL#3 — ulcerative colitis remission maintenance
- L. reuteri DSM 17938 — infant colic (3+ RCTs show benefit)
- B. lactis BB-12 — general immune support (moderate evidence)
What are prebiotics and which foods contain them?
Prebiotics are non-digestible fibers that selectively feed beneficial gut bacteria. The best food sources are garlic, onions, leeks, asparagus, bananas, Jerusalem artichokes, chicory root, oats, and legumes.
When beneficial bacteria ferment prebiotic fibers, they produce short-chain fatty acids (SCFAs) — particularly butyrate, propionate, and acetate. Butyrate is the primary energy source for colon cells, maintains the gut barrier, reduces inflammation, and may have anti-cancer properties. Higher SCFA production is consistently associated with better gut and metabolic health.
Prebiotic fiber types include inulin (chicory root, garlic, onions — 5-10g/day recommended), fructo-oligosaccharides (bananas, asparagus, artichokes), galacto-oligosaccharides (legumes, beans), and resistant starch (cooled potatoes and rice, green bananas, oats). Starting with large prebiotic doses can cause bloating and gas — increase gradually over 2-3 weeks to allow your microbiome to adapt.
How do you choose a quality probiotic supplement?
Look for: (1) a specific strain designation (not just genus/species), (2) a CFU count guaranteed through expiration date, (3) third-party testing, and (4) evidence supporting that exact strain for your condition.
The probiotic supplement industry is poorly regulated. Many products don't contain what the label claims — a 2016 analysis found that only 2 of 16 tested products matched their label claims. Choose products that guarantee CFU count at expiration (not at manufacturing), use specific strain designations (e.g., L. rhamnosus GG, not just 'Lactobacillus'), and have third-party verification (USP, ConsumerLab, or NSF International).
More CFUs is not necessarily better. The effective dose depends on the strain and condition — Align (B. infantis 35624) is effective at 1 billion CFU, while VSL#3 requires 450-900 billion CFU. Using a strain at a dose not supported by evidence provides no advantage. Refrigeration requirements also vary — some strains require cold storage, others are shelf-stable. Always check the manufacturer's storage instructions.
Who should avoid probiotics?
People who are severely immunocompromised, have central venous catheters, short bowel syndrome, or are critically ill should avoid probiotic supplements unless directed by a physician. Otherwise, probiotics are generally safe for most adults.
While probiotics are safe for the vast majority of people, rare cases of probiotic bacteremia (bacteria entering the bloodstream) have been reported in severely immunocompromised patients, including those with HIV/AIDS, organ transplant recipients on immunosuppression, cancer patients on chemotherapy, and premature infants. In these populations, the risk-benefit balance should be evaluated individually by a physician.
For healthy adults, the main risks are minor digestive side effects (bloating, gas) that resolve within 1-2 weeks. Pregnant and breastfeeding women can safely take probiotics — a meta-analysis found no adverse effects. Children can benefit from specific strains (L. reuteri for colic, L. rhamnosus GG for diarrhea), though pediatrician guidance is recommended for children under 1 year.
