Probiotics

What probiotics are (and aren't)

Probiotics are live microorganisms that, when consumed in adequate amounts, produce a measurable health benefit. The WHO established this definition in 2001, and it contains two points worth remembering: the organisms must be alive at the time you swallow them, and the benefit must be proven for a specific strain at a specific dose. Lactobacillus rhamnosus GG and Lactobacillus rhamnosus HN001 belong to the same species, yet their clinical evidence is completely different. Strain identity matters more than the name on the label.

How probiotics work in the gut

Probiotics don't simply "add good bacteria." They act through several distinct mechanisms. They compete with harmful microbes for binding sites on the intestinal wall (a process called competitive exclusion), produce antimicrobial compounds like bacteriocins and organic acids, and strengthen the gut barrier by stimulating mucin production and tight junction proteins ^[1]. Beyond the gut itself, probiotics shape systemic immunity. They boost secretory IgA, help regulate the balance between pro-inflammatory Th17 cells and anti-inflammatory regulatory T cells, and shift cytokine profiles toward lower chronic inflammation ^[2]. A 2023 meta-analysis confirmed that probiotics significantly reduce circulating CRP, IL-6, and TNF-alpha, all markers tied to accelerated biological aging ^[3].

Strains with the strongest evidence

Lactobacillus rhamnosus GG (LGG) is the most studied probiotic strain in the world. Clinical trials support its use for preventing antibiotic-associated diarrhea, reducing acute infectious diarrhea duration in children, and lowering atopic dermatitis risk ^[4]. Saccharomyces boulardii, a probiotic yeast unaffected by antibiotics, has strong evidence against Clostridioides difficile infection and traveler's diarrhea. Bifidobacterium infantis 35624 has shown significant IBS symptom relief across all subtypes in multiple randomized controlled trials. VSL#3, a high-dose multi-strain mix, has been studied for ulcerative colitis maintenance. For cholesterol, a meta-analysis found that Lactobacillus acidophilus and L. plantarum reduced total cholesterol by about 7.8 mg/dL and LDL by 7.3 mg/dL over 3 to 12 weeks ^[4].

Probiotics, aging, and the microbiome

The gut microbiome loses diversity with age. Beneficial genera like Bifidobacterium and Faecalibacterium decline, while potentially harmful taxa increase, fueling the chronic low-grade inflammation known as "inflammaging" ^[5]. But this trajectory isn't fixed. Studies of centenarians consistently show higher microbial diversity and elevated levels of butyrate-producing bacteria compared to younger elderly populations ^[6]. Probiotics, especially when combined with prebiotic fiber and fermented foods, can partially restore this diversity. The 2021 Stanford fermented foods trial found that 6 daily servings of yogurt, kefir, kimchi, sauerkraut, and kombucha significantly increased microbial diversity and reduced 19 inflammatory markers, including IL-6, over just 10 weeks ^[7].

Dosing and practical guidance

Effective probiotic doses range widely. Some Bifidobacterium strains work at 1 to 5 billion CFU per day, while certain Lactobacillus strains need 10 to 20 billion CFU. For preventing antibiotic-associated diarrhea, ESPGHAN recommends at least 5 billion CFU/day of LGG or S. boulardii, started within 2 days of the first antibiotic dose ^[4]. Always look for products that name specific strains (not just species), guarantee CFU at expiration (not at manufacture), and cite clinical evidence for your particular concern. Take probiotics with or just before a meal containing some fat, which buffers stomach acid and improves bacterial survival. Allow 2 to 4 weeks for digestive effects and 8 to 12 weeks for immune or mood-related benefits.

Safety

Common probiotic genera (Lactobacillus, Bifidobacterium, Saccharomyces) have an excellent safety record in healthy people. Side effects are typically limited to temporary bloating or gas during the first few days. However, probiotics should be used cautiously in severely immunocompromised patients, those with central venous catheters, or critically ill individuals, where rare cases of bacteremia and fungemia have been reported ^[4].