The question of whether probiotics can prevent the “stomach flu” is common, driven by the desire to avoid the unpleasant symptoms of this highly contagious illness. The term “stomach flu” is misleading; it refers to viral gastroenteritis, an infection of the intestines characterized by watery diarrhea, vomiting, and abdominal cramps. People often look to probiotics—live microorganisms that confer a health benefit on the host—as a preventive measure against these viral invaders. Answering this requires understanding the nature of the infection and how these beneficial microbes interact with the digestive system.
Defining Gastroenteritis and Probiotics
Viral gastroenteritis is an acute infection of the intestines, caused most frequently by norovirus in adults and rotavirus in children. These viruses are highly contagious and spread through close contact, contaminated food, or water, leading to inflammation and disruption of the gut lining. The goal of prevention is to stop the viral pathogen from successfully infecting the intestinal cells.
Probiotics are live, non-pathogenic microorganisms that, when consumed in adequate amounts, provide a positive health effect. These beneficial microbes, often from the Lactobacillus and Bifidobacterium genera, balance and diversify the gut microbiota. Probiotics support the gut’s defenses on an ongoing basis, contrasting with the acute nature of gastroenteritis.
How Probiotics Support Gut Barrier Protection
Probiotics contribute to the body’s defense mechanisms by strengthening the physical and immune barrier of the gut lining. One mechanism is competitive exclusion, where beneficial bacteria physically compete with harmful pathogens for adhesion sites and nutrients within the intestinal tract. By adhering to the intestinal mucosa, probiotics make it difficult for invading pathogens to colonize.
These beneficial microbes also enhance the integrity of the intestinal barrier, which acts as a physical shield against pathogens. They stimulate the production of mucin and regulate the expression of tight junction proteins like occludin and claudin. Strengthening these seals between intestinal cells prevents viruses from passing through the lining into the body.
Probiotics also produce antimicrobial substances that directly inhibit pathogen growth. They ferment undigested fibers to produce short-chain fatty acids (SCFAs), such as butyrate and acetate, which lower the gut pH. This environment is less hospitable for many harmful bacteria and viruses. Probiotics also modulate the local immune response in the gut lining by interacting with immune cells.
Evidence for Prevention: What the Research Shows
Clinical research on probiotics for viral gastroenteritis shows a mixed picture, distinguishing between preventing the illness and reducing severity once it has begun. The most consistent evidence points to probiotics helping with the treatment of acute viral gastroenteritis, particularly in children. Systematic reviews suggest that certain strains can modestly reduce the duration of diarrhea by about 0.7 to 1 day.
However, the evidence for true prevention—stopping the infection before symptoms occur—is less robust, particularly against common viruses like norovirus. Large, rigorous trials have challenged earlier, smaller studies suggesting a benefit for preventing moderate-to-severe gastroenteritis in children. These trials found that the administration of a common probiotic strain did not significantly reduce the duration of symptoms or the rate of household transmission in children already suffering from acute gastroenteritis.
Results also vary significantly based on the pathogen and the host population being studied. While some strains show anti-viral activity in laboratory models against norovirus and rotavirus, translating this to reliable prevention in humans remains complex. The most promising area for prevention remains in specific contexts, such as preventing traveler’s diarrhea, which can be caused by viruses, or in animal models where specific probiotic cocktails significantly reduce the incidence of norovirus-induced diarrhea.
Specific Probiotic Strains for Prevention
The effectiveness of probiotics is highly strain-specific; benefits seen with one strain do not necessarily apply to others. For supporting gut health against viral challenges, the most widely researched strains are Lactobacillus rhamnosus GG (LGG) and the yeast Saccharomyces boulardii. These strains are specifically studied for their effects on diarrhea and intestinal health.
L. rhamnosus GG is known for its ability to survive the harsh stomach environment and adhere effectively to the intestinal wall. Saccharomyces boulardii, a probiotic yeast, is resilient, resistant to antibiotics, and has shown efficacy in reducing the risk of traveler’s diarrhea. When selecting a product, look for the full strain designation—such as LGG or S. boulardii CNCM I-745—to ensure the product contains the specific microbes tested in clinical trials.