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Updated: March 31, 2017

Intentionally eating bacteria may not seem like the most health-conscious choice, but probiotics—the “good” bacteria—have been shown to have a variety of beneficial effects, including alleviating gastrointestinal distress (primarily diarrhea and, to a lesser extent, inflammatory bowel disease), improving lactose intolerance, maintaining urogenital health in women, preventing incidence of allergic reactions and, ironically, improving and enhancing the immune response.

Related: Health Benefits of Probiotics

However, probiotic dosing is an inexact science, with clinical research only beginning to uncover which probiotic strains should be used for specific ailments. Even less clear is how you should to take. So, then, what are the side of effects of too much “good” bacteria, and is overdosing a risk?

Related: Probiotic Efficacy: What Works Best?

To start, probiotic foods and beverages have been consumed by human for more than 100 years and have generally been shown to be safe. In healthy individuals not on antibiotic therapy, minor side effects like gas, upset stomach, and diarrhea can appear with a dose of more than 1–2 billion probiotic bacteria (L. acidophilus), usually when more probiotic bacteria are consumed than necessary.

If taken by those who are immunocompromised or critically ill, however, they may provide more harm than benefit. In patients with impaired immune functioning or sensitivity to probiotics, side effects can be severe. They can include:

  • Sepsis. One clinical review notes that “probiotic sepsis [overwhelming immune response to bacterial infection] is the most feared complication related to probiotic administration.” However, only one documented case of probiotic sepsis was thought to have been fatal.
  • Heart infection. In a rare, but documented study, Lactobacillus lead to endocarditis, an infection of the inner lining of the heart, in adults.
  • Bacterial or fungal infection. Several studies have linked probiotic therapy to bacteremia (presence of viable bacteria in the blood) in adults and children. Similarly, several studies have linked Saccharmoyces bouldarii, a fungal probiotic, to fungemia.
  • May increase risk of complications in those with preexisting conditions. One clinical study noted that probiotic intervention increase mortality in patients with pancreatitis. Bowel ischemia was more common in the probiotic group than in the placebo group.
  • Extent or severity of immunomodulation is hard to predict. This will vary among individuals, with the major determining factors expected to be age and preexisting conditions (which may impact immune activity).This is of particular concern to neonates and pregnant women. Individuals with immune disorders also fall into this group; some patients suffering from Crohn’s Disease, an autoimmune disease, have previously shown worse outcomes after probiotic administration.
  • Effects on metabolism. Probiotics may theoretically affect metabolic activity, as intestinal microbiota play central roles in carbohydrate and lipid metabolism and glucose regulation.
  • Probiotics may interact and interfere with medications. Antibiotics, for example, may kill probiotic bacteria. Preliminary evidence suggests that probiotics may speed up the metabolism of sulfasalazine, a medication used to treat ulcerative colitis.
  • Severe allergic reaction. Although probiotics are suggested to help treat/prevent some types of allergic responses (expected to be due to immuno-modulation), other ingredients commonly included in probiotic formulations may offset allergy. Inulin, a prebiotic commonly found in probiotic supplements, has been linked to anaphylaxis in at least one reported study.

Although probiotics appear to be generally well-tolerated and safe in otherwise healthy individuals, there are no formal clinical trials assessing probiotic safety. Currently, relying on case studies is the only definitive way to understand the safety profiles of probiotic foods and supplements (which have an ever-growing variety of strain/dosage combinations).

If you’re considering taking probiotics, make sure you know how much you’re getting and discuss your dosing with your doctor. We found from testing 32 popular brands that products can range from having only 1% of the total viable bacteria they claim to +2400%. For more information, head over to labdoor.com for quality testing results here: https://labdoor.com/rankings/probiotics.

Sources

  1. Use of Probiotics in Gastrointestinal Disorder: What to Recommend? - Therapeutic Advances in Gastroenterology
  2. Probiotic Use in Clinical Practice: What are the Risks? - The American Journal of Clinical Nutrition
  3. Probiotics: What They Are and What They Can Do for You - American Gastroenterological Association
  4. Lactobacillus acidophilus - University of Maryland Medical Center
  5. Diseases and Conditions: Endocarditis - Mayo Clinic
     

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