Background: Diarrheal illness affects up to 50 percent of international travelers and is the most common health issue in this population. Although traveler's diarrhea is usually self-limited, bacterial sources including Escherichia coli and Shigella and Salmonella species are responsible for approximately 80 percent of cases. Preventive dietary measures often are recommended to prevent symptoms, but high nonadherence rates reduce the effectiveness of these measures.

The potential role of probiotics in preventing traveler's diarrhea has generated considerable interest. Several trials have reported some benefit from Lactobacillus GG (Culturelle) or Saccharomyces species. Studies examining Lactobacillus acidophilus have generated mixed results, and their findings have been questioned because of methodologic concerns.

The Study: Briand and colleagues conducted a double-blind, randomized controlled trial to study the effectiveness of L. acidophilus in the prevention of traveler's diarrhea. Participants were 18 years or older and planned on traveling to areas where the risk of diarrheal illness was intermediate or high. Travel duration was limited to fewer than 21 days. The 348 participants who met the inclusion criteria were randomly assigned to use placebo or an L. acidophilus preparation twice a day. Participants began therapy the day before travel and continued until developing diarrhea (defined as three or more unformed stools in 24 hours) or until three days after returning from travel. Each L. acidophilus dose contained 10¹⁰ organisms mixed with fermented culture medium. To prevent potential systemic Lactobacillus infection, the organisms were heated to render them nonviable while maintaining their antimicrobial activity.

Participants logged the number and consistency of stools and any related symptoms if diarrhea occurred. Participants included persons on organized tours or business trips and those who would be backpacking or staying with local residents.

Results: Among the 245 participants included in the intent-to-treat analysis (123 in the L. acidophilus group and 122 in the placebo group), diarrhea occurred in 28.5 percent of the L. acidophilus group compared with 23 percent of the placebo group. No relationship was found between risk of developing diarrhea and the geographic region visited, type of accommodation, mode of travel, or participant age or sex. After adjusting for travel duration, there was no statistically significant difference in development of diarrhea between the two groups.

Conclusion: The authors conclude that non-viable L. acidophilus is ineffective in the prevention of traveler's diarrhea; however, the potential benefit of other probiotic preparations has not been determined.