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Cranberries and urinary tract infections: how can the same evidence lead to conflicting advice?

Liska, D. J. Kern, H. J. Maki, K. C.
Advances in Nutrition; 2016. 7(3):498-506.

Cranberry has been used traditionally to prevent urinary tract infections (UTIs), primarily among generally healthy women prone to recurrent UTIs. Results from a number of published clinical studies have supported this benefit; however, meta-analyses on cranberry and UTI prevention have reported conflicting conclusions. This article explores the methodological differences that contributed to these disparate findings. Despite similar research questions, the meta-analyses varied in the studies that were included, as well as the data that were extracted. In the 2 most comprehensive systematic reviews, heterogeneity was handled differently, leading to an I2 of 65% in one and 43% in the other. Most notably, the populations influencing the conclusions varied. In one analysis, populations with pathological/physiological conditions contributed 75.6% of the total weight to the summary risk estimate (RR: 0.86; 95% CI: 0.71, 1.04); another weighted the evidence relatively equally across UTI populations (RR: 0.62; 95% CI: 0.49, 0.80); and a third included only women with recurrent UTIs (RR: 0.53; 95% CI: 0.33, 0.83). Because women with recurrent UTIs are the group to whom most recommendations regarding cranberry consumption is directed, inclusion of other groups in the efficacy assessment could influence clinical practice quality. Therefore, conclusions on cranberry and UTIs should consider differences in results across various populations studied when interpreting results from meta-analyses.