Podcast: Cranberries for preventing urinary tract infections

Back in October 1998, we published the first version of the Cochrane review of the effects of cranberry for preventing urinary tract infections. Nearly 25 years on, the fifth update was published in April 2023 and we asked lead author, Gabrielle Williams from University of Sydney in Australia to bring us up to date in this podcast.

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Mike: Hello, I'm Mike Clarke, podcast editor for the Cochrane Library. Back in October 1998, we published the first version of the Cochrane review of the effects of cranberry for preventing urinary tract infections. Nearly 25 years on, the fifth update was published in April 2023 and we asked lead author, Gabrielle Williams from University of Sydney in Australia to bring us up to date in this podcast.

Gabrielle: For decades, cranberry juice has been suggested for women with recurrent urinary tract infections, or UTIs, to try to prevent them developing further infections.
UTIs are very common. Each year about 10% of women experience one and most women have at least one during their lifetime. About 7% of children develop a UTI each year and, for elderly people living in long-term care facilities, UTIs are the most common infections they experience. Other groups are also especially susceptible, such as people with a spinal cord injury or people undergoing a procedure that requires a urinary catheter.
Treatment for UTIs usually involves antibiotics of varying durations and, sometimes, as little as one dose is sufficient. Unfortunately, though, in some people, UTIs recur. For example, about one in five children who have one UTI will have another one and the risk of recurrence is even higher in women, at closer to one in two. One option for people susceptible to recurrent infection is long term low dose antibiotics, but this is associated with a doubling of the risk of developing a UTI where the causative bacteria is resistant to the given antibiotic. Many people are also reluctant to take long term antibiotics.
Before the modern era, Native Americans reportedly ate cranberries for bladder problems and laboratory scientists have explored the content of cranberries to try to find what may have helped and how it might work. There are several explanations, with perhaps the most widely accepted being that a component, called proanthocyanidin or PAC, prevents the most common UTI-causing bacteria, E.coli, from sticking to the cells on the bladder wall.
In the previous update of this review in 2012, we included 24 trials with almost 4500 participants and showed that, in a mixed population, cranberry might be beneficial, but the precision of the estimate was too poor for confidence in the result. Analyses of separate groups; including women with recurrence, children, the elderly, and people with bladder emptying issues, showed that while each estimate suggested a reduced risk of UTI, all lacked precision.
This 2023 update added 26 news trials, taking the total to 50, with more than 8000 participants. In these new analyses, compared to placebo or no treatment, cranberry products reduced the risk of repeat UTI in women with recurrence, children with prior infection, and people undergoing an intervention on their bladder. In women with recurrence, there was a 26% reduction in the risk of repeat infection; in children with prior UTI, a 56% reduction in risk and in people undergoing a bladder intervention, a 53% reduced risk. Results for elderly people in residential care, pregnant women and people with neuromuscular bladder dysfunction, were inconclusive. 
Comparisons of cranberry products with antibiotic treatment suggested no difference, but with imprecision. While, compared to probiotics, we found that cranberry may reduce the risk of UTI recurrence but, with data from only 3 studies, confidence is low. 
Serious adverse events, such as death and hospitalisations, were not different in frequency between the cranberry and placebo groups and the most common side effects of cranberry treatment were gastrointestinal events, such as diarrhoea and gastrointestinal pain. 
Thinking about the type of cranberry product, trials in earlier versions of the review were twice as likely to use cranberry juice than those added for the 2023 update, with a recent switch to tablets. This may be important for adherence, because cranberry juice is quite sour and intolerable to some people which might mean that they don't drink it. However, many studies didn't report adherence, and for those that did, there was high variability.
Reporting of the apparent active ingredient, PAC, was also quite different between the previous and the latest review. In 2012, less than a third of trials reported a PAC dose whereas nearly two thirds of the trials we've added in 2023 did so. Overall, 31 studies used a tablet, one study used both juice and tablet, and most studies, 60%, supplied 2 tablets per day, while 30% gave 1 tablet a day. The amount of PAC in each tablet varied quite substantially, from 1mg to 108mg, and when we did a new analysis by PAC amount, it suggested a dose dependent response but with imprecision in the estimates. When we grouped trials into whether they'd used juice or juice concentrate, or a tablet or powder, we found that the tablet or powder option might reduce the risk of further UTI more than the juice or concentrate.
We also looked to see if having a commercial entity involved in the trial, in this case juice companies or health food groups, had an impact on the results. But we found no difference in results, suggesting that having help in funding from these entities didn't greatly bias results.
In summary, our updated review suggests that cranberries reduce urinary tract infections in women with recurrent infections, children with previous UTI and people undergoing a medical procedure on their bladder and the increased precision and change to statistical significance is probably due to inclusion of additional studies. However, despite this inclusion of many more studies, the optimal dose of a cranberry product remains unclear.
Finally, looking to the future, trials could add very valuable information to existing knowledge if they focused on using cranberry tablets with a known dose of PAC, making it clear to consumers how much they should take.

Mike: Thanks Gabrielle. If you'd like to read this latest update of the review, and watch for future updates, it's available online at Cochrane Library dot com with a simple search for 'cranberries for preventing UTI'.

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