“It’s a myth that cranberry juice can cure bladder infections,” the Daily Mail reported today. The story was based on a large review of research that looked at the effectiveness of cranberry juice and cranberry products such as capsules, in preventing bladder and other urinary tract infections (UTIs), which are most common in women.
The use of cranberry juice to protect against infection was first thought to have originated with Native Americans. It is now a popular home remedy, often recommended to women by their doctors to help prevent infections on the basis of previous research that suggested it could be helpful.
The theory is that cranberry juice contains substances that help prevent bacteria from sticking to the wall of the urinary tract. However, this new widely reported study found that overall, cranberry products did not reduce the occurrence of UTIs, such as cystitis, when compared with placebo, water or no treatment at all.
This updated review comes from the respected Cochrane Collaboration and its results are likely to be reliable. It suggests that while cranberry juice may be of small benefit to some women, it would need to be consumed in such large quantities as to make it unacceptable to many as a remedy.
Other cranberry products were also found to be ineffective, possibly due to the lack of potency of the active ingredients. Other methods women can use to reduce the risk of developing a UTI include drinking plenty of fluids and practising good hygiene.
Read more about preventing UTIs.
Where did the story come from?
The study was carried out by researchers from the Cochrane Collaboration, an independent international organisation that publishes regular systematic reviews on the effectiveness of healthcare interventions. There was no external funding. The study was published in the peer-reviewed latest edition of the Cochrane Library, which is available to everyone.
Generally, the media covered the study fairly. Several papers used the term “cystitis” as a shorthand for bladder and other urinary tract infections. Cystitis is a painful inflammation of the bladder that can cause an urgent need to urinate and pain on urinating. It is often, but not always, caused by a bacterial infection. Infection may also affect other parts of the urinary tract including the urethra, kidneys and ureter, when it is more serious.
What kind of research was this?
This was a systematic review to assess the effectiveness of cranberry products in preventing urinary tract infections in susceptible people, including:
- women with recurrent UTIs
- people with certain bladder problems, such as kidney stones that make them more vulnerable to infection
- older people
UTIs are very common, more so in women than in men, possibly because women have a shorter urethra that may allow bacteria to get into the bladder more easily. It is estimated that half of all women in the UK will have a UTI at least once in their life. Some women seem to be particularly susceptible, with an average of two or three infections annually. Antibiotics are often advised as a preventative measure in people at risk of recurrent UTIs.
Children can also get UTIs, though less commonly, and elderly people are also vulnerable. Other at-risk groups are patients with spinal cord injuries and/or catheters, and those with certain chronic diseases such as diabetes or HIV.
The researchers point out that cranberries have been widely used for several decades to prevent and treat UTIs. Research suggests that they contain two constituents that may prevent bacteria (particularly E. coli, thought to be the bacterium that commonly causes bladder infections) from sticking to the bladder lining. The two substances are fructose and proanthocyanidins (PACs).
Cranberry products include juice, syrup, capsules and tablets. A commonly recommended amount for UTI prevention is 300ml of a cranberry juice “cocktail” containing 36mg of PACs.
However, the researchers point out that the processing involved in making cranberry products such as tablets and capsules may result in little PACs being left in the final product.
What did the research involve?
Researchers set out to test whether cranberry juice and other products were more effective than either placebo or no treatment, or any other treatment in the prevention of UTIs in susceptible populations. They also wished to test whether different cranberry products differed in how well they prevented UTIs.
They searched a number of electronic databases for all randomised controlled trial (RCT) and quasi-RCTs (RCT-type studies that did not meet the expected standard due to flaws or limitations into how the study was conducted, such as not being properly blinded) on the effectiveness of cranberry products in the prevention of UTIs.
They also contacted companies involved in the promotion and distribution of cranberry products for information on both published and unpublished studies, and searched reference lists of review articles and relevant studies.
They also searched by hand a number of specialist journals and the proceedings of major relevant conferences. Non-English language studies were included.
The studies included were of the following groups:
- those with a history of recurrent UTI (more than two episodes in the previous 12 months)
- elderly people
- people needing catheterisation (a risk factor for UTI), either permanently or intermittently
- pregnant women
- people with abnormalities of the urinary tract
- children with a UTI
They excluded any studies in which cranberry products were tested as a treatment for UTI, and studies of any urinary tract condition not caused by a bacterial infection.
Participants in the trials took cranberry products for at least one month. The authors looked primarily at whether cranberry products affected the number of UTIs experienced, as confirmed by laboratory analysis of urine specimens. They also looked at how far participants in the trials stuck to therapy, and potential side effects.
Two of the authors independently assessed the quality of all the studies eligible, using a validated tool to assess the risk of bias, and extracted the relevant data on study methods, participants, interventions and outcomes.
Using standard statistical methods they performed a meta analysis to calculate a summary measure for how well cranberry products prevent recurrent UTIs.
What were the basic results?
The review includes 24 studies with a total of 4,473 participants, comparing cranberry products with control or alternative treatments. Ten of the studies had been included in a previous review carried out by the researchers in 2009 into the same question. Studies included seven of women with recurrent UTIs, four of elderly men and women, three of patients needing catheterisation, two of pregnant women and three of children at risk. The main findings are below:
- Compared with placebo, water or no treatment, cranberry products did not significantly reduce the occurrence of UTI overall (relative risk (RR) 0.86, 95% confidence interval (CI) 0.71 to 1.04)
- Nor did cranberry products reduce UTIs in any of the individual subgroups: women with recurrent UTIs (RR 0.74, 95% CI 0.42 to 1.31); older people (RR 0.75, 95% CI 0.39 to 1.44); pregnant women (RR 1.04, 95% CI 0.97 to 1.17); children with recurrent UTI (RR 0.48, 95% CI 0.19 to 1.22); cancer patients (RR 1.15 95% CI 0.75 to 1.77); people with bladder abnormalities or spinal injury (RR 0.95, 95% CI 0.75 to 1.20).
- Overall heterogeneity – that is, the differences in results between trials – was moderate (I² = 55%).
- The effectiveness of cranberry was not significantly different to antibiotics for women (RR 1.31, 95% CI 0.85, 2.02) and children (RR 0.69 95% CI 0.32 to 1.51).
- Cranberry products were not associated with any adverse effects compared with placebo/no treatment (RR 0.83, 95% CI 0.31 to 2.27).
- Many studies reported low compliance and high withdrawal/dropout problems, which they attributed to palatability/acceptability of the products, primarily the cranberry juice.
- Most studies of other cranberry products (tablets and capsules) did not report how much of the “'active” ingredient the product contained, and therefore the products may not have had enough potency to be effective.
How did the researchers interpret the results?
The researchers say that the new review shows that cranberry juice does not appear to have a significant benefit in preventing urinary tract infections. Although some of the smaller studies demonstrated a small benefit for women with recurrent UTIs, there were no statistically significant differences when the results of a much larger study were included.
They also argue that low compliance and high dropout rates in the studies show that drinking the juice may be unacceptable in the long term. Cranberry products were also ineffective (although they had the same effect as taking antibiotics preventatively), possibly due to lack of potency of the potential “active ingredient”.
From the evidence they conclude it is unlikely that cranberry in its juice form is going to be an acceptable and effective intervention. Even if its “anti-adhesion” qualities could be proven, they calculate that to maintain levels of the PACs thought necessary to prevent bacteria sticking to the bladder lining, people would have to drink 150ml of the juice twice a day for an indefinite length of time.
“If a woman only has two UTIs a year she would have to drink the juice twice a day for a year to potentially have one less UTI. Although for some women this regime may be acceptable (i.e. those who have a high rate of occurrence), others may find that the price, the calories in the juice, and the taste may make it less appealing”.
They argue that other preparations need to be quantified using standardised methods to ensure they contain enough of the potential 'active' ingredient, before being evaluated in clinical studies or recommended for use.
These findings conflict with those that the researchers presented in their earlier 2009 review.
But revising or rejecting theories when new evidence becomes available (as opposed to trying to make the evidence "fit" with pre-existing theories) is in the best tradition of evidence-based medicine.
This updated review comes from the well-respected Cochrane Collaboration and its results are likely to be reliable. As the authors point out, although their previous review of 11 studies found some evidence that cranberry juice may reduce UTIs, the addition of 14 new studies suggest cranberry juice is less effective than previously indicated.
Some people find pure cranberry juice a bit sour, so if you only drank it for your health and not for the taste, it may be time to switch to a tastier alternative. If you do enjoy drinking cranberry juice, then it may be a good way to get your 5 a Day (although you should be aware that many cranberry ‘juice drinks’ contain a lot of sugar to reduce the sourness). At the moment it seems doubtful that there is any way of knowing if other cranberry products contain enough of the potential “active ingredient” to be properly evaluated.