Food and diet

Cranberries not a proven cure for bladder infection

"Cranberry juice really does prevent bladder infections," reports the Mail Online, suggesting that it's "not an old wives' tale" after all.

But the piece of research this story is based on only tested the effects of cranberries in the laboratory and not in humans. Crucially, human studies can differ from laboratory findings. The research found that cranberry powder made from whole cranberries had antibiotic properties against a bacterium called Proteus mirabilis, one of the common causes of urinary tract infections (UTIs).

The powder used in the research disrupted the movement of the bacteria (motility). It also inhibited the production of an enzyme called urease, thought to be key to the bacteria's virulence (its ability to cause illness).

Previous research found a small trend towards fewer UTIs in people taking cranberry products. However, this was not a statistically significant finding, meaning it may have been down to chance, and many people in these studies stopped drinking the juice early, suggesting it may not be an acceptable intervention.

This research adds to the biological understanding of a specific bacteria involved in some UTIs and the potential antibiotic role of cranberries, but it does not prove that cranberries prevent bladder infections in people. Other, more robust evidence that looked at this directly found that the effect is either very small or non-existent.

Where did the story come from?

The study was carried out by researchers from the Department of Chemical Engineering at McGill University in Canada and was funded by the Natural Sciences and Engineering Research Council of Canada, the Canada Research Chair Program, and, tellingly, the Wisconsin Cranberry Board and The Cranberry Institute.

The study was published in the peer-reviewed Canadian Journal of Microbiology.

It is normal for research publications to clearly state how the funders were involved in the research (for example, whether they just provided the cranberries or were involved in the interpretation of the results) and for the authors to declare any conflicts of interest.

Both important elements were noticeably absent from this publication, despite the clear potential conflict of interest from the funders, who appear to have interests in promoting cranberries.

The Cranberry Institute's website describes the organisation as "dedicated to the scientific discovery of the cranberry's health benefits", while the Wisconsin Cranberry Board website says it is responsible for Wisconsin's cranberry marketing order, among other promotions related to cranberries.

The Mail Online over-egged the study's findings, using phrases such as, "Drinking cranberry juice really can cure bladder infections", even though the research was about cranberry powder disrupting bacteria in a laboratory. Researchers did not actually test whether drinking cranberry juice cures or prevents bladder infections.

More useful sources about these issues are two Cochrane systematic reviews of randomised control trials, discussed in the conclusion of this article.

What kind of research was this?

This was a laboratory study looking at how cranberry powder affected a bacteria (Proteus mirabilis) known to be involved in urinary tract infections.

Cranberries have long been considered to have protective properties against urinary tract infections. But the science behind this notion is not clear-cut, with some saying it's nothing more than an "old wives' tale".

This is medically important, because complicated urinary tract infections often affect people who have been fitted with a catheter (a thin tube that drains urine from the bladder). Catheters can allow bacteria to enter the body more easily, causing a bladder or urinary tract infection.

If cranberries did have antibacterial properties, they could be harnessed to combat these infections without the need for antibiotics, which are currently used by doctors to treat these infections. Any method that can help avoid the use of antibiotics is always welcome, as this helps reduce the growing burden of antibiotic resistance.

A laboratory study is a very useful way of trying to better understand the biological processes of urinary tract infections and the effect cranberry extracts have on the bacteria involved.

However, it is important to recognise that what happens in the controlled environment of the laboratory and petri dish is not always exactly what happens in the body.

Finding that cranberry powder has an effect on bacteria in the laboratory environment will probably not tell us too much about the effect drinking cranberry juice has in preventing urinary tract infections in people, although there is clearly a relationship there. 

What did the research involve?

The research involved a large range of laboratory tests centring on how different concentrations of cranberry powder affected the growth, movement and behaviour of a bacteria involved in urinary tract infections. The cranberry powder was made from different concentrations of whole cranberries.

The growth and movement of these bacteria is key to how infectious they are (their virulence). Also crucial to the virulence of the bacteria is the expression of an enzyme called urease, which the researchers also measured. Alongside this, they measured any changes in the bacteria's genetics that could explain any change in the movement or growth.

What were the basic results?

The results showed that the movement of the bacteria, as well as the "swarming" movement of the bacteria in a group, were partially inhibited by the cranberry powder.

This was underpinned by genetic changes related to the bacteria's flagella. The flagella is a long, tail-like structure that propels the bacteria around, enabling it to move.

They also found that cranberry powder decreased the levels of urease the bacteria produced.

How did the researchers interpret the results?

The researchers concluded that taken together, their results demonstrated that cranberry powder reduced the movement of Proteus mirabilis and the expression of important virulence factors such as urease.


This laboratory study showed that cranberry powder reduced the movement and expression of virulence factors in Proteus mirabilis, a bacterium involved in some urinary tract infections. This raises the prospect that a cranberry extract could be developed as an antibacterial to prevent infections in medical practice.

However, this was exploratory research performed in the laboratory. More relevant research in people has already been done and was reviewed in two previous Cochrane systematic reviews of randomised control trials (see further reading for more information).

It was unclear from the laboratory study whether any reduction in motility and urease expression would actually result in lower levels of urinary tract infections in people if given as prevention, or shorter infections if it was used as a treatment.

There is also an obvious potential conflict of interest in this study, as some of the funding sources had cranberry-related interests. It was not stated what role these funders had in the conduct and publication of the research.

Finally, the study tested the effect of cranberry powder on just one of many bacteria involved in urinary tract infections. Cranberry powder may not work on other types of bacteria, even just in the laboratory, so it may not be a potential magic bullet to cure all urinary tract infections.

But it is possible to reduce your risk of contracting a UTI by:

  • treating constipation promptly
  • going to the toilet as soon as you feel the need to urinate, rather than holding it in
  • wiping from front to back after going to the toilet
  • practicing good hygiene by washing your genitals every day and before having sex
  • emptying your bladder after having sex
  • drinking plenty of fluids

NHS Attribution