Using cranberries to treat urinary tract infections (UTIs) goes back at least as far as the Native American Indians, but now there is solid scientific evidence to go with this traditional remedy.
UTIs can be incredibly painful and trigger a variety of bladder problems. If untreated, it can move to the kidneys and cause further complications, including sepsis in some rare cases. While most common in women, UTIs can also affect men and children.
In a new study, researchers reviewed 50 previous trials covering 8,857 participants in total. These earlier trials looked at how UTI risk was linked to cranberries delivered in juice, tablet, or powder form.
"The studies we looked at included a range of methods to determine the benefits of cranberry products," says epidemiologist Jacqueline Stephens from Flinders University in Australia.
"The vast majority compared cranberry products with a placebo or no treatment for UTI and determined drinking cranberries as a juice or taking capsules reduced the number of UTIs in women with recurrent cases, in children and in people susceptible to UTIs following medical interventions such as bladder radiotherapy."
The risk of developing a UTI was reduced by more than a quarter in women with recurrent cases of infection, by over a half in children, and by just over a half for people likely to get UTIs after medical procedures.
Only a few people across the studies reported side effects, with stomach pain being the most common. However, there are gaps in the data: Very few trials compared cranberry products directly to antibiotics or probiotics alone, for example, and these treatments may be as effective as the fruit.
No benefits of cranberries were shown for older adults, pregnant women, or people with bladder emptying problems, so this isn't a finding that applies across the board. However, it shows that for some groups of people, the reduced risk is significant.
"This incredible result didn't really surprise us, as we're taught that when there's more and better evidence, the truth will ultimately come out," says epidemiologist Gabrielle Williams from The Children's Hospital at Westmead in Australia.
"UTIs are horrible and very common; about a third of women will experience one, as will many elderly people and also people with bladder issues from spinal cord injury or other conditions."
While many women have already been using cranberry juice to ward off UTIs, up until this point, the scientific evidence for its effectiveness hasn't been particularly conclusive.
As per earlier studies, it's thought that compounds inside cranberries called proanthocyanidins (PACs) help stop Escherichia coli bacteria from attaching to the cells lining the bladder, giving the fruits their protective benefits. It's estimated that E. coli accounts for around 90 percent of UTIs.
It remains unclear what the optimal dosage of cranberry PACs should be, but a 2021 study recommended around 36 milligrams of PACs a day. The researchers responsible for this most recent review caution that "no conclusions could be drawn from these analyses as to the relative efficacy of different doses of PAC."
This is the fifth in a series of reviews (the first one was published in 1998), and each time the researchers can add more data to the pile. With each update, the potential benefits of cranberries are becoming more apparent.
"This is a review of the totality of the evidence, and as new evidence emerges, new findings might occur," says epidemiologist Jonathan Craig from Flinders University.
"In this case, the new evidence shows a very positive finding that cranberry juice can prevent UTI in susceptible people."
The research has been published in the Cochrane Database of Systematic Reviews.