Researchers with the highly respected Cochrane Collaboration — experts who identify and evaluate studies of health care interventions — say that scientific evidence supports daily consumption of cranberry products to reduce the likelihood of urinary tract infections (UTIs). The preventive effect is strongest in women with a history of recurrent UTIs — that is, three or more a year.
Cranberry juice has been used for years to treat and prevent UTIs, which are roughly 50 times more common in women than in men. UTIs can affect any part of the urinary tract but occur most often in the bladder (cystitis), producing symptoms of frequent, urgent, or painful urination, and sometimes abdominal pain or blood in the urine. Most UTIs are caused by Escherichia coli, or E. coli, which can travel from the anus to the urethra during activities such as using the toilet or having sexual intercourse. Antibiotics are very effective in preventing and treating these infections, but occasionally can cause diarrhea, nausea, and yeast infections of the vagina or mouth.
Research results showed that over a 12-month period, cranberry products reduced the overall incidence of UTIs by 35%. Within these results, cranberries appeared to be more effective at preventing UTIs in women with recurrent symptomatic infections than in elderly populations — and ineffective in people with bladder problems that involved catheterization. Among women with recurrent symptomatic UTIs, cranberries cut the annual rate of new infections by 39%.
One of the drawbacks of the studies being reviewed was their lack of consistency in dosing and in the concentration of cranberries. Also, the chemical composition of the cranberry products was not standardized, and the bioequivalence between the juice, capsules, or tablets was unknown. So it's hard to know exactly which and how much of the various cranberry components people were getting, or what level of intake was effective. (The traditional recommendation is a glass or two of 100% cranberry juice a day.)
How do cranberries help?
Cranberries contain several compounds that could confer health benefits, including salicylic acid (the active ingredient in aspirin) and plant chemicals called polyphenols, which have antioxidant properties. No exact mechanism of action has been established, but in laboratory tests, cranberry extracts have been shown to hamper the ability of E. coli to stick to the type of cells that line the urinary tract.
A possible limitation of cranberry therapy is that regular cranberry juice is fairly high in calories (130 to 140 calories per 8 ounces, about 50% more than orange juice and even colas). Also, the flavor may not be agreeable to some people, especially over the long term. Fortunately, there are many other things you can do to help stave off UTIs. Drink plenty of water, to help increase urine output. Don't resist the urge to urinate; go when you need to. Take showers instead of tub baths. Urinate before and after sexual intercourse. If you continue to have problems, talk to your clinician about taking an antibiotic daily, after intercourse, or for a few days at the first sign of symptoms.
May 2008 update