A mental_floss staffer who shall remain nameless asks, “Why is cranberry juice good for treating UTIs? I will not tell you why this question had been on my mind.”
Bacterial infections in the urinary tract and its organs are pretty common, especially in adult women, and more than seven million cases are reported by doctors every year. Among women who get one, about a third will experience them recurrently.
A popular folk remedy for the infections is cranberry juice. How the juice relieves UTIs, and even if it actually does help, isn’t entirely clear. Research on its effectiveness for UTI prevention has produced mixed results, and studies assessing the use of cranberry juice in UTI treatment are few. The strongest evidence available for prevention is among adult women with previous UTIs. In this group, results repeatedly show about a 50% reduction in disease recurrence with regular juice consumption. In elderly and pediatric patients, and some patients with bladder problems, the effect is less pronounced.
A caveat made in many of the prevention studies is that the cranberry juice’s effects seem to be dose-dependent, and that the amount a person would need to drink daily to have a real preventative effect is more than what most people are willing to drink. In some studies, as many as half of the participants withdrew before the studies' completion, suggesting that prolonged, regular guzzling of cranberry juice is too much to bear, either because of stomach aches, the calorie load, or adverse interactions between the juice and certain medications.
Overall, the evidence suggests that cranberry juice helps prevent UTIs. But how?
The Compound Equation
Cranberries contain quinic, malic, and citric acids, and for a long time researchers thought that the acidity of this mix had a bacteriostatic effect that kept bacteria from reproducing and gave cranberry juice its usefulness. Further studies showed that the amount of acid in the juice and the low amounts that people would tolerate drinking weren’t enough for the acid to do much good, though. More recent research suggests that the juice’s real benefit is that certain compounds in it keep bacteria from setting up shop in the urinary tract.
E. coli and other common UTI-causing bacteria have hairlike appendages called fimbria that they use to adhere to the walls of the urinary tract. A few different studies have found that two compounds in cranberries—fructose and proanthocyanidin—inhibit bacteria’s fimbria from sticking to anything, preventing the bacteria from colonizing and multiplying. The compounds' antiadherent effects start within two hours after someone drinks cranberry juice and persist for up to 10 hours after ingestion, keeping an infection from taking hold.
Other research has found that cranberry juice additionally alters and increases certain thermodynamic properties of bacteria in the urinary tract—including the amount of energy that that they have to expend before they can attach to tissue—creating an energy barrier that they can’t overcome, and therefore preventing them from latching on. If the bacteria can’t stick to the walls of the urinary tract, then they’re vulnerable to being flushed out and away by urine.