Cranberry Overview

The cranberry plant is a close relative of the common blueberry. Native Americans used it both as food and for the treatment of bladder and kidney diseases. The Pilgrims learned about cranberry from local tribes and quickly adopted it for their own use. Subsequent physicians used it for bladder infections, for "bladder gravel" (small bladder stones), and to remove "blood toxins."

In the 1920s, researchers observed that drinking cranberry juice makes the urine more acidic. Since common urinary tract-infection bacteria such as E. colidislike acidic surroundings, physicians concluded that they had discovered a scientific explanation for the traditional uses of cranberry. This discovery led to widespread medical use of cranberry juice for treating bladder infections. Cranberry fell out of favor with physicians after World War II, but it became popular again during the 1960s—as a self-treatment.

Bladder Infection

Probably the best evidence for the use of cranberry juice for preventing bladder infections comes from a 1-year, double-blind, placebo-controlled study of 150 sexually active women that compared placebo against both cranberry juice (8 ounces 3 times daily) and cranberry tablets. ¹ The results showed that both forms of cranberry significantly reduced the number of episodes of bladder infections; cranberry tablets were more cost-effective.

A double-blind study of 376 hospitalized seniors attempted to determine whether a low dose of cranberry juice (as cranberry juice cocktail, 10 ounces daily—a very low dose compared to the previous study) would help prevent acute infections. ² It failed to find benefit, perhaps in part due to the dosage of cranberry as well as the low number of infections that developed overall.

Another double-blind study evaluated cranberry juice cocktail for the treatmentof chronic bladder infections. ³ This trial followed 153 women with an average age of 78.5 years for a period of 6 months. Many women of this age group have what are called chronic asymptomatic bladder infections: signs of bacteria in the urine without any symptoms. Half of the participants were given 10 ounces a day of a standard commercial cranberry cocktail drink, the other a placebo drink prepared to look and taste the same. Both treatments contained the same amount of vitamin C to eliminate the possible antibacterial influence of that supplement. Despite the weak preparation of cranberry used, the results showed a 58% decrease in the incidence of bacteria and white blood cells in the urine.

A year-long open trial of 150 women found that regular use of a cranberry juice/lingonberry combination reduced the rate of urinary tract infection as compared to a probiotic drink or no treatment. ⁴ However, because this study was not double-blind, the results are unreliable. (For information on why double-blind studies are so important, see Why Does This Database Rely on Double-blind Studies? )

A review of 10 studies investigated the benefits of cranberry juice or tablets compared to a placebo control in patients susceptible to urinary tract infections. Among 1,049 participants, the researchers found the cranberry products reduced the incidence of urinary tract infections by 35%, a statistically significant amount, over a 12-month period. The effect was most notable in those with recurrent infections. However, many subjects dropped out of the studies early, suggesting that continuous consumption of cranberries is not well tolerated. ⁵ On the negative side, 3 other double-blind, placebo-controlled studies evaluated the effectiveness of cranberry extract for eliminating bacteria in the urine of people with bladder paralysis (neurogenic bladder). ⁶ The results showed no benefit. However, a subsequent study of 47 patients with neurogenic bladder from spinal cord injuries found that the use of cranberry extract tablets over 6 months significantly reduced the risk of urinary tract infection. ⁷

The bacteria Helicobacter pyloriplays a major role in the initiation and maintenance of peptic ulcers , ulcers in the stomach and duodenum. A 90-day, double-blind, placebo-controlled study performed in China tested the effects of daily consumption of cranberry juice in individuals who were chronically infected with helicobacter(but who did not necessarily have ulcers). ⁸ The results indicated that use of cranberry significantly decreased levels of helicobacterin the stomach, presumably by causing some of the detached bacteria to be “washed away.” Another study involving 295 children with helicobacter but without ulcer symptoms also demonstrated the ability of cranberry to reduce the levels of the stomach bacteria. ⁹ While this was a promising finding on a theoretical level, it did not directly address treatment or prevention of ulcers. A more practical study evaluated the use of cranberry as a support to standard therapy. ¹⁰ This double-blind trial enrolled 177 people with ulcers who were undergoing treatment with a common triple-drug therapy used to eradicate H. pylori(omeprazole, amoxicillin and clarithromycin, known as OAC). All participants received this drug treatment for one week. During this week and for two weeks after, they were additionally given either placebo or cranberry juice. Researchers also looked at a third group attending the same clinic, who received only OAC.

The results were somewhat promising. In the study group at large, OAC plus cranberry was no more effective than OAC plus placebo or OAC alone. However, among female participants in the study, use of cranberry was associated with a significantly increased rate of helicobactereradication as compared to placebo or no treatment.

Does this mean that women undergoing ulcer treatment may benefit from cranberry? Perhaps, but not necessarily. When a treatment fails to produce benefit in the entire group studied, researchers may, after the fact, go on a hunt for a subgroup who did benefit. The laws of chance alone ensure that they can almost always find one. Therefore, it is not clear whether cranberry actually did provide benefit, or whether this finding was merely a statistical fluke.

The usual dosage of dry cranberry juice extract is 300 mg to 400 mg twice daily or 8 to 16 ounces daily of pure cranberry juice (not cranberry juice cocktail.)

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