Cranberry Simply Explained

1. What It Is About

Cranberry is the fruit of the large-fruited cranberry. The plant is an evergreen shrub from the heath family. Raw berries are rarely eaten; they are processed into juice, dried fruit, powder or extract. An extract is a concentrated preparation from the berries.

In herbal medicine the berry has long been used for urinary tract complaints. Its constituents include organic acids, sugars, vitamins and polyphenols, a group of plant substances. The polyphenols include anthocyanins, flavonols and proanthocyanidins.

Of particular research interest is a specific type of these proanthocyanidins, the A-type. It can prevent certain bacteria (Escherichia coli) from adhering to the cells of the bladder wall. These bacteria often cause urinary tract infections. The composition of a berry varies considerably according to variety, cultivation, processing and dosage form. This makes comparison of studies difficult.

369 evaluated papers are available on cranberry, including 145 controlled human studies. Controlled means: one group receives the preparation, another receives something for comparison.

2. What Studies Show

The best-researched question is whether cranberry can prevent recurrent urinary tract infections.

A 2001 study followed 150 women who had previously had a urinary tract infection caused by Escherichia coli for six months. The women drank either a juice concentrate made from cranberries and lingonberries, a drink containing a lactic acid bacterium or received no treatment. After six months, significantly fewer women in the cranberry and lingonberry group had a recurrent infection than in the untreated group 2. The study was not blinded: all participants knew who received which treatment.

Two large reviews summarise many individual studies. One pools 13 controlled studies with 1616 participants; across all studies the risk of recurrent urinary tract infection was lower with cranberry than without, especially in women with frequently recurring infections 5. The Cochrane review by Jepson and Craig evaluated 24 studies and points in a similar direction 1.

There is also a study on the heart and circulation involving 44 people with coronary heart disease. They drank cranberry juice or a placebo drink, that is a comparison beverage without the berry, for four weeks. The stiffness of the aorta, a measure of vascular health, was assessed. The artery became more elastic under the juice but not under the placebo 3.

In 32 volunteers, urine was tested for how well bacteria can adhere to bladder-wall cells. After cranberry powder they adhered less well, and the effect lasted longer than after placebo 4. This is a laboratory measurement, not a count of actual infections.

3. What Remains Unclear

The largest single prevention study in 2011 was negative: it found no benefit. Another study from the same year concluded that cranberry was not inferior to an antibiotic. Reviews from 2007, 2008 and 2012 reach differing assessments. The evidence for protection is therefore not yet confirmed.

One reason is that product, preparation and participants differ markedly. Many studies are small, short and measure intermediate values such as bacterial adhesion rather than the number of infections. Such studies are valuable pointers and often the trigger for larger investigations; they cannot prove a benefit.

For vessels, inflammatory markers, blood glucose and gut bacteria the research is even earlier. The possible mechanism via adhesion in the urine remains a hypothesis because no study has directly linked it to fewer infections.

4. What This Means in Everyday Life

The effect is easiest to assess in women with frequently recurring urinary tract infections. Several large summaries exist and point in a similar direction. It is still not confirmed: the largest single study found no benefit.

For all other topics – vessels, inflammation, blood glucose, gut bacteria – the studies are small and short. They measure intermediate values, not diseases. A changed value in blood or urine does not yet indicate whether someone falls ill less often.

Much also depends on the product. Juice, powder and extract differ in composition, and even the same berry juice can vary according to variety and processing. This explains part of the conflicting results.

The next step would be to clarify whether a large, blinded study confirms the protection in women with recurrent infections – and whether adhesion in the urine really correlates with fewer infections.

All details with study numbers and sources: detailed page on cranberry (large-fruited cranberry). For children: cranberry (large-fruited cranberry) for children.