Sham vs. Wham has already presented scientific evidence in support of cranberry's effect on urinary tract health. This one is a no-brainer, because cranberry's traditional use is now backed up by a considerable number of clinical trials or research papers. Though the exact mechanism of its action has not been well understood in the past, a new study by researchers at Worcester Polytechnic Institute reveals that the juice actually changes the thermodynamic properties of bacteria; it creates an "energy barrier," preventing these microorganisms from getting close enough to latch onto cells and initiate an infection.
The study, published in the journal Colloids and Surfaces, was conducted by Prof. Terri Camesano (Chemical Engineering) and a team of graduate students, including PhD candidate Yatao Liu. They exposed two varieties of E. coli bacteria, one with hair-like projections that help latch onto cells and one without, to different concentrations of cranberry juice.
For the bacteria with the hair-like projections, they found that even at low concentrations, cranberry juice altered two properties that serve as indicators of the ability of bacteria to attach to cells. The first factor is a measure of the amount of energy that must be expended before a bacterium can attach to a cell. Without cranberry juice, this value was a negative number, indicating that energy would be released and attachment was highly likely. With cranberry juice the number was positive and it grew steadily as the concentration of juice increased, making attachment to urinary tract cells increasingly unlikely.
Surface free energy also rose, suggesting that the presence of cranberry juice creates an energy barrier that repels the bacteria. The researchers also placed the bacteria and urinary tract cells together in solution. Without cranberry juice, the bacteria attached readily to the cells. As increasing concentrations of cranberry juice were added to the solution, fewer and fewer attachments were observed.
Cranberry juice had no discernible effect on bacteria without these hair-like projections. That's a good thing, because the nasty bacteria in the urinary tract are those with the projections, and other bacteria can be very beneficial in the urinary tract.
"Our results show that, at least for urinary tract infections, cranberry juice targets the right bacteria—those that cause disease—but has no effect on non-pathogenic organisms, suggesting that cranberry juice will not disrupt bacteria that are part of the normal flora in the gut," Camesano says. "We have also shown that this effect occurs at concentrations of cranberry juice that are comparable to levels we would expect to find in the urinary tract."
The nasty bacteria will regain the ability to attach to cells if the cranberry is not taken. While the work above was done using both no-sugar-added juice and juice "cocktail," I'd stick with the former, or supplement capsules. Who needs all the sugar of a "10% juice" beverage!