European settlers first put cranberries on the Thanksgiving table because the local fruit lasted through winter and enhanced the flavor of gamy meat. The settlers had picked up on the berry's culinary potential from Native Americans, who survived cold winters by filling up on pemmican, a cake of cranberries, nuts and dried venison or bear meat.
Both groups also prescribed cranberries for fevers, gastrointestinal problems and dropsy -- a term used to describe any swelling or inflammation. Turns out, they were onto something. In the last few decades, scientists have begun to confirm and explain the cranberry's ability to fight infections of the urinary tract and gut and its potential to fight gum disease, heart disease and cancer.
"For over a hundred years, women have known that cranberry juice can prevent urinary tract infections," says Amy Howell, associate research scientist at the U.S. Department of Agriculture-supported Marucci Center for Blueberry and Cranberry Research at Rutgers University in Camden, N.J. "They thought it was due to acidity, but that's actually not the case."
Cranberry's antibacterial properties are due to a class of chemical compounds called proanthocyanidins. Ten years ago, Howell's research group isolated the compounds and demonstrated how they work: Proanthocyanidins bind to harmful bacteria such as E. coli, forming a "Teflon-like" coating around them. The coating prevents the bacteria from sticking to gastrointestinal and urinary tract walls, impeding infections.
The nonstick properties of proanthocyanidins may explain the results of several clinical trials that showed that cranberry juice can reduce the frequency of urinary tract infections.
For example, a study published in the British Medical Journal in 2001 showed that women who drank a couple of ounces of cranberry juice daily for six months had a 20% lower risk of urinary tract infections, compared with women in a control group. A study published in the Canadian Journal of Urology in 2002 showed that just 20% of women who drank three glasses of cranberry juice daily for a year experienced urinary tract infection symptoms, compared with 32% of women who drank a placebo.
And last month, a study in the journal Urology found that two glasses of cranberry juice a day reduced the frequency of urinary tract infections by 41% among pregnant women.
Proanthocyanidins also appear to keep the bacterium H. pylori, which causes ulcers, from sticking to the linings of the stomach and intestines. A 2005 study of 189 adults with H. pylori infections in the journal Helicobacter, showed that two glasses of cranberry juice daily for three months reduced the degree of infections, compared with those who drank a placebo.
And a study in the journal Nutrition this year showed that a daily glass of cranberry juice eliminated H. pylori infections in 16% of infected children; a placebo eliminated only 1.5%.
Other research suggests that the compounds could keep plaque-forming bacteria at bay. In lab experiments, cranberry proanthocyanidins stopped oral streptococci and other bacteria from sticking to surfaces. But researchers warn against using the juice as a mouthwash because of its sugar content and acidity.
Cranberries are high in vitamins A, E and C, iron, calcium, potassium and antioxidants. The last may explain the fruit's possible anti-cancer and anti-heart-disease effects. Cranberry impedes the growth of liver and breast cancer cells in lab dishes, says Jie Sun, a scientist at General Mills who previously researched the fruit's anti-cancer effects at Cornell University.
And in 2006, Canadian researchers published suggestive findings in the British Journal of Nutrition showing that drinking a glass of cranberry juice a day increased concentrations of good HDL cholesterol by 8% in overweight men. (The study was funded by the Canadian Cranberry Growers Coalition.)
But the tart red berries may not be for everyone. Gorging on too many or guzzling too much juice can result in an upset stomach or diarrhea. A couple of reports indicate that cranberry juice may increase the risk of kidney stones in people prone to them. And there's conflicting evidence that cranberries may interfere with blood thinning drugs, such as warfarin.
The common cranberry's benefits still seem to outweigh its drawbacks, but despite this, most Americans limit their consumption to a single day of the year. The reason for this may have been best summed up by writer and naturalist Henry David Thoreau more than 150 years ago: Cranberries, he wrote, were easy to harvest, but their taste was "a little bitterish."
Given the amount of sugar in most cranberry sauce recipes, most Americans, it seems, would agree.