Here at the Pundit, it is fair to say that we are inundated by promoters who submit studies purporting to prove that various fruits and vegetables have specific medicinal benefits.
We rarely write about these studies because most are too limited to prove much of anything. The number of participants is too small… there is no control group… the number of variables being tested at once obscures the results, etc.
The New York Times just ran a piece, titled Reputation of a Berry Is Difficult To Confirm by Abigail Zuger, M.D., that deals with recent efforts to confirm that the cranberry, long thought to be effective in preventing urinary infections, is actually effective in this manner:
For decades cranberry juice has enjoyed a reputation as an effective way to prevent bladder infections. Scientists have doggedly tried to confirm this well-known folk truth with dozens of studies, some in test tubes and some in people.
The latest results are now in, and the answer is conclusive: This field is all bogged down.
Some older studies found the juice worked. Some found it didn’t. All were too small to be definitive. In 1998 a substance presumed to be the active component in the cranberry was identified with some fanfare, and two years ago another study suggested that a cranberry extract containing this substance was almost as powerful as an antibiotic.
Now a large, impeccably designed and executed study of cranberry juice has found that the presumed active compound apparently has no effect. And yet the newest study closed no doors. It may simply mean that the juice works, but by an unknown mechanism.
The article goes on to explain that cranberries have hundreds of active substances and that old theories about why cranberries help have been disproven while newer theories have focused on the role of proanthocyanidin, a chemical related to tannin, which is in blueberry juice and in cranberry juice but not in other juices.
Finally, though, the article explains that an effort was made to resolve the matter once and for all:
…armed with a grant from the National Center for Alternative Medicine and cranberry juice cocktail from Ocean Spray, researchers in Michigan had set out in 2004 to do the definitive placebo-controlled study.
They enrolled young, healthy women who had just recovered from a bladder infection. Statistics predicted that about 30 percent of them could be expected to get another infection within six months.
Half the 319 subjects were assigned to drink 16 ounces of low-calorie cranberry juice cocktail daily. The others were given a placebo drink manufactured by Ocean Spray to look and taste the same, but with no cranberry content.
The results were published this month in Clinical Infectious Diseases. After six months, the women in the placebo group had 23 new infections and those in the cranberry group had 31, a statistically insignificant difference. The juice apparently offered no protection.
But that didn’t tell the whole story. The researchers had expected that 30% of the women would get a bladder infection again and both the control and the tested group came in well below that number. In fact, only 17% of the women had a recurring bladder infection. This raised the obvious question: Was there something in the placebo that gave women the same protection that the cranberry juice did?
In an interview, the senior author, Betsy Foxman, a professor of epidemiology at the University of Michigan School of Public Health, offered some ideas for what could be going on.
Both the juice drink and the placebo contained vitamin C — could that have prevented recurrent infection? Could just drinking two extra cups of fluid per day have helped? Could it have something to do with the coloring used to tint the placebo — after all, antibiotics were first identified by the German dye industry.
“It is still a big question mark,” Dr. Foxman said. Her research into the cranberry is continuing.
In other words, even in a carefully designed study, simple variables such as the amount of fluid people drink can obscure the cause and effect.
This is why studies typically need to be done many times, by many researchers, in many different ways before we can have confidence in the results.
There is reasonably good research on broad categories of diet, such as the impact of eating fewer calories, and it is reasonable to surmise that it is easier to do that by eating cabbage than chicken fat, but the arguments that specific produce items offer specific medicinal benefits are mostly standing on research too weak to confirm the hypothesis.
Until more and better research is done, most of these claims fall into the category of “interesting, if true.”