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If a beverage manufacturer or industry group funded the research, the finding may be biased, researchers report.
"When a food company sponsors a study, it is much more likely to be positive" about the health effects of the product, said Dr. David Ludwig. He's the study's senior author and director of the Optimal Weight for Life program at Children's Hospital Boston, the pediatric teaching hospital for Harvard Medical School.
Ludwig and his colleagues analyzed 206 articles from medical journals that evaluated the health benefits or effects of soft drinks, juice and milk. The studies were published from 1999 to 2003.
Of the 206 studies, 111 supplied information on funding. To prevent bias in Ludwig's review, one researcher selected the articles for inclusion in the study. Another two researchers who were not told the funding sources classified each study as favorable, not favorable or neutral toward the beverage studied. A fourth researcher who didn't know the conclusions of the study determined the funding source and classified the studies, based on whether they would be beneficial, negative or neutral to the funder's bottom line.
In all, 22 percent of the studies were funded totally by industry, while 32 percent had both industry and independent funding.
"We found when a food company pays for a study, the results are about eight times more likely to be favorable to the company's financial interest than when the studies are funded independently," Ludwig said. "It is a strong association. It raises concern for bias."
The same association has been found in studies of medications funded by drug companies, Ludwig said. But, he added, bias in studies of beverages could have a greater impact because nearly everyone drinks milk, juices or soft drinks.
"This is the first time this issue has been investigated systematically in the area of nutrition," Ludwig said. "More research needs to be done. No one study can prove an issue. This [conclusion] argues for the need for more independent funding" of research.
The results of Ludwig's study are published in the Jan. 9 online issue of the journal PloS Medicine.
Susan K. Neely, president and chief executive officer of the American Beverage Association, took exception to the study. "This is yet another attack on industry by activists who demonstrate their own biases in their review by looking only at the funding source and not judging the research on its merits. The science is what matters -- nothing else," she said in a prepared statement.
In an accompanying perspective article in the journal, Martijn Katan, professor of nutrition at Vrije Universiteit Amsterdam in the Netherlands, noted that a "blanket condemnation of industry-supported research" isn't the answer. He said collaboration with industry allowed him "to discover things that I could not have found otherwise." For instance: "We discovered the effects of trans fatty acids on heart-disease risk thanks to the expertise of Unilever, and the cholesterol-raising factor in unfiltered coffee thanks to Nestle."
In the Netherlands, Katan wrote, "The Royal Netherlands Academy of Sciences has put forth an innovative proposal on how to supervise relations between researchers and their sponsors."
Until more guidance is available, another expert, Connie Diekman, director of university nutrition at Washington University in St. Louis, has a suggestion. "The [Ludwig] study reminds consumers and the media that all research studies should be viewed as one part of the puzzle about food and health and not conclusive answers to questions." Any conclusions must come from several studies, not one, she added.
SOURCES: Connie Diekman, R.D., director of university nutrition, Washington University, St. Louis; David S. Ludwig, M.D., Ph.D., associate professor of pediatrics, Harvard Medical School, and director of the Optimal Weight for Life program at Children's Hospital Boston; Jan. 8, 2007, news release, American Beverage Association, Washington, D.C.; Jan. 9, 2007, PLoS Medicine, online
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