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Observational Studies Often Overshadow Clinical Research

First findings seem to stick, even when controlled trials later contradict them

TUESDAY, Dec. 4 (HealthDay News) -- Even when there's strong contradictory evidence from randomized trials, refuted claims based on observational studies often persist in scientific circles, Greek researchers claim.

Randomized trials, where participants are randomized to receive one intervention or another and then followed prospectively, are considered by many experts the "gold standard" for research.

But in a look at specific health claims involving vitamin E, beta carotene and estrogen, the Greek team noted that claims from observational studies of the cardiovascular benefits of vitamin E often continue to be supported in the medical literature, even though randomized trials have produced the opposite conclusion. The same is true for claims about the protective effects of beta carotene on cancer, and estrogen on Alzheimer's disease.

The University of Ioannina School of Medicine focused on two 1993 epidemiological studies that suggested an association between vitamin E and improved cardiovascular health. These studies were later strongly contradicted by evidence from large, randomized clinical trials. Even so, many articles in the medical literature still contain positive references to the findings from the epidemiological studies.

For example, in 2005, more than 50 percent of articles citing the two studies did so in favorable way, the Greek researchers found.

Initial claims about the effectiveness of beta carotene for preventing cancer and of estrogen for preventing Alzheimer's disease have been contradicted, but still appear in medical literature articles.

The researchers examined articles on beta carotene/cancer and estrogen/Alzheimer's published in 2006. For beta-carotene, 10 citing articles (62.5 percent) were favorable, three (18.8 percent) were equivocal, and three (18.8 percent) were unfavorable. For estrogen, 29 citing articles (61.7 percent) were favorable, 14 (29.8 percent) equivocal, and four (8.5 percent) unfavorable.

The study appears in the Dec. 5 issue of the Journal of the American Medical Association.

"Counterarguments defending vitamin E or estrogen included diverse selection and information biases and genuine differences across studies in participants, interventions, co-interventions, and outcomes. Favorable citations to beta carotene, long after evidence contradicted its effectiveness, did not consider the contradicting evidence," the Greek researchers wrote.

They said "it can be difficult to discern whether perpetuated beliefs are based on careful consideration of all evidence and differential interpretation, inappropriate entrenchment of old information, lack of dissemination of newer data, or purposeful silencing of their existence. Regardless of the reasons, better communication between research specialists and evidence-based clinical science may improve this situation and may lead to more rational and concerted translational efforts in basic, pre-clinical, and clinical research."

In a statement released Tuesday, the Council for Responsible Nutrition, which represents the dietary supplement industry, had this comment on the JAMA finding: "...the reality is that science doesn't always move forward there is some back and forth and while research may contradict itself, that should not be interpreted to mean one type of study trumps another, particularly when different studies ask and answer different questions. Seemingly conflicting data can exist side by side, when one understands that not all studies are asking the same questions in the same populations."

More information

The Family Caregiver Alliance offers advice on evaluating medical research and clinical trials.

-- Robert Preidt

SOURCE: Journal of the American Medical Association, news release, Dec. 4, 2007

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