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Observational Studies Often Overshadow Clinical Research
Date:12/4/2007

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 estr
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