9/30/08 12:46 PM
Medical Studies Vary in Validity of Findings - NYTimes.com
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September 30, 2008
Searching for Clarity: A Primer on Medical Studies
Everyone, it seemed, from the general public to many scientists, was enthralled by the idea that
would protect against
. In the early 1990s, the evidence seemed compelling
that this chemical, an antioxidant found in fruit and vegetables and converted by the body to
vitamin A, was a key to good health.
There were laboratory studies showing how beta carotene would work. There were animal studies
confirming that it was protective against cancer. There were observational studies showing that
the more fruit and vegetables people ate, the lower their cancer risk. So convinced were some
scientists that they themselves were taking beta carotene supplements.
Then came three large, rigorous clinical trials that randomly assigned people to take beta carotene
pills or a placebo. And the beta carotene hypothesis crumbled. The trials concluded that not only
did beta carotene fail to protect against cancer and heart disease, but it might increase the risk of
It was “the biggest disappointment of my career,” said one of the study researchers, Dr. Charles
Hennekens, then at
Brigham and Women’s Hospital
But Frankie Avalon, a ’50s singer and actor turned supplement marketer, had another view. When
the bad news was released, he appeared in an infomercial. On one side of him was a huge stack of
papers. At his other side were a few lonely pages. What are you going to believe, he asked, all
these studies saying beta carotene works or these saying it doesn’t?
That, of course, is the question about medical evidence. What are you going to believe, and why?
Why should a few clinical trials trump dozens of studies involving laboratory tests, animal studies
and observations of human populations? The beta carotene case is unusual because much of the
time when laboratory studies, animal studies and observational studies point in the same
direction, clinical trials confirm these results.
There are exceptions, notably the Women’s Health Initiative, a huge study begun in 1991 by the