It has been asserted that clinical trials hold the answer to questions about the role of nutrients in preventing chronic diseases. This is not the case. Clinical trials give us rigorous answers to restricted questions. Rarely can more than one or two substances be tested, usually at a single dose. Subjects usually have to be persons with precancerous conditions or an extremely high risk of the disease in question.
Rarely can any diseases other than the most common ones be studied. Most important, clinical trials test the efficacy of an agent that is administered for a limited time, beginning fairly late in life. Few trials will tell us anything about whether dietary amounts of nutrients might contribute to prevention of long-term chronic diseases. They also tell us nothing about whether agents at high doses might reduce disease risk if taken throughout the lifetime.
Furthermore, they tell us nothing about other antioxidants, other combinations, or other doses. Clinical trials were developed for therapeutic situations to determine which treatment was better for curing a specific disease. However, the questions about prevention that are of interest may involve persons with no unusual risk of disease, lifetimes of exposure, enormously complex interactions among nutrients, and the effects of these nutrients on hundreds of often uncommon disease conditions.
Clinical trials simply cannot answer these questions. Only a solid examination of the laboratory and epidemiologic evidence can approximate the answers to most of the questions of interest.
School of Public Health, University of California at Berkeley 94720, USA.
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