ViewpointThose confounded vitamins: what can we learn from the differences between observational versus randomised trial evidence?
Section snippets
Why did observational studies and randomised controlled trials come up with different answers?
Several reasons have been proposed for the disparity between the results of observational epidemiological studies and trials. First, antioxidants might be useful only for primary prevention of cardiovascular disease, and not protective once atherosclerosis is established.6 However, trials4, 7 found that antioxidants did not reduce cardiovascular disease risk in participants who had no evidence of this disorder at randomisation. Second, in many of the trials the choice of antioxidant regimen has
Better design of observational studies
Randomised controlled trials provide the most robust estimates of causal effects. However, a more careful approach to the implementation and interpretation of observational studies is needed, since trials are not always feasible and observational studies often generate hypotheses that are later tested in randomised controlled trials. In view of the expense of randomised controlled trials, only candidate agents with a high probability of being causal factors can be tested in this way. Here we
Conclusions
The disparity between observational studies and randomised trial evidence of the health effects of antioxidant vitamins is probably explained by a failure to appreciate the complex and important differences between adults with high vitamin concentrations and those with lower concentrations. High intake of antioxidant vitamins might not be causally related to cardiovascular and other diseases, but rather serves as a proxy indicator of a host of factors that protect against these diseases.
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