n p the subjects with lower a- and p-carotene levels tended to be older and were smoking more in comparison to those with higher plasma levels of a- and p-carotenes.
Other observational trials [20-33] are also listed in table 1.There are several well-known limitations of the epidemiological studies. Often the confounding variables are difficult to identify to be able to make the necessary statistical adjustments. Vitamin intake in these studies is usually determined through notoriously unreliable food questionnaires and dietary recalls. Furthermore, it is also difficult to isolate the exact component of the diet that might have had an impact on a particular outcome.
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