The danger of making public policy based on epidemiological studies

Scientific American recently ran an interesting story on antioxidants.  For a while, it seems, experts promoted antioxidants based on epidemiological studies that seemed to suggest they increased longevity.  It is a good thing these experts didn't convince policy makers to subsidize or mandated more vitamins and antioxidants in food years ago (although we do have mandated vitamin D milk and iodine in salt), only to discover this:

Vitamins Kill Epidemiological studies show that people who eat lots of fruits and vegetables, which are rich in vitamins and other antioxidants, tend to live longer and are less likely to develop cancer compared with those who do not.  So it seemed obvious that supplementing diet with antioxidants should lead to better health.  But the results of the most rigorously designed studies do not support that assumption.  Indeed, the evidence shows that some people who take certain supplements are actually more likely to develop life-threatening illnesses, such as lung cancer and heart disease.

There are many epidemiological studies showing correlations across people in the intake of one food (e.g., meat, chocolate, blueberries, wine) and some undesirable or desirable health outcome (e.g., cancer, heart disease, longevity, etc.).  But, it cannot be repeated enough: correlation is not causation.