Normally in these studies they do attempt to control for other variables.
It's true, most people who take megadoses of C also take other vitamins,
and may have a healthier diet and lifestyle.
But these factors are not perfectly correlated. Some people rely on
swallowing pills and eat like pigs. Others follow an ascetic diet
and exercise regimen but refuse to take "unnatural" vitamins. As a
result when you do your statistical analysis you do have some basis
for disentangling the various factors (at least, the ones you know to
check for).
With a large enough population that it should be possible to draw
conclusions about the various factors and their effects. The statistical
tests used by the researchers should show how successful they have been
in isolating the various factors affecting health and longevity.
Hal
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