I would wager that Vegans typically appear (ok, maybe "are") healthier than their meat-eating counterparts due to a significant number of other lifestyle changes that accompany such a belief system.
It is likely that were a fair survey done, you'd see increased percentages of people exercising regularly, taking vitamins and getting more sleep who were vegans, just based on the lifestyle they choose.
If my assumption is correct, then the theory that choosing to not eat meat leads to being a healthier person is flawed (if not downright incorrect).
An experiment could be performed, but isolating the single variable (to eat meat, or not to eat meat) would be challenging. Different ages, races and other genetic factors would likely effect the results. This means such an experiment would demand the honesty and integrity of a large, diverse group of people. Human nature threatens the accuracy of the potential experiment, even before it can begin.
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