There was a very good summary published 2 years ago in the Proceedings of the Nutrition Society which basically looked at all the major empirical studies regarding vegetarians. The interesting findings were as follows:
Recent German study actually found that non-vegetarians who have healthy habits and excercise a lot actually had LOWER mortality rates than vegetarians. And again, they found that there was no lower mortality rate among vegetarians.
I never did much research in this area but even upon a brief look at the medical journals, it seems that the consensus is NOT that vegetarianism has been shown to reduce mortality in perhaps any way, much less a significant way. At least when the studies are done well.
Vegetarian and vegan diets can vary widely, but the empirical evidence largely relates to the nutritional content and health effects of the average diet of well-educated vegetarians living in Western countries, together with some information on vegetarians in non-Western countries.
...
Cohort studies of vegetarians have shown a moderate reduction in mortality from IHD but little difference in other major causes of death or all-cause mortality in comparison with health-conscious non-vegetarians from the same population. Studies of cancer have not shown clear differences in cancer rates between vegetarians and non-vegetarians.
Recent German study actually found that non-vegetarians who have healthy habits and excercise a lot actually had LOWER mortality rates than vegetarians. And again, they found that there was no lower mortality rate among vegetarians.
Both vegetarians and nonvegetarian health-conscious persons in this study have reduced mortality compared with the general population. Within the study, low prevalence of smoking and moderate or high level of physical activity but not strictly vegetarian diet was associated with reduced overall mortality. The nonsignificant reduction in mortality from ischemic heart diseases in vegetarians compared with health-conscious persons could be explained in part by avoidance of meat intake.
I never did much research in this area but even upon a brief look at the medical journals, it seems that the consensus is NOT that vegetarianism has been shown to reduce mortality in perhaps any way, much less a significant way. At least when the studies are done well.