It should be stressed that the meat intake in the non-vegetarian group in most of these studies was significantly lower than that of the general population. As described in a different review, the selection criteria for the non-vegetarians in these studies generally included being affiliated with vegetarians in some way or another, likely explaining their relatively low meat intakes. Furthermore, evidence suggests that many of the self-proclaimed vegetarians in these studies actually consumed meat on a regular basis, resulting in minimal differences in meat intake between the groups.10Considering the lack of difference in meat intake between the vegetarian and non-vegetarian groups, it would only be expected that these studies would not have the statistical power to demonstrate a significant benefit of a vegetarian diet. Two studies which may be considered as especially having limited statistical power due to such limitations were the Heidelberg Study and Health Food Shoppers Study. In the Heidelberg Study, the non-vegetarian group were predominantly semi-vegetarians, while in the Health Food Shoppers Study, a validity assessment of the survey used to classify the participants vegetarian status suggested that 34% of the participants classified as vegetarians actually consumed meat. In fact, a slightly greater percentage of participants classified as vegetarians in the Health Food Shoppers Study were found to consume meat 3 or more times a week than the non-vegetarians in the Heidelberg Study (7.6% and 6.9%, respectively).14 18 Taking this into consideration, I performed a sensitivity analysis excluding either the Heidelberg Study or Health Food Shoppers Study from the meta-analysis. Excluding the Health Food Shopper Study alone reduced heterogeneity and strengthened the association between vegetarian status and a reduced risk of mortality from all-causes (RR 0.91 [95% CI, 0.87-0.94], and 0.92 [95% CI, 0.86-0.98] using the fixed effects and random effects model, respectively).

Most of these studies did not provide separate data for length of adherence to a vegetarian diet. As already described earlier in this review, evidence from several of these studies suggest a stronger effect on mortality would have been observed if the analysis was restricted to long-term vegetarians.10 Another important limitation was that most studies also did not verify changes to vegetarian status of the participants throughout the follow-up, which may in part explain why the association between a vegetarian diet and a reduced risk of mortality weakened over time in several of the studies.9 10

Another important potential limitation of these studies described in detail in the next section, is that some of the participants likely adopted a vegetarian diet in order to improve poor health, such as symptoms of an undiagnosed or developing illness that would ultimately became fatal. Each of these limitations described are expected to have either biased these findings towards null, or even in favor of the non-vegetarian group, suggesting that the findings of this meta-analysis may have significantly underestimated the benefits of an appropriately planned vegetarian diet.

In order to determine which factors may have contributed to the observed reduced risk of death in vegetarians, I also performed separate meta-analyses for the major causes of death. This included mortality from coronary heart disease and cardiovascular disease, and the incidence of all cancers combined. For mortality from coronary heart disease, based on 7 cohorts, the vegetarian group had a statistically significant 25% and 24% reduced risk, using the fixed effects and random effects models, respectively (Fig. 3).13 14 15 16 17 For mortality from cardiovascular disease, based on 7 cohorts, the vegetarian group had a statistically significant 17% and 14% reduced risk, using the fixed effects and random effects models, respectively (Fig. 4).13 14 15 16 17 For the Adventist Mortality Study and Adventist Health Study, mortality from cardiovascular disease was derived from pooling the relative risk for mortality from coronary heart disease and stroke.17 Excluding the Adventist Mortality Study and the Adventist Health Study, the vegetarian group had a statistically significant 8% reduced risk of mortality from cardiovascular disease, using both the fixed effects or random effects model (0.92 [95% CI, 0.85-0.99]).

Figure 3. Coronary heart disease mortality for vegetarians compared to health conscious non-vegetarians in a meta-analysis using the fixed effects model.  *Indicates that participants classified as semi-vegetarians were included in the vegetarian group

Figure 4. Cardiovascular disease mortality for vegetarians compared to health conscious non-vegetarians in a meta-analysis using the fixed effects model.  *Indicates that participants classified as semi-vegetarians were included in the vegetation group

As described in a previous review, the degree of reduction in risk of mortality from coronary heart disease observed in vegetarians in these cohort studies was generally in proportion to the expected reduced risk based on the differences in levels of total and non-HDL cholesterol, and blood pressure. There is a plethora of evidence, not only from epidemiological studies, but also clinical trials that plant-based diets and nutrients have favorable effects on total and LDL cholesterol, blood pressure, among several other factors which are established risk factors for cardiovascular and all-cause mortality.19 20 21 22

In the Oxford Vegetarian Study, high compared to low intake of saturated animal fat was associated with a nearly 3-fold increased risk of coronary heart disease mortality.23 Similarly, in a meta-analysis of 11 cohort studies, high compared to low intake of saturated fat was associated with a 32% increased risk of coronary heart disease mortality, despite the inclusion of over-adjustments for dietary and serum lipids.24 It was also found in the Oxford Vegetarian Study that high compared to low intake of total animal fat and dietary cholesterol was associated with a greater than 3-fold increased risk of coronary heart disease mortality.23 Furthermore, evidence from thousands of experiments carried out over the last century have shown that the feeding of dietary cholesterol and saturated fat has accelerated the development of atherosclerosis in virtually every animal species in which researchers were able to find a method to sufficiently elevate cholesterol concentrations. This includes herbivores, omnivores and carnivores from mammalian, avian and fish species, and over one dozen different species of nonhuman primates.

In both the Oxford Vegetarian Study and the Adventist Mortality Study, high compared to low intake of eggs was associated with an increased risk of coronary heart disease mortality.23 25 However, for ill-defined reasons, these studies were excluded from several recent meta-analyses. In the Adventist Mortality Study and Heidelberg Study, high compared to no intake of meat was associated with a 50% and almost 5-fold increased risk of coronary heart disease mortality, respectively.14 25 Similarly, in the Adventist Health Study, high compared to no intake of beef was associated with a greater than 2-fold increased risk for men.26 In addition, recent meta-analyses of prospective cohorts found that an increment of 1 mg/day of heme iron, found only in animal tissue, is associated with a 16% and 27% increased risk increased risk of type II diabetes and coronary heart disease, respectively.27 28 Therefore, the totality of evidence strongly suggests that the observed greater longevity of vegetarians can be explained, at least in part, by the reduced risk of cardiovascular disease as the result of the replacement of animal foods with minimally processed plant foods.

For incidence of all cancers combined, based on 5 cohorts, the vegetarian group had a statistically significant 8%, and borderline significant 6% reduced risk, using the fixed effects and random effects model, respectively (Fig. 5).13 14 29 30 Excluding the Health Food Shoppers Study removed evidence of heterogeneity and strengthened these findings (RR 0.90 [95% CI, 0.85-0.99] using both the fixed effects and random effects models).

Figure 5. Cancer incidence for vegetarians compared to health conscious non-vegetarians in a meta-analysis using the fixed effects model.  *Indicates that participants classified as semi-vegetarians were included in the vegetation group

The finding of a decreased risk of cancer in vegetarians may also be explained, in part, by a diet devoid in heme iron. Controlled feeding trials have established that NOCs (N-nitroso compounds) arising from heme iron in meat forms potentially cancerous DNA adducts in the human digestive tract, likely in part, explaining the significant association between heme iron and an increased risk of colorectal cancer in recent meta-analyses of prospective cohort studies.31 32 33 Heme iron has also been associated with numerous other cancers. These lines of evidence also provide confidence in the validity of the findings of greater longevity in vegetarians.

Why Some People Choose to Become Health Conscious

In the chapter Herbivore’s Dilemma, Minger reviews several prospective cohort studies that compared the mortality rates of vegetarians to health conscious non-vegetarians, carefully selecting only those studies in which vegetarians were not found to live longer. These studies were the Oxford Vegetarian Study, the Health Food Shoppers Study and the Heidelberg Study, all included in my meta-analysis. Minger emphasizes the lack of reduced risk of mortality in the vegetarian groups, while neglecting to mention that the difference in intake of meat between the groups was relatively small. Minger especially focuses on the Heidelberg Study, happening to be the smallest study, which found a non-significant increased risk of all-cause mortality in the vegetarian group. Minger points out that in this particular cohort, the vegetarians had greater levels of physical activity, consumed less alcohol and smoked less. Minger then states “Mortality and disease rates, in this case, might be expected to turn up in favor of the vegetarian crowd, even though the opposite ended up happening”[p.270]. This statement is outright misleading as all these factors were controlled for in the analysis. One could also correctly point out that the vegetarians tended to be older than the meat eaters, but suggesting that this would be expected to turn up in favor of the meat eaters would also be misleading, as age was also controlled for. Either way, the differences in mortality between the groups did not even come close to being statistically significant.

There is strong evidence that plant-based diets, including those rich in whole-grains and legumes reduces the risk of premature death from chronic and degenerative diseases, which in-turn helps explain the longevity of a number of plant-based populations. Contrary to the findings for plant-based diets, there is a plethora of evidence demonstrating the harms of popular carbohydrate restricted diets. For example, meta-analyses of clinical trials have found that low-carbohydrate diets elevate LDL cholesterol and impair endothelial function.39 Furthermore, a recent meta-analysis of prospective cohort studies with more than 272,000 participants found that low-carbohydrate diets, particularly those rich in animal foods are associated with an increased risk of all-cause mortality.40 Evidence also shows that the hazardous effects of diets rich in animal foods are also applicable to that of organic, grass-fed animal foods. These, and other lines of evidence described throughout this review appear to have been almost entirely ignored by proponents of these diets, perhaps because they find it too difficult to explain. 

Although there is strong evidence that plant-based diets promote longevity, it is important to replace animal foods with minimally refined plant foods in order to achieve the maximum benefits. This may in part explain why the benefits of a vegetarian diet were more evident in the Adventists than other populations.9 In conclusion, the totality of evidence supports the hypothesis that appropriately planned whole foods, plant-based diets promote longevity.