Kids Healthy Food Biography
In a meta-analysis including 7 prospective cohort studies, vegetarians had a statistically significant 9% lower risk of cancer incidence compared to health conscious omnivores (Fig. 1).5 6 7 8 9 It is important to note that meat intake was relatively low in the omnivorous group in these studies, especially taking into account that a significant portion of the omnivorous subjects were actually classified as semi-vegetarians. This suggests the difference in cancer incidence may be greater when compared to regular meat eaters.
The researchers suggested that if anything, it was not a flesh free diet that caused a higher rate of a number of health problems, but rather that it was poor health that caused these subjects to adopt a flesh free diet. This is similar to the phenomenon where former smokers report poorer perceived health than current smokers, because they quit smoking with the intention of alleviating poor health.2 This phenomenon is often referred to as reverse causality.
Unfortunately, Benjamin Fearnow, the author of the article in the CBS Atlanta ignored the evidence suggesting that these results were the result of reverse causality, and instead suggested that a flesh free diet was actually the cause of a number of health problems:
...the vegetarian diet — characterized by a low consumption of saturated fats and cholesterol that includes increased intake of fruits, vegetables and whole-grain products — carries elevated risks of cancer, allergies and mental health disorders.
It is important to note that the Austrian Health Interview Survey did not measure food intake in actual detail. Subjects who reported consuming a flesh free diet were simply assumed to be consuming a diet poor in dietary cholesterol and saturated fat. However, in this study 36% of the vegetarian subjects were classified as lacto-ovo vegetarians, and 55% pescetarians (allowing fish, dairy and eggs). Only 9% were classified as vegans.1 Therefore, up to 91% of the subjects classified as vegetarians consumed dairy and eggs, being the richest sources of saturated animal fat and cholesterol, respectively. The CBS Atlanta failed to mention even the definition of a vegetarian diet used in this study, yet alone the breakdown of subjects in each category of vegetarian diet.
At the time of the report, it was observed that 4.8% of the subjects of the Austrian Health Interview Survey classified as vegetarians had cancer, as opposed to 1.8% of the subjects following an omnivorous diet rich in meat. Unfortunately, no details were provided as to what portion of the studied population adopted a flesh free diet after diagnosis. However, data from previous studies suggest that cancer patients are highly motivated to adopt a plant based diet. As described previously:
The results of a recent study from the Netherlands illustrates the critical importance of considering reverse causality in research on plant-based diets. The researchers found that 75% of the vegetarian participants with cancer adopted a vegetarian diet after diagnosis, consistent with previous research which found that cancer survivors are highly motivated to adopt a more plant-based diet with the intention of improving poor health.3 4
If the 75% figure from the study from the Netherlands is to be considered representative of this Austrian population, this would suggest that only 1.2% of the vegetarians adopted a flesh free diet prior to diagnosis of cancer. This is lower than the 1.8% figure for omnivores following a meat rich diet, but similar to that of the omnivores following a diet low in meat. Unfortunately, due to the lack of reliable data these estimates should be taken with a grain of salt.
Prospective (forward-looking) studies which measure diet before diseases are diagnosed are much less likely to be complicated by reverse causality than cross-sectional studies, and therefore considered to be more appropriate for determining causality. I previously carried out a meta-analysis of 5 prospective cohort studies comparing the rates of cancer incidence in vegetarians compared to health conscious omnivores. For this review, I updated the meta-analysis to include the rates of major cancers in the Adventist Mortality and Adventist Health studies. In addition, I limited the inclusion criteria to studies that provided estimates specifically for subjects classified as either vegans, or lacto-ovo vegetarians.