Vegetarian Diets and Perceived Health: Cause or Effect?
Potential limitations of our results are due to the fact that the survey was based on cross-sectional data. Therefore, no statements can be made whether the poorer health in vegetarians in our study is caused by their dietary habit or if they consume this form of diet due to their poorer health status. We cannot state whether a causal relationship exists, but describe ascertained associations.
Our results have shown that vegetarians report chronic conditions and poorer subjective health more frequently. This might indicate that the vegetarians in our study consume this form of diet as a consequence of their disorders, since a vegetarian diet is often recommended as a method to manage weight and health.
...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.
Vegetarian Diets and Cancer
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
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.
The finding of a decreased risk of cancer in vegetarians may 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.10 11 12 Heme iron has also been associated with numerous other cancers.
Vegetarian Diets and Heart Disease
In the Austrian Health Interview Survey, it was suggested that subjects classified as vegetarians were more likely to have had a history of heart attacks. It is important to note however, that, plant-based diets, poor in saturated fat and cholesterol have for long been adopted by individuals at risk of coronary heart disease. For example, it is known that in studies carried out as far back as the late 1950s, subjects with unfavorable blood cholesterol levels tended to limit intake of dietary cholesterol and saturated fat in order to improve cardiovascular risk factor.13
I previously carried out a meta-analysis of 7 prospective cohort studies comparing the rate of death of coronary heart disease of vegetarians compared to health conscious omnivores. For this review, I examined the incidence of coronary heart disease, and limited the inclusion criteria to studies that provided estimates specifically for subjects classified as either vegans, or lacto-ovo vegetarians. In a meta-analysis including 7 prospective cohort studies, vegetarians had a statistically highly significant 24% lower risk of coronary heart disease compared to health conscious omnivores (Fig. 2).5 6 7 14 15
|FIGURE 2. Risk ratios and 95% CIs for fully adjusted random-effects models examining associations between vegetarian diets in relation to coronary heart disease incidence. VEG, vegetarian diet.|
Vegetarian Diets and Mental Heath
The findings from a number of clinical trials cast doubt on the hypothesis that an appropriately designed flesh free diet has adverse effects on, and that flesh rich diets, poor in carbohydrate have beneficial effects on overall mental health.
- Sacks and colleagues carried out a crossover trial to examine the effects of adding 250 g/day of beef isocalorically to the diet on blood cholesterol of vegetarians. As expected, during the meat phase total cholesterol and systolic blood pressure increased significantly. However, it was also observed that the participants experienced increased anger, anxiety, confusion, depression, and fatigue and less vigor compared to the vegetarian phase.17
- Beezhold and Johnston compared the mood scores of participants assigned to either a vegetarian diet, excluding all animal foods except dairy to participants assigned to either a omnivorous diet, or a diet that included fish, but excluded meat and poultry. The researchers found that the vegetarian group demonstrated significantly improved mood scores compared to both the omnivorous and fish groups.18
- Schweiger and colleagues compared the effects of a vegetarian diet and an omnivorous diet on global mood scores. They found that the vegetarian group demonstrated significantly better global mood, and that carbohydrate intake associated with better global mood.19
- Kieldsen-Kragh examined the effects of a vegetarian diet on rheumatoid arthritis. The researchers hypothesized that the participants may find the vegetarian diet too restrictive, and that therefore adherence to the diet would impose psychological distress on the them. However, contrary to their expectations, the vegetarian group demonstrated significantly improved physiological health, and were less anxious and depressed compared to the omnivorous group.20
- Brinkworth and colleagues examined the effects of a very low-carbohydrate diet and a low-fat diet on body weight and mood and cognitive function. Although there was no statistical difference in terms of weight loss between the groups, the participants assigned to the low-fat group demonstrated significantly improved mood scores compared to the participants assigned to the low-carbohydrate diet.21
- Holloway and colleagues carried out a crossover trial to examine the effects of a high-fat, low-carbohydrate diet on alterations to heart and brain function. The researchers found that the participants not only demonstrated significantly impaired cardiac health, but also impaired attention, memory recall speed, and mood while following the high-fat, low-carbohydrate diet.22
- Halyburton and colleagues examined the effects of a low and high-carbohydrate diet on mood and cognitive function. Although, unlike other studies, the researchers found that mood was similar in both groups, participants assigned to the low-fat diet demonstrated improved speed of processing compared to the participants assigned to the low-carbohydrate group.23
Mass Media as a Source of Health Information
Although there is convincing evidence of the health benefits of an appropriately planned diet that either excludes or significantly limits the intake of flesh, such findings cannot be extrapolated to all diets that exclude flesh. The definition of a vegetarian diet only provides information as to what foods an individual restricts, and not which foods are included. This is why the emphasis of a healthy diet also needs be on which foods are included, not only on those that are excluded. Future research in this area should address what foods vegetarians are substituting meat with, the length of adherence to a vegetarian diet, and whether subjects adopted a vegetarian diet in order to alleviate poor health. This would allow for a considerably more meaningful interpretation of the effects of vegetarian diets.