Are There Health Benefits to Being A Vegetarian?

Diet

The perception of the effects of a vegetarian diet on health has gone through a transformation in the last 20 years. In 1980 the American Dietetic Association (ADA) had serious doubts about whether a vegetarian diet was nutritionally sound [1]. Twenty years later the ADA’s position is that appropriately planned vegetarian diets (including vegan diets) are not only nutritionally adequate but may help prevent certain diseases [2]. Support for the view that vegetarianism reduces the risk of certain diseases comes from a variety of studies.  I’ll review some of these to give an idea of what is currently known about the health benefits of being a vegetarian.

However, to begin this discussion it is important to define what a vegetarian is. There are many types of vegetarians but the classic definition is a person who does not eat meat (including fowl) or seafood, or products containing these foods [2]. People, however, who limit their consumption of red meat, chicken, or fish sometimes refer to themselves as vegetarians. In studies of vegetarians these individuals are described as ‘semi-vegetarians’ because consumption of these food groups is infrequent or much less than is typically seen in the general population. There are also subgroups of people who meet the classic definition of vegetarian but include certain non-meat but animal-derived categories of food (dairy and egg products).  These generally are ovo- (eat egg or egg products), lacto- (eat dairy or dairy products) or ovo-lacto (eat egg and dairy products) vegetarians, and those who do not eat meat but do consume fish are pesco-vegetarians. Vegans are vegetarians who do not eat meat, fish, dairy or eggs.  Fruitarians are a sub-group of vegans who further restrict their food intake to fruits, nuts and seeds [2]. Table 1 lists the various types of vegetarians and the broad categories of foods they eat. If nothing else, this table highlights the fact that not all vegetarians are comparable in their diets, and even within a particular category there may be quite a bit of variation in nutrition intake, with the possible exception of vegans and fruitarians who have relatively restricted diets.

Table 1: Types of Vegetarians and Their Consumption Preferences (X indicates food products consumed)

Vegetarian Type % US popn’t* Red Meat (Beef) Poultry Fish/ Seafood Dairy Eggs
Vegan 1-2%
Frutarian <1%
Ovo-Vegetarian 2-5% X
Lacto-Vegetarian X
Ovo-Lacto Vegetarian X X
Pesco-Vegetarian ? X X X
Semi-Vegetarian ? X X X X X

*[2, 3]

It is also important to mention that the studies included in this paper were all done in the United States and Europe. The vegetarian diets in these populations may be quite different from those in other areas of the world so the findings may not be generalizable to other populations. Furthermore, vegetarians in these studies are compared to non-vegetarians and their diet, which is a Western omnivore diet that contains significant amounts of meat and meat products. This may also not reflect the non-vegetarian diets of other populations of the world.

And finally, I should mention something about vegetarian study subjects. In the United States many studies of vegetarians use cohorts of followers of the Seventh-day Adventist Church, a Christian denomination that promotes, among other things, a wholesome and healthy lifestyle. Many Seventh-Day Adventists also avoid alcohol and smoking. Given attributes of this study population it is important to keep in mind that research findings may not be generalizable to other vegetarian populations.

Photo credit: vegetarianfighter.com

Photo credit: vegetarianfighter.com

Vegetarian versus Non-Vegetarian diets

Vegetarian diets, given their restricted nature, are different from omnivore, non-vegetarian diets in many ways. They tend to have higher proportions of whole grains, nuts, fruits and vegetables, and as a result have higher dietary amounts of fiber, antioxidants, vitamins C and E, potassium, and magnesium.  They are also generally lower in calories, saturated fats, cholesterol, long-chain n-2 (omega-3) fatty acids, eicosapentaenoic acid (EPA), decosahexaenoic acid (DHA), iron, zinc and vitamin B-12.  Meats and meat products are a rich source of these nutrients.

Vegetarian diets that restrict dairy or egg consumption are also lower in vitamin B-2 and D, and calcium [3]. This lack of calcium may put vegans who do not supplement with calcium at risk for bone fractures;  A comparative study of vegetarians and omnivores showed no difference in risk for bone fractures between omnivores and ovo-lacto vegetarians, but vegans had a 30% higher risk [4]. In fact lacto- and ovo-lacto vegetarians may consume more calcium in their diets than non-vegetarians.

A review of vegetarian diets based on ten studies found that vegetarians had lower intake than omnivores for only a few nutrients: vitamins B-12 and C, calcium and zinc [2]. Overall conclusions were that vegetarian diets can be nutritionally adequate. Any nutritional deficits can be addressed by taking supplements, eating fortified foods, or preparing and combining foods to enhance absorption of vitamins and minerals (for example, soaking and sprouting beans and grains, or taking iron or zinc-rich foods with fruits rich in vitamin C to enhance iron and zinc absorption) [2]. B-12 is of particular concern to vegetarians, especially vegans, as there is no unfortified plant food that contains any significant amounts of it [2]. B-12 is found in dairy foods and eggs so ovo-lacto vegetarians generally consume adequate amounts. For those who don’t, however, B-12 fortified foods are available including soy and rice beverages, and some cereals.

Differences between Vegetarians and the General Population

Vegetarians also tend to differ from the general population in ways not related to diet intake. Studies show that vegetarians, on average, are more physically active, drink less alcohol, and smoke less than the general population, and also have higher socioeconomic status and a lower prevalence of being overweight or obese [5].

Health Benefits of being a vegetarian

A number of studies dating back to the 1950s have been done to examine diet-disease associations and many of these looked specifically at the effects of a vegetarian diet on health.  These studies focused primarily on prevalent conditions that have high mortality and morbidity, including different cancers, heart disease, and diabetes [6]. In addition, studies have also examined if vegetarians have lower rates of mortality compared to non-vegetarian counterparts [7]. Taken together these studies suggest that vegetarian diets (that are appropriately planned and nutritionally adequate) may lower the risk of certain chronic diseases, notably cardiovascular disease, diabetes and hypertension. I’ll go into the studies related to these conditions in a little more depth.

Cardiovascular Disease:

There appears to be strong evidence for the association between vegetarianism and lower risks for cardiovascular disease (CVD). A combined analysis of 5 prospective studies involving two cohorts of Seventh Day Adventists [8, 9] and a cohort of British [10] and German vegetarians [11] showed that  mortality from heart disease was 24% lower in vegetarians compared to non-vegetarians after adjusting for sex, smoking status and age [12]. These studies categorized individuals by degree of vegetarianism and showed that different groups had different rates of mortality from heart disease: pesco and lacto-ovo vegetarians had a 34% lower rate of mortality and vegans had a 26% lower rate. Vegetarians in the studies as a whole had higher nut and whole grain consumption, foods that are associated with lower CVD risk, and had lower total or LDL cholesterol levels, which are also associated with lower CVD risks.  Furthermore, in each of the studies the vegetarian subjects had lower body mass indexes compared to their non-vegetarian counterparts; The association between body mass and blood pressure, and studies of hypertension in vegetarians [13, 14]suggest that the vegetarians in these studies also had a lower prevalence of hypertension, another risk factor for CHD, although blood pressure was not measured in these cohorts.

Another comparison study between vegetarians (Californian Seventh-Day Adventists) and non-vegetarians (Californians from five cities) examining risks of having a first-time, fatal heart attack showed that the risks (adjusted for age) were about half that for vegetarians compared to their non-vegetarian counterparts [15].

While these results support a conclusion that a vegetarian diet may protect against heart disease, it can be argued that it is not the restriction of meat from the diet that provides the benefit but an increased consumption of whole grains, nuts, fruits and vegetables. Diets rich in these foods are associated with higher fiber content, lower cholesterol levels, a lower prevalence of obesity, lower blood pressure levels and a higher antioxidant status, all things that may reduce the risk of heart disease [5, 16].

Diabetes:

Most studies also agree on the association between a vegetarian diet and diabetes, with vegetarians having lower rates of diabetes. A large study showed that the risk of diabetes (adjusted for age) was about twice as high for non-vegetarians as for vegetarians  [14]. Another large study looking at the prevalence of self-reported diabetes and diabetes-related mortality showed that non-vegetarians reported having diabetes 1 ½ to 2 times more often than vegetarians, and that non-vegetarian men were about twice as likely to die from a diabetes-related cause then vegetarian men, after adjusting for age and weight. No difference in diabetes-related mortality, however was noted between vegetarian and non-vegetarian women in the study [17].

Lower rates of diabetes in vegetarians can be ascribed to having lower body weights. A study of body mass index (BMI), diabetes and hypertension in different types of vegetarians showed that non-vegetarians have the highest average BMIs (average of 28.26) and vegans have the lowest (average of 23.13). BMIs between 18.5 and 24.9 are considered to reflect a healthy weight [18].  In fact BMI tracks along the gradient of vegetarian food restriction, with those with the most restricted diets having the lowest BMIs [6]. Of note, hypertension and diabetes also track with BMI and vegetarian diet restriction, with those having the most restricted diet (vegans) having the lowest levels of hypertension and diabetes- about a quarter of the levels of diabetes or hypertension that is seen in non-vegetarians [14].

Cancer:

Many studies have looked at the association between a vegetarian diet and the risk of different cancers  [6]. A noteworthy association has only been found with colorectal cancer, but the evidence is mixed. A large six year study of individuals with no reported history of cancer showed that those who regularly ate red meat had nearly twice the risk for colon cancer compared to those who did not eat red meat [19]. In addition an inverse association was found with legume intake (legumes include foods like peas, beans, and lentils):  A higher colon cancer rate was associated with a lower legume intake. In addition a positive association with BMI was noted.  Therefore, in summary, overweight meat eaters who did not eat legumes had the highest rates of colon cancer.

However, these results are not supported by other studies. A study of 34,000 subjects showed that the risk of colon, breast, lung, prostate and uterine cancer, adjusted for age, sex and smoking, were no different between vegetarians and non-vegetarians [14]. In addition, in a pooled analysis of five larger studies looking at cancer mortality rates in vegetarians and non-vegetarians, no differences were found between the two groups after adjusting for age  and smoking status in mortality rates of colorectal, lung, stomach, prostate or breast cancer [12].

All cause mortality:

Finally, many studies have looked at the association between vegetarianism and the risk of mortality (due to all causes). These give mixed results.  The thought is that if a vegetarian diet is healthier than non-vegetarian diets then mortality rates for vegetarians should be lower compared to non-vegetarians. One study suggests that the association between mortality and vegetarianism differs depending on the age group (an age modification effect) with regular meat consumption only associated with increased mortality up until age 60 years. After age 60 years the mortality rates between the two groups (vegetarians and non-vegetarians) does not differ significantly [6]. Other studies, however, differ and suggest that there are no differences between vegetarians and non-vegetarians in terms of all-cause mortality, regardless of age. In a British study of 64,000 individuals 20-89 years old, the risk of mortality for both vegetarians and non-vegetarians was identical after adjusting for age, sex, smoking and alcohol consumption [7]. These results, in turn, are at odds with results of an eleven year follow up study of about 2,000 German vegetarians that showed that all-cause mortality was lower among vegetarians than non-vegetarians. However BMI, socioeconomic and smoking status were not controlled for in this study and it is known that similar low mortality rates are found in non-vegetarian populations that have a low prevalence of smoking and alcohol consumption (notably Mormons) [20]. Furthermore, to confuse the situation even more, the healthiest Europeans as judged by longevity- the Swiss, Swedes, Norwegians and Icelanders-  consume large amount of animal products, although primarily fish, dairy and poultry [5].

In summary there is evidence that a vegetarian diet protects against cardio-vascular disease, particularly heart disease, and there may be some health benefits related to diabetes and colon cancer. Evidence is lacking, however, for any benefits related to other cancers, other conditions, or mortality rates. Whether the benefits are related to not consuming meat/meat products or having a higher consumption of whole grains, nuts, fruits and vegetables is not clear. In addition, the fact that vegetarians generally have lower BMIs, lower blood pressure, lower alcohol intake, smoke less, have higher levels of physical activity and have higher socioeconomic levels confounds the issue. The take home message is that there may be health benefits to being a vegetarian, but these may be due mostly to eating a balanced diet, exercising regularly, limiting alcohol consumption and not smoking.

Nathalie McIntosh is a doctoral candidate at BUSPH.

References:

1.            ADA, Position paper on the vegetarian approach to eating. American Dietetic Association Reports, 1980. 77: p. 61-69.

2.            Craig, W., Mangels, AR, Position of the American Dietetic Association: Vegetarian Diets. Journal of the American Dietetic Association, 2009. 109(7): p. 1266-1282.

3.            Craig, W., Health effects of vegan diets. American Journal of Clinical Nutrition, 2009. 89(suppl): p. 1627S-1633S.

4.            Appleby, P., Roddam, A, Allen, N, Key, T, Comparative fracture risk in vegetarians and nonvegetarians in EPIC-Oxford. European Journal of Clinical Nutrition, 2007. 61(12): p. 1400-1406.

5.            Ginter, E., Vegetarian diets, chronic diseases and longevity. Bratisl Lek Listy, 2008. 109(10): p. 463-466.

6.            Fraser, G., Vegetarian diets: what do we know of their effects on common chronic diseases? American Journal of Clinical Nutrition, 2009. 89(suppl): p. 1607S-1612S.

7.            Key, T., Appleby, PN, Spencer, EA, Travis, RC, Roddam, AW, Allen, NE, Mortality in British vegetarians: results from the European Prospective Investigation into Cancer and Nutrition (EPIC-Oxford). American Journal of Clinical Nutrition, 2009. 89(suppl): p. 1613S-1619S.

8.            Snowdon, D., Animal product consumption and mortality because of all causes combined, coronary heart disease, stroke, diabetes, and cancer in Seventh-day Adventists. American Journal of Clinical Nutrition, 1988. 48(suppl): p. 739-748.

9.            Beeson, W., Mills, PK, Phillips, RL, Chronic disease among Seventh-day Adventists, a low-risk group. Cancer, 1989. 64: p. 570-581.

10.          Burr, M., Butland, BK, Heart disease in British vegetarians. American Journal of Clinical Nutrition, 1988. 48: p. 830-832.

11.          Frentzel-Beyme, R., Claude, J, Eilber, U, Mortality among German vegetarians: first results after five years of follow up. Nutrition and Cancer, 1988. 11: p. 117-126.

12.          Key, T., Fraser, GE, Thorogood, M, Appleby, PN, Beral, V, Reeves, G, Burr, ML, Chang-Claude, J, Frentzel-Beyme, R, Kuzma, JW, Mann, J, McPherson, K, Mortality in vegetarians and nonvegetarians: detailed findings from a collaborative analysis of 5 prospective studies. American Journal of Clinical Nutrition, 1999. 70(suppl): p. 516S-524S.

13.          Appleby, P., Davey, GK, Key, TJ, Hypertension and blood pressure among meat eaters, fish eaters, vegetarians and vegans in EPIC-Oxford. Public Health Nutrition, 2002. 5: p. 645-654.

14.          Fraser, G., Associations between diet and cancer, ischemic heart disease, and all-cause mortality in non-Hispanic white California Seventh-day Adventists. American Journal of Clinical Nutrition, 1999. 70(suppl): p. 532S-538S.

15.          Fraser, G., A comparison of first event coronary heart disease rates in two contrasting California populations. Journal of Nutrition, Health and Aging, 2005. 9(1): p. 53-58.

16.          Fraser, G., Sabate, J, Beeson, WL, Strahan, TM, A possible protective effect of nut consumption on risk of coronary heart disease. Archives of Internal Medicine, 1992. 152: p. 1416-1424.

17.          Snowdon, D., Phillips, RL, Does a vegetarian diet reduce the occurrence of diabetes? American Journal of Public Health, 1985. 75: p. 507-512.

18.          NHLBI.  2010  [cited 2010 December 4]; Available from: http://www.nhlbisupport.com/bmi/.

19.          Singh, P., Fraser, GE, Dietary risk factors for colon cancer in a low-risk population. American Journal of Epidemiology, 1998. 148: p. 761-774.

20.          Chang-Claude, J., Frentzel-Beyme, R, Eilber, U, Mortality pattern of German vegetarians after 11 years of follow up. Epidemiology, 1992. 3: p. 395-401.