Association between Vegan, Vegetarian, and Omnivorous Diets and Heart Disease Morbidity and Mortality
Michelle D. Adams
Short Title: Diet and Heart Disease
Heart disease is the leading cause of death in the United States. This chronic disease has a genetic component but is largely the result of poor lifestyle habits which may include an inadequate diet. Vegans, vegetarians, and omnivores consume different diets by definition and thereby ingest different levels of micronutrients and macronutrients. The aim of this literature review was to determine whether or not there was an association between following one of the aforementioned diets and the risk of developing and/or dying from heart disease. Based on epidemiological data, vegetarians and vegans show lower rates of death and hospitalization due to heart disease. Being a pesco-vegetarian or vegan may have an added protective effect. Intervention studies also indicate that vegetarian diets can reduce known risk factors of heart disease such as total cholesterol, LDL cholesterol, and blood pressure. The apparent protective effect of following a diet low or devoid in meat may be attributed to increased intake of fruits, vegetables, dietary fiber and nuts and decreased consumption of saturated fat and cholesterol. Further research should be conducted on the subject, separating out vegans and pesco-vegetarians from lacto-ovo-vegetarians in order to determine if there are any significant health outcomes within these groups due to their different dietary profiles.
Key Words: vegan, vegetarian, omnivore, diet, heart disease, risk, morbidity, mortality
According to the Center for Disease Control and Prevention (CDC), heart disease is the number one cause of death for men and women of all races in the United States and it claimed almost 600,000 lives in 2010 alone (1). It is well known that the incidence of heart disease is largely related to one’s diet, and the primary objective of this review is to ascertain whether or not people who are vegan, vegetarian, or omnivorous have an increased/decreased/equal chance of becoming another statistic for this grave public health issue.
Heart Disease: Definition and Risk Factors
Coronary heart disease affects both the heart and the blood vessels and is closely tied to atherosclerosis, which is the accumulation of plaque on arterial walls. This causes a narrowing of the arteries, which restricts blood flow. The flow of blood may stop completely if a blood clot forms and blocks the arteries. If this results in circulation being cut off from the heart, the result is a heart attack. When it occurs in the brain, it is an ischemic stroke (2). Risk factors for heart disease include both uncontrollable genetic components as well as modifiable lifestyle components. Family history, advancing age, and gender fall into the former category, while smoking, leading a sedentary lifestyle, and following an unhealthy diet fall into the latter category (3). High blood cholesterol and high blood pressure are other multi-factorial risk factors. This review will focus on one factor – diet.
Definitions, Benefits and Drawbacks of Vegan, Vegetarian, and Omnivorous diets
There are three broad dietary categories that most people in this country can classify themselves as: vegan, vegetarian, and omnivorous. A vegetarian diet is one that generally excludes animal products with some exceptions. Lacto-ovo-vegetarians include dairy products and eggs in their diets and pesco-vegetarians consume fish and other seafood. The only type of vegetarian that truly excludes all animal derived products is the vegan. An omnivore is anyone who receives their nourishment from both plant and animal sources. Animal foods provide cholesterol, which plant foods do not. Vegetarian diets when compared to omnivorous diets tend to offer more magnesium, potassium, folic acid, vitamin E, vitamin C, fiber and myriad phytochemicals while having a higher ratio of unsaturated to saturated fat (4). Of all the vegetarians, vegans in particular usually consume more dietary fiber, vitamin E, vitamin C, magnesium, iron, folic acid, and phytochemicals and they ingest less calories, cholesterol, saturated fat, vitamin D, zinc, calcium, long chain n-3 fatty acids, and vitamin B-12. The fact that they consume less of these nutrients puts them at risk for being deficient and as such they must either take a supplement or seek foods that are fortified with these vitamins, minerals, and fatty acids. Non-heme iron, found in plant foods, is also less bioavailable than the heme-iron that comes from animal products so it is another nutrient of concern for vegans.
Epidemiologic Data: Observed Differences in Heart Disease Morbidity and Mortality
It is no secret that the foods we nourish our bodies with on a daily basis have an undeniable effect on our health and well-being. The common saying “you are what you eat” makes sense, because the food we eat affects everything from liver cell division to hair growth, but what about our risk for heart disease?
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A cohort study involving 96,469 American and Canadian Seventh-day Adventists used very detailed food-frequency questionnaires to collect data on diet and the National Death Index for cause-specific mortality over a period of about 5.79 years (5). Researchers standardized for sex, age, race, smoking status, exercise level, educational level, alcohol consumption, region, and marital status and placed people into five dietary categories: vegan, lacto-ovo-vegetarian, pesco-vegetarian, semi-vegetarian (ate meat 1+ time per month but not more than once per week), and non-vegetarian. According to the hazard ratios (95% confidence intervals) for ischemic heart disease with non-vegetarian as the reference value 1, vegan, lacto-ovo, pesco, semi, and non-vegetarians died at the following respective rates: 0.90 (0.60-1.33), 0.82 (0.62-1.06), 0.65 (0.43-0.97), 0.92 (0.57-1.51), and 1. The hazard ratios for cardiovascular disease following the same order were as follows: 0.91 (0.71-1.16), 0.90 (0.76-1.06), 0.80 (0.62-1.03), 0.85 (0.63-1.16), and 1. Overall deaths from these diseases were lower for all categories of vegetarians, which is consistent with the aforementioned data, and pesco-vegetarians followed by vegans had the lowest mortality of all groups. Another large cohort study which included 15,000 vegetarians and 30,000 omnivores and adjusted for education, exercise level, physical activity, age, alcohol, and tobacco use found a 32% decreased risk of hospitalization or death as a direct result of ischemic heart disease among British vegetarians (HR: 0.68, 95% CI 0.58-0.81) versus non-vegetarians (6).
The association between a meat-less or meat-reduced diet and decreased risk of heart disease has been shown not only in North America but also in the UK, the Netherlands, Germany, and Japan. This indicates that the protective effects of a vegetarian diet may cross ethnic bounds. Pooled data from seven prospective cohort studies conducted across 5 countries with a grand total of 124,706 individuals found that the risk of dying from ischemic heart disease was 29% lower in vegetarians (risk ratio = 0.71, 95% CI = 0.56-0.87) compared to non-vegetarians (7). The high level of statistical significance provided by the compiled data indicates that the observed differences in mortality were very likely due to diet. However, a second look at the same compiled data with the addition of the Adventist Health Study 2 for a total of 8 studies looked at the weight of all the data and found that the Adventist studies showed a much greater effect than those conducted on non-Adventist populations (8). For all cardiac events including ischemic heart disease, the risk ratio for risk of death among vegetarians versus non-vegetarians for the three Adventist studies combined was 0.60 (95% CI = 0.43-0.83), and for four of the non-Adventist studies it was 0.84 (95% CI = 0.74-0.96). The risk ratios are both <1, which indicates that vegetarians have a lower risk of cardiac death, however it is clear that in the Adventist subgroup the protective effect is greater. This begs the question of possible confounds, such as lifestyle differences observed in Adventist populations, which may include avoidance of smoking, alcohol, and caffeine as well as a better chance of staying married (8).
Dietary practices are also linked to blood pressure and consequentially, heart disease (9). Among 11,000 British males and females from the ages of 20-78, vegans had the lowest blood pressure. This is consistent with the results of the EPIC-Oxford study, which showed that vegetarians on average had a systolic blood pressure that was 3.3 mmHg lower than the omnivores (6). This was translated into a 10% decrease in risk for ischemic heart disease. In addition two known risk factors for the buildup of atherosclerotic plaque – which is a precursor for high blood pressure – were found to be decreased by the adoption of a vegetarian diet in a cross-over study of 15 Americans ages 21-34 years (10). There was a significant decrease in total cholesterol (vegetarian 140.0 mg/dl +/- 20.8, omnivorous 160.0 mg/dl +/- 21.6, p<0.01) and LDL cholesterol (vegetarian 93.2 mg/dl +/- 18.1, omnivorous 109.2 mg/dl +/- 22.8, p<0.025). A similar outcome was duplicated in a study involving approximately 100 American adults with Type II diabetes where half of them were placed on a low-fat vegan diet and the remainder followed the American Diabetes Association (ADA) guidelines (11). A vegan diet decreased total cholesterol by -33.5 mg/dl while the decrease in the ADA group was only -19.0 mg/dl (p=0.0125). Likewise, LDL cholesterol decreased by -22.6 mg/dl for the vegans and -10.7 mg/dl for those in the ADA group (p=0.023).
Based on the largest and most recent epidemiologic studies done on the relationship between diet and chronic disease outcomes, it was found that vegetarians are less likely to suffer and die from heart disease when compared to omnivores. Among all vegetarians pesco-vegetarians may fare the best, followed by vegans and then lacto-ovo-vegetarians. Vegans also tend to have lower blood pressure and cholesterol, which is protective against heart disease. Results of intervention studies suggest that adoption of a vegetarian/vegan diet can reduce the risk of heart disease by reducing known risk factors such as total cholesterol, LDL cholesterol and blood pressure.
Discussion: Possible Explanations for Observed Patterns
While healthy diet plays an inextricable role in one’s likelihood of developing heart disease, it should also be noted that observed lower BMI, lower alcohol consumption, and higher levels of physical activity – all common among vegetarians – also contribute to better health (9) (8). Nevertheless, the observed patterns are very likely connected to differences in fruit and vegetable intake. The “National Health and Nutrition Examination Survey Epidemiologic Follow-up Study,” which first took place in 1971, used food-frequency questionnaires to collect information about the 9,608 adult participants’ dietary patterns (12). The participants, who ranged from 25-74 year olds, were followed for a mean of 19 years. Those who consumed fruits and vegetables at least 3 times each day versus those who consumed less than 1 piece of fruit/vegetable on a typical day experienced 27% decreased death from cardiovascular disease and 24% decreased death from ischemic heart disease. This implies that eating more fruits and vegetables may be protective against developing and dying from heart disease.
Is it safe to say that all types of vegetarians consume more fruits and vegetables than their meat-eating counterparts? Data collected by the USDA as part of the Continuing Survey of Food Intake by Individuals (CSFII) demonstrated that as a matter of fact, they do (13). People who self-identified and vegetarian generally ate more vegetables, dry and regular fruit, brown rice, pasta, pecans, lentils, and walnuts. These are all good sources of dietary fiber, which has an inverse association with risk of heart disease (14).
Given that the consumption of nuts, particularly walnuts, is higher among vegetarians, this may also be a significant matter in discerning why they have lower rates of mortality from heart disease. A prospective cohort study conducted on about 31,000 Seventh-day Adventists in California, USA found a clear inverse association with occurrence of myocardial infarction and death due to coronary heart disease (CHD) with nut consumption. To make the results even more convincing, the results were on a gradient, meaning that the more nuts one consumed, the better their health outcome. Compared to people who did not eat nuts, those who ate nuts 1-4 times each week enjoyed a 22% decreased heart attack morbidity and 27% decreased mortality from CHD, and those who at nuts 5 times or more each week enjoyed a 51% decreased heart attack morbidity and 53% decreased mortality from CHD (15).
Recommendations: Shortcomings and Further Research
One common shortcoming of research that compares health outcomes based on diet is overly-simplified categorization of study subjects. It is easy to state a comparison between vegetarians and non-vegetarians, but within vegetarianism there are a variety of sub-categories which means dietary composition and therefore health outcomes can vary widely between people who consider themselves vegetarian (7). As described above, there are lacto-ovo-vegetarians, pesco-vegetarians, and vegans. Research looking specifically at dietary patterns of a representative sample of Americans who consider themselves vegetarians found that about 66% of them reported eating fish, poultry, and other meats (13). Vegetarianism is a self-defined way of eating and people who only occasionally consume meat, sometimes called “flexitarians,” are often put in the same category as vegans who follow more restricted dietary practices. It is necessary for more research to be done that takes into account the uniqueness of each category instead of simply lumping them together.
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Among all types of vegetarians vegans are at the greatest risk of vitamin B12 deficiency, and this can interrupt metabolic functions and lead to a spike in blood homocysteine, a biomarker for cardiovascular disease (16). This is another reason why vegetarians should be assessed categorically, and studies that include vegans should indicate whether or not participants took a supplement given that vitamin B12 deficiency may affect the results of the study.
As previously mentioned, studies conducted in Adventist populations seemed to prove a greater protective effect of a vegetarian diet (8). Many studies typically controlled for obvious confounds like age and gender, but no studies were found that controlled for religion. Future studies should include religious beliefs/practices as a control factor.
Another issue is the use of food-frequency questionnaires. These are relatively simply research tools for investigators who need to collect dietary information from a large group of people, but unfortunately they are subject to reporting bias (13). People may under-report the amount of food they actually ate and vegetarians in particular may over-report having eaten certain foods based on how healthy those foods are perceived to be. This is all due to what has been termed social desirability bias.
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