The case for vegetarianism in the fight against heart disease

Multiple studies show that just about any low-meat or no-meat diet reduces almost all controllable risk factors for cardiovascular disease.

The case for vegetarianism in the fight against heart disease
The case for vegetarianism in the fight against heart disease

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Here is an alarming statistic: The Centers for Disease Control and Prevention (CDC) reports that more than 1 in 3 adults nationwide has one or more types of cardiovascular disease.1 In fact, cardiovascular disease (CVD) is the leading cause of death among men and women in the United States.1 

Several studies suggest that a vegetarian diet is effective at reducing almost all controllable risk factors for heart disease, which include total cholesterol, low-density lipoprotein (LDL) cholesterol, systolic blood pressure, diabetes, and obesity. 

Other established predictors of CVD include age, smoking status, high-density lipoprotein (HDL) cholesterol, and body mass index (BMI).2 Although there are a number of variables to consider, vegetarian diets as a way to treat and prevent CVD should be emphasized in patient treatment plans.

Types of vegetarian diets

“Vegetarianism” is an umbrella term for many dietary variations, some of which permit meat consumption and others of which entirely exclude or significantly limit the consumption of meat (Table 1).

Cholesterol

The higher a person's cholesterol, the greater his or her risk for CVD. A study of 800 persons that examined the effect of diet on lipid metabolism revealed that with the exception of HDL, vegetarians had significantly lower plasma lipid levels than did non-vegetarians.3

Considering that long-term vegetarians tend to have low plasma lipid levels, an important question remains: can non-vegetarian individuals who adopt a vegetarian diet expect a reduction in plasma lipid levels? And if so, how long does it take for those changes to become apparent?


The results of one randomized controlled trial suggest that the maximum benefit from a cholesterol-lowering diet involves more than simply reducing dietary intake of saturated fat and cholesterol: The amount of vegetables, legumes, and whole grains consumed seems to play a marked role in decreasing cholesterol levels.4 

Researchers compared 2 diets of participants with moderately elevated cholesterol levels over the course of 4 weeks: Diet A, a low-fat diet with a few vegetables, legumes, and whole grains; and Diet B, a low-fat, plant-based diet consisting largely of vegetables, legumes, and whole grains. In terms of cholesterol, saturated fat, and total fat intake, Diets A and B were identical. 

TABLE 1: Various vegetarian diets

Vegan Exclusively plant-based foods
Lacto-ovo vegetarian Eggs, milk, and milk products, but no meat
Pesco-vegetarian Same as lacto-ovo, plus fish or seafood
Semi-vegetarian Primarily plant-based diet with occasional meat consumption

Both diets were associated with significant reductions in LDL cholesterol and total cholesterol, but participants following Diet B, the plant-based diet, showed greater overall reductions. Diet B yielded decreases in total cholesterol of 17.6 mg/dL and in LDL cholesterol of 9 mg/dL, whereas Diet A reduced total cholesterol by 13.8 mg/dL and LDL cholesterol by 7 mg/dL.

Reductions in serum cholesterol levels can occur within a relatively short time period after initiating a vegetarian diet, as evidenced by one study in which participants were randomized into 3 groups that consumed vegetarian diets for 4 weeks.5 

A control group consumed a low-fat dairy and whole-wheat-cereal diet; one intervention group consumed the same diet plus 20 mg of lovastatin; and the second intervention group consumed a diet high in plant sterols, soy protein, viscous fiber, and almonds. Significant decreases in cholesterol were seen as early as 2 weeks after diet initiation, with reductions at 4 weeks in LDL levels of 8.0% in the control group, 30.9% in the statin group, and 28.6% in the final group. 

These results suggest that the efficacy of dietary modification in the treatment of hyperlipidemia is not only similar to that of a statin, but also that the lipid-lowering benefit of dietary modification can be evident as early as 2 weeks after implementing dietary change.

Another randomized controlled trial found that people with untreated high cholesterol attained a reduction in serum LDL levels and risk for coronary heart disease (CHD) that was nearly half of what might be expected from statin therapy when they were placed on a vegetarian diet emphasizing plant sterols, soy protein, viscous fiber, and nuts.6 

Specifically, researchers found that plant sterol intake lowered serum LDL by 5% (0.94 g plant sterols per 1,000-kcal diet), viscous fiber intake by 4% (9.8 g viscous fiber per 1,000-kcal diet), soy protein intake by 2% (22.5 g soy protein per 1,000-kcal diet), and nuts by 2% (22.5 g nuts per 1,000-kcal diet). 

The study measured serum LDL levels (LDL-C) and total cholesterol/HDL ratio (TC/HDL) over the course of 6 months in 351 participants with untreated high cholesterol who were assigned either to a control group, in which members followed a low-saturated-fat diet and received 2 consultations with dietitians; or to 1 of 2 intervention groups (low-intensity and high-intensity, receiving 2 and 7 consultations with dietitians, respectively), in which participants followed a vegetarian diet emphasizing plant sterols, soy protein, viscous fiber, and nuts. Significant reductions were seen in both the low-intensity group and the high-intensity group in terms of LDL-C (13.1% and 13.8%, respectively) and TC/HDL (8.2% and 6.6%, respectively). 

The risk for CHD was significantly lowered, by 10.8% in the low-intensity intervention group and by 11.3% in the high-intensity group, whereas the control group showed insignificant reductions in CHD risk.

Many popular diets that encourage low carbohydrate and high protein intake may be effective in terms of weight reduction, but they have not been shown to improve an individual's cholesterol profile. In one study, a low-carbohydrate, plant-based diet significantly reduced LDL and total cholesterol over 4 weeks while producing weight loss similar to that seen with low-carbohydrate, high-protein diets.7

For patients who are hesitant to take medication for high cholesterol, determining ways of modifying their diet would be worth exploring. 

As practitioners, we should encourage patients to consider cholesterol-lowering foods, such as plant sterols and nuts. In addition, we should help them explore ways of cutting back in other areas of their diet, such as saturated fats.

Blood pressure

Research indicates that a vegetarian diet protects against the development of hypertension. One study of 500 persons found significantly lower systolic and diastolic blood pressures among vegetarians compared with omnivorous subjects.8 Not surprisingly, the vegetarians also were less likely to use any blood pressure medications. 

Vegans in particular were less likely to have hypertension compared with omnivores (odds ratio [OR], 0.37), lacto-ovo vegetarians (OR, 0.57) and partial vegetarians (OR, 0.92). Meat-eaters self-reported high blood pressure most often (men, 15%; women, 12.1%) and vegans least often (men, 5.8%; women, 7.7%), even after adjusting for age.9 Approximately 9% of fish-eaters and vegetarians reported hypertension, regardless of gender.

As with changes in cholesterol, changes in blood pressure can happen fairly quickly with dietary modification, and certain combinations of foods yield greater benefits than others. A diet rich in fruits and vegetables decreased blood pressure in one randomized, controlled trial, but the reduction was greater when the diet also included low-fat dairy products and less saturated and total fat10

The 459 adults in the study spent the first 3 weeks on a control diet that was low in fruits, vegetables, and dairy products and had a fat content that was consistent with the average U.S. diet. For 8 weeks after that, half of the participants followed a diet rich in fruits and vegetables while the other half was assigned a “combination” diet that was rich in fruits and vegetables but also included low-fat dairy products and reduced saturated and total fat.

Although both groups had lower systolic and diastolic blood pressure by the end of the intervention, these measurements were significantly lower in the combination-diet group than in the control group (systolic, 11.4 mmHg less; diastolic, 5.5 mmHg less). It is important to note that body weight was maintained during the study, which excluded the possibility that the effects on blood pressure were due to an associated weight loss.

Diabetes

Research indicates that vegetarian diets protect against the development of type 2 diabetes and metabolic syndrome and that these diets can improve insulin sensitivity in patients who already have diabetes. One large-scale study of 22,434 men and 38,469 women indicated that adherence to a vegetarian diet could lower the risk of developing diabetes to approximately half that of a non-vegetarian (vegans: OR, 0.51; lacto-ovo vegetarians: OR, 0.54).11

The risk for diabetes was also less for pesco-vegetarians (OR, 0.70) and semi-vegetarians (OR, 0.76) compared with non-vegetarians, even after adjusting for BMI, lifestyle, and socioeconomic factors.

In other research, vegetarians had significantly lower insulin resistance and fasting glucose and insulin concentrations than did non-vegetarians, independent of age.12 In contrast, non-vegetarians aged 30 years and older showed significant increases in insulin resistance. All the subjects were healthy nonsmokers, had a BMI between 18.6 and 25.0 kg/m2, and reported similar levels of physical activity, education, energy intake, and main nutrient intake.

Many practitioners encourage patients with a new diagnosis of type 2 diabetes to adopt a “diabetes diet” as part of a comprehensive treatment plan. Although there is value in such an eating plan, some studies suggest that a vegetarian diet may be more beneficial for patients. 

When Barnard and associates compared the effect of a low-fat vegan diet to a diet compliant with the American Diabetes Association (ADA) guidelines on glycemic control, they found that after 22 weeks of the intervention, both groups used fewer diabetes medications, lost weight, and had lower hemoglobin A1c (HbA1c) values.13

However, the low-fat vegan diet group demonstrated greater improvements during the study period in all categories: 43% of those participants used fewer diabetes medications (vs. 26% of those on the diabetes diet), average weight loss was 6.5 kg (vs. 3.1 kg), and, among participants who did not change medications, HbA1c decreased by 1.23 points (vs. 0.38). 

A subsequent study examined nutrient intake and the Alternate Healthy Eating Index score (AHEI), a 9-category measure of overall diet quality and nutrient adequacy, in the same group of participants.14 The AHEI score is used as a predictor of risk for chronic disease, particularly CVD—the higher the score, the lower the risk. 

The vegan-diet group showed significant improvements in AHEI score from baseline to 22 weeks; the score in the diabetes-diet group did not change significantly. In terms of nutrient intake, the two groups had comparable changes in macronutrient intake, but the vegan group reported significant increases in micronutrient (such as beta carotene and vitamin C) intake.

A randomized, controlled trial involving 74 persons with type 2 diabetes evaluated the effects of a calorie-restricted vegetarian diet compared with a calorie-restricted diabetes diet on insulin resistance, visceral fat, and oxidative stress markers.15 

Both groups showed increased insulin sensitivity, which was estimated as the metabolic clearance rate of glucose, over 24 weeks, but the vegetarian group had greater increases. The vegetarian group also experienced greater weight loss, reduction in waist circumference, reduction in HbA1c, and increase in the plasma concentration of oxidative stress markers (such as vitamin C and superoxide dismutase) and adipokines. 

In another significant finding, 43% of the vegetarian-diet group was able to use fewer regular diabetes medications, compared with 5% of the diabetes-diet group.

Although research has shown that a vegetarian diet is equally, if not more, effective than a diabetes diet, an important question still remains: Will patients adhere to a vegetarian diet, and what sorts of barriers will patients face when transitioning diets?

Barnard and colleagues found that adherence, diet acceptability, and cravings were not significantly different between a low-fat vegan diet and diet following ADA guidelines during a 74-week intervention.16 Both groups were similar in increased dietary restraint, reduced hunger, reduced disinhibition, and fewer cravings for fatty foods.

With comparable adherence and acceptability, it may be advantageous to recommend a vegetarian diet to patients with diabetes as an alternative to the conventional diabetes diet.

Alternatives for non-vegetarians

Realistically, most patients who eat meat regularly likely will not be interested in eliminating all meat products from their diet. It would be advantageous, however, for those patients to make small—but significant—dietary changes. Different options have proven to be beneficial: reducing the amount of red meat consumption, or ideally, reducing meat consumption and substituting other protein sources for red meat.

In one study, the risk for CHD was 25% higher for people who consume 3.5 oz of meat per day (relative risk [RR], 1.25).17 On the other hand, a prospective study of 37,698 men and 83,644 women found that the risk for mortality was lowered by 7% (hazard ratio [HR], 0.93) to 19% (HR, 0.81) in people who replaced a serving of red meat with a serving of fish, poultry, nuts, legumes, low-fat dairy products, or whole grains once a day.18 

If the substitution was in place of processed red meat, such as bacon or hot dogs, the effect was even more significant: The risk for mortality decreased by 10% to 22%. The researchers estimated that if participants had consumed less than a half a serving of red meat per day, death could have been prevented in 9.3% of men and 7.6% of women. The HRs for a one-serving-per-day increase in red meat and risk of cardiovascular disease mortality were 1.18 and 1.21 for unprocessed red meat and for processed red meat, respectively.

Similar health risks were associated with red meat consumption in a prospective study of 500,000 persons aged 50 to 71 years.19 The cohort was broken down into 5 groups based on the amount of red meat consumed. The investigators found that 11% and 16% of overall deaths in men and women, respectively, likely would have been prevented if the entire study population had reduced the amount of red meat consumed to the amount consumed by the population in the first quintile. 

This finding, extrapolated to the effect on cardiovascular mortality, would have decreased mortality 11% among men and 21% among women. Additionally, there was a small decrease in total mortality for both men and women with higher white-meat consumption.

An alternative to eliminating meat from one's diet is to eat more fruits and vegetables, which alone may decrease CHD risk. A recent study of the association between the quantity and type of fruit and vegetable intake and the risk of CHD demonstrated a 17% lower risk of CHD for participants who consumed approximately 4 to 5 servings of fruits or vegetables per day.20 There was a linear association between increased fruit and vegetable consumption and decreased CHD risk up to roughly 5 servings per day, at which point there was no added benefit from additional servings per day. 

Notably, the association was independent of other risk factors for heart disease. Citrus fruits, green leafy vegetables, and foods rich in beta-carotene or vitamin C were all found to have the most impact on risk reduction.

Adopting a vegetarian diet may be too drastic of a lifestyle change for most people. However, making recommendations based on this clinical data, such as substituting white meat or other healthy, protein-rich foods for a serving of red meat every day or adding more fruits and vegetables to the diet could greatly improve patients' cardiovascular health.

Lifestyle intervention

Patients willing to commit to lifestyle changes can significantly improve their health status, and changes in CHD risk factors can occur relatively quickly. In a study of 5,070 participants, Rankin and coauthors learned that a 30-day lifestyle modification program that included a plant-based diet (with an emphasis on fruits, vegetables, legumes, and grains), 30 minutes of daily aerobic exercise, and lifestyle counseling yielded significant reductions in all cardiovascular risk factors, including a reduction in the percentage of subjects requiring medication for blood pressure and/or cholesterol.21 

Participants had significant reductions in weight, blood pressure, plasma lipids, and fasting glucose. Specifically, the cohort lost an average of 6.1 lb of total body mass and had reductions in systolic blood pressure of 6.5 mmHg, in diastolic blood pressure of 4.2 mmHg, in total cholesterol of 21.3 mg/dL, in LDL of 17 mg/dL, in triglycerides of 11 mg/dL, and in fasting plasma glucose of 6.2 mg/dL. 

The authors noted that the reduction in cholesterol was similar to the results expected with a statin medication. Using the Framingham Heart Study2 score, a 1.8% decrease was predicted for the likelihood of participants having a coronary event during the 10 years postintervention. Overall, the results of the intervention showed that subjects with higher cardiovascular risk benefited more than subjects with lower risk.

The remarkable results of a prior intervention, the Lifestyle Heart Trial, demonstrated the true potential of how comprehensive lifestyle changes can affect cardiovascular health.22 The intervention involved 1 year of a strict low-fat vegetarian diet, moderate exercise, smoking cessation, and stress management. The control group continued standard medical therapies. 

In the experimental group as a whole, anginal symptoms decreased by 91% in frequency, 42% in duration, and 28% in severity—each statistically significant—compared with the control group. In the control group, angina symptom frequency increased by 165%, duration by 95%, and severity by 39%. The intervention participants also experienced a decrease in total cholesterol of 24.3% and in LDL of 37.4%. 

The degree of coronary artery stenosis was averaged for each group and fell from 40.0% to 37.8% for the experimental group but increased from 42.7% to 46.1% in the control group, as documented by pre- and post-coronary angiography. By actually reversing CHD, the intervention was substantial: It not only caused a change in the health status of participants, but also changed their functional status.

Mortality

Further research builds upon the association of a vegetarian diet and lower risk of CVD and suggests that people adhering to a vegetarian diet have lower CVD mortality rates than do non-vegetarians. One prospective study involving 44,561 men and women from England and Scotland found that the vegetarian segment of the study population had a 32% (HR, 0.68) lower risk of ischemic heart disease (IHD) than did the non-vegetarians in the study, even after adjusting for factors such as age, smoking, alcohol, and physical activity.23 

After adjusting for BMI, vegetarians still showed a 28% (HR, 0.72) lower risk of IHD than did non-vegetarians. The probability of hospitalization or death due to IHD was 4.6% for vegetarians, compared with 6.8% for non-vegetarians.

In congruence with those data, a meta-analysis of 5 prospective studies found that vegetarians had a 29% (RR, 0.71) lower mortality rate from IHD than did non-vegetarians.24  In another example, a study of 73,308 men and women found that all types of vegetarians (semi-vegetarians, pesco-vegetarians, lacto-ovo vegetarians, and vegans) had significantly lower mortality rates compared with non-vegetarians.25 

A strength of this study was that all the subjects followed the same conservative religious principles of the Seventh-day Adventist Church, thereby reducing variables outside of their diet (such as tobacco and alcohol use) that may have led to poor health outcomes. 

The group was almost evenly split: 51.8% were vegetarian and 48.2% were non-vegetarian. The non-vegetarian mortality rate was 6.61 deaths per 1,000 person-years; pesco-vegetarians had the lowest mortality rate at 5.33 deaths per 1,000 person-years. Mortality from CVD and IHD were significantly reduced for male vegetarians.

Conclusion

Numerous studies have proven that a vegetarian diet can decrease a person's risk of heart disease by reducing risk factors such as high cholesterol, high blood pressure, diabetes, and obesity, and can even reverse lesions of occlusive coronary artery disease. 

Moreover, patients at risk of CHD who adopt a vegetarian diet or modify their current diet by reducing meat intake can often reduce or even prevent the need for medications, thereby avoiding related adverse side effects.

Given the shift in our health-care model toward preventive medicine, interventions such as dietary modification to reduce the risk of CVD will be vitally important. Patients should be encouraged at every opportunity to make positive lifestyle changes. Those who are serious about changing their diets as part of a comprehensive treatment plan would likely benefit from referral to a registered dietitian. 

In addition, several websites serve as good resources for information on vegetarian diets, including the Vegetarian Nutrition Dietetic Practice Group of the Academy of Nutrition and Dietetics (vegetariannutrion.net), The Vegetarian Resource Group (www.vrg.org), the Jesse & Julie Rasch Foundation's nutritionfacts.org, and the Mayo Clinic's “Nutrition and healthy eating” page (www.mayoclinic.org/healthy-living/nutrition-and-healthy-eating/in-depth/vegetarian-diet/art-20046446).

Allison Nichols, PA-C, is a resident in the Physician Assistant Residency Program for Cardiothoracic Surgery at St. Joseph Mercy Hospital in Ann Arbor, Mich.

John Grosel, MD, is an associate professor in the Physician Assistant Program at Marietta College in Marietta, Ohio, and a radiologist with Riverside Radiology and Interventional Associates, Inc., in Columbus, Ohio.

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All electronic documents accessed July 18, 2014.

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