Impact of Vegetarian Diet on Heart Disease, 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 showed significant increases in insulin resistance. All the subjects were healthy nonsmokers, had a BMI between 18.6–25.0kg/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 6.5kg (vs. 3.1kg), 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 nine-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 3.5oz 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 50–71 years.19 The cohort was broken down into five 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 4–5 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 five 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 6.1lb of total body mass and had reductions in systolic blood pressure of 6.5mmHg, in diastolic blood pressure of 4.2mmHg, in total cholesterol of 21.3mg/dL, in LDL of 17mg/dL, in triglycerides of 11mg/dL, and in fasting plasma glucose of 6.2mg/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 one 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.

This article originally appeared on Clinical Advisor