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Unlike fad diets that come and go, plant-based diets have been and will continue to be practiced by millions of people worldwide. Despite the debate and controversy surrounding many dietary programs, the human health benefits, especially cardiovascular health benefits, provided by diets rich in fruits, vegetables, whole grains, and legumes are generally undisputed.
How Do Plants Benefit Cardiovascular Health?
A great deal of research has linked plant-based diets to improvements in a variety of cardiovascular risk factors. Plant-based diets have been shown to reduce body mass index, systolic and diastolic blood pressure, triglyceride levels, total cholesterol, low-density lipoprotein cholesterol, fasting blood glucose, and insulin resistance.1 Carotid intima-media thickness (IMT), an early measure of atherosclerotic disease progression, has also been reported to be reduced in vegetarians when compared with omnivores.1
Further, the positive impact of adopting a plant-based diet may be clinically evident within relatively short periods of time. One clinical study demonstrated that a low-fat vegetarian diet in combination with other heart healthy lifestyle changes reverses severe coronary atherosclerosis after just one year, without the need of lipid-reducing medications.2 Biochemically, additional benefits may be provided through reductions in oxidative stress and inflammation.3-7
What do all of these factors have in common? They all have been shown to impact blood viscosity.
Plant-Based Diets Improve Blood Flow – Resurging Knowledge From The Past
Recently, Neal Barnard, MD, a nutrition researcher at George Washington University School of Medicine and Health Sciences and the founder of the Physicians Committee for Responsible Medicine, proposed that plant-based diets improve cardiovascular health by improving blood viscosity and overall blood flow.
Barnard and an international team of researchers performed a meta-analysis of studies linking vegetarian diets and blood pressure. They concluded, like many previous studies have, that vegetarian diets are associated with reduced blood pressure.8 When interviewed by The Atlantic, Barnard stated, “…However, I think there’s a more important factor: viscosity, how thick your blood is.”9
He substantiated this claim, citing a World Health Organization statement linking saturated fats, particularly those in animal fats, to viscous blood. 10 He said, “If you’re eating animal fat, your blood is actually thicker and has a hard time circulating. So the heart has to push harder to get the blood to flow. If you’re not eating meat, your blood viscosity drops and your blood pressure drops.”9
This seemingly simple yet revolutionary explanation is nothing new, however. Nearly 30 years ago, Ernst et al. published research comparing blood viscosity between 48 vegetarians and 41 matched controls following an omnivorous diet. Ultimately, vegetarians had lower viscosity – both native blood viscosity and hematocrit-standardized blood viscosity - likely secondary to plasma viscosity reduction, compared to their omnivorous counterparts.11 Vegetarian subgroups with the lowest intake of animal products had the most favorable blood flow indices, in addition to lower blood pressure and lower cholesterol levels, suggesting a dose-response effect of animal fats on blood viscosity. The authors concluded that, “These observations are in agreement with the fact that other low-cardiovascular-risk groups show better than average blood fluidity.”11
A few years prior, Dintenfass reported improvements in a variety of blood flow measures, notably blood viscosity, after 2-3 weeks of a daily walking exercise regimen combined with a plant-based whole-food diet.12 On average, blood viscosity was reduced by about 10%, an amount shown to benefit patients with intermittent claudication treated with hemodilution.13 Dintenfass, like Ernst et al., also noted a reduction in hematocrit, about 7%.12 It is unclear whether exercise, a factor known to improve blood fluidity, or the plant-based diet was responsible for the improved viscosity profile in Dintenfass’ study. However, Ernst’s findings highlight the potential impact of diet alone.
Plant-Based Diets Reduce Cardiovascular Mortality – A Brief Review Of The Literature
It is difficult to ascertain the true relationship between dietary factors as standalone lifestyle interventions and specific health effects and outcomes. Whole foods are complex in content, function, and interactions with the body, drugs, and other foods. Further, it is difficult to distinguish the beneficial cardiovascular effects of other lifestyle modifications such as exercise, stress reduction, and smoking cessation from those which are conveyed by diet. In spite of these challenges to research study design, a number of scientific publications have proposed a link between plant-rich diets and cardiovascular risk reduction through improvements in blood flow and reductions in atherogenesis.
A recent meta-analysis and systematic review investigating cardiovascular disease (CVD) mortality in vegetarians and non-vegetarians found that, among 124,706 total subjects, coronary heart disease (CHD) mortality was significantly lower in vegetarians than non-vegetarians (RR = 0.71; 95% CI, 0.56–0.87).14 Similarly, vegetarians benefited from a 16% lower risk of circulatory disease mortality (RR = 0.84; 95% CI, 0.54–1.14), in addition to a 12% reduced risk of cerebrovascular disease mortality (RR = 0.88; 95% CI, 0.70–1.06).
One large observational study of European men and women found that each additional portion of fruits and vegetables (80 g) reduced risk of fatal CHD by 4% (RR = 0.96, 95% CI, 0.92-1.00, p < 0.05). 15
Additional evidence can be found in large studies conducted in the United States. A prospective cohort study of 73,308 Seventh-Day Adventist men and women showed significantly lower all-cause mortality in all vegetarians when compared to non-vegetarians, adjusted hazard ratio 0.88 (95% CI, 0.80-0.97).15 Further, cardiovascular mortality was also lower in those with vegetarian diets.
Another prospective study of the Cancer Prevention Study II Nutrition Cohort in the United States showed similar results. The study, involving 38,180 older men and 60,289 older women, revealed that those with the highest dietary intake of flavanoids, antioxidant compounds within plants, had a lower risk of fatal CVD than those with the least total flavanoid intake (RR = 0.82; 95% CI, 0.73–0.92; p < 0.05).16 Secondly, after 7 years of follow up, 5 of the 7 flavanoid classes evaluated were independently associated with a lower overall risk of fatal CVD (p < 0.05).
Perhaps the most important finding of this study was that, overall, the amount of flavanoid consumption required to show these benefits was relatively small. Research suggests that the cardioprotective effects of fruits and vegetables are likely due to their content of mono- and polyunsaturated fatty acids, omega-3 fatty acids, antioxidants, minerals, polyphenols, flavanoids, fiber, and plant protein.17 Indeed, overall quantity rather than variety or subtypes of fruit and vegetable intake is likely to provide CHD risk reduction.18
Even plant-rich diets that allow for small-to-moderate amounts of red meat, fish, and dairy have been shown to reduce the risk of developing diseases such as CHD and type-2 diabetes.19,20 This was highlighted by the PREDIMED trial, one of the largest long-term randomized controlled clinical trials involving diet as an intervention. This study showed that the Mediterranean diet supplemented with either extra-virgin olive oil or nuts, significantly reduced the composite incidence of major cardiovascular events (myocardial infarction, stroke, or cardiovascular death) in 7,447 adults aged 55-80 years with high cardiovascular risk when compared to control subjects who were advised to reduce dietary fat intake.21 Similar risk reductions can be found in other population studies.22
The Mediterranean diet is rich in fruits, vegetables, whole grains, potatoes, beans, nuts, and seeds. Low to moderate amounts of dairy, fish, and poultry are consumed in this diet while red meat is rarely eaten.
It is important to consider the internal benefits of heart-healthy lifestyles. While fad diets and drastic lifestyle changes may result in rapid weight loss and changes in physique, they may be potentially dangerous or harmful. Likewise, any major changes in diet or exercise, especially in those with specific dietary needs or chronic medical conditions, may warrant a discussion with a healthcare provider.
1. Yang SY, Li XJ, Zhang W, et al. Chinese lacto-vegetarian diet exerts favorable effects on metabolic parameters, intima-media thickness, and cardiovascular risks in healthy men. Nutrition in clinical practice : official publication of the American Society for Parenteral and Enteral Nutrition. Jun 2012;27(3):392-398.
7. Middleton E, Jr., Kandaswami C, Theoharides TC. The effects of plant flavonoids on mammalian cells: implications for inflammation, heart disease, and cancer.Pharmacological reviews. Dec 2000;52(4):673-751.
9. Hamblin J. Vegetarians and Their Superior Blood. http://www.theatlantic.com/health/archive/2014/02/vegetarians-and-their-superior-blood/284036/. Accessed May 27, 2014.
10. World Health Organization. World Health Day 2013 - Diet, nutrition and hypertension. http://www.emro.who.int/world-health-days/2013/nutrition-hypertension-factsheet-whd-2013.html. Accessed May 27, 2014.
14. Huang T, Yang B, Zheng J, Li G, Wahlqvist ML, Li D. Cardiovascular disease mortality and cancer incidence in vegetarians: a meta-analysis and systematic review. Annals of nutrition & metabolism. 2012;60(4):233-240.
16. McCullough ML, Peterson JJ, Patel R, Jacques PF, Shah R, Dwyer JT. Flavonoid intake and cardiovascular disease mortality in a prospective cohort of US adults. Am J Clin Nutr. Feb 2012;95(2):454-464.
20. He FJ, Nowson CA, Lucas M, MacGregor GA. Increased consumption of fruit and vegetables is related to a reduced risk of coronary heart disease: meta-analysis of cohort studies. J Hum Hypertens. Sep 2007;21(9):717-728.
22. Mitrou PN, Kipnis V, Thiebaut AC, et al. Mediterranean dietary pattern and prediction of all-cause mortality in a US population: results from the NIH-AARP Diet and Health Study. Arch Intern Med. Dec 10 2007;167(22):2461-2468.