Another Win for Veggies

Study Overview

Objective. To determine the association between a vegetarian diet and blood pressure (BP).

Design. Systematic review and meta-analysis of controlled clinical trials and observational studies.

Setting and participants. MEDLINE and Web of Science were respectively searched for English articles published between 1946 to October 2013 and 1900 to November 2013. Inclusion criteria were age > 20 years and vegetarian diet. This included a vegan diet (omitting all animal products), ovo/lacto/pesco vegetarian diet (including eggs/dairy/fish), or semi-vegetarian diet (meat or fish rarely). Exclusion criteria included studies of twins, a multipronged intervention, describing only categorical BP, and reliance on a case series. A total of 258 records were identified. Seven clinical trials and 32 observational studies met inclusion criteria. The 7 clinical trials encompassed 311 participants (median, 38; range, 11–113) with a mean age of 44.5 years (range, 38.0–54.3). All were open-label, 6 were randomized, and 6 provided food to participants. The 32 observational studies included 21,604 participants (median, 152; range, 20–9242) with a mean age of 46.6 years (range, 28.8–68.4 years). Fifteen of these studies included mixed diet types (vegan, lacto, ovolacto, pesca, and/or semivegetarian).

Main outcome measures. The primary outcome was BP. Differences in systolic BP (SBP) and diastolic BP (DBP) between groups consuming vegetarian or comparison diets were pooled using a random effects model. The study compared clinical trials and observational studies separately. Funnel plots, the Egger test, and the trim-and-fill method were all used to assess and correct for publication bias.

Results. Vegetarian diets in clinical trials were associated with a mean SBP reduction of −4.8 mm Hg (95% confidence interval [CI], −6.6 to −3.1; P < 0.001; I² = 0; P = 0.45 for heterogeneity) and DBP reduction of −2.2 mm Hg (95% CI, −3.5 to −1.0; P < 0.001; I² = 0; P= 0.43 for heterogeneity) when compared with omnivorous diets. Observational studies had larger reductions but significant heterogeneity: SBP −6.9 mm Hg (95% CI, −9.1 to −4.7; P < 0.001; I² = 91.4; P < 0.001 for heterogeneity) and DBP −4.7 mm Hg (95% CI, −6.3 to −3.1; P < 0.001; I² = 92.6; P < 0.001 for heterogeneity). This heterogeneity was best explained by proportion of men (β −0.03; P < 0.001), baseline SBP (β −0.13; P = 0.003), baseline DBP (β −0.30; P < 0.001), sample size (β 0.001; P< 0.001), and BMI (β −0.46; P = 0.02). This suggests that vegetarian diets and lower BP are more strongly associated in men and those with higher baseline BP and BMI.

Subgroup analysis included stratification by age, gender, BMI, diet type, sample size, diet duration, BP medication use, baseline BP, and geographic region. In subgroup analysis of clinical trials, no statistically significant difference between group variation or heterogeneity existed. In comparison, subgroup analysis of observational studies reduced heterogeneity and often effect size. For example, lower SBP was evident in the majority male subgroups (mean SBP/DBP: –18.5 mm Hg/–10.1 mm Hg).

Publication bias existed for both clinical trials and observational studies. According to trim-and-fill methodology, 3 clinical trials of smaller size and larger BP reduction likely were missing (Egger P = 0.04). Their addition shifted mean SBP reduction from −4.8 mm Hg (−6.6 to −3.1) to −5.2 mm Hg (−6.9 to −3.5). Observational studies lacked medium sized negative trials and were overrepresented by larger positive trials (Egger P < 0.001), although this was not confirmed by trim-and-fill (yet this method performs less well under heterogeneous conditions) [1].

Conclusion. Vegetarian diets, when compared with omnivorous diets, are associated with reductions in BP.

Commentary

Several studies show that dietary modifications are effective in preventing and managing hypertension [2,3]. Landmark randomized trials, including the DASH diet [4], DASH-sodium diet [5], and OmniHeart diets [6], all of which emphasize fruit and vegetable intake but are not vegetarian, have led to SBP and DBP reductions ranging from 5.5 to 9.5 mm Hg and 3.0 to 5.2, respectively. However, the impact of a vegetarian diet still remains debated, particularly given disparate findings among randomized controlled trials (RCTs). For example, findings in the early- and mid-1980s of small RCTs with ovolactovegetarians (a vegetarian who consumes eggs and dairy products but not animal flesh) suggested reductions similar to the pooled SBP reduction of –4.8 mm Hg Yokoyama et al report [7,8]. In contrast, one RCT comparing ovolactovegetarian with lean meat diets failed to show a BP benefit [9]. Striking is the dearth of RCTs in the last 20 years to assist in better estimating this impact, particularly given its continual recommendation in the scientific [10] and lay communities [11]. To the authors’ credit, this is the first meta-analysis and second systematic review of this important relationship [12].

A vegetarian diet likely supports BP reductions through a variety of mechanisms, most notably via an abundance of potassium [13]. Potassium likely promotes vasodilation, which facilitates glomerular filtration, allowing decreased renal sodium reabsorption and decreased platelet aggregation. Other more controversial hypotheses include decreased energy density leading to reduced BMI [14], decreased sodium intake [15], reduced blood viscosity [16], and high polyunsaturated with low saturated fat content [17].

Strengths of this analysis include the large observational sample size, the separate analysis of clinical trials and observational studies, the lengthy search time-frame, the subgroup analyses, and the adjustment for publication bias. Although the overall association was robust throughout, we agree with the authors that large heterogeneity among observational studies, small clinical trial sample sizes, and the variation in what “vegetarian” represents throughout the world and in individual studies all represent limitations. The participants in many of the observational studies could have technically eaten meat with unclear and undefined frequency, and this may have explained the heterogeneity of these studies. Correspondingly, the lack of heterogeneity observed in the clinical trials may be due to the fact that participants were provided meals in 6 out of 7 studies.

It was surprising that only 7 clinical trials were found. The authors utilized 2 databases, but perhaps searching additional databases such as EMBASE or CINAHL would have yielded other pertinent studies. The authors also did not use a bias assessment tool such as that proposed by the Cochrane bias methods group, which could have better discriminated high- from low-quality trials and made for useful subgroup analyses [1]. Similarly, reporting on both attrition and adherence could have assisted in decreasing heterogeneity during subgroup analyses and determining high-quality from low-quality studies. For example, adherence in Ferdowsian et al (vegan diet) was determined by unannounced dietician phone calls and found that only 57% of participants abstained from animal products. This may have been secondary to the study’s design, in that “providing meals” included simply making them an available option at the company cafeteria instead of requiring consumption of a study-specific vegetarian meal [18].

Applications for Clinical Practice

In this meta-analysis, vegetarian diets were associated with –4.8 mm Hg SBP and −2.2 mm Hg DBP reductions, indicating that providers can recommend a vegetarian diet as on par with other lifestyle changes, including low-sodium diet, weight loss, and exercise. A vegetarian diet may be comparable to pharmacologic therapy in magnitude of BP change. Short- and long-term pharmacologic therapy is associated with respective SBP/DBP reductions of –8.3/–3.8 and –5.4/–2.3, not altogether different from reductions seen with vegetarian or vegetable-heavy diets [19].

Although there are barriers to a vegetarian diet, including provider attitudes [20], cost [21], poor culinary skill [22], palatability, and adherence, pharmacologic BP treatment also presents barriers: adherence to BP medications is estimated to be 50% to 70% [23], and harm due to side effects can preclude use. Thus, providers can present a vegetarian diet as a potentially effective option, depending on patient preference and ability to adhere.

—David M. Levine, MD, MA, New York University
School of Medicine, and Melanie Jay, MD, MS

References

1. Higgins JPT, Green S, editors. Cochrane handbook for systematic reviews of interventions version 5.1.0 [updated March 2011]. The Cochrane Collaboration, 2011. Available at http://handbook.cochrane.org/chapter_10/10_4_4_2_trim_and_fill.htm.

2. Eckel RH, Jakicic JM, Ard JD, et al. 2013 AHA/ACC guideline on lifestyle management to reduce cardiovascular risk: a report of the American College of Cardiology/American Heart Association Task Force on Practice Guidelines. Circulation 2013 Nov 12. [Epub ahead of print]

3. James PA, Oparil S, Carter BL, et al. 2014 evidence-based guideline for the management of high blood pressure in adults: report from the panel members appointed to the Eighth Joint National Committee (JNC 8). JAMA 2014;311:507–20.

4. Appel LJ, Moore TJ, Obarzanek E, et al. A clinical trial of the effects of dietary patterns on blood pressure. N Engl J Med 1997;336:1117–24.

5. Sacks FM, Svetkey LP, Vollmer WM, et al. Effects on blood pressure of reduced dietary sodium and the dietary approaches to stop hypertension (DASH) diet. N Engl J Med 2001;344:3–10.

6. Appel LJ, Sacks FM, Carey VJ, et al. Effects of protein, monounsaturated fat, and carbohydrate intake on blood pressure and serum lipids. JAMA 2005;294:2455–64.

7. Rouse IL, Beilin LJ, Armstrong BK, Vandongen R. Blood-pressure–lowering effect of a vegetarian diet: controlled trial in normotensive subjects. Lancet 1983;321:5–10.

8. Margetts BM, Beilin LJ, Vandongen R, Armstrong BK. Vegetarian diet in mild hypertension: a randomised controlled trial. BMJ 1986;293:1468–71.

9. Kestin M, Rouse IL, Correll RA, Nestel PJ. Cardiovascular disease risk factors in free-living men: comparison of two prudent diets, one based on lactoovovegetarianism and the other allowing lean meat. Am J Clin Nutr 1989;50:280–7.

10. Alpert JS. Nutritional advice for the patient with heart disease: what diet should we recommend for our patients? Circulation 2011;124:e258–e260.

11. Gordinier J. Making vegan a new normal. New York Times. 26 Sept 2012. Page D1.

12. Berkow SE, Barnard ND. Blood pressure regulation and vegetarian diets. Nutr Rev 2005;63:1–8.

13. Aburto NJ, Hanson S, Gutierrez H, et al. Effect of increased potassium intake on cardiovascular risk factors and disease: systematic review and meta-analyses. BMJ 2013;346:f1378.

14. Berkow SE, Barnard N. Vegetarian diets and weight status. Nutr Rev 2006;64:175–88.

15. Larsson CL, Johansson GK. Dietary intake and nutritional status of young vegans and omnivores in Sweden. Am J Clin Nutr 2002;76:100–6.

16. Ernst E, Pietsch L, Matrai A, Eisenberg J. Blood rheology in vegetarians. Br J Nutr 1986;56:555–60.

17. Iacono JM, Dougherty RM. Effects of polyunsaturated fats on blood pressure. Annu Rev Nutr 1993;13:243–60.

18. Ferdowsian HR, Barnard ND, Hoover VJ, et al. A multicomponent intervention reduces body weight and cardiovascular risk at a GEICO corporate site. Am J Health Promot 2010;24:384–7.

19. Brugts JJ, Ninomiya T, Boersma E, et al. The consistency of the treatment effect of an ACE-inhibitor based treatment regimen in patients with vascular disease or high risk of vascular disease: a combined analysis of individual data of ADVANCE, EUROPA, and PROGRESS trials. Eur Heart J 2009;30:1385–94.

20. Berman BM, Singh BB, Hartnoll SM, et al. Primary care physicians and complementary-alternative medicine: training, attitudes, and practice patterns. Am Board Fam Pract 1998;11:272–81.

21. Drewnowski A, Darmon N. The economics of obesity: dietary energy density and energy cost. Am J Clin Nutr 2005;82(1 Suppl):265S-273S.

22. Lea EJ, Crawford D, Worsley A. Public views of the benefits and barriers to the consumption of a plant-based diet. Eur J Clin Nutr 2006;60:828–37.

23. Schroeder K, Fahey T, Ebrahim S. How can we improve adherence to blood pressure–lowering medication in ambulatory care? Systematic review of randomized controlled trials. Arch Intern Med 2004;164:722–32.

Study Overview

Objective. To determine the association between a vegetarian diet and blood pressure (BP).

Design. Systematic review and meta-analysis of controlled clinical trials and observational studies.

Setting and participants. MEDLINE and Web of Science were respectively searched for English articles published between 1946 to October 2013 and 1900 to November 2013. Inclusion criteria were age > 20 years and vegetarian diet. This included a vegan diet (omitting all animal products), ovo/lacto/pesco vegetarian diet (including eggs/dairy/fish), or semi-vegetarian diet (meat or fish rarely). Exclusion criteria included studies of twins, a multipronged intervention, describing only categorical BP, and reliance on a case series. A total of 258 records were identified. Seven clinical trials and 32 observational studies met inclusion criteria. The 7 clinical trials encompassed 311 participants (median, 38; range, 11–113) with a mean age of 44.5 years (range, 38.0–54.3). All were open-label, 6 were randomized, and 6 provided food to participants. The 32 observational studies included 21,604 participants (median, 152; range, 20–9242) with a mean age of 46.6 years (range, 28.8–68.4 years). Fifteen of these studies included mixed diet types (vegan, lacto, ovolacto, pesca, and/or semivegetarian).

Main outcome measures. The primary outcome was BP. Differences in systolic BP (SBP) and diastolic BP (DBP) between groups consuming vegetarian or comparison diets were pooled using a random effects model. The study compared clinical trials and observational studies separately. Funnel plots, the Egger test, and the trim-and-fill method were all used to assess and correct for publication bias.

Results. Vegetarian diets in clinical trials were associated with a mean SBP reduction of −4.8 mm Hg (95% confidence interval [CI], −6.6 to −3.1; P < 0.001; I² = 0; P = 0.45 for heterogeneity) and DBP reduction of −2.2 mm Hg (95% CI, −3.5 to −1.0; P < 0.001; I² = 0; P= 0.43 for heterogeneity) when compared with omnivorous diets. Observational studies had larger reductions but significant heterogeneity: SBP −6.9 mm Hg (95% CI, −9.1 to −4.7; P < 0.001; I² = 91.4; P < 0.001 for heterogeneity) and DBP −4.7 mm Hg (95% CI, −6.3 to −3.1; P < 0.001; I² = 92.6; P < 0.001 for heterogeneity). This heterogeneity was best explained by proportion of men (β −0.03; P < 0.001), baseline SBP (β −0.13; P = 0.003), baseline DBP (β −0.30; P < 0.001), sample size (β 0.001; P< 0.001), and BMI (β −0.46; P = 0.02). This suggests that vegetarian diets and lower BP are more strongly associated in men and those with higher baseline BP and BMI.

Subgroup analysis included stratification by age, gender, BMI, diet type, sample size, diet duration, BP medication use, baseline BP, and geographic region. In subgroup analysis of clinical trials, no statistically significant difference between group variation or heterogeneity existed. In comparison, subgroup analysis of observational studies reduced heterogeneity and often effect size. For example, lower SBP was evident in the majority male subgroups (mean SBP/DBP: –18.5 mm Hg/–10.1 mm Hg).

Publication bias existed for both clinical trials and observational studies. According to trim-and-fill methodology, 3 clinical trials of smaller size and larger BP reduction likely were missing (Egger P = 0.04). Their addition shifted mean SBP reduction from −4.8 mm Hg (−6.6 to −3.1) to −5.2 mm Hg (−6.9 to −3.5). Observational studies lacked medium sized negative trials and were overrepresented by larger positive trials (Egger P < 0.001), although this was not confirmed by trim-and-fill (yet this method performs less well under heterogeneous conditions) [1].

Conclusion. Vegetarian diets, when compared with omnivorous diets, are associated with reductions in BP.

Commentary

Several studies show that dietary modifications are effective in preventing and managing hypertension [2,3]. Landmark randomized trials, including the DASH diet [4], DASH-sodium diet [5], and OmniHeart diets [6], all of which emphasize fruit and vegetable intake but are not vegetarian, have led to SBP and DBP reductions ranging from 5.5 to 9.5 mm Hg and 3.0 to 5.2, respectively. However, the impact of a vegetarian diet still remains debated, particularly given disparate findings among randomized controlled trials (RCTs). For example, findings in the early- and mid-1980s of small RCTs with ovolactovegetarians (a vegetarian who consumes eggs and dairy products but not animal flesh) suggested reductions similar to the pooled SBP reduction of –4.8 mm Hg Yokoyama et al report [7,8]. In contrast, one RCT comparing ovolactovegetarian with lean meat diets failed to show a BP benefit [9]. Striking is the dearth of RCTs in the last 20 years to assist in better estimating this impact, particularly given its continual recommendation in the scientific [10] and lay communities [11]. To the authors’ credit, this is the first meta-analysis and second systematic review of this important relationship [12].

A vegetarian diet likely supports BP reductions through a variety of mechanisms, most notably via an abundance of potassium [13]. Potassium likely promotes vasodilation, which facilitates glomerular filtration, allowing decreased renal sodium reabsorption and decreased platelet aggregation. Other more controversial hypotheses include decreased energy density leading to reduced BMI [14], decreased sodium intake [15], reduced blood viscosity [16], and high polyunsaturated with low saturated fat content [17].

Strengths of this analysis include the large observational sample size, the separate analysis of clinical trials and observational studies, the lengthy search time-frame, the subgroup analyses, and the adjustment for publication bias. Although the overall association was robust throughout, we agree with the authors that large heterogeneity among observational studies, small clinical trial sample sizes, and the variation in what “vegetarian” represents throughout the world and in individual studies all represent limitations. The participants in many of the observational studies could have technically eaten meat with unclear and undefined frequency, and this may have explained the heterogeneity of these studies. Correspondingly, the lack of heterogeneity observed in the clinical trials may be due to the fact that participants were provided meals in 6 out of 7 studies.

It was surprising that only 7 clinical trials were found. The authors utilized 2 databases, but perhaps searching additional databases such as EMBASE or CINAHL would have yielded other pertinent studies. The authors also did not use a bias assessment tool such as that proposed by the Cochrane bias methods group, which could have better discriminated high- from low-quality trials and made for useful subgroup analyses [1]. Similarly, reporting on both attrition and adherence could have assisted in decreasing heterogeneity during subgroup analyses and determining high-quality from low-quality studies. For example, adherence in Ferdowsian et al (vegan diet) was determined by unannounced dietician phone calls and found that only 57% of participants abstained from animal products. This may have been secondary to the study’s design, in that “providing meals” included simply making them an available option at the company cafeteria instead of requiring consumption of a study-specific vegetarian meal [18].

Applications for Clinical Practice

In this meta-analysis, vegetarian diets were associated with –4.8 mm Hg SBP and −2.2 mm Hg DBP reductions, indicating that providers can recommend a vegetarian diet as on par with other lifestyle changes, including low-sodium diet, weight loss, and exercise. A vegetarian diet may be comparable to pharmacologic therapy in magnitude of BP change. Short- and long-term pharmacologic therapy is associated with respective SBP/DBP reductions of –8.3/–3.8 and –5.4/–2.3, not altogether different from reductions seen with vegetarian or vegetable-heavy diets [19].

Although there are barriers to a vegetarian diet, including provider attitudes [20], cost [21], poor culinary skill [22], palatability, and adherence, pharmacologic BP treatment also presents barriers: adherence to BP medications is estimated to be 50% to 70% [23], and harm due to side effects can preclude use. Thus, providers can present a vegetarian diet as a potentially effective option, depending on patient preference and ability to adhere.

—David M. Levine, MD, MA, New York University
School of Medicine, and Melanie Jay, MD, MS

References

1. Higgins JPT, Green S, editors. Cochrane handbook for systematic reviews of interventions version 5.1.0 [updated March 2011]. The Cochrane Collaboration, 2011. Available at http://handbook.cochrane.org/chapter_10/10_4_4_2_trim_and_fill.htm.

2. Eckel RH, Jakicic JM, Ard JD, et al. 2013 AHA/ACC guideline on lifestyle management to reduce cardiovascular risk: a report of the American College of Cardiology/American Heart Association Task Force on Practice Guidelines. Circulation 2013 Nov 12. [Epub ahead of print]

3. James PA, Oparil S, Carter BL, et al. 2014 evidence-based guideline for the management of high blood pressure in adults: report from the panel members appointed to the Eighth Joint National Committee (JNC 8). JAMA 2014;311:507–20.

4. Appel LJ, Moore TJ, Obarzanek E, et al. A clinical trial of the effects of dietary patterns on blood pressure. N Engl J Med 1997;336:1117–24.

5. Sacks FM, Svetkey LP, Vollmer WM, et al. Effects on blood pressure of reduced dietary sodium and the dietary approaches to stop hypertension (DASH) diet. N Engl J Med 2001;344:3–10.

6. Appel LJ, Sacks FM, Carey VJ, et al. Effects of protein, monounsaturated fat, and carbohydrate intake on blood pressure and serum lipids. JAMA 2005;294:2455–64.

7. Rouse IL, Beilin LJ, Armstrong BK, Vandongen R. Blood-pressure–lowering effect of a vegetarian diet: controlled trial in normotensive subjects. Lancet 1983;321:5–10.

8. Margetts BM, Beilin LJ, Vandongen R, Armstrong BK. Vegetarian diet in mild hypertension: a randomised controlled trial. BMJ 1986;293:1468–71.

9. Kestin M, Rouse IL, Correll RA, Nestel PJ. Cardiovascular disease risk factors in free-living men: comparison of two prudent diets, one based on lactoovovegetarianism and the other allowing lean meat. Am J Clin Nutr 1989;50:280–7.

10. Alpert JS. Nutritional advice for the patient with heart disease: what diet should we recommend for our patients? Circulation 2011;124:e258–e260.

11. Gordinier J. Making vegan a new normal. New York Times. 26 Sept 2012. Page D1.

12. Berkow SE, Barnard ND. Blood pressure regulation and vegetarian diets. Nutr Rev 2005;63:1–8.

13. Aburto NJ, Hanson S, Gutierrez H, et al. Effect of increased potassium intake on cardiovascular risk factors and disease: systematic review and meta-analyses. BMJ 2013;346:f1378.

14. Berkow SE, Barnard N. Vegetarian diets and weight status. Nutr Rev 2006;64:175–88.

15. Larsson CL, Johansson GK. Dietary intake and nutritional status of young vegans and omnivores in Sweden. Am J Clin Nutr 2002;76:100–6.

16. Ernst E, Pietsch L, Matrai A, Eisenberg J. Blood rheology in vegetarians. Br J Nutr 1986;56:555–60.

17. Iacono JM, Dougherty RM. Effects of polyunsaturated fats on blood pressure. Annu Rev Nutr 1993;13:243–60.

18. Ferdowsian HR, Barnard ND, Hoover VJ, et al. A multicomponent intervention reduces body weight and cardiovascular risk at a GEICO corporate site. Am J Health Promot 2010;24:384–7.

19. Brugts JJ, Ninomiya T, Boersma E, et al. The consistency of the treatment effect of an ACE-inhibitor based treatment regimen in patients with vascular disease or high risk of vascular disease: a combined analysis of individual data of ADVANCE, EUROPA, and PROGRESS trials. Eur Heart J 2009;30:1385–94.

20. Berman BM, Singh BB, Hartnoll SM, et al. Primary care physicians and complementary-alternative medicine: training, attitudes, and practice patterns. Am Board Fam Pract 1998;11:272–81.

21. Drewnowski A, Darmon N. The economics of obesity: dietary energy density and energy cost. Am J Clin Nutr 2005;82(1 Suppl):265S-273S.

22. Lea EJ, Crawford D, Worsley A. Public views of the benefits and barriers to the consumption of a plant-based diet. Eur J Clin Nutr 2006;60:828–37.

23. Schroeder K, Fahey T, Ebrahim S. How can we improve adherence to blood pressure–lowering medication in ambulatory care? Systematic review of randomized controlled trials. Arch Intern Med 2004;164:722–32.

Study Overview

Objective. To determine the association between a vegetarian diet and blood pressure (BP).

Design. Systematic review and meta-analysis of controlled clinical trials and observational studies.

Setting and participants. MEDLINE and Web of Science were respectively searched for English articles published between 1946 to October 2013 and 1900 to November 2013. Inclusion criteria were age > 20 years and vegetarian diet. This included a vegan diet (omitting all animal products), ovo/lacto/pesco vegetarian diet (including eggs/dairy/fish), or semi-vegetarian diet (meat or fish rarely). Exclusion criteria included studies of twins, a multipronged intervention, describing only categorical BP, and reliance on a case series. A total of 258 records were identified. Seven clinical trials and 32 observational studies met inclusion criteria. The 7 clinical trials encompassed 311 participants (median, 38; range, 11–113) with a mean age of 44.5 years (range, 38.0–54.3). All were open-label, 6 were randomized, and 6 provided food to participants. The 32 observational studies included 21,604 participants (median, 152; range, 20–9242) with a mean age of 46.6 years (range, 28.8–68.4 years). Fifteen of these studies included mixed diet types (vegan, lacto, ovolacto, pesca, and/or semivegetarian).

Main outcome measures. The primary outcome was BP. Differences in systolic BP (SBP) and diastolic BP (DBP) between groups consuming vegetarian or comparison diets were pooled using a random effects model. The study compared clinical trials and observational studies separately. Funnel plots, the Egger test, and the trim-and-fill method were all used to assess and correct for publication bias.

Results. Vegetarian diets in clinical trials were associated with a mean SBP reduction of −4.8 mm Hg (95% confidence interval [CI], −6.6 to −3.1; P < 0.001; I² = 0; P = 0.45 for heterogeneity) and DBP reduction of −2.2 mm Hg (95% CI, −3.5 to −1.0; P < 0.001; I² = 0; P= 0.43 for heterogeneity) when compared with omnivorous diets. Observational studies had larger reductions but significant heterogeneity: SBP −6.9 mm Hg (95% CI, −9.1 to −4.7; P < 0.001; I² = 91.4; P < 0.001 for heterogeneity) and DBP −4.7 mm Hg (95% CI, −6.3 to −3.1; P < 0.001; I² = 92.6; P < 0.001 for heterogeneity). This heterogeneity was best explained by proportion of men (β −0.03; P < 0.001), baseline SBP (β −0.13; P = 0.003), baseline DBP (β −0.30; P < 0.001), sample size (β 0.001; P< 0.001), and BMI (β −0.46; P = 0.02). This suggests that vegetarian diets and lower BP are more strongly associated in men and those with higher baseline BP and BMI.

Subgroup analysis included stratification by age, gender, BMI, diet type, sample size, diet duration, BP medication use, baseline BP, and geographic region. In subgroup analysis of clinical trials, no statistically significant difference between group variation or heterogeneity existed. In comparison, subgroup analysis of observational studies reduced heterogeneity and often effect size. For example, lower SBP was evident in the majority male subgroups (mean SBP/DBP: –18.5 mm Hg/–10.1 mm Hg).

Publication bias existed for both clinical trials and observational studies. According to trim-and-fill methodology, 3 clinical trials of smaller size and larger BP reduction likely were missing (Egger P = 0.04). Their addition shifted mean SBP reduction from −4.8 mm Hg (−6.6 to −3.1) to −5.2 mm Hg (−6.9 to −3.5). Observational studies lacked medium sized negative trials and were overrepresented by larger positive trials (Egger P < 0.001), although this was not confirmed by trim-and-fill (yet this method performs less well under heterogeneous conditions) [1].

Conclusion. Vegetarian diets, when compared with omnivorous diets, are associated with reductions in BP.

Commentary

Several studies show that dietary modifications are effective in preventing and managing hypertension [2,3]. Landmark randomized trials, including the DASH diet [4], DASH-sodium diet [5], and OmniHeart diets [6], all of which emphasize fruit and vegetable intake but are not vegetarian, have led to SBP and DBP reductions ranging from 5.5 to 9.5 mm Hg and 3.0 to 5.2, respectively. However, the impact of a vegetarian diet still remains debated, particularly given disparate findings among randomized controlled trials (RCTs). For example, findings in the early- and mid-1980s of small RCTs with ovolactovegetarians (a vegetarian who consumes eggs and dairy products but not animal flesh) suggested reductions similar to the pooled SBP reduction of –4.8 mm Hg Yokoyama et al report [7,8]. In contrast, one RCT comparing ovolactovegetarian with lean meat diets failed to show a BP benefit [9]. Striking is the dearth of RCTs in the last 20 years to assist in better estimating this impact, particularly given its continual recommendation in the scientific [10] and lay communities [11]. To the authors’ credit, this is the first meta-analysis and second systematic review of this important relationship [12].

A vegetarian diet likely supports BP reductions through a variety of mechanisms, most notably via an abundance of potassium [13]. Potassium likely promotes vasodilation, which facilitates glomerular filtration, allowing decreased renal sodium reabsorption and decreased platelet aggregation. Other more controversial hypotheses include decreased energy density leading to reduced BMI [14], decreased sodium intake [15], reduced blood viscosity [16], and high polyunsaturated with low saturated fat content [17].

Strengths of this analysis include the large observational sample size, the separate analysis of clinical trials and observational studies, the lengthy search time-frame, the subgroup analyses, and the adjustment for publication bias. Although the overall association was robust throughout, we agree with the authors that large heterogeneity among observational studies, small clinical trial sample sizes, and the variation in what “vegetarian” represents throughout the world and in individual studies all represent limitations. The participants in many of the observational studies could have technically eaten meat with unclear and undefined frequency, and this may have explained the heterogeneity of these studies. Correspondingly, the lack of heterogeneity observed in the clinical trials may be due to the fact that participants were provided meals in 6 out of 7 studies.

It was surprising that only 7 clinical trials were found. The authors utilized 2 databases, but perhaps searching additional databases such as EMBASE or CINAHL would have yielded other pertinent studies. The authors also did not use a bias assessment tool such as that proposed by the Cochrane bias methods group, which could have better discriminated high- from low-quality trials and made for useful subgroup analyses [1]. Similarly, reporting on both attrition and adherence could have assisted in decreasing heterogeneity during subgroup analyses and determining high-quality from low-quality studies. For example, adherence in Ferdowsian et al (vegan diet) was determined by unannounced dietician phone calls and found that only 57% of participants abstained from animal products. This may have been secondary to the study’s design, in that “providing meals” included simply making them an available option at the company cafeteria instead of requiring consumption of a study-specific vegetarian meal [18].

Applications for Clinical Practice

In this meta-analysis, vegetarian diets were associated with –4.8 mm Hg SBP and −2.2 mm Hg DBP reductions, indicating that providers can recommend a vegetarian diet as on par with other lifestyle changes, including low-sodium diet, weight loss, and exercise. A vegetarian diet may be comparable to pharmacologic therapy in magnitude of BP change. Short- and long-term pharmacologic therapy is associated with respective SBP/DBP reductions of –8.3/–3.8 and –5.4/–2.3, not altogether different from reductions seen with vegetarian or vegetable-heavy diets [19].

Although there are barriers to a vegetarian diet, including provider attitudes [20], cost [21], poor culinary skill [22], palatability, and adherence, pharmacologic BP treatment also presents barriers: adherence to BP medications is estimated to be 50% to 70% [23], and harm due to side effects can preclude use. Thus, providers can present a vegetarian diet as a potentially effective option, depending on patient preference and ability to adhere.

—David M. Levine, MD, MA, New York University
School of Medicine, and Melanie Jay, MD, MS

References

1. Higgins JPT, Green S, editors. Cochrane handbook for systematic reviews of interventions version 5.1.0 [updated March 2011]. The Cochrane Collaboration, 2011. Available at http://handbook.cochrane.org/chapter_10/10_4_4_2_trim_and_fill.htm.

2. Eckel RH, Jakicic JM, Ard JD, et al. 2013 AHA/ACC guideline on lifestyle management to reduce cardiovascular risk: a report of the American College of Cardiology/American Heart Association Task Force on Practice Guidelines. Circulation 2013 Nov 12. [Epub ahead of print]

3. James PA, Oparil S, Carter BL, et al. 2014 evidence-based guideline for the management of high blood pressure in adults: report from the panel members appointed to the Eighth Joint National Committee (JNC 8). JAMA 2014;311:507–20.

4. Appel LJ, Moore TJ, Obarzanek E, et al. A clinical trial of the effects of dietary patterns on blood pressure. N Engl J Med 1997;336:1117–24.

5. Sacks FM, Svetkey LP, Vollmer WM, et al. Effects on blood pressure of reduced dietary sodium and the dietary approaches to stop hypertension (DASH) diet. N Engl J Med 2001;344:3–10.

6. Appel LJ, Sacks FM, Carey VJ, et al. Effects of protein, monounsaturated fat, and carbohydrate intake on blood pressure and serum lipids. JAMA 2005;294:2455–64.

7. Rouse IL, Beilin LJ, Armstrong BK, Vandongen R. Blood-pressure–lowering effect of a vegetarian diet: controlled trial in normotensive subjects. Lancet 1983;321:5–10.

8. Margetts BM, Beilin LJ, Vandongen R, Armstrong BK. Vegetarian diet in mild hypertension: a randomised controlled trial. BMJ 1986;293:1468–71.

9. Kestin M, Rouse IL, Correll RA, Nestel PJ. Cardiovascular disease risk factors in free-living men: comparison of two prudent diets, one based on lactoovovegetarianism and the other allowing lean meat. Am J Clin Nutr 1989;50:280–7.

10. Alpert JS. Nutritional advice for the patient with heart disease: what diet should we recommend for our patients? Circulation 2011;124:e258–e260.

11. Gordinier J. Making vegan a new normal. New York Times. 26 Sept 2012. Page D1.

12. Berkow SE, Barnard ND. Blood pressure regulation and vegetarian diets. Nutr Rev 2005;63:1–8.

13. Aburto NJ, Hanson S, Gutierrez H, et al. Effect of increased potassium intake on cardiovascular risk factors and disease: systematic review and meta-analyses. BMJ 2013;346:f1378.

14. Berkow SE, Barnard N. Vegetarian diets and weight status. Nutr Rev 2006;64:175–88.

15. Larsson CL, Johansson GK. Dietary intake and nutritional status of young vegans and omnivores in Sweden. Am J Clin Nutr 2002;76:100–6.

16. Ernst E, Pietsch L, Matrai A, Eisenberg J. Blood rheology in vegetarians. Br J Nutr 1986;56:555–60.

17. Iacono JM, Dougherty RM. Effects of polyunsaturated fats on blood pressure. Annu Rev Nutr 1993;13:243–60.

18. Ferdowsian HR, Barnard ND, Hoover VJ, et al. A multicomponent intervention reduces body weight and cardiovascular risk at a GEICO corporate site. Am J Health Promot 2010;24:384–7.

19. Brugts JJ, Ninomiya T, Boersma E, et al. The consistency of the treatment effect of an ACE-inhibitor based treatment regimen in patients with vascular disease or high risk of vascular disease: a combined analysis of individual data of ADVANCE, EUROPA, and PROGRESS trials. Eur Heart J 2009;30:1385–94.

20. Berman BM, Singh BB, Hartnoll SM, et al. Primary care physicians and complementary-alternative medicine: training, attitudes, and practice patterns. Am Board Fam Pract 1998;11:272–81.

21. Drewnowski A, Darmon N. The economics of obesity: dietary energy density and energy cost. Am J Clin Nutr 2005;82(1 Suppl):265S-273S.

22. Lea EJ, Crawford D, Worsley A. Public views of the benefits and barriers to the consumption of a plant-based diet. Eur J Clin Nutr 2006;60:828–37.

23. Schroeder K, Fahey T, Ebrahim S. How can we improve adherence to blood pressure–lowering medication in ambulatory care? Systematic review of randomized controlled trials. Arch Intern Med 2004;164:722–32.