Preventing and Treating Hypertension with Diet (Part 5)

Interventional Studies on Diet and Blood Pressure

laboratory

We have looked at the observational evidence and potential mechanisms by which food influences the development of hypertension. However, only interventional studies can give us an approximate clue of the causal relationship between diet and blood pressure. Which diet is best for preventing and treating hypertension?

As shown before, observational studies indicate that a high intake of certain food groups such as whole grains, fruits, nuts, eggs, and dairy is inversely associated with risk of hypertension, whereas the consumption of red and processed meat and sugar-sweetened beverages increases the risk of elevated blood pressure. According to a meta-analysis of 28 prospective cohort studies, the association follows a dose-response curve1)Schwingshackl L, Schwedhelm C, Hoffmann G, et al. Food Groups and Risk of Hypertension: A Systematic Review and Dose-Response Meta-Analysis of Prospective Studies. Adv Nutr. 2017;8(6):793-803. doi:10.3945/an.117.017178.

Many findings from observational studies and their meta-analyses are corroborated by clinical trials and corresponding meta-analyses. However, some results are inconclusive and hence demand for further research. In addition, even though interventional studies can give us a hint to the underlying causality between diet and blood pressure, they cannot put a certain dietary element into the context of mortality risk. An interventional study might show a blood pressure increasing or lowering effect of a food group or dietary pattern, but it does not say anything about cardiovascular or mortality risk reduction in general, omitting possible beneficial or detrimental effects on overall health. That is the reason why we want to view the evidence in the context of total mortality risk, derived from major well-designed observational studies.

The most recent systematic analysis of the Global Burden of Diseases, Injuries, and Risk Factors Study (GBD) 2017 estimated the effect of 15 foods and nutrients across 195 countries on mortality from non-communicable disease and quantified the overall impact of poor dietary habits on mortality2)Afshin A, Sur PJ, Fay KA, et al. Health effects of dietary risks in 195 countries, 1990–2017: a systematic analysis for the Global Burden of Disease Study 2017. The Lancet. 2019;0(0). doi:10.1016/S0140-6736(19)30041-8. The authors state that in 2017, 11 million deaths and 255 million DALYs were attributable to dietary risk factors. Among those, a diet high in sodium, low in whole grains, and low in fruits were the leading risk factors in many countries, followed by low intake of vegetables, legumes, nuts and seeds, milk, fiber, calcium, and seafood omega-3 fatty acids, and a high intake of red and processed meat, sugar-sweetened beverages, and trans fatty acids.

We will refer to this study by speaking of “the LANCET study”.

The following sections will now give a summary of the recent evidence regarding the most important food groups and specific diets.

Whole Grains

Two randomized controlled trials (RCTs) could show reductions in blood pressure when a whole-grain diet was compared to a diet high in refined grains3)Kirwan JP, Malin SK, Scelsi AR, et al. A Whole-Grain Diet Reduces Cardiovascular Risk Factors in Overweight and Obese Adults: A Randomized Controlled Trial. J Nutr. 2016;146(11):2244-2251. doi:10.3945/jn.116.2305084)Tighe P, Duthie G, Vaughan N, et al. Effect of increased consumption of whole-grain foods on blood pressure and other cardiovascular risk markers in healthy middle-aged persons: a randomized controlled trial. Am J Clin Nutr. 2010;92(4):733-740. doi:10.3945/ajcn.2010.29417. Whole grains are a source of nutrients, such as magnesium, potassium and fiber, which could exert antihypertensive effects, as already discussed in the ‘Mechanisms’ section. However, other studies could not show an effect, even when high doses of whole grains (up to 120 g/d) were examined5)Andersson A, Tengblad S, Karlström B, et al. Whole-grain foods do not affect insulin sensitivity or markers of lipid peroxidation and inflammation in healthy, moderately overweight subjects. J Nutr. 2007;137(6):1401-1407. doi:10.1093/jn/137.6.14016)Brownlee IA, Moore C, Chatfield M, et al. Markers of cardiovascular risk are not changed by increased whole-grain intake: the WHOLEheart study, a randomised, controlled dietary intervention. Br J Nutr. 2010;104(1):125-134. doi:10.1017/S0007114510000644.

Regarding mortality risk, a diet rich in whole grains lowers all-cause mortality risk, according to the LANCET study7)Afshin A, Sur PJ, Fay KA, et al. Health effects of dietary risks in 195 countries, 1990–2017: a systematic analysis for the Global Burden of Disease Study 2017. The Lancet. 2019;0(0). doi:10.1016/S0140-6736(19)30041-8. An optimal range of intake is considered to be 100-150 grams per day. This is consistent with a meta-analysis by Zhang and colleagues, stating that the risk reduction comparing the highest intake of whole grain with the lowest category is 0.84 (95% CI: 0.81, 0.88) for total mortality and 0.83 (95% CI: 0.79, 0.86) for CVD mortality8)Zhang B, Zhao Q, Guo W, Bao W, Wang X. Association of whole grain intake with all-cause, cardiovascular, and cancer mortality: a systematic review and dose-response meta-analysis from prospective cohort studies. Eur J Clin Nutr. 2018;72(1):57-65. doi:10.1038/ejcn.2017.149.

Fruits, Vegetables, Nuts and Pulses

As fruits, vegetables, legumes, and nuts are a good source of antioxidants, fiber, micronutrients, and anti-inflammatory substances, a lowering effect on blood pressure seems likely.

In contrast to the beforementioned meta-analysis by Schwingshackl and colleagues9)Schwingshackl L, Schwedhelm C, Hoffmann G, et al. Food Groups and Risk of Hypertension: A Systematic Review and Dose-Response Meta-Analysis of Prospective Studies. Adv Nutr. 2017;8(6):793-803. doi:10.3945/an.117.017178 that could not establish a significant association between the intake of vegetables and hypertension, a Cochrane review of 10 intervention trials including 1,730 participants found a reduction in systolic blood pressure after increasing fruit and vegetable intake10)Hartley L, Igbinedion E, Holmes J, et al. Increased consumption of fruit and vegetables for the primary prevention of cardiovascular diseases. Cochrane Database Syst Rev. 2013;(6):CD009874. doi:10.1002/14651858.CD009874.pub2. However, the evidence was limited due to a low amount of available studies, which also exhibited heterogenous study designs and only short-term interventions.

Another meta-analysis by Shin and colleagues11)Shin JY, Kim JY, Kang HT, Han KH, Shim JY. Effect of fruits and vegetables on metabolic syndrome: a systematic review and meta-analysis of randomized controlled trials. International Journal of Food Sciences and Nutrition. 2015;66(4):416-425. doi:10.3109/09637486.2015.1025716 showed a reduction of diastolic blood pressure associated with fruit and vegetable intake in patients with metabolic syndrome. Systolic blood pressure was not influenced by this dietary intervention.

A systematic review and meta-analysis of 8 dietary pulse intervention trials by Jayalath and colleagues12)Jayalath VH, de Souza RJ, Sievenpiper JL, et al. Effect of dietary pulses on blood pressure: a systematic review and meta-analysis of controlled feeding trials. Am J Hypertens. 2014;27(1):56-64. doi:10.1093/ajh/hpt155 showed that legumes significantly lower both systolic and mean arterial blood pressure when exchanged isocalorically for other foods.  A median of around 162 g/d of dietary pulses significantly lowered systolic blood pressure by 2.25 mm Hg and mean arterial blood pressure by 0.75 mm Hg over a median 10-week follow-up in middle-age participants with or without hypertension.

Regarding nuts (including leguminous peanuts and soy nuts), a systematic review and meta-analysis of RCTs conducted by Mohammadifard and colleagues13)Mohammadifard N, Salehi-Abargouei A, Salas-Salvadó J, Guasch-Ferré M, Humphries K, Sarrafzadegan N. The effect of tree nut, peanut, and soy nut consumption on blood pressure: a systematic review and meta-analysis of randomized controlled clinical trials. The American Journal of Clinical Nutrition. 2015;101(5):966-982. doi:10.3945/ajcn.114.091595 found that nut consumption leads to a significant reduction in systolic blood pressure in participants without type 2 diabetes. Pistachios seemed to have the strongest effect on both systolic and diastolic blood pressure, while mixed nuts also reduce diastolic blood pressure.

According to the LANCET study, a diet low in vegetables, fruits, nuts, and legumes is considered to be a dietary risk factor for total mortality. An optimal level of intake to reduce mortality risk should be 250 g/d of fruits, 260 g/d of vegetables, 60 g/d of legumes, and 21 g/d of nuts and seeds.

Another systematic review and dose-response meta-analysis of prospective studies found a relative risk per 200 g/d for fruits and vegetables combined of 0.97 (95% CI: 0.95, 0.99) cardiovascular disease and 0.90 (95% CI: 0.87, 0.93) for all-cause mortality14)Aune D, Giovannucci E, Boffetta P, et al. Fruit and vegetable intake and the risk of cardiovascular disease, total cancer and all-cause mortality-a systematic review and dose-response meta-analysis of prospective studies. Int J Epidemiol. 2017;46(3):1029-1056. doi:10.1093/ije/dyw319. Risk reductions were observed up to 800 g/day.

For legumes, similar results were found. Li and colleagues15)Li H, Li J, Shen Y, Wang J, Zhou D. Legume Consumption and All-Cause and Cardiovascular Disease Mortality. Biomed Res Int. 2017;2017:8450618. doi:10.1155/2017/8450618 found a relative risk of 0.96 (95% CI: 0.86, 1.06) for cardiovascular disease mortality and 0.93 (95% CI: 0.87, 0.99) for all-cause mortality, comparing highest with lowest intake. For nuts, Chen and colleagues similarly found a risk reduction comparing high with low nut consumption of 0.81 (95% CI: 0.78, 0.84) for all-cause mortality and 0.75 (95% CI: 0.71, 0.79) for cardiovascular disease mortality.

Dairy

Concerning intervention trials with dairy, results are inconclusive, which corresponds with the similarly inconsistent observational evidence16)Wang H, Fox CS, Troy LM, Mckeown NM, Jacques PF. Longitudinal association of dairy consumption with the changes in blood pressure and the risk of incident hypertension: the Framingham Heart Study. Br J Nutr. 2015;114(11):1887-1899. doi:10.1017/S0007114515003578vs.(Dauchet L, Kesse-Guyot E, Czernichow S, et al. Dietary patterns and blood pressure change over 5-y follow-up in the SU.VI.MAX cohort. Am J Clin Nutr. 2007;85(6):1650-1656. doi:10.1093/ajcn/85.6.1650))17)Ralston RA, Lee JH, Truby H, Palermo CE, Walker KZ. A systematic review and meta-analysis of elevated blood pressure and consumption of dairy foods. Journal of Human Hypertension. 2012;26(1):3-13. doi:10.1038/jhh.2011.3. Some clinical trials report an inverse relationship between the consumption of dairy products and blood pressure18)van Meijl LEC, Mensink RP. Low-fat dairy consumption reduces systolic blood pressure, but does not improve other metabolic risk parameters in overweight and obese subjects. Nutr Metab Cardiovasc Dis. 2011;21(5):355-361. doi:10.1016/j.numecd.2009.10.00819)Tanaka S, Uenishi K, Ishida H, et al. A randomized intervention trial of 24-wk dairy consumption on waist circumference, blood pressure, and fasting blood sugar and lipids in Japanese men with metabolic syndrome. J Nutr Sci Vitaminol. 2014;60(5):305-312. doi:10.3177/jnsv.60.305, whereas others could not see any changes after the intervention20)Crichton GE, C Howe PR, Buckley JD, Coates AM, Murphy KJ. Dairy consumption and cardiometabolic health: outcomes of a 12-month crossover trial. Nutr Metab (Lond). 2012;9:19. doi:10.1186/1743-7075-9-19.

Reasons for the inconsistent results could be that different types of milk products (e.g. full-fat and low-fat milk, cheese, yogurt, etc.) exert different effects and hereby produce a heterogeneity among studies looking at various dairy products. For example, it might be that only certain peptides in specific dairy products exert antihypertensive effects. Interventional studies have shown that daily administration of fermented milk (150 ml) over 21 weeks reduced hypertensive subjects’ blood pressure by an average of 6.7 ± 3.0 mm Hg (systolic) and 3.6 ± 1.9 mm Hg (diastolic) compared to the control group. The milk of the intervention group contained two bioactive peptides with the properties of an ACE inhibitor during the fermentation process21)Seppo L, Jauhiainen T, Poussa T, Korpela R. A fermented milk high in bioactive peptides has a blood pressure-lowering effect in hypertensive subjects. Am J Clin Nutr. 2003;77(2):326-330. doi:10.1093/ajcn/77.2.326.

Moreover, the majority of interventional (and also epidemiological) studies reporting antihypertensive effects of dairy products is funded financially by dairy companies, which may or may not be an influencing parameter (for example,22)Drouin-Chartier J-P, Gigleux I, Tremblay AJ, Poirier L, Lamarche B, Couture P. Impact of dairy consumption on essential hypertension: a clinical study. Nutr J. 2014;13. doi:10.1186/1475-2891-13-8323)van Meijl LEC, Mensink RP. Low-fat dairy consumption reduces systolic blood pressure, but does not improve other metabolic risk parameters in overweight and obese subjects. Nutr Metab Cardiovasc Dis. 2011;21(5):355-361. doi:10.1016/j.numecd.2009.10.008 24)Tanaka S, Uenishi K, Ishida H, et al. A randomized intervention trial of 24-wk dairy consumption on waist circumference, blood pressure, and fasting blood sugar and lipids in Japanese men with metabolic syndrome. J Nutr Sci Vitaminol. 2014;60(5):305-312. doi:10.3177/jnsv.60.30525)Machin DR, Park W, Alkatan M, Mouton M, Tanaka H. Hypotensive effects of solitary addition of conventional nonfat dairy products to the routine diet: a randomized controlled trial. Am J Clin Nutr. 2014;100(1):80-87. doi:10.3945/ajcn.114.085761. Thus, they need to be interpreted with caution, and recommendations based on these findings need to be under constant reconsideration.

A meta-analysis conducted by Ding and colleagues26)Ding M, Huang T, Bergholdt HK, et al. Dairy consumption, systolic blood pressure, and risk of hypertension: Mendelian randomization study. BMJ. 2017;356:j1000. doi:10.1136/bmj.j1000 looked at both observational studies and clinical trials, using data collected from 32 studies with 197,332 participants. They applied the method of Mendelian randomization using a single nucleotide polymorphism coding for lactase persistence as an instrumental variable that is strongly associated with dairy intake to establish a quasi-causal relationship between dairy intake and systolic blood pressure. When the lactase persistence gene polymorphism was compared to the one indicating a complete lactase deficiency, the researchers found no association with systolic blood pressure or risk of hypertension. Moreover, a meta-analysis of several clinical trials conducted in the same study showed that higher dairy intake has no significant effect on systolic blood pressure for interventions over one month to 12 months.

The most recent randomized controlled crossover trial led by Roy and colleagues27)Roy SJ, Lapierre SS, Baker BD, Delfausse LA, Machin DR, Tanaka H. High dietary intake of whole milk and full-fat dairy products does not exert hypotensive effects in adults with elevated blood pressure. Nutr Res. 2019;64:72-81. doi:10.1016/j.nutres.2019.01.003 found that a 4-week diet high in full-fat dairy products (4 servings/d of full-fat dairy products + regular diet) did not change blood pressure in adults with hypertension or pre-hypertension (120-159/<99 mm Hg) compared to a no-dairy control.

This is in contrast to a previous trial showing a hypotensive effect of non-fat dairy products in a similar study design28)Machin DR, Park W, Alkatan M, Mouton M, Tanaka H. Hypotensive effects of solitary addition of conventional nonfat dairy products to the routine diet: a randomized controlled trial. Am J Clin Nutr. 2014;100(1):80-87. doi:10.3945/ajcn.114.085761. A four-week diet high in non-fat dairy products (4 additional servings of conventional nonfat dairy products/d) reduced average systolic blood pressure from 135 ± 1 to 127 ± 1 mm Hg. However, no changes in diastolic blood pressure could be observed.

The LANCET study describes a diet low in milk as a dietary risk factor for mortality and recommends a daily intake of 435 g29)Afshin A, Sur PJ, Fay KA, et al. Health effects of dietary risks in 195 countries, 1990–2017: a systematic analysis for the Global Burden of Disease Study 2017. The Lancet. 2019;0(0). doi:10.1016/S0140-6736(19)30041-8.

The so-called Golestan Cohort study, a prospective cohort study launched in Golestan Province, Iran, showed that the highest quintile of total dairy consumption compared to the lowest was associated with a lower all-cause mortality risk by 19% (95% CI: 0.72, 0.91), and a lower cardiovascular disease mortality risk by 28% (95% CI: 0.6, 0.86). However, higher intake of high-fat dairy products and milk was not associated with all-cause or cardiovascular disease mortality. The association was mainly established by consumption of fermented dairy products such as yogurt and cheese30)Farvid MS, Malekshah AF, Pourshams A, et al. Dairy Food Intake and All-Cause, Cardiovascular Disease, and Cancer Mortality: The Golestan Cohort Study. Am J Epidemiol. 2017;185(8):697-711. doi:10.1093/aje/kww139.

In contrast, a dose-response meta-analysis of prospective cohort studies found no associations for high-fat/low-fat dairy and milk with all-cause mortality or cardiovascular disease. However, there was an inverse association between total fermented dairy with mortality (RR 0.98, 95% CI: 0.97, 0.99) and cardiovascular disease risk (RR 0.98, 95% CI: 0.97, 0.99)31)Guo J, Astrup A, Lovegrove JA, Gijsbers L, Givens DI, Soedamah-Muthu SS. Milk and dairy consumption and risk of cardiovascular diseases and all-cause mortality: dose-response meta-analysis of prospective cohort studies. Eur J Epidemiol. 2017;32(4):269-287. doi:10.1007/s10654-017-0243-1.

Fish and Meat

Dietary interventions examining the effect of fish or meat on blood pressure are scarce.

There are two clinical trials investigating the inclusion of lean red meat into the DASH diet. Nowson and colleagues32)Nowson CA, Wattanapenpaiboon N, Pachett A. Low-sodium Dietary Approaches to Stop Hypertension-type diet including lean red meat lowers blood pressure in postmenopausal women. Nutr Res. 2009;29(1):8-18. doi:10.1016/j.nutres.2008.12.002 found that a low dietary acid load DASH-type diet containing 6 servings of 100 g cooked lean red meat per week lowered SBP by 5.6 mm Hg compared to a reduction of 2.7 mm Hg in the control group. The difference was not significant. In addition, the control diet was a higher acid load reference diet.

Another trial by Roussell and colleagues33)Roussell MA, Hill AM, Gaugler TL, et al. Effects of a DASH-like diet containing lean beef on vascular health. J Hum Hypertens. 2014;28(10):600-605. doi:10.1038/jhh.2014.34 compared four isocaloric diets in a randomized, crossover study design in normotensive patients: a healthy American diet (20 g beef per day), the DASH diet (see below for more information; 28 g beef per day), the beef in an optimal lean diet (BOLD, 113 g beef per day), and the BOLD+ diet (153 g beef per day). Interestingly, the study showed that the BOLD+ diet reduced systolic blood pressure in normotensive adults by 4.2 mm Hg when compared with a healthy American diet that was lower in overall calories (10%) and in protein, and higher in carbs and saturated fat. BOLD and DASH diet showed nonsignificant reductions of 1.6 and 2.8 mm Hg, respectively.

Importantly, both trials received financial funds from the meat industry (“Meat & Livestock Australia”; “The Beef Checkoff”).

Regarding fish consumption (as a whole), a clinical trial investigated the effect of 3 weekly fish meals containing trout rich in polyunsaturated fatty acids for 6 months on blood pressure in hypertensive patients34)Colussi G, Catena C, Dialti V, Pezzutto F, Mos L, Sechi LA. Fish meal supplementation and ambulatory blood pressure in patients with hypertension: relevance of baseline membrane fatty acid composition. Am J Hypertens. 2014;27(3):471-481. doi:10.1093/ajh/hpt231. The intervention decreased 24-hour systolic and diastolic blood pressure in such patients who showed an increased content of polyunsaturated fatty acids in their red blood cell membranes (-5/3 mm Hg). A previous clinical study had shown that compared to eating no fish, eating 125 g/d of salmon significantly decreased mean systolic blood pressure by 5 mm Hg and mean diastolic blood pressure by 3 mm Hg35)Lara JJ, Economou M, Wallace AM, et al. Benefits of salmon eating on traditional and novel vascular risk factors in young, non-obese healthy subjects. Atherosclerosis. 2007;193(1):213-221. doi:10.1016/j.atherosclerosis.2006.06.018

The LANCET study considers a diet high in red and processed meat as a dietary risk factor36)Afshin A, Sur PJ, Fay KA, et al. Health effects of dietary risks in 195 countries, 1990–2017: a systematic analysis for the Global Burden of Disease Study 2017. The Lancet. 2019;0(0). doi:10.1016/S0140-6736(19)30041-8. This is corroborated by multiple meta-analyses of epidemiological studies37)Wang X, Lin X, Ouyang YY, et al. Red and processed meat consumption and mortality: dose-response meta-analysis of prospective cohort studies. Public Health Nutr. 2016;19(5):893-905. doi:10.1017/S136898001500206238)Larsson SC, Orsini N. Red meat and processed meat consumption and all-cause mortality: a meta-analysis. Am J Epidemiol. 2014;179(3):282-289. doi:10.1093/aje/kwt261 39)Schwingshackl L, Schwedhelm C, Hoffmann G, et al. Food Groups and Risk of Hypertension: A Systematic Review and Dose-Response Meta-Analysis of Prospective Studies. Adv Nutr. 2017;8(6):793-803. doi:10.3945/an.117.017178. Regarding fish, the Lancet study only makes a statement concerning seafood omega-3 fatty acids which are considered beneficial. A large prospective cohort study found no association between fish consumption and overall and ischaemic heart disease mortality40)Engeset D, Braaten T, Teucher B, et al. Fish consumption and mortality in the European Prospective Investigation into Cancer and Nutrition cohort. Eur J Epidemiol. 2015;30(1):57-70. doi:10.1007/s10654-014-9966-4. However, a meta-analysis by Schwingshackl and colleagues41)Schwingshackl L, Schwedhelm C, Hoffmann G, et al. Food Groups and Risk of Hypertension: A Systematic Review and Dose-Response Meta-Analysis of Prospective Studies. Adv Nutr. 2017;8(6):793-803. doi:10.3945/an.117.017178 showed an inverse association for the highest compared with the lowest fish intake category (RR: 0.95; 95% CI: 0.92, 0.98).

Micronutrients

Most interventional studies confirm the data for sodium, potassium and magnesium obtained from observational studies.

A meta-analysis by Aburto and colleagues42)Aburto NJ, Ziolkovska A, Hooper L, Elliott P, Cappuccio FP, Meerpohl JJ. Effect of lower sodium intake on health: systematic review and meta-analyses. BMJ. 2013;346:f1326. doi:10.1136/bmj.f1326 showed that a dietary sodium reduction significantly reduced systolic blood pressure by 3.39 mm Hg (95% CI: 2.46, 4.31) and resting diastolic blood pressure by 1.54 mm Hg (0.98, 2.11). When sodium intake of less than 2 g/day was compared to an intake of at least 2 g/day, systolic blood pressure was reduced by 3.47 mm Hg (0.76, 6.18) and diastolic blood pressure by 1.81 mm Hg (0.54, 3.08). Interestingly, a decreased sodium intake had no significant adverse effect on blood lipids, catecholamine levels, or renal function.

Another meta-analysis of randomized controlled trials, in which subjects ate salt-reduced diets for four or more weeks, showed that reducing urinary sodium excretion by 100 mmol/d (equating to 6 g salt/d) resulted in a reduced systolic/diastolic blood pressure by 2.42 and 1.0 mm Hg, respectively, in healthy volunteers, and by 5.39 and 2.82 mm Hg, respectively, in hypertensive volunteers43)He FJ, Li J, Macgregor GA. Effect of longer term modest salt reduction on blood pressure: Cochrane systematic review and meta-analysis of randomised trials. BMJ. 2013;346:f1325. doi:10.1136/bmj.f1325.

The LANCET study considers a diet high in sodium a risk factor for high mortality risk and recommends a daily intake of 1-5 grams per day44)Afshin A, Sur PJ, Fay KA, et al. Health effects of dietary risks in 195 countries, 1990–2017: a systematic analysis for the Global Burden of Disease Study 2017. The Lancet. 2019;0(0). doi:10.1016/S0140-6736(19)30041-8.

Regarding potassium, several randomized controlled trials and a meta-analysis of these studies indicate that high dietary potassium intake is associated with reduced blood pressure in both hypertensive and normotensive subjects45)Whelton PK, He J. Potassium in preventing and treating high blood pressure. Semin Nephrol. 1999;19(5):494-49946)Geleijnse JM, Kok FJ, Grobbee DE. Blood pressure response to changes in sodium and potassium intake: a metaregression analysis of randomised trials. J Hum Hypertens. 2003;17(7):471-480. doi:10.1038/sj.jhh.1001575. Also, potassium supplementation moderately but significantly lowers blood pressure, according to a meta-analysis by Poorolajal and colleagues47)Poorolajal J, Zeraati F, Soltanian AR, Sheikh V, Hooshmand E, Maleki A. Oral potassium supplementation for management of essential hypertension: A meta-analysis of randomized controlled trials. PLoS ONE. 2017;12(4):e0174967. doi:10.1371/journal.pone.0174967.

In the context of sodium intake, a systematic review of randomized controlled and observational studies suggests that the ratio of sodium to potassium for hypertensive patients is actually more relevant to the level of blood pressure than the respective electrolytes alone48)Perez V, Chang ET. Sodium-to-potassium ratio and blood pressure, hypertension, and related factors. Adv Nutr. 2014;5(6):712-741. doi:10.3945/an.114.006783.

For magnesium, a meta-analysis of 34 randomized, controlled trials showed a significantly reduced blood pressure of 2.0/1.78 mm Hg for an average magnesium supplementation of 368 mg/d for an average of three months49)Zhang X, Li Y, Del Gobbo LC, et al. Effects of Magnesium Supplementation on Blood Pressure: A Meta-Analysis of Randomized Double-Blind Placebo-Controlled Trials. Hypertension. 2016;68(2):324-333. doi:10.1161/HYPERTENSIONAHA.116.07664.

Regarding calcium, a Cochrane review of 16 randomized controlled trials was able to show that supplementing calcium in a dosage of 1500 mg/d resulted in an average decrease in systolic and diastolic blood pressure of 2.79 mm Hg and 1.43 mm Hg, respectively50)Cormick G, Ciapponi A, Cafferata ML, Belizán JM. Calcium supplementation for prevention of primary hypertension. Cochrane Database Syst Rev. 2015;(6):CD010037. doi:10.1002/14651858.CD010037.pub2.

Fatty acids also have an impact on blood pressure. A meta-analysis of 70 randomized controlled trials showed that the consumption of omega-3 fatty acids (0.3-15 g per day for 4-26 weeks) reduced blood pressure by 1.25/-0.99 mm Hg (95% CI: -2.25, -0.79; -1.54, -0.44) compared to placebo. The effect was highest in untreated hypertensive participants (-4.51/3.05 mm Hg;  95% CI: -6.12, -2.83; -4.35, -1.74)51)Miller PE, Van Elswyk M, Alexander DD. Long-chain omega-3 fatty acids eicosapentaenoic acid and docosahexaenoic acid and blood pressure: a meta-analysis of randomized controlled trials. Am J Hypertens. 2014;27(7):885-896. doi:10.1093/ajh/hpu024.

Omega-9 fatty acids, in particular the monounsaturated fatty acid oleic acid contained in olive oil as part of a Mediterranean diet, are associated with a reduction of blood pressure52)Psaltopoulou T, Naska A, Orfanos P, Trichopoulos D, Mountokalakis T, Trichopoulou A. Olive oil, the Mediterranean diet, and arterial blood pressure: the Greek European Prospective Investigation into Cancer and Nutrition (EPIC) study. Am J Clin Nutr. 2004;80(4):1012-1018. doi:10.1093/ajcn/80.4.1012. In one study, blood pressure was 8.0/6.0 mm Hg lower in a group supplemented with monounsaturated fatty acids from olive oil compared to a control group that received rich sunflower oil in omega-6 fatty acids53)Ferrara LA, Raimondi AS, d’Episcopo L, Guida L, Dello Russo A, Marotta T. Olive oil and reduced need for antihypertensive medications. Arch Intern Med. 2000;160(6):837-842.

Regarding vitamins and antioxidants, studies have shown promising results.

A meta-analysis of clinical trials, in which hypertensive subjects were administered a vitamin C supplement (500 mg) for an average duration of eight weeks, showed that systolic blood pressure decreased by 4.8 ± 1.2 mm Hg. However, this supplementation had no influence on diastolic blood pressure54)Juraschek SP, Guallar E, Appel LJ, Miller ER. Effects of vitamin C supplementation on blood pressure: a meta-analysis of randomized controlled trials. Am J Clin Nutr. 2012;95(5):1079-1088. doi:10.3945/ajcn.111.027995.

Dark chocolate and high polyphenol cocoa, both rich in antioxidants, have been shown to lower blood pressure55)Hooper L, Kay C, Abdelhamid A, et al. Effects of chocolate, cocoa, and flavan-3-ols on cardiovascular health: a systematic review and meta-analysis of randomized trials. Am J Clin Nutr. 2012;95(3):740-751. doi:10.3945/ajcn.111.02345756)Ried K, Sullivan T, Fakler P, Frank OR, Stocks NP. Does chocolate reduce blood pressure? A meta-analysis. BMC Med. 2010;8:39. doi:10.1186/1741-7015-8-39. A meta-analysis that included 173 hypertensive patients showed that adding cocoa for an average of two weeks reduced blood pressure by 4.7/2.8 mm Hg57)Taubert D, Roesen R, Schömig E. Effect of cocoa and tea intake on blood pressure: a meta-analysis. Arch Intern Med. 2007;167(7):626-634. doi:10.1001/archinte.167.7.626.

The more epicatechin is absorbed from cocoa, the larger the blood pressure lowering effect. In a meta-analysis, an intake of 25 mg epicatechin via cocoa products achieved an average reduction in blood pressure of 4.1/2.0 mm Hg58)Ellinger S, Reusch A, Stehle P, Helfrich H-P. Epicatechin ingested via cocoa products reduces blood pressure in humans: a nonlinear regression model with a Bayesian approach. Am J Clin Nutr. 2012;95(6):1365-1377. doi:10.3945/ajcn.111.029330.

The administration of a polyphenol-rich grape seed extract (300 mg/d) over eight weeks, however, according to a study by Ras and colleagues59)Ras RT, Zock PL, Zebregs YEMP, Johnston NR, Webb DJ, Draijer R. Effect of polyphenol-rich grape seed extract on ambulatory blood pressure in subjects with pre- and stage I hypertension. Br J Nutr. 2013;110(12):2234-2241. doi:10.1017/S000711451300161X, had no significant effect on blood pressure in untreated hypertensives (grade I and II), although a trend of – 3/1.4 mm Hg was observed.

Finally, numerous intervention studies have shown a hypotensive effect of nitrate-rich beetroot juice. A 2013 meta-analysis of 16 RCTs found that red beet juice reduced average blood pressure by 4.4/1.1 mm Hg (95% CI: 2.8, 5.9 | 0.1, 2.2, respectively)60)Siervo M, Lara J, Ogbonmwan I, Mathers JC. Inorganic nitrate and beetroot juice supplementation reduces blood pressure in adults: a systematic review and meta-analysis. J Nutr. 2013;143(6):818-826. doi:10.3945/jn.112.170233. Green leafy vegetables also contain nitrate and therefore lower blood pressure and improve endothelial function in studies61)Ashworth A, Mitchell K, Blackwell JR, Vanhatalo A, Jones AM. High-nitrate vegetable diet increases plasma nitrate and nitrite concentrations and reduces blood pressure in healthy women. Public Health Nutr. 2015;18(14):2669-2678. doi:10.1017/S136898001500003862)Jonvik KL, Nyakayiru J, Pinckaers PJ, Senden JM, van Loon LJ, Verdijk LB. Nitrate-Rich Vegetables Increase Plasma Nitrate and Nitrite Concentrations and Lower Blood Pressure in Healthy Adults. J Nutr. 2016;146(5):986-993. doi:10.3945/jn.116.229807.

Fiber Intake

In clinical studies and two meta-analyses63)Whelton SP, Hyre AD, Pedersen B, Yi Y, Whelton PK, He J. Effect of dietary fiber intake on blood pressure: a meta-analysis of randomized, controlled clinical trials. J Hypertens. 2005;23(3):475-48164)Streppel MT, Arends LR, van ’t Veer P, Grobbee DE, Geleijnse JM. Dietary fiber and blood pressure: a meta-analysis of randomized placebo-controlled trials. Arch Intern Med. 2005;165(2):150-156. doi:10.1001/archinte.165.2.150, dietary fiber was found to reduce blood pressure. Whelton and colleagues65)Whelton SP, Hyre AD, Pedersen B, Yi Y, Whelton PK, He J. Effect of dietary fiber intake on blood pressure: a meta-analysis of randomized, controlled clinical trials. J Hypertens. 2005;23(3):475-481 showed that dietary fiber intake reduced blood pressure by 1.15/1.65 mm Hg, although only the reduction of diastolic blood pressure was statistically significant. Hypertensive patients even achieved a significant reduction of 5.95/4.2 mm Hg.

A recent meta-analysis on more than twenty RCTs by Khan and colleagues66)Khan K, Jovanovski E, Ho HVT, et al. The effect of viscous soluble fiber on blood pressure: A systematic review and meta-analysis of randomized controlled trials. Nutr Metab Cardiovasc Dis. 2018;28(1):3-13. doi:10.1016/j.numecd.2017.09.007 revealed that viscous fiber supplementation from oats and barley, guar gum, konjac, pectin, and psyllium reduced mean systolic blood pressure by 1.59 mm Hg (95% CI: -2.72, -0.46), and mean diastolic blood pressure by 0.39 mm Hg (95% CI: -0.76, -0.01) at a median dose of 8.7 g/d over a median follow-up of 7 weeks.

Several dietary interventions with foods high in fiber could reduce blood pressure, such as trials using lupin flour bread67)Lee YP, Mori TA, Puddey IB, et al. Effects of lupin kernel flour-enriched bread on blood pressure: a controlled intervention study. Am J Clin Nutr. 2009;89(3):766-772. doi:10.3945/ajcn.2008.26708 or wine grape pomace68)Urquiaga I, D’Acuña S, Pérez D, et al. Wine grape pomace flour improves blood pressure, fasting glucose and protein damage in humans: a randomized controlled trial. Biol Res. 2015;48:49. doi:10.1186/s40659-015-0040-9. However, it is not clear whether the antihypertensive effects of these foods can be attributed solely to their fiber content.

Complex Dietary Patterns

There are several complex dietary patterns with various execution protocols that have been examined in randomized controlled trials and compared to different control diets. This makes it difficult to perform a comparing analysis that can make a valid statement on which diet pattern is most effective in reducing blood pressure.

The most recent systematic review, tackling the difficulty of comparing heterogenous study designs, was conducted by Schwingshackl and colleagues69)Schwingshackl L, Chaimani A, Schwedhelm C, et al. Comparative effects of different dietary approaches on blood pressure in hypertensive and pre-hypertensive patients: A systematic review and network meta-analysis. Crit Rev Food Sci Nutr. May 2018a:1-14. doi:10.1080/10408398.2018.1463967. Looking at 67 trials published until June 2017, they addressed the question of which dietary approach is most effective in treating pre-hypertension or hypertension. The authors clustered the different dietary protocols into 13 approaches in total: DASH (dietary approaches to stop hypertension), low-fat, moderate-carbohydrate, high-protein, low-carbohydrate, Mediterranean, Paleolithic, vegetarian, low-glycemic index/low-glycemic load, low-sodium, Nordic, Tibetan, and control.

The review used a network meta-analysis as an extension of the standardly applied pairwise meta-analysis. This enabled the researchers to compare multiple interventions simultaneously while preserving the internal randomization of the individual trials.

According to this network meta-analysis, DASH, Mediterranean, low-carbohydrate, Palaeolithic, high-protein, low-sodium, and low-fat dietary approaches are significantly more effective in reducing systolic blood pressure (SBP) (-8.73 to -2.32 mm Hg) and diastolic blood pressure (DBP) (-4,85 to -1.27 mm Hg) compared to a control diet.

The review used the so-called “surface under the cumulative ranking (SUCRA) curve” method to find the most effective type of diet. According to this method, the DASH diet ranked highest as being most effective, followed by the Palaeolithic, and the low-carbohydrate diet (in terms of SBP) or the Mediterranean diet (in terms of DBP).

However, the credibility of evidence was only very low to moderate for most comparisons. Only when the DASH diet was compared to a low-fat diet or a control diet, the evidence was rated high-quality.

Interestingly, the DASH diet reduced the blood pressure in the same amount as drug monotherapy in this meta-analysis70)Elmer PJ, Obarzanek E, Vollmer WM, et al. Effects of comprehensive lifestyle modification on diet, weight, physical fitness, and blood pressure control: 18-month results of a randomized trial. Ann Intern Med. 2006;144(7):485-495.

The DASH-Diet

The DASH-diet is a diet high in whole grains, fruits, vegetables, and low-fat dairy products and low in red and processed meat as well as in sugar sweetened foods and beverages, which makes it a diet low in saturated fat, refined sugar, and cholesterol. Furthermore, it contains beans, nuts, poultry and fish. DASH stands for Dietary Approaches to Stop Hypertension, and it’s a diet promoted by the U.S.-based National Heart, Lung, and Blood Institute.

Several other RCTs examining this diet have been performed, and some other meta-analyses apart from the one by Schwingshackl and colleagues71)Schwingshackl L, Chaimani A, Schwedhelm C, et al. Comparative effects of different dietary approaches on blood pressure in hypertensive and pre-hypertensive patients: A systematic review and network meta-analysis. Crit Rev Food Sci Nutr. May 2018a:1-14. doi:10.1080/10408398.2018.1463967 showed a strong reduction in SBP and DBP by this diet compared to a control diet72)Ndanuko RN, Tapsell LC, Charlton KE, Neale EP, Batterham MJ. Dietary Patterns and Blood Pressure in Adults: A Systematic Review and Meta-Analysis of Randomized Controlled Trials. Adv Nutr. 2016;7(1):76-89. doi:10.3945/an.115.00975373)Saneei P, Salehi-Abargouei A, Esmaillzadeh A, Azadbakht L. Influence of Dietary Approaches to Stop Hypertension (DASH) diet on blood pressure: a systematic review and meta-analysis on randomized controlled trials. Nutr Metab Cardiovasc Dis. 2014;24(12):1253-1261. doi:10.1016/j.numecd.2014.06.00874)Siervo M, Lara J, Chowdhury S, Ashor A, Oggioni C, Mathers JC. Effects of the Dietary Approach to Stop Hypertension (DASH) diet on cardiovascular risk factors: a systematic review and meta-analysis. Br J Nutr. 2015;113(1):1-15. doi:10.1017/S0007114514003341.

The first trial from 199775)Appel LJ, Moore TJ, Obarzanek E, et al. A clinical trial of the effects of dietary patterns on blood pressure. DASH Collaborative Research Group. N Engl J Med. 1997;336(16):1117-1124. doi:10.1056/NEJM199704173361601 was successful in proving the blood-pressure lowering effect of this diet. It demonstrated that compared with control, DASH reduced systolic blood pressure by 5.5 mm Hg in general and by 11.3 mm Hg among hypertensive patients. Originally, the DASH diet was tested independent of sodium intake, and sodium level of this trial was about 3 g per day.

In a secondary analysis of the so-called DASH-sodium trial76)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. DASH-Sodium Collaborative Research Group. N Engl J Med. 2001;344(1):3-10. doi:10.1056/NEJM200101043440101, in which an independent BP-lowering effect of a low-sodium diet was shown, Juraschek and colleagues77)Juraschek SP, Miller ER, Weaver CM, Appel LJ. Effects of Sodium Reduction and the DASH Diet in Relation to Baseline Blood Pressure. J Am Coll Cardiol. 2017;70(23):2841-2848. doi:10.1016/j.jacc.2017.10.011 confirmed that the combination of both reduced sodium intake and the DASH diet is able to reduce blood pressure even more. Among persons with a systolic blood pressure ≥150 mm Hg, mean reduction was even more than 20 mm Hg when a low-sodium DASH diet was compared to a high sodium-control diet.

To place this result in context, standard drug effects appear to be lower. For example, compared to placebo, beta blockers reduce systolic blood pressure by 13 mm Hg, angiotensin-converting enzyme inhibitors reduce it by 12 mm Hg, and calcium channel blockers reduce it by 16 mm Hg78)Manisty CH, Hughes AD. Meta-analysis of the comparative effects of different classes of antihypertensive agents on brachial and central systolic blood pressure, and augmentation index. Br J Clin Pharmacol. 2013;75(1):79-92. doi:10.1111/j.1365-2125.2012.04342.x.

A possible explanation for the superiority of the DASH diet compared to the Mediterranean diet as shown in Schwingshackl and colleagues79)Schwingshackl L, Chaimani A, Schwedhelm C, et al. Comparative effects of different dietary approaches on blood pressure in hypertensive and pre-hypertensive patients: A systematic review and network meta-analysis. Crit Rev Food Sci Nutr. May 2018a:1-14. doi:10.1080/10408398.2018.1463967 could be that the Mediterranean diet does not consider sodium in the diet80)Trichopoulou A, Costacou T, Bamia C, Trichopoulos D. Adherence to a Mediterranean diet and survival in a Greek population. N Engl J Med. 2003;348(26):2599-2608. doi:10.1056/NEJMoa025039. Referring to our previous Mechanisms section (LINK), a low‐sodium DASH diet decreases oxidative stress and improves vascular function, which contributes to its blood pressure lowering effect in salt-sensitive subjects81)Al-Solaiman Y, Jesri A, Zhao Y, Morrow JD, Egan BM. Low-Sodium DASH reduces oxidative stress and improves vascular function in salt-sensitive humans. J Hum Hypertens. 2009;23(12):826-835. doi:10.1038/jhh.2009.32.

A systematic review and meta-analysis of 11 cohort studies examining the DASH diet found a pooled reduced relative risk by 20% (95% CI: 0.79, 0.82) for all-cause mortality, and by 20% (95% CI: 0.77, 0.84) for cardiovascular disease mortality82)Schwingshackl L, Bogensberger B, Hoffmann G. Diet Quality as Assessed by the Healthy Eating Index, Alternate Healthy Eating Index, Dietary Approaches to Stop Hypertension Score, and Health Outcomes: An Updated Systematic Review and Meta-Analysis of Cohort Studies. Journal of the Academy of Nutrition and Dietetics. 2018b;118(1):74-100.e11. doi:10.1016/j.jand.2017.08.024.

The Mediterranean Diet

The Mediterranean diet is characterized by a high consumption of fruit, vegetables, olive oil, legumes, unrefined cereals, a moderate to high consumption of fish, a moderate consumption of dairy products, in particular cheese and yogurt, a moderate intake of red wine during meals, and a low consumption of meat.

In contrast to the widely confirmed effectiveness of the DASH diet, results concerning the Mediterranean Diet are heterogenous. The mentioned meta-analysis by Schwingshackl and colleagues83)Schwingshackl L, Chaimani A, Schwedhelm C, et al. Comparative effects of different dietary approaches on blood pressure in hypertensive and pre-hypertensive patients: A systematic review and network meta-analysis. Crit Rev Food Sci Nutr. May 2018a:1-14. doi:10.1080/10408398.2018.1463967 demonstrated a blood pressure lowering effect of the Mediterranean diet compared to a control diet, but it was less effective than the DASH diet. However, it ranked third in regard of a reduction in diastolic blood pressure.

Another meta-analysis of six trials, including more than 7,000 individuals, found insufficient evidence to suggest that the Mediterranean diet decreases blood pressure84)Nissensohn M, Román-Viñas B, Sánchez-Villegas A, Piscopo S, Serra-Majem L. The Effect of the Mediterranean Diet on Hypertension: A Systematic Review and Meta-Analysis. J Nutr Educ Behav. 2016;48(1):42-53.e1. doi:10.1016/j.jneb.2015.08.023. This result is in contradiction to a meta-analysis by Ndanuko and colleagues85)Ndanuko RN, Tapsell LC, Charlton KE, Neale EP, Batterham MJ. Dietary Patterns and Blood Pressure in Adults: A Systematic Review and Meta-Analysis of Randomized Controlled Trials. Adv Nutr. 2016;7(1):76-89. doi:10.3945/an.115.009753 that included three trials and demonstrated that the Mediterranean diet both reduced systolic and diastolic blood pressure.  Another meta-analysis by Gay and colleagues86)Gay HC, Rao SG, Vaccarino V, Ali MK. Effects of Different Dietary Interventions on Blood Pressure: Systematic Review and Meta-Analysis of Randomized Controlled Trials. Hypertension. 2016;67(4):733-739. doi:10.1161/HYPERTENSIONAHA.115.06853 only showed a significant reduction in diastolic but not systolic blood pressure.

A meta-analysis, analyzing eight prospective cohort studies, showed that a two point increase in the adherence score of a Mediterranean diet was associated with a reduced risk of all-cause mortality (pooled relative risk 0.92; 95% CI: 0.90, 0.94), and a reduced risk of cardiovascular incidence or mortality (pooled relative risk 0.90; 95% CI: 0.87, 0.93)87)Sofi F, Abbate R, Gensini GF, Casini A. Accruing evidence on benefits of adherence to the Mediterranean diet on health: an updated systematic review and meta-analysis. Am J Clin Nutr. 2010;92(5):1189-1196. doi:10.3945/ajcn.2010.29673.

The Paleolithic Diet

The Paleolithic diet is oriented towards the presumed eating habits of humans during the Paleolithic era (2.6 million to ∼10,000 years ago). A Paleolithic diet typically includes vegetables, fruits, nuts, roots, eggs, fish and lean meat. Foods such as dairy products, grains, sugar, legumes, salt, sweets and processed foods are excluded.

A systematic review and meta-analysis examined four RCTs that involved 159 participants88)Manheimer EW, van Zuuren EJ, Fedorowicz Z, Pijl H. Paleolithic nutrition for metabolic syndrome: systematic review and meta-analysis12. Am J Clin Nutr. 2015;102(4):922-932. doi:10.3945/ajcn.115.113613. Compared to control diets, adherence to a paleolithic diet resulted in a reduction in systolic (−3.64 mm Hg; 95% CI: −7.36, 0.08 mm Hg) and diastolic blood pressure (−2.48 mm Hg; 95% CI: −4.98, 0.02 mm Hg). However, both parameters had wide confidence intervals, and the differences were small, which points out that the changes probably aren’t clinically relevant. Manheimer and colleagues also did not compare the studies with one another but only analyzed changes from baseline, which is supposed to be a misleading approach89)Bland JM, Altman DG. Best (but oft forgotten) practices: testing for treatment effects in randomized trials by separate analyses of changes from baseline in each group is a misleading approach. Am J Clin Nutr. 2015;102(5):991-994. doi:10.3945/ajcn.115.119768

The analysis by Schwingshackl and colleagues90)Schwingshackl L, Chaimani A, Schwedhelm C, et al. Comparative effects of different dietary approaches on blood pressure in hypertensive and pre-hypertensive patients: A systematic review and network meta-analysis. Crit Rev Food Sci Nutr. May 2018a:1-14. doi:10.1080/10408398.2018.1463967 found that the Paleolithic diet ranked second as the most effective dietary approach in reducing systolic blood pressure, but this result should also be interpreted with caution, since only two RCTs were examined91)Jönsson T, Granfeldt Y, Ahrén B, et al. Beneficial effects of a Paleolithic diet on cardiovascular risk factors in type 2 diabetes: a randomized cross-over pilot study. Cardiovasc Diabetol. 2009;8:35. doi:10.1186/1475-2840-8-3592)Mellberg C, Sandberg S, Ryberg M, et al. Long-term effects of a Palaeolithic-type diet in obese postmenopausal women: a 2-year randomized trial. Eur J Clin Nutr. 2014;68(3):350-357. doi:10.1038/ejcn.2013.290.

Regarding cardiovascular disease mortality and all-cause mortality, an analysis of the prospective Iowa Women’s Health Study (1986-2012) did not show an association of an evolutionary-concordance diet (i.e. Paleolithic diet) with mortality93)Cheng E, Um CY, Prizment A, Lazovich D, Bostick RM. Associations of evolutionary-concordance diet, Mediterranean diet and evolutionary-concordance lifestyle pattern scores with all-cause and cause-specific mortality. Br J Nutr. December 2018:1-10. doi:10.1017/S0007114518003483. A previous longitudinal cohort study conducted by Whalen and colleagues94)Whalen KA, Judd S, McCullough ML, Flanders WD, Hartman TJ, Bostick RM. Paleolithic and Mediterranean Diet Pattern Scores Are Inversely Associated with All-Cause and Cause-Specific Mortality in Adults. J Nutr. 2017;147(4):612-620. doi:10.3945/jn.116.241919 showed a reduced mortality risk for the Paleolithic diet. The all-cause mortality hazard ratio was 0.77 (95% CI: 0.67, 0.89). For cardiovascular disease mortality, it was not significant.

Low Carb and High Protein Diets

Low carb diets are characterized by a replacement of carbohydrates by fat and/or protein. Carbohydrate intake is usually less than 20-25% of total energy intake. This diet often implicates a high intake of fat and of animal and/or plant protein. Low carb diets have been proven successful for weight reduction when compared to low fat diets95)Brehm BJ, Seeley RJ, Daniels SR, D’Alessio DA. A randomized trial comparing a very low carbohydrate diet and a calorie-restricted low fat diet on body weight and cardiovascular risk factors in healthy women. J Clin Endocrinol Metab. 2003;88(4):1617-1623. doi:10.1210/jc.2002-02148096)Bueno NB, de Melo ISV, de Oliveira SL, da Rocha Ataide T. Very-low-carbohydrate ketogenic diet v. low-fat diet for long-term weight loss: a meta-analysis of randomised controlled trials. Br J Nutr. 2013;110(7):1178-1187. doi:10.1017/S0007114513000548. High protein diets include an (animal and/or plant) protein intake higher than 20% of total energy intake97)Schwingshackl L, Hoffmann G. Long-term effects of low-fat diets either low or high in protein on cardiovascular and metabolic risk factors: a systematic review and meta-analysis. Nutr J. 2013b;12:48. doi:10.1186/1475-2891-12-48.

The low carb diet ranked third in reducing systolic blood pressure in the meta-analysis by Schwingshackl and colleagues98)Schwingshackl L, Chaimani A, Schwedhelm C, et al. Comparative effects of different dietary approaches on blood pressure in hypertensive and pre-hypertensive patients: A systematic review and network meta-analysis. Crit Rev Food Sci Nutr. May 2018a:1-14. doi:10.1080/10408398.2018.1463967. Both the low carb and the high protein diets were more effective in reducing blood pressure compared to control diets. However, the effects compared to low-fat diets were not statistically significant.

Previous meta-analyses have yielded inconsistent results. A meta-analysis of data obtained in more than 1,000 obese patients showed that a low carb diet is associated with a significant reduction of systolic (-4.81 mm Hg [95% CI -5.33, -4.29]) and diastolic blood pressure (-3.10 mm Hg [95% CI: -3.45, -2.74])99)Santos FL, Esteves SS, da Costa Pereira A, Yancy Jr WS, Nunes JPL. Systematic review and meta-analysis of clinical trials of the effects of low carbohydrate diets on cardiovascular risk factors. Obesity Reviews. 2012;13(11):1048-1066. doi:10.1111/j.1467-789x.2012.01021.x compared to heterogenous comparison diets. These results contradict two other meta-analyses100)Hession M, Rolland C, Kulkarni U, Wise A, Broom J. Systematic review of randomized controlled trials of low-carbohydrate vs. low-fat/low-calorie diets in the management of obesity and its comorbidities. Obes Rev. 2009;10(1):36-50. doi:10.1111/j.1467-789X.2008.00518.x101)Nordmann AJ, Nordmann A, Briel M, et al. Effects of low-carbohydrate vs low-fat diets on weight loss and cardiovascular risk factors: a meta-analysis of randomized controlled trials. Arch Intern Med. 2006;166(3):285-293. doi:10.1001/archinte.166.3.285 which could not observe any effects on blood pressure when compared to low-fat/energy-restricted diets. However, the analysis by Santos and colleagues was criticized for several methodological errors102)Schwingshackl L, Hoffmann G. Low-carbohydrate diets and cardiovascular risk factors. Obes Rev. 2013a;14(2):183-184. doi:10.1111/j.1467-789X.2012.01060.x.

The ketogenic diet is a type of low-carbohydrate diet that is high in fat and adequate in protein. In this diet, fats are supposed to be the body’s primary energy source. A typical ketogenic diet includes primarily animal products such as meat, fish, eggs, high-fat dairy products, and some fruits and vegetables that hardly contain any carbohydrates, such as berries and leafy greens.

A study conducted by Samaha and colleagues103)Samaha FF, Iqbal N, Seshadri P, et al. A low-carbohydrate as compared with a low-fat diet in severe obesity. N Engl J Med. 2003;348(21):2074-2081. doi:10.1056/NEJMoa022637 showed a slight but not significant reduction in systolic and/or diastolic blood pressure. Another study found an improvement in both systolic and diastolic blood pressure in obese participants under a ketogenic dietary intervention for 48 weeks when compared to a low-fat diet plus the lipase-inhibitor orlistat104)Mayer SB, Jeffreys AS, Olsen MK, McDuffie JR, Feinglos MN, Yancy WS. Two diets with different haemoglobin A1c and antiglycaemic medication effects despite similar weight loss in type 2 diabetes. Diabetes Obes Metab. 2014;16(1):90-93. doi:10.1111/dom.12191. Finally, another study105)Cicero AFG, Benelli M, Brancaleoni M, Dainelli G, Merlini D, Negri R. Middle and Long-Term Impact of a Very Low-Carbohydrate Ketogenic Diet on Cardiometabolic Factors: A Multi-Center, Cross-Sectional, Clinical Study. High Blood Press Cardiovasc Prev. 2015;22(4):389-394. doi:10.1007/s40292-015-0096-1 could show a reduction in systolic blood pressure after three months of a ketogenic diet (-10.5 ± 6.4 mm Hg).

Low-carbohydrate diets are associated with a higher risk of all-cause mortality. A systematic review and meta-analysis of observational studies found an elevated risk of all-cause mortality for this dietary pattern: the pooled relative risk (95% CI) was 1.31 (95% CI: 1.07, 1.59). Cardiovascular disease mortality was not statistically increased106)Noto H, Goto A, Tsujimoto T, Noda M. Low-carbohydrate diets and all-cause mortality: a systematic review and meta-analysis of observational studies. PLoS ONE. 2013;8(1):e55030. doi:10.1371/journal.pone.0055030.

A more recent prospective cohort study and meta-analysis published in the Lancet found that both high (>70%) and low (<40%) percentages of carbohydrate diets are associated with increased mortality (pooled hazard ratio 1.23 for high-carb (95% CI: 1.11, 1.36); 1.20 for low-carb (95% CI: 1.09, 1.32)). However, only low carbohydrate patterns rich in animal-derived protein were associated with higher mortality when carbohydrates were exchanged for protein, whereas plant-derived protein and fat substitutions decreased mortality107)Seidelmann SB, Claggett B, Cheng S, et al. Dietary carbohydrate intake and mortality: a prospective cohort study and meta-analysis. The Lancet Public Health. 2018;3(9):e419-e428. doi:10.1016/S2468-2667(18)30135-X.

The Vegetarian Diet

A vegetarian diet is characterized by the exclusion of meat and fish. Instead, it emphasizes foods of plant origin, especially vegetables, grains, legumes, and fruits. Most vegetarian diets also contain dairy products and eggs, which differentiates them from vegan diets.

Observational studies have shown that a vegetarian diet is associated with a lower prevalence of hypertension. However, evidence regarding clinical trials is inconclusive. Some randomized trials have found that the adoption of a vegetarian diet compared to an omnivorous diet reduces blood108)Rouse IL, Beilin LJ, Armstrong BK, Vandongen R. Blood-pressure-lowering effect of a vegetarian diet: controlled trial in normotensive subjects. Lancet. 1983;1(8314-5):5-10109)Margetts BM, Beilin LJ, Vandongen R, Armstrong BK. Vegetarian diet in mild hypertension: a randomised controlled trial. Br Med J (Clin Res Ed). 1986;293(6560):1468-1471, whereas others can’t corroborate these results110)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(2):280-287. doi:10.1093/ajcn/50.2.280111)Nicholson AS, Sklar M, Barnard ND, Gore S, Sullivan R, Browning S. Toward improved management of NIDDM: A randomized, controlled, pilot intervention using a lowfat, vegetarian diet. Prev Med. 1999;29(2):87-91. doi:10.1006/pmed.1999.0529.

A meta-analysis by Yokoyama and colleagues112)Yokoyama Y, Nishimura K, Barnard ND, et al. Vegetarian diets and blood pressure: a meta-analysis. JAMA Intern Med. 2014;174(4):577-587. doi:10.1001/jamainternmed.2013.14547 reported blood pressure reductions following vegetarian dietary interventions. In 7 controlled trials (a total of 311 participants), consumption of vegetarian diets was associated with a reduction in mean systolic BP (−4.8 mm Hg; 95% CI: −6.6, −3.1; P < .001) and diastolic BP (−2.2 mm Hg; 95% CI: −3.5, −1.0; P < .001) compared with the consumption of omnivorous diets. These effect sizes are approximately half the magnitude of those observed with pharmaceutical therapy, such as administration of angiotensin-converting enzyme inhibitors113)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(11):1385-1394. doi:10.1093/eurheartj/ehp103.

The meta-analysis by Schwingshackl and colleagues114)Schwingshackl L, Chaimani A, Schwedhelm C, et al. Comparative effects of different dietary approaches on blood pressure in hypertensive and pre-hypertensive patients: A systematic review and network meta-analysis. Crit Rev Food Sci Nutr. May 2018a:1-14. doi:10.1080/10408398.2018.1463967 could not confirm these results regarding hypertensive or pre-hypertensive patients. However, it included only one study which makes it impossible to draw a conclusion based on this meta-analysis.

A special type of diet, the so-called Portfolio diet, is a plant-based diet emphasizing a “portfolio” of cholesterol-lowering foods such as soluble fiber (e.g. from oatmeal, barley, beans, lentils), nuts, soy products (such as tofu, soy meat), and margarines enriched with plant sterols. A systematic review and meta-analysis on 7 study comparisons found that the portfolio dietary pattern lowered systolic blood pressure by 1% (mean difference = -1.75 mm Hg [95% CI: −3.23, −0.26 mm Hg], p = 0.02) and diastolic blood pressure by 2% (mean difference = −1.36 mm Hg [95% CI: −2.33, −0.38 mm Hg], p = 0.006)115)Chiavaroli L, Nishi SK, Khan TA, et al. Portfolio Dietary Pattern and Cardiovascular Disease: A Systematic Review and Meta-analysis of Controlled Trials. Progress in Cardiovascular Diseases. 2018;61(1):43-53. doi:10.1016/j.pcad.2018.05.004.

A very recent meta-analysis116)Lopez PD, Cativo EH, Atlas SA, Rosendorff C. The Effect of Vegan Diets on Blood Pressure in Adults: A Meta-Analysis of Randomized Controlled Trials. Am J Med. March 2019. doi:10.1016/j.amjmed.2019.01.044 analyzed 11 clinical trials comparing a vegan diet to any less restrictive diet. The researchers found no significant change in systolic (-1.33 mm Hg; 95% CI: -3.50, 0.84) or diastolic (-1.21 mm Hg; 95% CI: -3.06, 0.65) blood pressure. Only when a subgroup analysis of studies with baseline systolic blood pressure ≥ 130 mm Hg was performed, a vegan diet was shown to lead to a mean decrease in systolic (-4.10 mm Hg; 95% CI: -8.14, -0.06) and diastolic (-4.01 mm Hg; 95% CI: -5.97, -2.05) blood pressure.

Summary

The mentioned meta-analyses and RCTs point towards a low-sodium DASH-diet for optimal blood pressure reduction. If followed correctly, there’s even a larger reduction of blood pressure possible than might be achieved by blood pressure medication.

However, so far it is not clear what the major blood pressure reducing element of this diet is. Is it the plant-based foods, is it the sodium reduction, is it the dairy? As vegetarian, vegan or Portfolio diets also yield promising results, further research comparing the effectiveness of diets rich or low in animal products could be a meaningful future research area, especially with regard to a holistic view of the health aspects of animal-derived foods. 

This article is part of the series “Preventing and Treating Hypertension with Diet“.

References   [ + ]

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