Preventing and Treating Hypertension with Diet (Part 5)
Interventional Studies on Diet and Blood Pressure
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).
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). 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.
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)4). 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)6).
Regarding mortality risk, a diet rich in whole grains lowers all-cause mortality risk, according to the LANCET study7). 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).
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) 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). 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) 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) 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) 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). Risk reductions were observed up to 800 g/day.
For legumes, similar results were found. Li and colleagues15) 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.
Concerning intervention trials with dairy, results are inconclusive, which corresponds with the similarly inconsistent observational evidence16)vs.17)18). Some clinical trials report an inverse relationship between the consumption of dairy products and blood pressure19)20), whereas others could not see any changes after the intervention21).
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 process22).
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 parameter23)24)25)26). 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 colleagues27) 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 colleagues28) 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 design29). 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 g30).
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 cheese31).
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)32).
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 colleagues33) 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 colleagues34) 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 patients35). 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 Hg36).
The LANCET study considers a diet high in red and processed meat as a dietary risk factor37). This is corroborated by multiple meta-analyses of epidemiological studies38)39)40). 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 mortality41). However, a meta-analysis by Schwingshackl and colleagues42) showed an inverse association for the highest compared with the lowest fish intake category (RR: 0.95; 95% CI: 0.92, 0.98).
Most interventional studies confirm the data for sodium, potassium and magnesium obtained from observational studies.
A meta-analysis by Aburto and colleagues43) 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 volunteers44).
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 day45).
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 subjects46)47). Also, potassium supplementation moderately but significantly lowers blood pressure, according to a meta-analysis by Poorolajal and colleagues48).
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 alone49).
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 months50).
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, respectively51).
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)52).
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 pressure53). 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 acids54).
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 pressure55).
Dark chocolate and high polyphenol cocoa, both rich in antioxidants, have been shown to lower blood pressure56)57). 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 Hg58).
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 Hg59).
The administration of a polyphenol-rich grape seed extract (300 mg/d) over eight weeks, however, according to a study by Ras and colleagues60), 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)61). Green leafy vegetables also contain nitrate and therefore lower blood pressure and improve endothelial function in studies62)63).
In clinical studies and two meta-analyses64)65), dietary fiber was found to reduce blood pressure. Whelton and colleagues66) 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 colleagues67) 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 bread68) or wine grape pomace69). 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 colleagues70). 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-analysis71).
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 colleagues72) showed a strong reduction in SBP and DBP by this diet compared to a control diet73)74)75).
The first trial from 199776) 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 trial77), in which an independent BP-lowering effect of a low-sodium diet was shown, Juraschek and colleagues78) 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 Hg79).
A possible explanation for the superiority of the DASH diet compared to the Mediterranean diet as shown in Schwingshackl and colleagues80) could be that the Mediterranean diet does not consider sodium in the diet81). Referring to our previous Mechanisms section, a low‐sodium DASH diet decreases oxidative stress and improves vascular function, which contributes to its blood pressure lowering effect in salt-sensitive subjects82).
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 mortality83).
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 colleagues84) 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 pressure85). This result is in contradiction to a meta-analysis by Ndanuko and colleagues86) that included three trials and demonstrated that the Mediterranean diet both reduced systolic and diastolic blood pressure. Another meta-analysis by Gay and colleagues87) 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)88).
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 participants89). 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 approach90).
The analysis by Schwingshackl and colleagues91) 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 examined92)93).
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 mortality94). A previous longitudinal cohort study conducted by Whalen and colleagues95) 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 diets96)97). High protein diets include an (animal and/or plant) protein intake higher than 20% of total energy intake98).
The low carb diet ranked third in reducing systolic blood pressure in the meta-analysis by Schwingshackl and colleagues99). 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])100) compared to heterogenous comparison diets. These results contradict two other meta-analyses101)102) 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 errors103).
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 colleagues104) 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 orlistat105). Finally, another study106) 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 increased107).
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 mortality108).
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 blood pressure109)110), whereas others can’t corroborate these results111)112).
A meta-analysis by Yokoyama and colleagues113) 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 inhibitors114).
The meta-analysis by Schwingshackl and colleagues115) 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)116).
A very recent meta-analysis117) 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.
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“.
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