Impact of the Level of Adherence to the DASH Diet on Blood Pressure: A Systematic Review and Meta-Analysis

Introduction: the objective of our study was to systematically review the current literature and perform a meta-analysis to evaluate the effect of the level of adherence to the DASH diet on blood pressure. Methods: The identification of relevant studies, data extraction and critical appraisal of the included studies were performed independently by two reviewers. A random-effects model was employed to synthesize the available evidence using the standardized mean difference (SMD) as the appropriate effect size. Results: A total of 37 and 29 articles were included in the qualitative and quantitative analysis, respectively. The pooled effect for systolic blood pressure was SMD = −0.18 (95%CI: −0.32 to −0.04; I2 = 94%; PI: −0.93 to 0.57) and for diastolic blood pressure it was SMD = −0.13 (95%CI: −0.19 to −0.06; I2 = 94%; PI: −0.42 to 0.17). Conclusions: Our findings showed that greater adherence to the DASH diet has a beneficial effect on blood pressure compared to the lowest adherence. Increased compliance with DASH diet recommendations might also have a positive effect on cardiometabolic factors and overall health status. Future studies should aim to standardize the tools of adherence to the DASH diet and utilize rigorous study designs to establish a clearer understanding of the potential benefits of the level of adherence to the DASH diet in blood pressure management.


Introduction
Hypertension, defined as the consistently high pressure of blood flow within vessels, is the leading cause of cardiovascular events and all-cause mortality worldwide.Hypertension bears a correlation with the incidence of cardiovascular and renal detriment [1].As of 2010, nearly one third of adults worldwide had hypertension.The increasing prevalence of hypertension is mainly attributed to the growing number of elderly people, the preference for unhealthy food options (diets rich in sodium and poor in potassium), smoking and the absence of exercise [2].
According to the current literature, the cornerstone of hypertension treatment includes anti-hypertensive drugs [3], as well as lifestyle alterations that consist of salt moderation, the restriction of alcohol and cigarettes, body weight diminution, exercise and dietary approaches [4].Specifically, the Dietary Approaches to Stop Hypertension (DASH) diet, which comprises fruits, vegetables, fiber and low-fat dairy products in abundance, has been recommended as an efficient diet for regulating normal blood pressure measurements [5,6].On the other hand, adherence to the DASH diet can be defined as the extent to which an individual may follow nutritional recommendations according to the DASH dietary pattern [7].Accordingly, the DASH Score is calculated using information obtained from validated food frequency questionnaires in which low and high scores indicate poor and good adherence, respectively.
Several studies have demonstrated that the DASH diet holds a pivotal role in decreasing blood pressure, taking into consideration that people must be able and inclined to ensue this dietary pattern [5,8].Therefore, proper adherence to the DASH diet is important in the prevention and treatment of elevated blood pressure measurements.Recently, a considerable number of observational studies have been conducted, regarding the effect of the DASH diet on cardiovascular events, including blood pressure measurements [9].Nevertheless, the results of the available studies are contradictory.
Thus, the aim of our study was to systematically review the current literature and perform a meta-analysis to evaluate the effect of the level of adherence to the DASH diet on blood pressure values.

Quality Appraisal
The quality appraisal regarding the methodological validity of all included studies was evaluated by two independent researchers using the checklists developed by the Joanna Briggs Institute (JBI).Checklists were employed according to the study design of each included record (cohort, case-control and cross-sectional studies).The quality assessment was completed by answering the 11 questions of the JBI tool related to the study design, methodological validity and reliability.The risk of bias (RoB 2.0) tool and the Critical Appraisal Skills Programme (CASP) for the randomized controlled trial checklist were used to evaluate the quality of interventional studies.Any disagreement was resolved by a third reviewer.

Statistical Analysis
A meta-analysis was conducted for our outcome of interest.Blood pressure measurements were considered as a continuous variable.We used the mean, standard deviation and number of participants in each arm.When the included studies reported standard errors or 95% confidence intervals (95% CI) we transformed them to standard deviation following the guidelines by Cochrane.Furthermore, median values were transformed to mean values according to Wan and colleagues' [12] approach.A random effects model was employed due to the expected heterogeneity between the included studies.Standardized mean differences (SMDs) and 95% confidence intervals (Cis) were used to present our findings.Heterogeneity was measured using tau-square (τ 2 ) and the I 2 index and estimated using the restricted maximum likelihood method.Funnel plots and Egger's test were used for the evaluation of publication bias.We also performed subgroup and sensitivity analyses to explain heterogeneity and assess the robustness of our findings, respectively.All of the analyses were performed in the statistical software R Studio (version 2022.12.0 + 353) using the meta package.

Quality of the Evidence
The quality of our findings was assessed using the Grading of Recommendations Assessment, Development and Evaluation (GRADE), as recommended by the Cochrane handbook [13].Domains such as the risk of bias, publication bias, heterogeneity, imprecision of the results and indirectness of the evidence were taken into consideration for the total evaluation.

Study Selection
Through the electronic database search, a total of 4319 records were identified, and after the removal of duplicates, 628 articles were reviewed for eligibility.Of those, 527 were excluded based on the title and abstract, 21 were removed due to a lack of access to the full-text articles and a total of 80 records remained for full-text assessment.In the qualitative synthesis, we included 37 papers, and of which, 3 were randomized trials, 20 were cohort studies, 1 was a case-control study and 14 were cross-sectional studies, while only 29 studies were included in the quantitative analysis (Figure 1).
Regarding patient characteristics, all details can be found in Table 2.In only one study [15] were participants disease-free, while in the remaining 35 studies [14,[46][47][48][49][50], participants were diagnosed with depression, insomnia and cardiometabolic diseases including diabetes, obesity, hypertension, dyslipidemia, metabolic syndrome (MetS), chronic kidney disease (CKD), hyperuricemia, atrial fibrillation or diabetic nephropathy or had undergone surgery for leg amputation; for two [30,45] studies, relevant details were not provided.The mean BMI of all individuals ranged from 23.1 to 32.8 kg/m 2 , the mean SBP ranged from 102.5 to 154.1 mmHg and the DBP ranged from 45.8 to 88.8 mmHg.

Subgroup Analysis
There was a difference between the two groups regarding both SBP and DBP levels according to the use of drug prescription for hypertension.More specifically, high adherence to the DASH diet was associated with SBP values compared to low adherence for the participants that did not receive any anti-hypertensive medication (SMD = −0.14;95%CI −0.22 to −0.06,I 2 = 91%) (Supplementary Figure S1).Furthermore, a similar association was also observed for DBP values (SMD = −0.23;95%CI −0.34 to −0.13,I 2 = 84%) (Supplementary Figure S2).
Furthermore, a subgroup analysis according to the study design of the included studies was conducted.There was no difference between the high and low adherence to the DASH diet on SBP when cohort or cross-sectional studies were pooled together.On the other hand, there was a significant difference favoring high adherence to the DASH diet based on the randomized controlled trial (Supplementary Figure S3).As far as DBP is concerned, a difference was observed when cohort or cross-sectional studies were synthesized.In contrast, a difference was absent in the randomized controlled trial (Supplementary Figure S4).
Lastly, we performed a subgroup analysis for subsets of studies such as different continents for the SBP and DBP outcomes.There was no difference in SBP between high and low adherence to the DASH diet when studies performed in North America, Europe and South America were synthesized.A significant difference was observed in one study, which was a multicenter one, and in the studies from Asia (Supplementary Figure S5).Regarding DBP, a difference between the two groups was present in the studies that were conducted in North and South America, as well as in the multicenter one (Supplementary Figure S6).

Subgroup Analysis
There was a difference between the two groups regarding both SBP and DBP levels according to the use of drug prescription for hypertension.More specifically, high adherence to the DASH diet was associated with SBP values compared to low adherence for the participants that did not receive any anti-hypertensive medication (SMD = −0.14;95%CI −0.22 to −0.06,I 2 = 91%) (Supplementary Figure S1).Furthermore, a similar association was also observed for DBP values (SMD = −0.23;95%CI −0.34 to −0.13,I 2 = 84%) (Supplementary Figure S2).
Furthermore, a subgroup analysis according to the study design of the included studies was conducted.There was no difference between the high and low adherence to the DASH diet on SBP when cohort or cross-sectional studies were pooled together.On the other hand, there was a significant difference favoring high adherence to the DASH diet based on the randomized controlled trial (Supplementary Figure S3).As far as DBP is concerned, a difference was observed when cohort or cross-sectional studies were synthesized.In contrast, a difference was absent in the randomized controlled trial (Supplementary Figure S4).
Lastly, we performed a subgroup analysis for subsets of studies such as different continents for the SBP and DBP outcomes.There was no difference in SBP between high and low adherence to the DASH diet when studies performed in North America, Europe and South America were synthesized.A significant difference was observed in one study, which was a multicenter one, and in the studies from Asia (Supplementary Figure S5).Regarding DBP, a difference between the two groups was present in the studies that were conducted in North and South America, as well as in the multicenter one (Supplementary Figure S6).

Sensitivity Analysis
To explore high heterogeneity, we conducted a leave-one-out analysis for both of our outcomes.The findings of this sensitivity analysis showed that regarding SBP there was no significant change in heterogeneity values when omitting one study each time (Supplementary Figure S7).The same findings apply to the DBP outcome (Supplementary Figure S8).

Risk of Bias Assessment
As depicted in Supplementary Tables S4-S6, almost all cohort and cross-sectional studies successfully performed the recruitment process of participants, identified the potential confounding factors, and used valid methods for measuring the exposures and outcomes.However, information on the sufficient follow-up time, the potential reasons regarding incomplete follow-up, and information on the implementation of strategies for addressing this matter were either missing or were not described clearly.With reference to the interventional studies (Supplementary Table S7), the overall quality was rated as having "some concerns", according to the RoB 2.0 tool.

Publication Bias
According to the funnel plots, there were no signs of publication bias in our review (Supplementary Figures S9-S10).Moreover, Egger's test for the SBP was p = 0.355 and for DBP it was p = 0.232, indicating the absence of publication bias.

Certainty of Findings
Based on the GRADE approach, the certainty of our evidence was judged as being very low for both of our outcomes of interest.

Discussion
The present systematic review and meta-analysis aimed to evaluate the impact of the level of adherence to the DASH diet on blood pressure based on synthesizing the available data from observational and interventional studies.Our findings demonstrate a difference in the reported values of SBP and DBP between participants in the highest and lowest adherence group.
The results of our review support the notion that higher adherence to the DASH diet may have a favorable effect on SBP.However, they should be interpreted with caution due to the high heterogeneity among the included studies.This beneficial effect of the DASH diet could be attributed to its dietary characteristics and the combination of various foods including the high consumption of fruits, vegetables, whole grains, and nuts and the limited salt intake, which have been associated with numerous studies with a reduction in blood pressure [58].
With regard to SBP, high adherence to the DASH diet had a beneficial effect compared to low adherence.It should be stated that few of the included studies presented a mean SBP > 140 mmHg, while in parallel, the majority of them presented a mean DBP < 130 mmHg.This finding is essential, as it supports the protective role of high adherence to the DASH diet in SBP even in subjects with normal SBP.
Regarding DBP, the level of adherence to the DASH diet led to a difference between the highest and lowest adherence group.It should be noted that none of the included studies presented a mean DBP > 90 mmHg, while in parallel, the majority of them presented a mean DBP < 80 mmHg.This finding is of great importance, as it supports that high adherence to the DASH diet could reduce DBP values even in subjects with normal DBP.
In line with our results, published systematic reviews and meta-analyses investigating the effectiveness of the DASH diet provided as an intervention, compared to the usual diet group, showed that the DASH diet is effective in reducing both systolic and diastolic blood pressure [58][59][60].Furthermore, the DASH diet is also effective in lessening other cardiovascular risk factors such as the concentrations of total and LDL cholesterols.HbA1c and insulin concentrations as well as body weight were also reduced in participants assigned to the dietary intervention group compared to the control group, as demonstrated by an umbrella review of systematic reviews and meta-analyses [61].
It should be noted that the DASH diet given exclusively as a dietary intervention to individuals might promote different health outcomes compared to those that emerged from simply measuring adherence to the DASH diet with the use of specific tools.It is possible for dietary interventions to not enhance compliance with a particular dietary pattern as they also require participants' adherence.On the contrary, dietary adherence demonstrates the degree of compliance to a diet that is directly related to individuals' preferences, without corresponding to the consumption of a specified dietary plan.In addition, the level of diet adherence may be affected by various factors, including socioeconomic status, medical history, self-efficacy, level of education, religion, and place of residence, as well as psychological factors and individuals' attitudes [62].
The DASH diet is not only effective in reducing cardiometabolic outcomes, but there are also published syntheses demonstrating that higher adherence to the DASH diet has a protective role in developing type 2 diabetes mellitus [63] and cardiovascular diseases (CVDs) [64] such as coronary heart disease and stroke [65], and also leads to a significant reduction in all-cause, cancer, and CVD mortality [64].Lastly, a recently published protocol (PROSPERO 2022 CRD42022344686) of a systematic review and meta-analysis aimed to evaluate adherence to the DASH diet and hypertension risk [66].The authors found that higher adherence to the DASH diet was associated with a reduced risk of hypertension incidence compared to the lowest adherence to the DASH diet.
To the best of our knowledge, this is the first systematic review and meta-analysis that has investigated the association between adherence to the DASH diet and blood pressure levels.It is also worth noting that our study had certain limitations.Firstly, the study design of the majority of the included studies, i.e., observational studies, limits the confidence in our findings.Furthermore, we are unable to establish causality between adherence to the DASH diet and blood pressure outcomes using observational studies.Secondly, the high heterogeneity observed among the included studies could affect the reliability of the findings; hence, they should be cautiously interpreted.Lastly, we used data from crude models as our outcome of interest was not reported in adjusted analyses.
In conclusion, our findings showed that greater adherence to the DASH diet has a significant effect on blood pressure levels compared to the lowest adherence.Increased compliance with DASH diet recommendations might also have a positive effect on cardiometabolic factors and overall health status.Future studies should aim to standardize the tools of adherence to the DASH diet and utilize rigorous study designs to establish a clearer understanding of the potential benefits of the level of adherence to the DASH diet in blood pressure management and monitoring.

Figure 1 .
Figure 1.Flow diagram of the eligibility process.Figure 1. Flow diagram of the eligibility process.

Figure 1 .
Figure 1.Flow diagram of the eligibility process.Figure 1. Flow diagram of the eligibility process.

Table 1 .
Study characteristics included in the systematic review.

Table 2 .
Patients' health characteristics of the included studies.