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Curr Hypertens Rep. Author manuscript; available in PMC 2020 Apr 29.
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Published online 2020 Feb 29. doi:10.1007/s11906-020-1029-5
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Abstract
Purpose of Review
We identified and quantified the results of randomized controlled trials (RCTs) that have assessed the impact of egg consumption on blood pressure in adults.
Recent Findings
We conducted a comprehensive search of medical bibliographic databases up to February 2019 for RCTs investigating the effect of egg consumption on blood pressure in adults. Fifteen RCTs were included with a total of 748 participants. Overall, egg consumption had no significant effect on systolic blood pressure (weighted mean difference (WMD) = 0.046 mmHg; 95% CI − 0.792, 0.884) and diastolic blood pressure (WMD = − 0.603 mmHg; 95% CI − 1.521, 0.315). Subgroup analyses had no effect on pooled results and no heterogeneity was found among included studies.
Summary
Egg consumption has no significant effects on systolic and diastolic blood pressure in adults. Due to several limitations among existing studies, general conclusions cannot be drawn regarding the beneficial or neutral impact of egg consumption on blood pressure in adults.
Keywords: Egg, Systolic blood pressure, Diastolic blood pressure, Meta-analysis
Introduction
Cardiovascular diseases (CVD) are the leading cause of death in the developed world [1]. Hypertension (HTN) is a key contributor to the global burden of CVD [2] and overall death, leading to 10.5 million deaths annually [3]. The prevalence of HTN in the adult population is estimated to reach 1.56 billion by 2025 [4]. However, HTN is a modifiable risk factor for CVD, and as such, with proper treatment and control, risk of CVD is significantly reduced [2]. Various antihypertensive medications are often used for treatment of HTN, but many of these medications have adverse side effects and require lifelong adherence [5].
In recent years, the use of food for prevention and management of HTN has become a growing interest [6]. Among these foods, eggs have received much attention. Eggs are an economic and nutritious food commodity, which contain highly bioavailable proteins, essential fatty acids, antioxidants, choline, essential vitamins, and minerals [7•]. Despite being an excellent source of nutrients, the cholesterol content of eggs has been a matter of concern [8]. The current (2015–2020) Dietary Guidelines for Americans have removed the recommendation of limiting cholesterol intake and have explicitly stated that cholesterol is not a nutrient of concern for overconsumption; however, the current Dietary Guidelines for Americans also state that individuals should consume as little dietary cholesterol as possible [9]. A recent meta-analysis of ten prospective cohort studies revealed that higher dietary cholesterol intake was not associated with an increased risk of CVD [10]. Additionally, another meta-analysis of prospective cohort studies demonstrated no association between daily egg consumption and risk of coronary heart disease (CHD) [11•]. Interestingly, daily egg consumption was actually accompanied with a 12% reduction in stroke risk [11•].
A recent systematic review and meta-analysis of three prospective cohort studies by Zhang et al. showed the egg consumption was associated with lower risk of HTN (relative risk (RR) = 0.79; 95% CI 0.68–0.91; P = 0.001) [12••]. In contrast, the results of human randomized controlled trials (RCTs) investigating the effect of egg consumption on blood pressure have been inconsistent [13–27] with some studies showing beneficial effects of egg consumption on systolic blood pressure (SBP) and/or diastolic blood pressure (DBP) [16, 23], while others have shown no effect [13–15, 17–22, 24–27]. The primary purpose of the current study was to conduct a systematic review and meta-analysis of the published RCTs to comprehensively evaluate and quantify the effects of egg consumption on blood pressure in adults. The secondary purpose was to establish evidence that could potentially inform future dietary guidelines for the management of HTN.
Methods
Protocol and Registration
The current systematic review and meta-analysis was carried out and reported in accordance with the preferred reporting items for systematic review and meta-analyses (PRISMA) guidelines [28] and the study protocol was registered in the international prospective register of systematic reviews (PROSPERO) database as CRD42019127500.
Search Strategy
We performed a search of PubMed, Ovid Medline, Scopus, and the Cochrane Central Register of Controlled Trials (Central) databases from inception up to 25 February 2019 for studies that described the effects of egg consumption on blood pressure in adults using the keywords egg and dietary cholesterol which were paired with hypertension or blood pressure (supplementary Table S1). The search was restricted to clinical trials. No limit was placed on language or date of publication during the literature search. We also reviewed reference lists of all retrieved publications again to obtain further relevant articles. Attempts were also made to contact investigators for additional information and the full-text article if the article could not be retrieved during the original search.
Study Selection
Studies were included in this meta-analysis if they met the following criteria: (1) the study was a RCTwith either a parallel or a crossover design; (2) the participants used whole egg matched by a suitable control arm; (3) the study reported baseline and post intervention values (or their differences) for SBP, DBP or both with SDs, SEMs, or 95% CIs for each group; and (4) the study was conducted on an adult population. Study exclusion criteria were as follows: (1) non-RCT studies; (2) nonhuman studies; (3) trial conducted on pregnant women or children; (4) studies which used eggs enriched with different nutrients in the experimental group but not in the control group; and (5) studies that evaluated egg components such as egg yolk, egg white, or bioactive peptides derived from eggs.
Data Extraction
Two authors (RKM, MM) independently assessed the abstract and the full text of eligible articles and extracted the following data: (1) study characteristics (first author’s name, country, year of publication, sample size, study design, type and amount (number of eggs consumed per day) of intervention, specific diet adherence, and study duration); (2) population information (sex, age, BMI, and health status); and (3) mean ± SD for baseline, post intervention, and changes from baseline for SBP and DBP in experimental and control groups. Any disagreement in abstracted data was resolved through discussion and consultation with a third author (SS).
All study durations were converted to weeks from days or months. In the case where results were reported separately for hypo and hyper-responders to dietary cholesterol intake, we combined both groups using a fix model to retrieve an overall effect.
Quality Assessment and Quality of Evidence
Two independent authors (MM and NP) assessed the quality of included trials using the Cochrane Collaboration’s tool for the following domains: (1) random sequence generation; (2) allocation concealment; (3) blinding (participants, personnel, and outcome assessors); (4) incomplete outcome data; (5) selective outcome reporting; and (6) other sources of bias and each RCT stratified as good, fair, or poor quality according to each domain [29].
The overall quality of the studies included in this meta-analysis was also evaluated by the use of NutriGrade scoring system which comprises the following items: (1) risk of bias, study quality, and study limitations (0–3 points); (2) precision (0–1 point); (3) heterogeneity (0–1 point); (4) directness (0–1 point); (5) publication bias (0–1 point); (6) funding bias (0–1 point); and (7) study design (+ 2 points). This scoring system recommends four categories to judge the meta-evidence as high meta-evidence (≥ 8 points); moderate meta-evidence (6–7.99 points); low meta-evidence (4–5.99 points); and very low meta-evidence (0–3.99 points) [30].
Statistical Analysis
Intervention effects were defined as weighted mean differences (WMD) and 95% CIs calculated for net changes in SBP and DBP values with the use of the random effects models [31]. Heterogeneity of studies was examined with the use of Cochran’s test and the inconsistency index I2 with an I2 of 25%, 50%, and 75% indicating low, medium, and high heterogeneity, respectively [32]. Pre-defined subgroup analysis based on sex (male, female, or both), population health status (healthy, type 2 diabetic and metabolic syndrome, patients with coronary artery disease (CAD) and hyperlipidemia), hypertension (patients who had SBP > 120, DBP > 80 mmHg or used antihypertensive drugs and who did not have hypertension as an inclusion criteria) and obesity status (nonobese or overweight/obese), duration (less than 12 weeks or ≥ 12 weeks) and design (parallel or crossover) of study, diet adherence (usual diet or specific/restricted diet), and amount of egg consumption in intervention group (one egg/day or greater than one egg/day) was conducted to detect potential sources of heterogeneity. Furthermore, the potential nonlinear effects of egg consumption (number/day) and study duration (week) on SBP and DBP were evaluated using fractional polynomial models [33]. Publication bias was tested using Begg’s and Egger’s tests [34]. Sensitivity analysis using the “leave one out” approach was also conducted to evaluate the impact of one study on overall pooled estimates and heterogeneity [35]. Alpha was set at 0.05 and significance was determined at P < 0.05. Data analyses were conducted using Stata version 13 (Stata Corp LP, College Station, TX, USA).
Results
Results of the Literature Search
Initial database search yielded 3589 articles, of which 79 articles were assessed in full text for inclusion in the meta-analysis. With the 79 articles assessed, fifteen trials were eligible for inclusion. Excluded studies are reported in Fig. 1.
Study Characteristics
The characteristics of included studies are displayed in Table 1. Fifteen articles with 748 participants and mean age range from 23.3 to 67.1 years old were included in this systematic review and meta-analysis. Included studies were published between 1984 and 2018 and were conducted in the USA (n = 10) [15, 16, 18–24, 27], Australia (n = 2) [17, 25], Mexico (n = 1) [13], Colombia (n = 1) [14], and Thailand [26] (n = 1). Ten studies used a crossover design [13, 16, 18–22, 24, 26, 27] and five studies used a parallel arm design [14, 15, 17, 23, 25]. Of the fifteen studies, one study was conducted only in women [18] and one conducted only in men [23]; all of the remaining studies were conducted in both men and women [13–17, 19–22, 24–27]. Four trials were conducted in participants with type 2 diabetes [13, 17, 24, 25], one trial conducted in participants with the metabolic syndrome [14], one trial included patients with hyperlipidemia [26], one trial selected patients with CAD [20], and eight studies used healthy participants [15, 16, 18, 19, 21–23, 27]. Four studies included participants who were taking blood pressure lowering medications or who had SBP/DBP > 120/80 mmHg [13, 17, 23, 25]. Nine studies included participants that were overweight and obese [13–15, 17, 19, 20, 23–25]. All included studies used whole egg as an intervention which varied from one egg/day to three eggs/day and duration of supplementation ranged from 3 to 12 weeks. In some studies, egg consumption in the intervention group was compared with less [17, 26] or no egg consumption [16, 24, 27] in the control group, while other studies compared egg consumption with egg substitute [14, 18, 23], egg beater [20], oatmeal [13, 19, 22], bagel [15], lean animal protein [25], and choline bitartrate supplementation [21] in the control group.
Table 1
Summary of RCTs that assessed the impact of egg consumption on systolic and diastolic blood pressure
| Author (year) [References] | Country | Sex | Mean age (year) | Study design | Egg group | Control group | Duration (week) | Population characteristic | Specific diet adherence | Outcome |
|---|---|---|---|---|---|---|---|---|---|---|
| Ballesteros (2015) [13] | Mexico | 19 F/10 M | 53.5 | Crossover | 1 egg/day | 40 g oatmeal/day | 5 | T2DM | Usual diet | ↔ SBP ↔ DBP |
| Blesso (2013) [14] | Colombia | 25 F/12 M | 51.9 | Parallel | 3 eggs/day | 3 eggs substitute/day | 12 | MetS | Carbohydrate-restricted diets | ↔ SBP ↔ DBP |
| Clayton (2015) [15] | USA | 25 F and M | 25.7 | Parallel | 2 eggs/day | 9 cm bagel/day | 12 | Healthy | Not reported | ↔ SBP ↔ DBP |
| DiMarco (2016) [16] | USA | 19 F/19 M | 24.1 | Crossover | 1 to 3 eggs/day | 0 egg/day | 4 | Healthy | Usual diet | ↔ SBP ↓ DBP |
| Fuller (2015) [17] | Australia | 77 F/63 M | 59.8 | Parallel | 12 eggs/week | < 2 eggs/week | 12 | Overweight and obese with T2DM | Usual diet | ↔ SBP ↔ DBP |
| Herron (2002) [18] | USA | 51 F | Not reported | Crossover | 3 eggs/day | 3 eggs substitute/day | 4 | Healthy | NCEP step I diet | ↔ SBP ↔ DBP |
| Katz (2005) [19] | USA | 19 F/30 M | 56 | Crossover | 2 eggs/day | 60 g oatmeal/day | 6 | Healthy | Usual diet | ↔ SB ↔ DBP |
| Katz (2015) [20] | USA | 6 F/26 M | 67.1 | Crossover | 2 eggs/day | 1/2 cup egg beaters | 6 | CAD | Usual diet | ↔ SBP ↔ DBP |
| Lemos (2018) [21] | USA | 16 F/14 M | 25.5 | Crossover | 3 eggs/day | 1.5 tablets choline bitartrate | 4 | Healthy | Usual diet | ↔ SBP ↔ DBP |
| Missimer (2017) [22] | USA | 26 F/24 M | 23.3 | Crossover | 2 eggs/day | 1 packet oatmeal/day | 4 | Healthy | Usual diet | ↔ SBP ↔ DBP |
| Mutungi (2008) [23] | USA | 31 M | Not reported | Parallel | 3 eggs/day | 3 eggs substitute/day | 12 | Overweight and obese | Carbohydrate-restricted diets | ↓ SBP ↓ DBP |
| Njike (2016) [24] | USA | 14 F/20 M | 64.5 | Crossover | 2 eggs/day | 0 egg/day | 12 | T2DM | Usual diet | ↔ SBP ↔ DBP |
| Pearce (2010) [25] | Australia | 65 F and M | 59.7 | Parallel | 2 eggs/day | 100 g lean animal protein | 12 | T2DM | Energy restricted diet | ↔ SBP ↔ DBP |
| Putadechakum (2013) [26] | Thailand | 63 F/8 M | 50.79 | Crossover | 3 eggs/day | One egg/day | 4 | Hyperlipidemia | Usual diet | ↔ SBP ↔ DBP |
| Sacks (1984) [27] | USA | 13F/4M | Not reported | Crossover | 1 egg/day | 0 egg/day | 3 | Healthy | Usual diet | ↔ SBP ↔ DBP |
RCTs randomized controlled trials; F female; M male; gr gram; T2DM type 2 diabetes mellitus; MetS metabolic syndrome; CAD coronary artery disease; ↔, no change; ↓, significant reduction
Effect of Egg Consumption on SBP and DBP
The overall effect of egg consumption on SBP was 0.046 mmHg (95% CI − 0.792, 0.884; P = 0.914) and there was no heterogeneity among studies (I2 = 0.0%, P = 0.453) (Fig. 2). The nonlinear dose-response analysis failed to show a significant effect of egg consumption (P-nonlinearity = 0.07) and study duration on SBP (P-nonlinearity = 0.61). The overall effect of egg consumption on DBP was − 0.603 mmHg (95% CI − 1.521, 0.315; P = 0.198) and there was no heterogeneity among studies (I2 = 38.1%, P = 0.067) (Fig. 3). Nonlinear dose-response meta-analysis did not reveal a significant effect of egg consumption (P-nonlinearity = 0.76) and study duration on DBP (P-nonlinearity = 0.63). Subgroup analyses based on study design, sex, population health status, obesity and hypertension status, diet adherence, study duration, and amount of egg consumption did not affect the overall effects of egg consumption on SBP and DBP values (supplementary Table S2 and Table S3).
Sensitivity Analysis and Publication Bias
Sensitivity analysis indicated that excluding each trial did not significantly change the overall observed effects of egg consumption on SBP and DBP values. No evidence of publication bias was seen in the meta-analyses of SBP (Begg’s P = 0.235, Eggers’s P = 0.433) and DBP (Begg’s P = 0.254, Eggers’s P = 0.267).
Data Quality
The quality of selected trials was diverse with one trial classified as high quality (score 6) [17], 7 trials were classified as fair (score 4) [13, 18–20, 23–25], and the remaining 7 trials were classified as poor quality (score = 2 or 3) [14–16, 21, 22, 26, 27]. Qualities of included studies are presented in supplementary Table S4. Additionally, the NutriGrade meta-evidence rating was 6.86 in this meta-analysis which suggests that there is a moderate confidence in effect estimate and further research may change the effect estimate (supplementary Table S5).
Discussion
The overall outcome of this meta-analysis from fifteen eligible RCTs showed no significant effect of egg consumption on blood pressure. Subgroup analyses showed that the pooled effects of egg consumption on SBP and DBP were not influenced by study design, sex, population health status, obesity and HTN status, diet adherence, study duration, and number of eggs consumed.
HTN is a strong risk factor for CVD and strokes [36]. The results from our study are in accordance with a meta-analysis of cohort studies that reported intake of one egg/day compared with low egg intake (approximately < 2 eggs/week) was not associated with increased risk of CHD [11•]. Similarly, a systematic review and meta-analysis by Shin et al. showed that high (≥ 1 egg/day) vs low egg consumption (< 1 egg/week or never) was not associated with an increased risk of CVD, ischemic heart disease (IHD), stroke, and cardiac mortality in the general population [37]. In relation to blood pressure, a systematic review and meta-analysis of three prospective cohort studies showed that egg consumption was associated with a reduced risk of HTN [12••]. In contrast, Wang et al. recently reported that consumption of > 4 whole eggs/week compared with < 4 whole eggs/week intake did not influence blood pressure in middle-aged and older adults [38].
The beneficial effects of egg consumption on blood pressure in humans may be attributed to the presence of nutrients with antioxidant and antihypertensive effects in whole eggs [7•, 39]. For example, beneficial effects of egg white derived peptides on blood pressure have been shown in animal studies. In these studies, orally administered IQW (Ile-Gln-Trp) and LKP (Leu-Lys-Pro) (tripeptides provided by enzymatic digestion of the ovotransferrin) decreased mean blood pressure in hypertensive rats [40]. Additionally, orally administered egg white hydrolysate [41] and an antihypertensive tripeptide Ile-Arg-Trp (IRW) [42•] reduced blood pressure in hypertensive rats. In these studies, reduction of blood pressure was concomitant with increased nitric oxide (NO) mediated vasodilatation, angiotensin converting enzyme (ACE) inhibition, reduced vascular inflammation and oxidative stress, reduced ACE and angiotensin II type 1 receptor expression, and increased ACE2 expression [40, 41, 42•].
We considered several possible explanations for the lack of association between egg consumption and blood pressure in the current meta-analysis. Firstly, blood pressure was not a primary objective in the majority of the trials included in this meta-analysis, which may have resulted in inadequate statistical power to detect significant changes regarding the effects of egg consumption on blood pressure. Secondly, it is possible that lifestyle factors associated with egg consumption may influence HTN, as higher egg consumption tends to be associated with smoking, physical inactivity, and increased consumption of red and processed meat [12••, 43]. These factors tend to exaggerate the association between egg consumption and HTN [43]. Thirdly, heterogeneity of control interventions is an important factor which can significantly alter blood pressure results. In three of the fifteen studies included in the current meta-analysis, egg consumption was compared with consumption of oatmeal [13, 19, 22]. A recent systematic review and meta-analysis demonstrated that ß-glucan can decrease SBP by 0.22 mmHg per gram consumed [44], and oatmeal intake in the studies included in the present systematic review and meta-analysis varied between 40 and 60 g. Additionally, two studies used lean animal protein [25] and choline bitartrate supplement [21] as control.
Fourthly, baseline characteristics of the studied populations must be considered. Thus, we performed subgroup analyses for baseline characteristics for all studies included in the present meta-analysis. However, due to small number of trials in each subgroup, we could not find association in this regard. Of the fifteen trials included in the current meta-analysis, thirteen trials found that egg consumption did not affect blood pressure [13–15, 17–22, 24–27], while in one trial, consumption of three eggs per day in overweight or obese participants significantly reduced SBP and DBP relative to the control group [23]. Additionally, one study demonstrated that consumption of three eggs per day in healthy participants significantly reduced DBP but had no effect on SBP [16]. Taken together, these data suggest that high egg intake, baseline characteristics of participants (healthy not sick), and intervention in the control group (low or no egg consumption or egg substitute) have an important role in the association between egg consumption and blood pressure. Finally, most of the included studies in this meta-analysis utilized a crossover design with short washout periods ranging from 0 to 6 weeks [13, 16, 18–22, 24, 26, 27]. Inadequate and/or inconsistent washout periods from one intervention to the next may lead to inaccurate results, potentially due to carryover effects from the first arm into the second.
To our knowledge, the current systematic review and meta-analysis of RCTs is the first to evaluate the effect of egg consumption on blood pressure in general adult population. We performed several subgroup analyses to identify the possible source of heterogeneity that may be influencing the relationship between egg consumption and blood pressure. Moreover, doing comprehensive literature search to include available RCTs is examining the effect of egg consumption on blood pressure. As with all systematic reviews and meta-analyses, the current analysis has some limitations. First, blood pressure was not a primary objective in almost all included studies, so it is possible that the study sample size was not sufficient to detect significant relationship. Second, the number of RCTs assessing the association between egg consumption and blood pressure was small and most studies were of low or fair quality, which can be attributed mostly to lack of blinding of participants, personnel, and outcome assessment. These issues should be addressed in future studies. Third, the different types of placebo in control groups may have imposed effects on our results. Fourth, most included studies did not adjust the effect of confounding factors including medications, physical activity, and dietary habit which may have affected the results regarding the relationship between egg consumption and blood pressure.
Conclusion
Overall our analysis has shown that egg consumption had no effect on blood pressure. However, high-quality RCTs with longer durations are needed to further confirm the effects of egg consumption on blood pressure.
Supplementary Material
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Acknowledgments
We thank Iran University of Medical Sciences for providing facilities to search in electronic databases.
Availability of Data and Material It was registered in the PROSPERO (international prospective register of systematic reviews) database under the number CRD42019127500.
Funding Information ZSC is currently supported by NIH T32 DK007135-44.
Abbreviations
| CVD | Cardiovascular diseases |
| HTN | Hypertension |
| CHD | Coronary heart disease |
| RR | Relative risk |
| RCTs | Randomized controlled trials |
| SBP | Systolic blood pressure |
| DBP | Diastolic blood pressure |
| PRISMA | Preferred reporting items for systematic review and meta-analyses |
| PROSPERO | Prospective register of systematic reviews |
| WMD | Weighted mean differences |
| CAD | Coronary artery disease |
| IHD | Ischemic heart disease |
| IQW | Ile-Gln-Trp |
| LKP | Leu-Lys-Pro |
| IRW | Ile-Arg-Trp |
| NO | Nitric oxide |
| ACE | Angiotensin converting enzyme |
Footnotes
Electronic supplementary material The online version of this article (https://doi.org/10.1007/s11906-020-1029-5) contains supplementary material, which is available to authorized users.
Conflict of Interest The authors declare that they have no conflict of interest.
Human and Animal Rights and Informed Consent This article does not contain any studies with human or animal subjects performed by any of the authors.
Publisher’s Note Springer Nature remains neutral with regard to jurisdictional claims in published maps and institutional affiliations.
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