Effect of nut consumption on vascular endothelial function: A systematic review and meta-analysis of randomized controlled trials

Clinical Nutrition xxx (2017) 1e9

Contents lists available at ScienceDirect

Clinical Nutrition journal homepage: http://www.elsevier.com/locate/clnu

Meta-analyses

Effect of nut consumption on vascular endothelial function: A systematic review and meta-analysis of randomized controlled trials Yunjun Xiao a, **, Wei Huang a, Chaoqiong Peng a, Jinzhou Zhang a, Carmen Wong b, Jean H. Kim b, Eng-kiong Yeoh b, Xuefen Su b, c, * a b c

Department of Nutrition and Food Hygiene, Shenzhen Center for Disease Control and Prevention, Shenzhen, China School of Public Health and Primary Care, Faculty of Medicine, The Chinese University of Hong Kong, Hong Kong, China The Chinese University of Hong Kong Shenzhen Research Institute, Shenzhen, China

a r t i c l e i n f o

s u m m a r y

Article history: Received 19 October 2016 Accepted 12 April 2017

Objective: nut consumption has consistently been found to be associated with a reduced risk of cardiovascular diseases (CVD) and mortality in prospective studies. However, its effect on endothelial function, a prognostic marker of CVD, is still controversial in clinical trials. This meta-analysis of randomized controlled trials (RCTs) aimed to quantitatively assess the effect of nuts on vascular endothelial function. Methods: Major electronic databases were searched for published RCTs that reported the effect of nuts on flow mediated dilation (FMD) as a measurement of endothelial function in the adult population (age eighteen years or over). We calculated the pooled estimates of weighted mean differences (WMDs) and their 95% confidence intervals (CIs) by using random-effects models. Results: A total of nine papers (10 trials) involving 374 participants were included. The pooled estimates found that nut consumption significantly improved FMD (WMD: 0.41%; 95% CI: 0.18%, 0.63%; P ¼ 0.001). Moderate and marginally significant heterogeneity was observed among the studies (I2 ¼ 39.5%, P ¼ 0.094). Subgroup analyses indicated that walnuts significantly improved FMD (WMD: 0.39%; 95% CI: 0.16%, 0.63%; P ¼ 0.001). In addition, nut consumption had a significant effect on FMD in the trials with study duration <18 weeks, nut dose <67 g/d, or subjects with baseline FMD 8.6%. Conclusions: Nut consumption significantly improved endothelial function. However, the beneficial effect was limited to walnuts. More studies examining the effect of other nuts on endothelial function are needed in the future. © 2017 Elsevier Ltd and European Society for Clinical Nutrition and Metabolism. All rights reserved.

Keywords: Nut Endothelial Flow-mediated dilation Randomized controlled trials Meta-analysis

1. Introduction Epidemiological studies have observed that consumption of nuts is associated with a reduced risk of type 2 diabetes, cardiovascular disease (CVD) and mortality [1,2]. Nuts are rich in unsaturated fatty acids [3] and are rich sources of bioactive compounds with potential benefits for CVD prevention, such as dietary fiber, vitamin E, folic acid, flavonoids, and polyphenols [4]. Nuts also * Corresponding author. School of Public Health and Primary Care, Faculty of Medicine, Room 508, School of Public Health, Prince of Wales Hospital, The Chinese University of Hong Kong, Shatin, New Territories, Hong Kong, China. Fax: þ852 2645 3098. ** Corresponding author. Department of Nutrition and Food Hygiene, Shenzhen Center for Disease Control and Prevention, Longyuan Road No 8, Nanshan District, Shenzhen, China. Fax: þ86 755 2561 7321. E-mail addresses: [email protected] (Y. Xiao), [email protected] (X. Su).

contain considerable amounts of L-arginine, which is the precursor amino acid of nitric oxide (NO). Among the different types of nuts, walnuts have been found to be particularly beneficial. In comparison with other nuts, walnuts have a higher content of aelinolenic acid (ALA), a plant n-3 fatty acid, which might have additional antiatherogenic effects [5]. Healthy diets which are rich in a variety of nuts, for example, a Mediterranean diet, have been found to reduce the risk factors of CVD, including lipids, inflammation, and blood pressure in previous clinical trials [6,7]. Two meta-analyses found that, compared with control diets, walnut-rich diets reduced the levels of low-density lipoprotein cholesterol (LDL-C) and total cholesterol in previous randomized controlled trials (RCTs) [8,9]. Endothelial dysfunction is an event in the early-stage of atherosclerosis and an independent predictor of future CVD events [10]. Endothelial function can be evaluated non-invasively by several methods, such as peripheral artery tonometry, or brachial

http://dx.doi.org/10.1016/j.clnu.2017.04.011 0261-5614/© 2017 Elsevier Ltd and European Society for Clinical Nutrition and Metabolism. All rights reserved.

Please cite this article in press as: Xiao Y, et al., Effect of nut consumption on vascular endothelial function: A systematic review and metaanalysis of randomized controlled trials, Clinical Nutrition (2017), http://dx.doi.org/10.1016/j.clnu.2017.04.011

2

Y. Xiao et al. / Clinical Nutrition xxx (2017) 1e9

artery ultrasound. In most studies, flow-mediated dilation (FMD) measured by brachial artery ultrasound has been widely used and found to be sensitive and accurate in measuring endothelial function [11]. Experimental studies found that vasodilation could be improved by dietary interventions, for example, fish oil n-3 fatty acid, flavonoids, polyphenols, and L-arginine [12]. The effect of nuts, including walnuts, almonds, pistachios, and hazelnut, which are rich in these compounds, on endothelial function has been investigated in previous clinical trials [13e28]. However, the results have not been consistent, likely due to the relatively small sample sizes, different duration, and types and doses of nut interventions of these trials. Therefore, the precise effect of nut intake on endothelial function has not been well established. In the present study, a meta-analysis of RCTs was performed to comprehensively assess the evidence and quantitatively estimate the effect of nut consumption on endothelial function measured by FMD. 2. Materials and methods 2.1. Data sources and search strategy We performed a systematic review and meta-analysis of the RCTs following the PRISMA criteria guidelines [29] (Fig. 1). Relevant articles were identified by searching the major electronic

databases, including PubMed (www.ncbi.nlm.nih.gov/pubmed), Cochrane database (http://www.cochrane.org), Embase (www. embase.com), and Google Scholar (www.scholar.google.com) through July 2016, with no language restriction. Three groups of keywords were used in the literature search: first group: “nut”, “almond”, “pistachio”, “hazelnut”, “walnut”, “cashew”, “ macadamia”, “pecan”, “peanut”, or “soy nut”; second group: “endothelial”, “endothelium”, “FMD”, or “flow-mediated dilatation”; and third group: “-randomized”, “intervention”, “controlled trial”, “random”, and “placebo”. The first keyword group was combined with both the second and the third groups to search relevant studies. 2.2. Study selection The following inclusion criteria were used: 1) studies were RCTs; 2) the RCTs focused on the effect on endothelial function and used FMD as a measurement of endothelial function; 3) the RCTs examined consumption of nuts, broadly defined as a hard-shelled dry fruit or seed with a separable rind or shell and interior kernel, including walnuts, almonds, pecans, peanuts, hazelnuts, pistachios, cashews, macadamia nuts, and soy nuts; 4) the RCTs explicitly reported baseline and follow-up values of FMD or the mean change between baseline and follow-up for each group, or the mean difference between intervention and control groups;

Fig. 1. Flowchart of study selection process.

Please cite this article in press as: Xiao Y, et al., Effect of nut consumption on vascular endothelial function: A systematic review and metaanalysis of randomized controlled trials, Clinical Nutrition (2017), http://dx.doi.org/10.1016/j.clnu.2017.04.011

Y. Xiao et al. / Clinical Nutrition xxx (2017) 1e9

and 5) the studies were conducted in samples of age 18 years. Two investigators (YX and WH) reviewed the titles abstracts, and screened full texts of the papers to indentify potentially eligible studies. The references of relevant papers and reviews were also screened to avoid missing potentially eligible studies and to ensure a full study list. If there were multiple papers published from the same study, we included the most recent or the most informative paper. Studies were excluded used: 1) if the duration was less than two weeks [17,19,23]; 2) studies that did not use FMD as a measurement of endothelial function; 3) studies that did not present mean changes of FMD, or these statistics could not be calculated from the data reported in the paper; and 4) articles that reported redundant results of the same RCT. 2.3. Data collection and quality assessment Two investigators independently performed data collection and quality assessment (YX and WH) by applying the inclusion criteria. Discrepancies were resolved through consensus. Data extracted included the first author, year of publication, study location, design of the RCTs (parallel or crossover), whether participants having relevant comorbidities (including hypercholesterolemia, dyslipidemia, diabetes, and coronary artery disease), mean (range) age, and the inclusion and exclusion criteria of participants, the sample size of the trials, the types and dosages of nuts (grams per day, and/ or % of total daily energy consumption), the control diets, study duration, the run in or wash out periods, and baseline and followup FMD levels or percent change in FMD. The quality of the studies was evaluated by using the Jadad score for RCTs [30]. Quality scores were assigned with one point each for whether randomization and blinding were used, participant withdrawals and dropouts, random number generation, and allocation concealment of treatment, on a scale from zero (the lowest level of quality) to five (the highest level) [31]. The method used to monitor participant compliance in these nine RCTs was also evaluated as an additional aspect of study quality.

3

plots and was tested statistically by using Egger's and Begg's tests [35,36]. Statistical significance was set as P < 0.05, unless otherwise indicated. All analyses were conducted with STATA 10.0 (Stata Corporation, College Station, TX). 3. Results 3.1. Search results Figure 1 showed the study selection process. A total of 938 papers were found by combining the literature searched through major electronic databases and by manual searching of the references in relevant reviews or articles. Among the 938 papers, 906 were excluded, as they were not human studies, not original investigations, not RCTs, or the study objectives did not meet the inclusion criteria of this meta-analysis, leaving 32 potentially eligible papers for full text assessment. After further evaluation, nine articles of 10 RCT studies were included. The other 23 articles were excluded, due to reasons as described using the PRISMA 2009 flow diagram [37] (Fig. 1). 3.2. Study characteristics Table 1 presented the characteristics and results of the nine included articles. Because Njike et al. [13] reported the results of nut intervention among the subjects with and without calorie adjusted diet separately, two effect estimates were extracted and included. Seven studies used a randomized crossover design [14,16,20e22,24,26], and three studies from two articles [13,28] used randomized parallel design. Six studies examined the effect of walnuts [13,14,16,20,21], two studied with pistachios [24,28], one with almonds [26], and one with hazelnuts, respectively [22]. The dosage of nut consumption varied from 37 to 128 g/d. The baseline FMD levels were from 3.4 ± 3.7% to 15.2 ± 5.37%. Supplemental Table 1 presented the inclusion and exclusion criteria of the included studies. 3.3. Study quality

2.4. Statistical analysis The primary outcome was the mean difference between nut group and control group in changes FMD from baseline to followup. If the change in FMD was not reported, we calculated it using the Cochrane Handbook for Systemic Review and Follman D's theory [32]. Equal variance was assumed between intervention and control groups in each trial and across different trials. Standard deviations (SDs) were derived from standard errors (SEs) and confidence intervals (CIs). For trials with multiple comparison groups, we included the group with the diet most similar to the nut group as the reference group. Studies that did not report SDs, SEs, CIs, or P values along with the mean values were excluded from the analysis. We calculated the overall estimates of weighted mean differences (WMDs) and their 95% CIs by using random effects analysis [33]. Heterogeneity among the included trials was assessed with Cochran's test, with P < 0.1 considered as statistically significant. The I2 statistic was also calculated, with I2 values greater than 50%, indicating large heterogeneity among the studies [34]. We explored the potential sources of heterogeneity by using meta-regression and stratified analyses by the number and age of participants, design and duration of the trials, nut types, nut dose, and baseline FMD levels. In addition, we performed sensitivity analysis by removing one study at a time and by excluding studies conducted in specific subject groups, for instance, diabetic or hypercholesterolemic patients. Publication bias was evaluated by using funnel

Table 2 reported the quality scores of the included articles. All studies were randomized, but only five reported the method of random number generation. Of these five studies, two [24,26] employed the simple randomization program of the randomization.com, and three [13,21] used a randomization table. Eight studies did not report allocation concealment. Because of the nature of the dietary intervention, subjects might be aware of their intervention diet, and none of the studies employed doubleblinded design. However, four studies [16,20,21,24] used singleblinded design. With the exception of three studies [14,21,22], all studies had some problem of participant drop-outs. All studies, except one [24], described methods used to monitor or verify subject compliance. The dietary assessment methods included 24-h dietary recalls [13], 7-day diet recalls [14], 3-day food records [16,20,22,26]. Four trials assessed the difference in serum fatty acids levels, a-tocopherol, or g-tocopherol [14,21,22,26] concentrations to evaluate achievement of dietary goals. Four studies [16,20,21,24] achieved the Jadad score of 4, and the other studies [13,14,22,26,28] had a score of 3. 3.4. Effects of nut consumption on FMD Four trials [13,16,20] directly presented the changes in FMD. The remaining 6 studies reported the baseline and follow up FMD levels for the intervention and control groups, and the changes of FMD were calculated for these six studies.

Please cite this article in press as: Xiao Y, et al., Effect of nut consumption on vascular endothelial function: A systematic review and metaanalysis of randomized controlled trials, Clinical Nutrition (2017), http://dx.doi.org/10.1016/j.clnu.2017.04.011

4

First author, year (Ref)

Country Study design

Participants

Mean No. of age subjects, (female %) (years)a

Nut types and dose Study duration (weeks)

Crossover RCT Type 2 diabetes Parallel RCT Participants had a high risk for diabetes Crossover RCT Hypercholesterolemic men and women

24 (58.3) 112 (72)

58.1 ± 9.2 8 54.9 ± 11.4 24

Walnut, 56 g/d Walnut, 56 g/d

20 (60)

55 (26e75) 4

crossover RCT Hypercholesterolemic subjects Crossover RCT Overweight adults

20 (NA)

49.3 ± 1.7

46 (60)

57.4 ± 11.9 8

Kasliwal et al., 2015 [28] India

Parallel RCT

56 (17)

39 ± 8

12

Sauder et al., 2015 [24]

USA

Crossover RCT Type 2 diabetes

30 (50)

56.1 ± 7.8

12

Chen et al., 2015 [26]

USA

Crossover RCT CAD patients

45 (60)

61.8 ± 8.6

22

Mediterranean diet þ walnuts, 40 e65 g/d, 18% of total energy Walnuts, 37 g/d, 16.4% of energy Shelled, unroasted English walnuts, 56 g/d Shelled pistachios, 40 g/d Pistachios, 59e128 g/d, 20% of total energy Almonds, 85 g/d

Orem et al., 2013 [22]

Turkey

Crossover RCT Hypercholesterolemic subjects

21 (14)

44.6 ± 10.4 12

Ma et al., 2010 [16] Njike et al., 2015 [13]

USA USA

Ros et al., 2004 [14]

Spain

West et al., 2010 [21]

Canada

Katz et al., 2012 [20]

USA

Dyslipidemia

18

Comparison group

Outcomes FMD or FMD changeb

Run in/Wash out (weeks)

Intervention group (n)

Control Group (n)

2.2 ± 1.7 (24) 1.94 ± 3.76 (26)e 2.21 ± 4.01 (23)f 5.9 ± 3.3 (18)

1.2 ± 1.6 (24) 4/8 1.54 ± 4.31 (26)e 2/12 1.44 ± 3.6 (26)f 3.6 ± 3.3 (18) 4/NA

Control Control

± ± ± ±

Control

4.6 ± 0.69

6.7 ± 3.4 (12)

6.1 ± 3.8 (12)

NA

Control

8.8 ± 2.4

1.4 ± 2.4 (40)

0.3 ± 1.5 (40)

4/4

Lifestyle modifications Control

8.0 ± 6.9 (21) 5.5 ± 4.7c 8.4 ± 4.9d 5.28 ± 0.41 4.89 ± 0.48 (30)

5.6 ± 6.5 (21)

NA

5.29 ± 0.47 (30)

2/2

Control

8.3 ± 3.8 (45) 7.7 ± 3.3c 7.8 ± 3.5d 15.2 ± 5.37 21.8 ± 6.99 (21)

7.5 ± 3.7 (45)

6/4

15.9 ± 4.19 (21)

1/NA

8.6 8.9 8.6 Mediterranean 3.4 diet

Hazelnut, 49e86 g/day, Control 18%e20% of total energy

CAD ¼ coronary artery disease; FMD ¼ flow-mediated dilation; NA ¼ not accessible; RCT ¼ randomized controlled trials. a Values are mean ± SDs or ranges in parentheses. b Values are mean ± SDs. c Values are nuts consumption group. d Values are control group. e Values of participants with calorie-adjusted diet. f Values of participants without calorie-adjusted diet.

Baseline FMD(%)b

4.3 3.1e 2.1f 3.7

Y. Xiao et al. / Clinical Nutrition xxx (2017) 1e9

Please cite this article in press as: Xiao Y, et al., Effect of nut consumption on vascular endothelial function: A systematic review and metaanalysis of randomized controlled trials, Clinical Nutrition (2017), http://dx.doi.org/10.1016/j.clnu.2017.04.011

Table 1 Characteristics of randomized controlled trials included in the meta-analysis.

Y. Xiao et al. / Clinical Nutrition xxx (2017) 1e9

5

Table 2 Quality assessment of the included studies. Reference

Randomized

Blind

Withdrawals and dropouts

Generation of random numbers

Allocation concealment of treatment

Method of monitoring participant compliance

Jadad score

Ma et al. Chen et al.

Yes Yes

Single-blind Not reported

Yes Yes

Not reported Randomized blocks of size 6 (total 10 blocks) (www.randomization. com)

Not reported Not reported

4 3

Njike et al.

Yes

Not reported

Yes

Not reported

Ros et al.

Yes

Not reported

No dropouts

A SASegenerated random table Not reported

3-day diet records 3 food recall questionnaires, package bags of almonds were collected to confirm compliance at the end of intervention, and plasma a-tocopherol and g-tocopherol were measured 24 h dietary recalls

Not reported

3

Kasliwal et al.

Yes

Open-label

Yes

Not reported

Not reported

West et al. Katz et al.

Yes Yes

Single-blind Single-blind

No dropouts Yes

Randomization table Not reported

Yes Not reported

Orem et al.

Yes

Not reported

No dropouts

Not reported

Not reported

Sauder et al.

Yes

Single-blind

Yes

Simple randomization (www.randomization. com)

Yes

7-day diet recalls, and serum g-tocopherol measurement Biweekly telephone calls, monthly visits to collect the next month's quota of pistachios Serum fatty acid measurement 3-day diet records and consumption log sheets 3-day diet records, serum fatty acid and a-tocopherol measurement Not reported

3

3

4 4 3 4

SAS ¼ statistical analysis system.

The random-effects model found that the change in FMD was significantly higher in the nut group than the control diet group (10 trials, 374 participants; WMD: 0.41%; 95% CI: 0.18%, 0.63%; P ¼ 0.001), with moderate and marginally significant heterogeneity (heterogeneity c2 ¼ 14.87, I2 ¼ 39.5%, P ¼ 0.094) (Fig. 2). In the meta-regression, non-significant inverse relationships were observed between nut dose (range: 37e93 g/d) (I2 ¼ 25.1%; b ¼ 0.01; 95% CI: 0.02, 0.003; P ¼ 0.128), intervention duration (range: 4e24 wks) (I2 ¼ 28.1%; b ¼ 0.02; 95% CI: 0.06, 0.01; P ¼ 0.137) and WMD estimates (Fig. 3A and B). A non-significant positive relationship was observed between baseline FMD levels (range: 3.4%e15.2%) and WMD (I2 ¼ 30.1%; b ¼ 0.05; 95% CI: 0.03, 0.15; P ¼ 0.168) (Fig. 3C). However, these factors did not fully explain the heterogeneity of the results.

In the stratified analysis by trial design, nut consumption significantly increased FMD levels in the seven crossover studies, but not in the three parallel studies (WMD: 0.42%; 95% CI: 0.14%, 0.71%; P ¼ 0.004) (Table 3). Walnuts (WMD: 0.40%; 95% CI: 0.16%, 0.63%; P ¼ 0.001), but not other nuts, significantly improved FMD levels. In addition, nut consumption had a significant effect on FMD if the study duration was <18 wks (below median) (WMD: 0.57%; 95% CI: 0.22%, 0.92%; P ¼ 0.002), or if the nut dose was <67 g/d (WMD: 0.46%; 95% CI: 0.24%, 0.67%; P ¼ 0.001), or if the baseline FMD levels were 8.6% (above median) (WMD: 0.48%; 95% CI: 0.18%, 0.78%; P ¼ 0.002). There was significant heterogeneity among studies with sample size <45 (below median) (I2 ¼ 60.2%, P ¼ 0.039), study duration <18 wks (I2 ¼ 55.6%, P ¼ 0.046), studies with other nuts as intervention diet (I2 ¼ 73.8%,

Fig. 2. Forest plot of the pooled analysis of nut intake on flow-mediated dilation. Note: weights are from random-effects analysis.

Please cite this article in press as: Xiao Y, et al., Effect of nut consumption on vascular endothelial function: A systematic review and metaanalysis of randomized controlled trials, Clinical Nutrition (2017), http://dx.doi.org/10.1016/j.clnu.2017.04.011

6

Y. Xiao et al. / Clinical Nutrition xxx (2017) 1e9

addition, exclusion of the trials conducted among type 2 diabetic patients [16,24] or those at a high risk of diabetes [13] did not alter the estimates (8 trials of non-diabetic subjects, WMD: 0.46%; 95% CI: 0.23%, 0.69%; P < 0.001; 6 trials of subjects who were nondiabetic patients or not at high risk of diabetes, WMD: 0.56%; 95% CI: 0.31%, 0.82%; P < 0.001). Nuts had a stronger effect on FMD among hypercholesterolemic subjects [14,21,22] (WMD: 0.41%; 95% CI: 0.18%, 0.64%; P ¼ 0.005; I2 ¼ 25.4%, P ¼ 0.262) than nonhypercholesterolemic subjects (WMD: 0.33%; 95% CI: 0.08%, 0.57%; P ¼ 0.01; I2 ¼ 37.2%, P ¼ 0.144). Although there was a slight asymmetry in the funnel plots (Fig. 5), no publication bias was observed (Begg's test, P ¼ 0.074; Egger's test, P ¼ 0.343). 4. Discussion

Fig. 3. Meta-regression analysis showing the relationships between nut dosages (A), study duration (B), baseline FMD levels (C), and the weighted mean differences (WMDs) of flow-mediated dilation.

P ¼ 0.009), and studies with high dose nuts (67 g/d) (I2 ¼ 76.4%, P ¼ 0.014). 3.5. Sensitivity analyses and publication bias In the sensitivity analysis, the WMD estimates ranged from 0.34% to 0.47%, with the lower limit of 95% CI ranging from 0.13% to 0.27%, and the upper limit from 0.55% to 0.70%, suggesting that removal of each single trial did not change the results (Fig. 4). In

To the best of our knowledge, this is the first meta-analysis and systematic review to analyze the effect of nut consumption on endothelial function assessed by FMD. We found that nut consumption significantly improved FMD levels, although a moderate and marginally significant heterogeneity was observed. Metaregression and subgroup analyses found that types and dosages of nuts, and duration of trials, were the potential sources of heterogeneity. Walnuts markedly improved FMD, whereas other nuts had no effect on FMD changes. This might be explained by the small number of trials which used other nuts in the intervention. Among 10 trials, six were conducted with walnuts, whereas one each with almonds and hazelnuts and 2 with pistachios. Another explanation may be due to the different nutrient profiles of walnuts than other nuts. In addition, nuts had significant beneficial effect on FMD if the study duration was <18 wks, or if the nut dose was <67 g/d. These paradoxical results may be explained by participants' lower compliance in long-term and high dose trials. Some studies reported that a minority of participants were unable to adhere throughout the entire trial, probably due to their intolerance to nuts [38,39]. Results from observational studies have shown that increased nut intake was significantly associated with a reduced risk of CVD [5]. Lipid profile and blood pressure are two traditional risk factors of CVD. Endothelial dysfunction has also been found as a marker of susceptibility to future cardiovascular events [10]. In addition, interactions have been found among these CVD risk factors. For instance, LDL cholesterol can directly induce endothelial dysfunction. Therefore, the cardio-protective effect of nuts may be at least partially mediated through its beneficial effects on these CVD risk factors. A number of clinical trials have been conducted to investigate the effect of nut intervention on lipid profiles and blood pressure to elucidate the underlying mechanisms [8,9,39,40]. A meta-analysis including 25 RCTs reported that a daily intake of 67 g nuts reduced total cholesterol levels by 10.9 mg/dl and triglycerides by 20.6 mg/dl [41]. Another meta-analysis of 13 RCTs showed that walnut intervention significantly lowered triglycerides and total or LDL cholesterol [8]. Furthermore, a recent pooled analysis including 21 RCTs found that nut intervention significantly reduced blood pressure, in particular systolic blood pressure, in subjects without type 2 diabetes [40]. Results of the current meta-analysis are consistent with the findings from previous meta-analyses and suggested nuts may have favorable effects on endothelial function, another important CVD risk factor. Several explanations have been proposed to elucidate the mechanisms through which nut consumption may improve endothelial function. In addition to improved lipid profile, nuts may have direct beneficial effects on other vascular factors. Nuts are rich in monounsaturated (MUFA) and polyunsaturated fatty acids (PUFA). In particular, walnuts contain greater amounts of plant n-3 fatty

Please cite this article in press as: Xiao Y, et al., Effect of nut consumption on vascular endothelial function: A systematic review and metaanalysis of randomized controlled trials, Clinical Nutrition (2017), http://dx.doi.org/10.1016/j.clnu.2017.04.011

Y. Xiao et al. / Clinical Nutrition xxx (2017) 1e9

7

Table 3 Stratified analyses for the effect of nut intervention on flow-mediated dilation. Subgroups

No. of studies

Effect (95% CI)

P

I2 (%)

P

Overall Study design Crossover study Parallel study Sample size <45, below median 45, above median Mean age <55 y, below median 55 y, above median Study duration <18 wks, below median 18 wks, above median Nut types Walnuts Other nuts Nut doses <67 g/d, low dose 67 g/d, high dose Baseline FMD levels <8.6%, below median 8.6%, above median

10

0.406 (0.177, 0.635)

0.001

39.5

0.094

7 3

0.424 (0.139, 0.710) 0.372 (0.091, 0.835)

0.004 0.115

45.3 47.8

0.089 0.147

5 5

0.452 (0.005, 0.899) 0.376 (0.129, 0.623)

0.048 0.003

60.2 16.0

0.039 0.312

5 5

0.477 (0.083, 0.870) 0.353 (0.059, 0.647)

0.018 0.019

48.9 39.6

0.098 0.157

6 4

0.565 (0.215, 0.916) 0.197 (0.071, 0.465)

0.002 0.150

55.6 0.0

0.046 0.978

6 4

0.398 (0.164, 0.632) 0.467 (0.061, 0.995)

0.001 0.083

0.0 73.8

0.639 0.009

7 3

0.456 (0.237, 0.673) 0.343 (0.270, 0.956)

0.001 0.273

0.0 76.4

0.493 0.014

5 5

0.332 (0.033, 0.698) 0.481 (0.179, 0.782)

0.075 0.002

49.0 33.7

0.097 0.197

Significant values are indicated in bold. CI ¼ confidence interval.

Fig. 4. Sensitivity analysis with exclusion of one study at a time from the analyses. Estimated effect size of weighted mean differences (WMDs) of flow-mediated dilation was plotted on the horizontal axis and studies (first author, year) on the vertical axis.

acid ALA than other nuts. In vitro studies showed that ALA might increase membrane fluidity of endothelial cells, promote NO synthesis, and reduce endothelial inflammation [42]. Clinical trials have found that intake of ALA significantly improved arterial compliance [43] and fish oil supplements rich in ALA ameliorated endothelial function [44]. Although high PUFA content of nuts might have deleterious effects on biomarkers of oxidative stress [45], other biologically active phytochemicals may counteract the pro-oxidant effect of PUFA on LDL [46]. a-tocopherol and g-

tocopherol are strong antioxidants and particularly abundant in walnuts and almonds. These tocopherols can reduce the oxidation of lipids, protein, and lipoproteins and decrease the severity of atherosclerosis [47,48]. The different nutrient profiles of walnuts than other nuts may at least partially explain that walnuts, but not other nuts, markedly improved FMD. In addition, L-arginine is another important amino acid constituent of nuts, is the substrate of NO synthase. L-arginine supplements improved endothelial function in subjects with impaired NO production, for example,

Please cite this article in press as: Xiao Y, et al., Effect of nut consumption on vascular endothelial function: A systematic review and metaanalysis of randomized controlled trials, Clinical Nutrition (2017), http://dx.doi.org/10.1016/j.clnu.2017.04.011

8

Y. Xiao et al. / Clinical Nutrition xxx (2017) 1e9

Fig. 5. Publication bias assessed by funnel plot of all studies in the meta-analysis. The y axis represented the weighted mean differences (WMDs) and the x axis represented the standard errors (SEs) of the WMDs. Graph symbols were scaled by weights.

patients with hypercholesterolemia or congestive heart disease [49], suggesting that the effect of nuts on endothelial function may be partially attributable to L-arginine. In addition to FMD, endothelial function can be measured by using a finger plethysmograph based on non-invasive peripheral artery tonometry (EndoPAT 2000) and by calculating the EndoPAT index or reactive hyperemia index (RHI). However, the results from studies that used EndoPAT index or RHI are highly inconsistent. Although a modestly beneficial effect was found in a 4week cross-over RCT of soy nuts using RHI [25], no significant improvement in endothelial function was found in an 8-week crossover RCT of walnuts using RHI [50], and two parallel RCTs of mixed nuts [15,51] using EndoPAT, or a cross-over study of pistachios using RHI and FMD [24]. The discrepant findings of these studies may be, at least in part, explained by the different measurement methods of endothelial function. The measurement by peripheral artery tonometry may not be sensitive enough to detect positive outcomes of endothelial function from nut intervention. In contrast, nut consumption significantly improved FMD in the current meta-analysis. This might be attributed to the fact that FMD measured by brachial artery ultrasound may be more sensitive and accurate in assessing endothelial function and obtaining positive outcomes than EndoPAT index or RHI measured by peripheral artery tonometry. The current meta-analysis has several limitations. Firstly, moderate and marginally significant heterogeneity was found among the 10 eligible RCTs. Differences in the intervention duration, sample size, types and dosages of nuts may be the possible sources of heterogeneity. However, the random effects analysis is a suitable method in the presence of heterogeneity studies. Secondly, six trials used either 24-h dietary recalls or 3-d or 7-d food records to assess compliance with nut intervention, although the limitations of these dietary assessment methods to validate nut consumption are well acknowledged. Only four studies used biochemical markers, e.g. serum a-tocopherol level as a biomarker of nut intake. Our findings of the meta-analysis should be interpreted with caution, considering the methodological limitations of the trials.

5. Conclusion The results of this systematic review and meta-analysis of 10 RCTs suggested that nut consumption may improve endothelial function measured by FMD. Among the various types of nuts, walnuts may have a beneficial effect, whereas other nuts did not show significant protective effect, possibly due to the small number of studies. Hence, more studies examining the effect of nuts other than walnuts on FMD are needed. Furthermore, large-scale randomized trials with a longer duration are warranted to assess the effect of nuts on cardiovascular disease risk.

Funding This study was supported by grants from the National Natural Science Foundation of China (81402672) and the Guangdong Provincial Medical Research Fund (B2014356).

Authorship Yunjun Xiao designed the study and drafted the manuscript; Yunjun Xiao and Wei Huang identified relevant articles and extracted and analyzed data; Chaoqiong Peng, Jinzhou Zhang, Carmen Wong, Jean H. Kim, Eng-kiong Yeoh, and Xuefen Su commented and revised the manuscript; and all the authors read and approved the final manuscript. Conflict of interest The authors declare no conflict of interest.

Appendix A. Supplementary data Supplementary data related to this article can be found at http:// dx.doi.org/10.1016/j.clnu.2017.04.011.

Please cite this article in press as: Xiao Y, et al., Effect of nut consumption on vascular endothelial function: A systematic review and metaanalysis of randomized controlled trials, Clinical Nutrition (2017), http://dx.doi.org/10.1016/j.clnu.2017.04.011

Y. Xiao et al. / Clinical Nutrition xxx (2017) 1e9

References [1] Afshin A, Micha R, Khatibzadeh S, Mozaffarian D. Consumption of nuts and legumes and risk of incident ischemic heart disease, stroke, and diabetes: a systematic review and meta-analysis. Am J Clin Nutr 2014;100(1): 278e88. [2] Luo C, Zhang Y, Ding Y, Shan Z, Chen S, Yu M, et al. Nut consumption and risk of type 2 diabetes, cardiovascular disease, and all-cause mortality: a systematic review and meta-analysis. Am J Clin Nutr 2014;100(1):256e69. [3] Ros E, Mataix J. Fatty acid composition of nutseimplications for cardiovascular health. Br J Nutr 2006;96(Suppl. 2):S29e35. [4] Li L, Tsao R, Yang R, Kramer JK, Hernandez M. Fatty acid profiles, tocopherol contents, and antioxidant activities of heartnut (Juglans ailanthifolia Var. cordiformis) and Persian walnut (Juglans regia L.). J Agric Food Chem 2007;55(4):1164e9. [5] Vinson JA, Cai Y. Nuts, especially walnuts, have both antioxidant quantity and efficacy and exhibit significant potential health benefits. Food Funct 2012;3(2):134e40. [6] Sabate J, Fraser GE, Burke K, Knutsen SF, Bennett H, Lindsted KD. Effects of walnuts on serum lipid levels and blood pressure in normal men. N. Engl J Med 1993;328(9):603e7. [7] Urpi-Sarda M, Casas R, Chiva-Blanch G, Romero-Mamani ES, ValderasMartinez P, Arranz S, et al. Virgin olive oil and nuts as key foods of the Mediterranean diet effects on inflammatory biomakers related to atherosclerosis. Pharmacol Res 2012;65(6):577e83. [8] Banel DK, Hu FB. Effects of walnut consumption on blood lipids and other cardiovascular risk factors: a meta-analysis and systematic review. Am J Clin Nutr 2009;90(1):56e63. [9] Del Gobbo LC, Falk MC, Feldman R, Lewis K, Mozaffarian D. Effects of tree nuts on blood lipids, apolipoproteins, and blood pressure: systematic review, metaanalysis, and dose-response of 61 controlled intervention trials. Am J Clin Nutr 2015;102(6):1347e56. [10] Yeboah J, Crouse JR, Hsu FC, Burke GL, Herrington DM. Brachial flow-mediated dilation predicts incident cardiovascular events in older adults: the Cardiovascular Health Study. Circulation 2007;115(18):2390e7. [11] Moens AL, Goovaerts I, Claeys MJ, Vrints CJ. Flow-mediated vasodilation: a diagnostic instrument, or an experimental tool? Chest 2005;127(6): 2254e63. [12] Brown AA, Hu FB. Dietary modulation of endothelial function: implications for cardiovascular disease. Am J Clin Nutr 2001;73(4):673e86. [13] Njike VY, Ayettey R, Petraro P, Treu JA, Katz DL. Walnut ingestion in adults at risk for diabetes: effects on body composition, diet quality, and cardiac risk measures. BMJ Open Diabetes Res care 2015;3(1):e000115. [14] Ros E, Nunez I, Perez-Heras A, Serra M, Gilabert R, Casals E, et al. A walnut diet improves endothelial function in hypercholesterolemic subjects: a randomized crossover trial. Circulation 2004;109(13):1609e14. [15] Lopez-Uriarte P, Nogues R, Saez G, Bullo M, Romeu M, Masana L, et al. Effect of nut consumption on oxidative stress and the endothelial function in metabolic syndrome. Clin Nutr 2010;29(3):373e80. [16] Ma Y, Njike VY, Millet J, Dutta S, Doughty K, Treu JA, et al. Effects of walnut consumption on endothelial function in type 2 diabetic subjects: a randomized controlled crossover trial. Diabetes Care 2010;33(2):227e32. [17] Kendall CW, West SG, Augustin LS, Esfahani A, Vidgen E, Bashyam B, et al. Acute effects of pistachio consumption on glucose and insulin, satiety hormones and endothelial function in the metabolic syndrome. Eur J Clin Nutr 2014;68(3):370e5. [18] Trimarco V, Izzo R, Stabile E, Rozza F, Santoro M, Manzi MV, et al. Effects of a new combination of nutraceuticals with Morus alba on lipid profile, insulin sensitivity and endotelial function in dyslipidemic subjects. A cross-over, randomized, double-blind trial. High Blood Press Cardiovasc Prev 2015;22(2):149e54. [19] Cortes B, Nunez I, Cofan M, Gilabert R, Perez-Heras A, Casals E, et al. Acute effects of high-fat meals enriched with walnuts or olive oil on postprandial endothelial function. J Am Coll Cardiol 2006;48(8):1666e71. [20] Katz DL, Davidhi A, Ma Y, Kavak Y, Bifulco L, Njike VY. Effects of walnuts on endothelial function in overweight adults with visceral obesity: a randomized, controlled, crossover trial. J Am Coll Nutr 2012;31(6):415e23. [21] West SG, Krick AL, Klein LC, Zhao G, Wojtowicz TF, McGuiness M, et al. Effects of diets high in walnuts and flax oil on hemodynamic responses to stress and vascular endothelial function. J Am Coll Nutr 2010;29(6):595e603. [22] Orem A, Yucesan FB, Orem C, Akcan B, Kural BV, Alasalvar C, et al. Hazelnutenriched diet improves cardiovascular risk biomarkers beyond a lipidlowering effect in hypercholesterolemic subjects. J Clin Lipidol 2013;7(2): 123e31. [23] Berryman CE, Grieger JA, West SG, Chen CY, Blumberg JB, Rothblat GH, et al. Acute consumption of walnuts and walnut components differentially affect postprandial lipemia, endothelial function, oxidative stress, and cholesterol efflux in humans with mild hypercholesterolemia. J Nutr 2013;143(6): 788e94. [24] Sauder KA, McCrea CE, Ulbrecht JS, Kris-Etherton PM, West SG. Effects of pistachios on the lipid/lipoprotein profile, glycemic control, inflammation, and endothelial function in type 2 diabetes: a randomized trial. Metabolism 2015;64(11):1521e9.

9

[25] Reverri EJ, LaSalle CD, Franke AA, Steinberg FM. Soy provides modest benefits on endothelial function without affecting inflammatory biomarkers in adults at cardiometabolic risk. Mol Nutr Food Res 2015;59(2):323e33. [26] Chen CY, Holbrook M, Duess MA, Dohadwala MM, Hamburg NM, Asztalos BF, et al. Effect of almond consumption on vascular function in patients with coronary artery disease: a randomized, controlled, cross-over trial. Nutr J 2015;14:61. [27] Spaccarotella KJ, Kris-Etherton PM, Stone WL, Bagshaw DM, Fishell VK, West SG, et al. The effect of walnut intake on factors related to prostate and vascular health in older men. Nutr J 2008;7:13. [28] Kasliwal RR, Bansal M, Mehrotra R, Yeptho KP, Trehan N. Effect of pistachio nut consumption on endothelial function and arterial stiffness. Nutrition 2015;31(5):678e85. [29] Moher D, Cook DJ, Eastwood S, Olkin I, Rennie D, Stroup DF. Improving the quality of reports of meta-analyses of randomised controlled trials: the QUOROM statement. Quality of Reporting of Meta-analyses. Lancet 1999;354(9193):1896e900. [30] Jadad AR, Moore RA, Carroll D, Jenkinson C, Reynolds DJ, Gavaghan DJ, et al. Assessing the quality of reports of randomized clinical trials: is blinding necessary? Control Clin Trials 1996;17(1):1e12. [31] Moher D, Pham B, Jones A, Cook DJ, Jadad AR, Moher M, et al. Does quality of reports of randomised trials affect estimates of intervention efficacy reported in meta-analyses? Lancet 1998;352(9128):609e13. [32] Follmann D, Elliott P, Suh I, Cutler J. Variance imputation for overviews of clinical trials with continuous response. J Clin Epidemiol 1992;45(7):769e73. [33] DerSimonian R, Laird N. Meta-analysis in clinical trials. Control Clin Trials 1986;7(3):177e88. [34] Higgins JP, Thompson SG, Deeks JJ, Altman DG. Measuring inconsistency in meta-analyses. BMJ 2003;327(7414):557e60. [35] Begg CB, Mazumdar M. Operating characteristics of a rank correlation test for publication bias. Biometrics 1994;50(4):1088e101. [36] Egger M, Davey Smith G, Schneider M, Minder C. Bias in meta-analysis detected by a simple, graphical test. BMJ 1997;315(7109):629e34. [37] Stewart LA, Clarke M, Rovers M, Riley RD, Simmonds M, Stewart G, et al. Preferred reporting items for systematic review and meta-analyses of individual participant data: the PRISMA-IPD statement. Jama 2015;313(16): 1657e65. [38] Zambon D, Sabate J, Munoz S, Campero B, Casals E, Merlos M, et al. Substituting walnuts for monounsaturated fat improves the serum lipid profile of hypercholesterolemic men and women. A randomized crossover trial. Ann Intern Med 2000;132(7):538e46. [39] Almario RU, Vonghavaravat V, Wong R, Kasim-Karakas SE. Effects of walnut consumption on plasma fatty acids and lipoproteins in combined hyperlipidemia. Am J Clin Nutr 2001;74(1):72e9. [40] Mohammadifard N, Salehi-Abargouei A, Salas-Salvado J, Guasch-Ferre 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. Am J Clin Nutr 2015;101(5):966e82. [41] Sabate J, Oda K, Ros E. Nut consumption and blood lipid levels: a pooled analysis of 25 intervention trials. Arch Intern Med 2010;170(9):821e7. [42] De Caterina R, Liao JK, Libby P. Fatty acid modulation of endothelial activation. Am J Clin Nutr 2000;71(1 Suppl.):213Se23S. [43] Nestel PJ, Pomeroy SE, Sasahara T, Yamashita T, Liang YL, Dart AM, et al. Arterial compliance in obese subjects is improved with dietary plant n-3 fatty acid from flaxseed oil despite increased LDL oxidizability. Arterioscler Thromb Vasc Biol 1997;17(6):1163e70. [44] Goodfellow J, Bellamy MF, Ramsey MW, Jones CJ, Lewis MJ. Dietary supplementation with marine omega-3 fatty acids improve systemic large artery endothelial function in subjects with hypercholesterolemia. J Am Coll Cardiol 2000;35(2):265e70. [45] Kimura Y, Sato M, Kurotani K, Nanri A, Kawai K, Kasai H, et al. PUFAs in serum cholesterol ester and oxidative DNA damage in Japanese men and women. Am J Clin Nutr 2012;95(5):1209e14. [46] Anderson KJ, Teuber SS, Gobeille A, Cremin P, Waterhouse AL, Steinberg FM. Walnut polyphenolics inhibit in vitro human plasma and LDL oxidation. J Nutr 2001;131(11):2837e42. [47] Murr C, Winklhofer-Roob BM, Schroecksnadel K, Maritschnegg M, Mangge H, Bohm BO, et al. Inverse association between serum concentrations of neopterin and antioxidants in patients with and without angiographic coronary artery disease. Atherosclerosis 2009;202(2):543e9. [48] Devaraj S, Leonard S, Traber MG, Jialal I. Gamma-tocopherol supplementation alone and in combination with alpha-tocopherol alters biomarkers of oxidative stress and inflammation in subjects with metabolic syndrome. Free Radic Biol Med 2008;44(6):1203e8. [49] Preli RB, Klein KP, Herrington DM. Vascular effects of dietary L-arginine supplementation. Atherosclerosis 2002;162(1):1e15. [50] Wu L, Piotrowski K, Rau T, Waldmann E, Broedl UC, Demmelmair H, et al. Walnut-enriched diet reduces fasting non-HDL-cholesterol and apolipoprotein B in healthy Caucasian subjects: a randomized controlled cross-over clinical trial. Metabolism 2014;63(3):382e91. [51] Lee YJ, Nam GE, Seo JA, Yoon T, Seo I, Lee JH, et al. Nut consumption has favorable effects on lipid profiles of Korean women with metabolic syndrome. Nutr Res 2014;34(9):814e20.

Please cite this article in press as: Xiao Y, et al., Effect of nut consumption on vascular endothelial function: A systematic review and metaanalysis of randomized controlled trials, Clinical Nutrition (2017), http://dx.doi.org/10.1016/j.clnu.2017.04.011