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J Am Coll Cardiol, 2006; 48:1666-1671, doi:10.1016/j.jacc.2006.06.057
(Published online 25 September 2006). © 2006 by the American College of Cardiology Foundation |
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* Departament de Medicina, Universitat Autònoma de Barcelona, Barcelona, Spain
Secció dEcografia, Centre de Diagnòstic per lImatge, Barcelona, Spain
Unitat de Lípids, Institut Clínic de Malalties Digestives i Metabòliques, Barcelona, Spain
Centre de Diagnòstic Biològic, Institut dInvestigacions Biomèdiques August Pi i Sunyer, Hospital Clínic, Barcelona, Spain.
Manuscript received March 22, 2006; revised manuscript received May 31, 2006, accepted June 6, 2006.
* Reprint requests and correspondence: Dr. Emilio Ros, Unitat de Lípids, Hospital Clínic, Villarroel 170, 08036 Barcelona, Spain. (Email: eros{at}clinic.ub.es).
| Abstract |
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BACKGROUND: Compared with a Mediterranean diet, a walnut diet has been shown to improve endothelial function in hypercholesterolemic patients. We hypothesized that walnuts would reverse postprandial endothelial dysfunction associated with consumption of a fatty meal.
METHODS: We randomized in a crossover design 12 healthy subjects and 12 patients with hypercholesterolemia to 2 high-fat meal sequences to which 25 g olive oil or 40 g walnuts had been added. Both test meals contained 80 g fat and 35% saturated fatty acids, and consumption of each meal was separated by 1 week. Venipunctures and ultrasound measurements of brachial artery endothelial function were performed after fasting and 4 h after test meals.
RESULTS: In both study groups, flow-mediated dilation (FMD) was worse after the olive oil meal than after the walnut meal (p = 0.006, time-period interaction). Fasting, but not postprandial, triglyceride concentrations correlated inversely with FMD (r = 0.324; p = 0.024). Flow-independent dilation and plasma ADMA concentrations were unchanged, and the concentration of oxidized low-density lipoproteins decreased (p = 0.051) after either meal. The plasma concentrations of soluble inflammatory cytokines and adhesion molecules decreased (p < 0.01) independently of meal type, except for E-selectin, which decreased more (p = 0.033) after the walnut meal.
CONCLUSIONS: Adding walnuts to a high-fat meal acutely improves FMD independently of changes in oxidation, inflammation, or ADMA. Both walnuts and olive oil preserve the protective phenotype of endothelial cells.
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Food intake is an important factor that affects vascular reactivity. Short-term feeding trials have shown the potential of food for improving endothelial function, either as isolated nutrients, such as n-3 polyunsaturated fatty acids (PUFA), L-arginine, and antioxidant vitamins, or as healthy food patterns (5). A high-fat meal is usually followed by transient endothelial dysfunction in association with raised triglyceride-rich lipoproteins (6). Abnormal vasoactivity after a fatty meal is attenuated by pretreatment with antioxidant phytochemicals (7) or addition of antioxidants to the meal (8,9), suggesting that postprandial oxidative stress plays an important role. Elevated concentrations of the endogenous NO inhibitor ADMA may also contribute to fatty meal-induced endothelial dysfunction (3).
Walnuts are a rich source of antioxidants, L-arginine, and
-linolenic acid (ALA), a plant n-3 PUFA. Recently we showed that, compared with a Mediterranean diet, a walnut diet improves endothelial function in hypercholesterolemic patients (10). To test the hypothesis that walnuts also would have acute favorable effects on vasoactivity, we examined the effects of adding walnuts or olive oil to a single high-fat meal on postprandial endothelial function of the brachial artery and markers of oxidation and endothelial activation in controls and hypercholesterolemic subjects.
| Methods |
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Test meals.
The meals were prepared at the hospitals kitchen and consisted of
1,200 kcal with 63% fat (35% saturated fatty acids [SFA]), 15% protein, 22% carbohydrate, and 120 mg cholesterol, for a total fat content of 80 g. They included a sandwich with 100 g white bread, 75 g salami, and 50 g fatty cheese, 125 g fat-rich (10%) yogurt, and water ad libitum. Additionally, participants consumed 25 ml olive oil soaked into the bread (olive oil meal) or 40 g shelled walnuts (walnut meal). The unsaturated fatty acid content of the olive oil and walnut meals differed: 38% and 23% monounsaturated fatty acids (MUFA), and 7% and 23% PUFA, respectively. Only the walnut meal contained ALA (5.4 g). The nutrient composition of the walnuts used in the study has been published previously (10). The olive oil used contained 78% oleic acid and 30 mg/100 g
-tocopherol.
Endothelial function testing. The methods for ultrasound evaluation of endothelial function in the brachial artery and the within-subject variability of FMD in our laboratory have been described (10). The operator was unaware of meal sequences. Suitable measurements were obtained in all tests.
Laboratory determinations.
Blood samples were centrifuged (1,500 g, 15 min, 4°C) immediately after collection, and serum and EDTA-plasma promptly stored at 80°C for later processing. Analytic techniques for serum glucose, total cholesterol, high-density lipoprotein (HDL) cholesterol, triglycerides, apolipoprotein B (apoB), lipoprotein separation by ultracentrifugation, and in vitro copper-induced LDL oxidation have been described (10). Analytes determined by subject in frozen samples of serum or EDTA-plasma were: insulin by radioimmunoassay (IRI-CIS, Gif-Sur-Yvette, France); free fatty acids by an enzymatic colorimetric method (Wako, Neuss, Germany);
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-tocopherol, carotenoids, and vitamin C by standard HPLC methods; oxidized LDL by a monoclonal antibody-based immunoassay (Mercodia, Uppsala, Sweden); soluble E-selectin, soluble intercellular adhesion molecule 1 (sICAM-1), soluble vascular cell adhesion molecule 1 (sVCAM-1), and total soluble tumor necrosis factor receptors (sTNF-R) by standard ELISA (Bender, Wien, Austria).
Statistical analyses. Changes in clinical outcomes were assessed using repeated-measures analysis of variance (ANOVA) with 4 factors: group (control vs. hypercholesterolemia), period (olive oil vs. walnut meals), time (baseline vs. postprandial), and treatment sequence, and all their interactions. Period and time were factors with repeated measures. Because no carryover effects between treatment sequences were observed, final analyses were performed with repeated-measures ANOVA for the 3 factors (group, period, and time) and all their interactions. Only significant interactions are reported. Linear regression analysis was used to determine relationships between continuous variables. Statistical significance was set at p < 0.05. The SPSS software (version 11.0; SPSS, Chicago, Illinois) was used. All values are mean ± SD.
| Results |
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Table 2 and Figure 1 show that postprandial FMD was impaired after the olive oil meal in both control (17%) and hypercholesterolemic (36%) subjects, whereas it was unchanged in the control group and increased by 24% in the hypercholesterolemic group after the walnut meal (timeperiod interaction: p = 0.006). No postprandial changes of FID were observed. Blood pressure and heart rate were unchanged, and hyperemic flow increased to a similar extent after both meals.
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-tocopherol, and
-tocopherol decreased significantly (p < 0.05) and to a similar extent in the 2 groups. The changes ranged from 2% to 14% and were unrelated to meal type. The
-carotene and ß-carotene levels were similar before and after either meal (data not shown). As shown in Table 3, baseline oxidized LDL was higher in hypercholesterolemic compared with control subjects and decreased postprandially in the 2 groups. In the copper-induced oxidizability experiments, opposite effects were observed in lag times of LDL-conjugated diene production depending on study group and meal type.
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| Discussion |
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The results confirm and extend those of an earlier feeding trial in hypercholesterolemic subjects showing that a walnut meal in a background walnut diet was associated with improved brachial artery vasoactivity in comparison with an olive oil meal during a Mediterranean diet (10). In that study, the benefit on endothelial function was mediated in part through an improved lipid profile. This mechanism cannot be implicated in our acute studies, where both meals induced postprandial hypertriglyceridemia to a similar extent. However, VLDL triglyceride/apoB ratios increased after the olive oil meal in comparison with the walnut meal and were related to impaired FMD. Because this ratio is an indirect measure of the presence of large triglyceride-rich VLDL particles that are highly atherogenic (14), our findings may explain in part the differential effects of the test meals on endothelial function. Abnormal vasoactivity after a lipid challenge has been related to accumulation of postprandial triglyceride-rich lipoproteins in other studies (6,9,15).
That a fatty meal induces endothelial dysfunction has been shown by most (59,13,15,16) but not all (5,16) studies. Because earlier studies were designed so that baseline testing was performed in the early morning and postprandial testing was done 3 to 4 h later, a reason for this discrepancy is that the physiologic improvement of FMD during the morning counteracted any changes due to the fat challenge (12,13). For the same reason, the deterioration of FMD after a fatty meal may be more pronounced than usually reported. This objection may also apply to studies of the acute vasomotor effects of unsaturated fats and could explain the inconsistent effects on FMD reported after meals rich in MUFA from olive oil or other sources (5,8,16,17) or the lack of effect of an n-3 PUFA-rich salmon meal (8). Recently, West et al. (17) reported increased FMD in diabetic patients after meals enriched with either marine n-3 PUFA or ALA, thus supporting the beneficial role of ALA-rich walnuts on endothelial function. Besides ALA, other cardioprotective constituents of walnuts, such as L-arginine and antioxidants (10), could favorably influence vasoactivity.
A role for oxidative stress in the causation of postprandial endothelial dysfunction is suggested by the protective effect of different antioxidants when added to fatty meals (5,79). Unexpectedly, oxidized LDL decreased postprandially, independently of group and meal type, and the susceptibility of LDL to oxidation was unaffected overall (Table 3). Although the supplemental foods provided vitamin E compounds (
-tocopherol in olive oil and
-tocopherol in walnuts), their plasma levels were slightly reduced after meals. Presumably, partial consumption of circulating antioxidants helped preserve the resistance of LDL to oxidation. These results provide further evidence that, in spite of a high PUFA content, walnut intake does not promote lipid peroxidation (10,11). They also suggest that FMD changes after a fatty meal are independent of lipoprotein oxidation (18,19).
Besides denoting impaired FMD, endothelial dysfunction comprises a specific state of endothelial activation that is characterized by enhanced expression and release into the circulation of inflammatory cytokines and adhesion molecules (1,2). A fatty meal thus activates the endothelium, and this process is counteracted by antioxidant vitamins (20). Given the positive effects of the walnut meal on FMD, the fact that the circulating levels of sTNF-R, soluble E-selectin, sICAM-1, and sVCAM-1 did not increase was not unexpected. However, a similar lack of endothelial activation was observed after the olive oil meal, in clear dissociation from its effects on FMD. Postprandially, inflammatory biomarkers decreased independently of meal type, except for soluble E-selectin, an adhesion molecule involved in the early steps of monocyte recruitment to the endothelium (21), which decreased more after the walnut meal than after the olive oil meal. We have previously shown that a walnut diet attenuates endothelial activation, as suggested by decreased sVCAM-1 levels (10). Our findings are consistent with the evidence of decreased expression of adhesion molecules by endothelial cells exposed to marine n-3 PUFA or oleic acid (21). A recent study (22) has shown that diets enriched in ALA from walnuts reduce the levels of inflammatory biomarkers. Taken together, these findings suggest that ALA shares the anti-inflammatory effects of marine n-3 PUFA.
The plasma ADMA level was unchanged after either meal (Fig. 3), suggesting that this factor was not involved in the changes of endothelial function.
Study limitations. Our study is limited by the small representation of women; therefore, the findings may only apply to men. Hypercholesterolemic subjects were slightly older than control subjects, but participants in both groups had no other nonlipid risk factors and showed similar hemodynamics and lack of carotid atherosclerosis. Besides, the study was crossover and focused on between-meal differences, not between-group ones. Another limitation is that we did not evaluate the effects of fat loads made up only of foods rich in SFA. Olive oil alone may impair endothelial function (8,16), thus it is not completely clear whether between-meal differences were due to a beneficial effect of walnuts or a detrimental effect of olive oil, or both.
Conclusions. The mechanisms underlying impaired endothelial function in the postprandial state are likely multifactorial (3,5,6,20). We showed that supplemental walnuts, but not olive oil, counteracted the detrimental changes in FMD associated with eating a fatty meal. Vasomotor changes, however, did not appear to occur through pro-oxidative, inflammation-sensitive, or ADMA-related mechanisms. Because of an increased delivery of ALA, improved FMD after the walnut meal could be mediated by increased membrane fluidity of endothelial cells promoting enhanced synthesis and/or release of NO, as has been postulated for marine n-3 PUFA (21). Nevertheless, unsaturated fatty acids and antioxidants in both olive oil and walnuts appear to preserve the protective phenotype of endothelial cells. Additional work is needed to prove these contentions and explore basic mechanisms for the vascular effects of different fats, such as vascular cell signaling and pro- or antiatherogenic gene expression.
| Acknowledgments |
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| Footnotes |
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1 The first two authors contributed equally to this work. ![]()
| References |
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-linolenic acid reduces inflammatory and lipid cardiovascular risk factors in hypercholesterolemic men and women J Nutr 2004;134:2991-2997.This article has been cited by other articles:
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M. Shah, B. Adams-Huet, L. Brinkley, S. M. Grundy, and A. Garg Lipid, Glycemic, and Insulin Responses to Meals Rich in Saturated, cis-Monounsaturated, and Polyunsaturated (n-3 and n-6) Fatty Acids in Subjects With Type 2 Diabetes Diabetes Care, December 1, 2007; 30(12): 2993 - 2998. [Abstract] [Full Text] [PDF] |
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P. Dandona, A. Chaudhuri, and P. Mohanty Macronutrients, Advanced Glycation End Products, and Vascular Reactivity Diabetes Care, October 1, 2007; 30(10): 2750 - 2751. [Full Text] [PDF] |
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