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3 Nutritional Sciences Division, King's College London, London SE1 9NH, United Kingdom and 4 Cardiovascular Division, King's College London School of Medicine, St. Thomas' Hospital, London SE1 7EH, United Kingdom
* To whom correspondence should be addressed. E-mail: sarah.e.berry{at}kcl.ac.uk.
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ABSTRACT |
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 TOP ABSTRACT IntroductionSubjects and MethodsResultsDiscussionLITERATURE CITED |
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) at fasting and 3 h following the test meals. The postprandial increase in plasma TAG was lower (66% lower incremental area under curve) following the SA meal [28.3 (9.7, 46.9)] than after the HO meal [83.4 (57.0, 109.8); P < 0.001] (geometric means with 95% CI, arbitary units). Following the HO meal, there was a decrease in FMD [–3.0% (–4.4, –1.6); P < 0.001] and an increase in plasma 8-isoprostane F2 [10.4ng/L (3.8, 16.9); P = 0.005] compared with fasting values, but no changes followed the SA meal. The changes in 8-isoprostane F2 and FMD differed between meals and were 14.0 ng/L (6.4, 21.6; P = 0.001) and 1.75% (0.10, 3.39; P = 0.02), respectively. The reductions in PWA and PWV c-f did not differ between meals. This study demonstrates that a stearic acid-rich fat attenuates the postprandial impairment in endothelial function compared with an oleic acid-rich fat and supports the hypothesis that postprandial lipemia impairs endothelial function via an increase in oxidative stress.
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Introduction |
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TOPABSTRACTIntroductionSubjects and MethodsResultsDiscussionLITERATURE CITED |
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The effects of different fats on the magnitude of postprandial lipemia have been extensively studied; however, there is a paucity of information on the effects of different fats on the degree of postprandial impairment of endothelial function and increase in oxidative stress. Currently, there is much interest in stearic acid-rich fats as replacements for trans fatty acids, because it has been proposed that stearic acid may be neutral with regard to CHD due to its lack of effect on serum cholesterol (17). Furthermore, a lower postprandial lipemia and factor VII activated concentration has been reported following a stearic acid-rich fat [shea butter blend (SA)] (18) and a structured stearic acid-rich triacylglycerol (TAG) compared with an oleic acid-rich fat (19). Previous research by our group (8) and others (6) has demonstrated impaired endothelial function using the flow-mediated dilatation (FMD) technique following meals rich in oleic acid compared with a low-fat/ high-carbohydrate meal. It was hypothesized that the lower level of lipemia previously observed following the SA (18) may attenuate the impairment of endothelial function following a high-fat meal. The primary objective of the current study was to compare the effect of a stearic acid-rich fat (SA) with an oleic acid-rich fat [high oleic sunflower oil (HO)] on postprandial changes in endothelial function. The secondary objective was to investigate whether changes in endothelial function were accompanied by increased oxidative stress using the highly specific marker of oxidative stress, 8-isoprostane F2 (19).
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Subjects and Methods |
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TOPABSTRACTIntroductionSubjects and MethodsResultsDiscussionLITERATURE CITED |
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Subjects. Seventeen men aged 18–40 y were recruited from among staff and students of King's College London. The subjects were healthy and exclusion criteria included history of cardiovascular disease, diabetes, BMI <20 or >35 kg/m2, plasma cholesterol >7.8 mmol/L, plasma TAG >3 mmol/L, current use of antihypertensive or lipid-lowering medication, and a self-reported intake of alcohol of >28 U/wk (1 U = 10 mL ethanol). Fasting plasma lipoprotein lipid concentrations, body weight, blood pressure (BP), blood cell count, and liver function were confirmed to be within the prescribed limits prior to entry into the study. Habitual nutrient intake was assessed from a 3-d food intake diary and nutrients were estimated using the Microdiet program (Downlee Systems Limited). Dietary intakes were close to recommended dietary guidelines for the UK (20). Subject details are shown in Table 1.
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Test meals.
The test meals consisted of 2 muffins (each containing 25 g test fat) and a milkshake and were formulated to provide 3.57 MJ, 50 g fat, 15 g protein, and 89 g carbohydrate. The milkshake consisted of 220 mL skimmed milk and 15 g Nesquick milkshake mix (Nestlé). The 2 muffins contained 50 g test fat, 28 g baking flour, 10 g corn flour, 28 g sugar, 38 mL skimmed milk, 4 g pasteurized egg white, 4 g vanilla essence, and 2 g baking powder. The test fats consisted of HO (Anglia Oils) and a SA. For the SA, shea butter (Britannia Food Ingredient) was refined and blended with a small amount of sunflower oil (Unilever Research) so that linoleic acid accounted for 
10% of the fatty acids, similar to the level present in HO. The fatty acid composition and solid fat content profiles of the test fats were measured by GLC and pulsed low resolution NMR, respectively, as previously described (18). The fatty acid composition of the SA and HO meals were: stearic acid, 26.7 and 0.8 g; oleic acid, 16.6 and 42.5 g; linoleic acid, 4.5 and 4.0 g, respectively. The proportion of solid fat at body temperature (37°C) of the SA was 22% and <1% for HO.
Collection and handling of blood samples.
Venous blood samples were collected using the vacutainer technique. Blood samples were processed within 15 min of blood collection. Blood for lipid analysis (plasma fatty acid and TAG concentrations) was collected into 10-mL EDTA-containing vacutainers and plasma was separated by centrifugation at 1500 g; 15 min at 4°C. Blood for glucose and insulin analysis was collected into 4-mL fluoride oxalate tubes and 4-mL lithium heparin tubes, respectively, and both were centrifuged at 1500 g; 15 min at 4°C. Blood for 8-isoprostane F2 analysis was collected at 0, 4, and 8 h into chilled 4.5-mL vacutainers containing 5 mL trisodium citrate (0.105 mol/L) to which 35 µL of 2 mmol/L indomethacin was added and the vacutainer left to stand at 4°C for 30 min. Samples were centrifuged at 1500 g; 15 min at 4°C, plasma was separated, and 5 mmol/L butylated hydroxytoluene ethanolic solution was added (final concentration 20 µmol/L) and the sample stored at –70°C until analyzed.
Hemodynamics measurements. All vascular measurements were performed with the subject supine in a quiet, darkened, and temperature-controlled (23°C) room. FMD measurements were performed on the right arm after 20 min of rest according to techniques described by the International Brachial Artery Reactivity Task Force (21). The right brachial artery was scanned longitudinally 5–10 cm above the antecubital fossa with a high-resolution ultrasound probe (7–10 MHz probe, Acuson Aspen, Siemens) held in a stereotactic clamp. Brachial artery FMD was induced by a 5-min inflation of a pneumatic cuff placed around the forearm immediately below the medial epicondyle (250 mm Hg) followed by rapid deflation with an automatic air regulator (Logan Research). Brachial artery diameter was measured with edge detection software (Brachial Tools) from electrocardiogram-triggered images captured every 3 s throughout an 11-min recording. FMD was expressed as maximal percentage change in vessel diameter from baseline. Blood flow was recorded continuously by pulsed-wave Doppler. Reactive hyperemia was calculated from the maximal flow within the first 15 s after deflation of the pneumatic cuff, relative to the baseline flow. Following the 3-h FMD measurement, endothelial independent dilation was determined in response to glycerol tri-nitrate (25 µg sublingually).
Radial PWA was measured following 5 min rest, in triplicate, 5 min apart. An automatic oscillometric digital BP monitor (OMRON) was used to measure BP before each pulse wave measurement according to British Hypertension Society guidelines (22). Applantaion tonometry of the radial artery was performed using a sensitive transducer (SphygomoCor, AtCor Medical) to detect waveform traces of the peripheral waveform. Data were recorded directly onto a computer using SphygomoCor version 7.01 (Scanmed). The central arterial waveform was derived from the peripheral waveform using a validated transfer function. Peripheral and central augmentation indices were calculated by dividing the difference between the 2nd systolic peak and the diastolic pressure by the difference between the 1st systolic peak and the diastolic pressure (x 100%). PWVc-f was measured following 5 min rest, in triplicate, 5 min apart, using the SyphgomoCor systems (23).
Analytical methods.
Plasma glucose, insulin, and TAG were analyzed using enzymatic assays as described elsewhere (18). Plasma concentrations of palmitic, stearic, oleic, and linoleic acid were determined by GLC using pentadecanoic acid (C15:0) as an internal standard (24) on a BP75 column (25 m x 220 µm x 0.25 µm, SGE Analytical Science) on an Agilent 6890 (Agilent Technologies). Total 8-isoprostane-F2 was analyzed as outlined elsewhere (25). Samples for each subject were analyzed in the same run to avoid between-assay variation.
Statistical analysis. Data that were not normally distributed were log transformed prior to analysis. Incremental area under the curves were calculated using GraphPad Prism (version 4.0) using the trapezoid rule. Statistical analysis of the data was conducted using repeated-measures ANOVA, with meal and time as within-subject factors using SPSS PC version 10. Baseline diameter was included as a covariate in the model for FMD analysis. Post hoc analysis using Bonferroni t tests was conducted, using GraphPad Prism, when main effects were identified by ANOVA. To investigate the relationship between variables, linear regression analysis was performed on changes from baseline values, with FMD as the dependent variable and baseline diameter and TAG as independent variables. Differences were considered significant at P < 0.05. Results are presented as means ± SD, geometric mean with 95% CI, or mean change from baseline (with 95% CI).
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Results |
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TOPABSTRACTIntroductionSubjects and MethodsResultsDiscussionLITERATURE CITED |
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concentrations (Table 2) did not increase from fasting following the SA meal but increased following the HO meal, and the changes differed between meals by 14.0 ng/L (95% CI 6.4, 21.6) (P = 0.005). Baseline brachial artery diameter (Table 2) did not differ between meals at fasting and 3 h postprandially and did not change from fasting values at 3 h following either meal. Postreactive hyperemia brachial artery diameter did not differ at fasting between meals but was lower at 3 h after the HO meal than after the SA meal (P = 0.013). There was a meal x time interaction (P = 0.039) for FMD, which was reduced from fasting values at 3 h following the HO meal (P < 0.001) but did not differ following the SA meal. The postprandial change in FMD at 3 h differed after the 2 meals by 1.75% (95% CI 0.10, 3.39; P < 0.05). Baseline brachial artery diameter did not affect FMD responses and there were no differences in endothelial independent vasodilatation measured following glycerol tri-nitrate between the SA [10.8% (95% CI 8.7, 12.9)] and HO meals [9.7% (95% CI 8.3, 11.2)]. There were no changes in large artery stiffness as indicated by PWVc-f. However, there was a time effect for the central augmentation index (CAIx) (P = 0.019), peripheral augmentation index (PAIx) (P < 0.001), and diastolic BP (P < 0.001) but not in heart rate or systolic BP; the former values decreased significantly following the meals but the changes did not differ significantly between meals. Linear regression analysis showed no significant relationship between the change in FMD at 3 h and the change in plasma TAG concentrations at 3 h or baseline brachial artery diameter following either test meal.
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Discussion |
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TOPABSTRACTIntroductionSubjects and MethodsResultsDiscussionLITERATURE CITED |
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The primary observation in the present study is that the postprandial change in FMD differed between the 2 test meals, which contained the same amount of fat. Several studies have reported that postprandial lipemia results in impaired endothelial function and some have reported a correlation between the magnitude of lipemia and the change in endothelial function (7,16,26). In the present study, although postprandial lipemia was greater following the oleic acid-rich meal and there was a greater decrease in FMD, there was no significant relationship between the extent of postprandial lipemia and the change in FMD between subjects, in agreement with Williams et al. (27) and Steer et al. (9). Although it is plausible that the different fatty acids may have differential direct stimulatory or inhibitory effects on the endothelium as observed in vitro (28,29,30), unrelated to the magnitude of postprandial lipemia, a likely explanation is a lack of statistical power to detect a significant relationship. Furthermore, it is noteworthy that postprandial peripheral vasodilatation may account for some of the conflicting reports of high-fat meals on endothelial function, because postprandial vasodilatation of the brachial artery would result in a decline in FMD when expressed as a percentage of the baseline diameter (31). However, despite reductions in PAIx, baseline brachial artery diameter did not differ between fasting and 3 h following either test meal. Differences in the antioxidant content (polyphenolic compounds and antioxidant vitamins) of test meals and/or timing of measurements may also explain some of the discrepancies between studies. Indeed, it has been reported that FMD responses exhibit diurnal variation, with lower levels of FMD in the morning compared with the afternoon (32). This may in part explain the lack of effect of a high monounsaturated fatty acid meal on FMD, observed by West et al. (33), together with differences in the antioxidant capacity of the meals.
There are several mechanisms by which elevations in plasma TAG may impair endothelial function, including production of the superoxide anion (which reacts with NO to produce peroxynitrite) and stimulatory effects of chylomicrons and/or free fatty acids on the proinflammatory and procoagulant signaling pathways, ultimately resulting in a reduced NO bioavailability (5). In the present study, we measured plasma 8-isoprostane F2 concentration using a highly specific GC-MS assay (34) to assess whether there was increased generation of ROS and found a significant difference in the postprandial increase between test meals. Our finding provides strong evidence to indicate that postprandial lipemia results in the increased generation of ROS, in agreement with studies showing that coadministration of antioxidant vitamins prevents the postprandial decline in FMD (14–16). Future studies should therefore ensure to report the level of antioxidant compounds in the test meals.
In this study, we also noted postprandial decreases in diastolic BP, CAIx, and PAIx, which are measures of arterial tone of small and large vessels and reflect vasodilatation caused by endothelial-dependent and -independent mechanisms. Previous studies feeding high-fat and high-carbohydrate meals have also reported postprandial peripheral vasodilatation (35,31,36). It has been proposed that this may be due to increased plasma insulin concentrations (37), which may induce systemic vasodilatation via nonendothelial-dependent mechanisms. Therefore, the similar reductions in PAIx and CAIx observed following both meals, despite differences in lipemia, may be explained by the similar increases in postprandial insulin concentrations.
Although the current study demonstrates the differential effects of a specific stearic acid-rich fat on acute changes in endothelial function and oxidative stress, these findings cannot be generalized to other stearic acid-rich fats such as cocoa butter, which may induce different degrees of postprandial lipemia (38).
In conclusion, this study demonstrates differences between a stearic and oleic acid-rich fat on oxidative stress and endothelial function. These differences parallel the previously reported postprandial activation of factor VII (18) and support the suggestion that large elevations of plasma TAG cause perturbations in hemostatic and hemodynamic indices that may influence risk of thrombosis and atherosclerosis. As the magnitude of impairment of endothelial function predicts adverse cardiovascular events and long-term outcomes (39) and a considerable part of the day is spent in the postprandial state, dietary factors that reduce the postprandial increase in endothelial dysfunction may therefore contribute toward the prevention of CHD.
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ACKNOWLEDGMENTS |
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FOOTNOTES |
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2 Supported by the Department of Health via the National Institute for Health Research (NIHR) comprehensive Biomedical Research Centre award to Guy's & St Thomas' NHS Foundation Trust in partnership with King's College London.
5 Abbreviations used: BP, blood pressure; CAIx, central augmentation index; CHD, coronary heart disease; HO, high oleic sunflower oil; FMD, flow-mediated dilatation; NO, nitric oxide; PAIx, peripheral augmentation index; PWA, pulse wave analysis; PWVc-f, carotid-femoral pulse wave velocity; ROS, reactive oxygen species; SA, shea butter blend; TAG, triacylglycerol.
Manuscript received 29 April 2008. Initial review completed 18 June 2008. Revision accepted 30 July 2008.
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LITERATURE CITED |
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TOPABSTRACTIntroductionSubjects and MethodsResultsDiscussionLITERATURE CITED |
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