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J Am Coll Cardiol, 2000; 36:417-422 © 2000 by the American College of Cardiology Foundation |
* Department of Cardiology, Royal Prince Alfred Hospital, Sydney, Australia
Department of Clinical Biochemistry, Royal Prince Alfred Hospital, Sydney, Australia
Department of Medicine, University of Sydney, Sydney, Australia
Department of Clinical Physiology, Turku University Central Hospital, Turku, Finland
Manuscript received October 25, 1999; revised manuscript received February 11, 2000, accepted March 30, 2000.
Reprint requests and correspondence: Dr. David S. Celermajer, Department of Cardiology, Royal Prince Alfred Hospital, Missenden Road, Camperdown NSW 2050, Sydney, Australia
davidc{at}card.rpa.cs.nsw.gov.au
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Abstract |
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We sought to study the effects of a fatty meal on vascular reactivity, including endothelial function and maximal vasodilation.
BACKGROUND
Recent reports regarding the physiological changes in peripheral vasculature after eating a fatty meal have been controversial.
METHODS
Twelve volunteers were studied before, 3 h after, and 6 h after a high-fat meal (1030 kcal, 61 g fat) rich in saturated fatty acids, and 10 were restudied after a similar meal rich in monounsaturated fatty acids. Endothelial function was assessed as flow-mediated dilatation (FMD) in the brachial artery using ultrasound. Resting and postischemic forearm blood flow (FBF) were recorded using venous occlusion strain-gauge plethysmography, before, and every 10 to 15 s after, 5 min upper arm ischemia.
RESULTS
Brachial artery basal diameter, resting FBF and postischemic hyperemia increased after high-fat meals (all p < 0.001), whereas FMD did not change. The increase in resting FBF correlated with increases in postprandial insulin (r = 0.80, p < 0.002) and triglyceride (r = 0.77, p < 0.005) levels.
CONCLUSIONS
We concluded that eating a fatty meal induces vasodilation and increases resting and stimulated FBF and that these observations are probably mediated by postprandial changes in insulin and/or triglyceride levels. The metabolic changes that occur after meals are not associated with impaired endothelial nitric oxide release in the conduit arteries.
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The acute effects of eating a high-fat meal include certain well-characterized plasma changes such as transient elevations in glucose, insulin and triglyceride levels (4); however, the possible effects on the arterial physiology are less well-examined. As humans exist predominantly in the postprandial state, there has been recent interest in the nature of the alterations of lipoprotein levels after meals and their role in the etiology of vascular disease.
Reports regarding the physiological changes in peripheral vasculature after eating a fatty meal have been controversial. Some investigators (5,6) have observed meal-induced changes in arterial reactivity suggestive of transient endothelial dysfunction and linked these changes to postprandial hypertriglyceridemia. Others have not reported alterations in vascular reactivity but have instead observed changes in resting arterial tone (7). The aim of this study was to examine the effects of different types of fatty meals on vascular physiology more comprehensively, both at the level of the conduit artery and the microcirculation, and to correlate any vascular changes with metabolic alterations observed postprandially.
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Methods |
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Study design. Subjects were tested in the fasting state (12 h overnight fast), 3 h after, and 6 h after eating a controlled fatty meal (see below) in a quiet, temperature-controlled laboratory in a supine position. The ultrasound study for brachial artery endothelial function was performed on the right arm, and, immediately after this, the plethysmography test was performed on the left side. The study meal (meal 1) included a sausage, two muffins and two hash browns cooked in 61 g of fresh tallow fat and had an energy content of 1,030 kcal (fatty acid profile: 48% saturated, 40% monounsaturated, 7.4% polyunsaturated and 4.6% trans fatty acids). Ten subjects were restudied at least one week later using the same protocol after eating a fatty meal (meal 2) (the other two were unable to participate due to work commitments). Meal 2 had similar constituents, fat content and energy amount, but a different fatty acid profile consisting mainly of monounsaturated fatty acids (85% monounsaturated, 10% saturated, 5% polyunsaturated). The test meals were prepared in our laboratory.
Plethysmography. Forearm blood flow (FBF) was measured from the left forearm by venous occlusion strain-gauge plethysmography, using calibrated mercury-in-silastic strain gauges (Hokanson, Bellevue, Washington) (8). The gauge was attached around the widest, most muscular segment of the forearm. Venous occlusion pressure averaged 45 mm Hg in the cuff placed around the upper arm. Circulation to the hand was prevented by inflating a pediatric cuff around the wrist to suprasystolic pressures (250 mm Hg). After completing the resting blood flow measurements (average of six acceptable flow curves), the upper arm cuff was inflated to 250 mm Hg for 5 min to induce ischemia. After the cuff release, two indicators of postischemic hyperemia were measured: the peak reactive hyperemia and the postischemic volume. The peak reactive hyperemia was determined by measuring the FBF immediately after the release of the upper arm cuff to venous occlusion (maximum flow after ischemia). Thereafter, the FBF was recorded for 350 s by using an automated cuff controller that measured blood flow every 10 s during the first 80 s, every 15 s between 80 and 200 s, and every 30 s between 200 and 350 s. The postischemic volume was determined as the area under flow versus time curve (between 0 to 350 s). Postischemic hyperemia is common to most human vascular beds, and it indicates the maximum functional vasodilator capacity of these tissues.
The flow curves were recorded using a computer based chart recorder (MacLab/8e System, ADInstruments, Castle Hill, Australia). Arterial inflow was measured by determining a straight regression line derived from the initial part of the upward flow curve during the first few pulses after the cuff release. The slope of that regression line reflects the forearm volume change per unit of time (8).
Ultrasound studies. All studies were performed using an Acuson 128XP/10 mainframe (Acuson, Mountain View, California) with a 7.0 MHz linear array transducer, as previously described (9,10). Brachial artery diameter was measured from B-mode ultrasound images. Scans were obtained at rest, during reactive hyperemia and again at rest. The artery was scanned in longitudinal sections 2 to 15 cm above the elbow. A resting scan was recorded, and arterial flow velocity was measured using a Doppler signal. Increased flow was induced by inflation of a pneumatic tourniquet placed around the forearm (distal to the scanned part of the artery) to a pressure of 250 mm Hg for 4.5 min, followed by release. A second scan was taken continuously for 30 s before and 90 s after cuff deflation, including a flow velocity recording for the first 15 s after the cuff was released. Thereafter, 10 to 15 min was allowed for vessel recovery, after which a further resting scan was taken. At the end of each study day (6 h after meal), a dose of sublingual nitroglycerin (glyceryl trinitrate spray 400 µm) was administered, and 3 to 4 min later the last scan was acquired to assess nitrate-mediated dilatation.
Vessel diameter was measured by two independent observers who were ‘blinded’ to the subject’s clinical details and stage of the experiment, as previously described (9,10). Measurements were taken from the anterior to the posterior ‘m’ line at end-diastole, incident with the R-wave on a continuously recorded electrocardiogram. For the reactive hyperemia scan, diameter measurements were taken 45 to 60 s after cuff deflation. The vessel diameter in scans after reactive hyperemia and nitroglycerin administration was expressed as the percentage relative to the average diameter of the artery in the two resting (control) scans (100%). This method has been previously shown to be accurate and reproducible for measurement of small changes in arterial diameter (11), with low interobserver error for measurement of flow-mediated dilation (FMD) and nitrate-induced arterial dilatation (9,11). Endothelial function tested by the described method in the brachial artery is due predominantly to nitric oxide release by the endothelium (12) and correlates well with coronary endothelial function (13).
Serum lipoproteins, homocysteine and insulin. Fasting serum total cholesterol and triglyceride concentrations were measured using standard enzymatic methods (Boehringer Mannheim GmbH, Germany) with a fully automated analyser (Hitachi 917 or 747; Hitachi Ltd., Tokyo, Japan). High-density lipoprotein cholesterol (HDL-cholesterol) was measured by a cyclodextrin-modified enzymatic method (Roche, Switzerland). Serum insulin concentration was measured by microparticles enzyme immunometric assay (MEIMA, Abbott, Japan). Homocysteine levels were measured using reversed phase high performance liquid chromatography using fluorescence detection (14).
Statistical methods. Descriptive data are presented as mean ± SD and significance inferred at two-tailed p < 0.05. The effect of eating was tested by two-way repeated measures analysis of variance. Spearman rank correlation analysis was used to examine the relationships between changes in vascular parameters and the metabolic variables. As the postprandial changes in insulin, triglyceride and HDL-cholesterol levels were all significantly correlated with each other, no attempt was made to analyze the independent predictors of the observed vascular changes by using multivariate models. With 12 subjects enrolled, this study had 80% power to detect a meal-related improvement of 2.5% in FMD at the p < 0.05 level. All statistical analyses were performed by using the Statistical Analysis System (SAS Institute Inc., Cary, North Carolina).
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The increases in FBF, brachial artery diameter and postischemic hyperemia that occurred after eating (0–3 h) were not significantly correlated with each other. The postprandial increase (0–3 h) in resting FBF was closely correlated with increases in serum insulin (r = 0.80, p < 0.002) (Fig. 1A) and triglyceride levels (r = 0.77, p < 0.005) (Fig. 1B). Change in postischemic hyperemia also correlated significantly with the changes in triglycerides (r = 0.69, p = 0.01) as well as HDL-cholesterol levels (r = –0.67, p < 0.02).
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Discussion |
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Eating and blood flow. The effects of eating on mesenteric flow characteristics are well established. There is a substantial increase in intestinal circulation postprandially (15), associated with increases in cardiac output (16) and heart rate (17); therefore, blood pressure is normally maintained. The effects of eating on peripheral blood flow, however, are not clearly established, and previous results in humans have been inconsistent, showing either no change (16,17), reduction (18) or increases (19) in postprandial limb blood flow. Differences in study meals, including physical form (liquid, solid), composition, size and time course of measurements (initial decrease in blood flow, late increase) may explain these paradoxical observations. In this study, we found a significant increase in resting and stimulated forearm blood flow postprandially, consistent with a generalized systemic vascular effect secondary to the metabolic changes occurring after ingestion of food. Changes in cardiac output, not measured in this study, may also have contributed to this observed effect.
Eating and postischemic hyperemia. Postischemic hyperemia after temporary interruption of blood flow is thought to result from an interplay between physical and several locally generated factors, such as prostaglandins, lactic acid, pH, adenosine, carbon dioxide, potassium and nitric oxide (8). It is not predominantly endothelium-dependent (20). Postischemic hyperemia is common to most human vascular beds and has been extensively studied in skeletal muscle and the coronary vasculature, as it indicates the maximum functional vasodilator capacity of these tissues. The effects of a high-fat meal on postischemic hyperemia, however, have not been previously examined. In this study, we measured two variables indicative of postischemic hyperemia: the peak reactive blood flow (maximum flow immediately after ischemia) and the postischemic volume (area under flow vs. time curve, also known as the "debt repaid" after ischemia). The peak flow tended to increase (p = 0.08) after the high-fat meals, and there was a highly significant and marked increase in the total volume repaid, suggesting enhanced meal-induced vasodilatory capacity. The increase in postischemic volume was not merely a consequence of increased basal FBF since these two variables were not significantly related.
High-fat diets and cardiovascular risk. Diets high in saturated fatty acids have typically been associated with increased risk of cardiovascular disease. By contrast, epidemiologic studies suggest that a diet high in monounsaturated fatty acids might be associated with reduced risk. The long-term use of monounsaturated fatty acids may result in lower serum cholesterol levels (21), but it is not known whether the short-term effects of monounsaturated fatty acids on postprandial changes in lipids or arterial physiology differ from those induced by saturated fatty acids. This possibility was examined in this study, demonstrating that both these high fat meals induced similar physiologic and metabolic changes.
Both meals were associated with the expected acute increase in the levels of serum triglycerides and insulin and with a decrease in the levels of free fatty acids, as well as a slight decrease in HDL-cholesterol. Such meal-induced changes in triglycerides and insulin have been well documented previously (4), while a decrease in HDL-cholesterol can be attributed to increased cholesteryl ester transfer in the postprandial phase (22). The small decrease in HDL-cholesterol, therefore, is logical and consistent with the close metabolic link between triglycerides and HDL metabolism (4,22). The postprandial changes in triglycerides, insulin and HDL-cholesterol levels were closely interrelated and separately associated with the increase in resting FBF. Insulin has been shown to increase blood flow and blood volume in human skeletal muscle (23), while the effects of transient changes in serum lipids on blood flow have not been studied.
Endothelial function and cardiovascular risk. Endothelial dysfunction is a key early event in atherosclerosis (9). After the development of noninvasive ultrasound testing to study conduit artery endothelial function (9), which is based on the measurement of flow-mediated endothelium-dependent arterial dilation, the number of investigations in this area has greatly increased. All major coronary risk factors are associated with impaired endothelial function (24). The effects of triglycerides on endothelial function have not been clearly established. Elevated triglycerides have been associated with impaired endothelial function in some (25), but not all, previous studies (26,27). Intralipid infusion, which transiently increases triglyceride and free fatty acid levels, has been shown to decrease both endothelium-dependent and endothelium-independent vascular reactivity in healthy volunteers (28). In patients with isolated hypertriglyceridemia without other lipid risk factors, however, arterial endothelial function remains unimpaired (26,27).
Eating and endothelial function. Observations regarding postprandial changes in endothelial function have been controversial. One group of investigators has shown acute impairment in endothelial function after ingestion of a fatty meal and suggested that this impairment is closely related to postprandial hypertriglyceridemia (5,6). Consistent with this, a recent in vitro study showed that remnant lipoproteins impair endothelium-dependent vasorelaxation in vascular rings (29). The study by Wilmink et al. (30) also demonstrated acute endothelial dysfunction in healthy volunteers after an oral fat load that was prevented by blockade of the renin-angiotensin system. Contrary to these findings, however, Djousse and coworkers (31) were unable to show significant effects of high-fat meals on endothelial function in healthy subjects. Preliminary results from one group have suggested that the impairment in FMD after a high fat meal may be an artefact due to postprandial change in basal arterial tone (7). Our results support this latter observation since both high fat meals were associated with increased brachial artery basal diameter accompanied by increased FBF but not with changes in FMD. A recent study by Williams and coworkers (32) showed impaired endothelial function after a meal rich in used cooking fat but not after unused fat (as in our study), despite similar increases in postprandial triglyceride levels. Therefore, changes in basal arterial tone and the use of potentially toxic heat modified fatty acids, rather than physiological hypertriglyceridemia, may explain some of the earlier observations, suggesting transient postprandial endothelial dysfunction.
We conclude that eating a fatty meal (rich in saturated or monounsaturated fats) induces vasodilation and increases resting and stimulated FBF and that these observations are probably mediated by postprandial changes in insulin or triglyceride levels. The metabolic changes that occur after meals, however, are not associated with impaired endothelial nitric oxide release in the conduit arteries.
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