Arterial Compliance in Obese Subjects Is Improved With Dietary Plant n-3 Fatty Acid From Flaxseed Oil Despite Increased LDL Oxidizability
Abstract The compliance or elasticity of the arterial system, an important index of circulatory function, diminishes with increasing cardiovascular risk. Conversely, systemic arterial compliance improves through eating of fish and fish oil. We therefore tested the value of high intake of α-linolenic acid, the plant precursor of fish fatty acids. Fifteen obese people with markers for insulin resistance ate in turn four diets of 4 weeks each: saturated/high fat (SHF), α-linolenic acid/low fat (ALF), oleic/low fat (OLF), and SHF. Daily intake of α-linolenic acid was 20 g from margarine products based on flax oil. Systemic arterial compliance was calculated from aortic flow velocity and aortic root driving pressure. Plasma lipids, glucose tolerance, and in vitro LDL oxidizability were also measured. Systemic arterial compliance during the first and last SHF periods was 0.42±0.12 (mean±SD) and 0.56±0.21 units based on milliliters per millimeter of mercury. It rose significantly to 0.78±0.28 (P<.0001) with ALF; systemic arterial compliance with OLF was 0.62±0.19, lower than with ALF (P<.05). Mean arterial pressures and results of oral glucose tolerance tests were similar during ALF, OLF, and second SHF; total cholesterol levels were also not significantly different. However, insulin sensitivity and HDL cholesterol diminished and LDL oxidizability increased with ALF. The marked rise in arterial compliance at least with α-linolenic acid reflected rapid functional improvement in the systemic arterial circulation despite a rise in LDL oxidizability. Dietary n-3 fatty acids in flax oil thus confer a novel approach to improving arterial function.
- Received May 22, 1996.
- Accepted October 2, 1996.
Of the conventional risk factors for coronary heart disease, overweight is the most notable risk factor to have become increasingly prevalent.1 Overweight is strongly associated with coronary heart disease mortality,2 3 especially when excess body fat is distributed preferentially within the abdominal region.4 5 Linked to this type of fatness, within the so-called metabolic syndrome, are insulin resistance, dyslipidemia, and hypertension.6 Although each of these is an established risk factor and therefore must contribute to the overall risk posed by overweight, there is a strong additional possibility of a specific effect of fatness on arterial function.
Aortic distensibility or elasticity or compliance is a potential index of arterial function that can now be measured noninvasively. It declines with age,7 increasing blood pressure,7 and diabetes8 as well as through the structural changes of atherosclerosis. From this laboratory, Cameron and Dart9 reported that systemic arterial compliance can be significantly improved after only 4 weeks of exercise training. A short period of fish oil supplementation has also been shown to improve arterial compliance in diabetic subjects.8 Furthermore, we had demonstrated previously that fish oil supplementation inhibited norepinephrine-mediated vasoconstriction,10 which also influences compliance.
The possibility therefore arises that dietary fatty acids, especially the n-3 variety, may improve compliance in overweight subjects in whom plasma lipids and blood pressure tend to be raised.
This may extend beyond fish oil fatty acids to ALA, the essential n-3 fatty acid of plant origin and the precursor of the n-3 fatty acids in fish. In a dietary intervention study in France in persons with coronary heart disease, the inclusion of a rich source of ALA led to significant reduction in cardiac deaths.11
The appropriate diet for subjects with metabolic syndrome is uncertain, apart from energy reduction when overweight is present. There is concern that restricting fat intake might raise plasma triglyceride levels and lower HDL cholesterol in persons already predisposed to such dyslipidemic features. Increasing the oleic acid content of the diet is an alternative strategy, currently under trial in the DELTA study.12 We have chosen to compare the effects of oleic acid and ALA within a fat-restricted diet by testing both against a habitual Australian diet comprising 35% energy fat, predominantly saturated fat.
The primary end point was arterial compliance, which is probably regulated in part by endothelial function.9 13 Because the latter is influenced by plasma cholesterol level14 and by circulating oxidized lipoproteins15 that might be altered by the dietary fatty acids, these aspects were also tested. The subjects were overweight men and women with markers for insulin resistance.
Fifteen middle-aged overweight subjects, 8 men and 7 postmenopausal women, were recruited through advertisement. Inclusion criteria were body mass index of 25 to 36 (kilograms per meter squared), age <65 years, absence of known metabolic disorders other than those resulting from fatness but excluding diabetes, pharmacotherapy that might affect arterial function or glucose metabolism (eg, antihypertensive or anti-inflammatory drugs, hormonal replacement therapy, etc), smoking, and alcohol intake >20 g/d.
The nature of the study was carefully explained, and informed consent was obtained. The study had the approval of the institutional Human Experimentation Committee. Relevant vital data are shown in Table 1⇓. Men and women were of similar age. The men were taller and heavier as expected but had waist-hip ratios similar to those of the women.
Four periods of about 4 weeks each constituted the study: there was a control period at either end and two dietary intervention studies. Because one dietary period (flaxseed oil) was high in ALA and the carryover effects of long-chain n-3 fatty acids extend over many weeks for some biological functions,16 we chose Sunola oil for the first test period in 12 subjects. Three subjects began with flaxseed oil followed by the Sunola oil period to test the extent of any carryover.
At the end of each period, blood was obtained on 2 consecutive days for measurement of fasting-state plasma lipids, glucose, and insulin concentrations. Systemic arterial compliance was measured on one of those 2 days, and this also provided a prolonged continual automated blood pressure reading. On the second day, a glucose tolerance test was carried out over 2 hours, but this was done only at the end of the first three periods.
The subjects maintained their normal activities and were encouraged to exercise consistently and similarly during each period. Supervision was maintained by at least two weekly visits to the unit, supplemented by frequent telephone calls and occasional home spot-checks.
The probable account of the nutrients eaten during the four periods is shown in Table 2⇓. Fat intake during the two control periods was 35% of energy, which is about the Australian average; the proportions of saturated, polyunsaturated (mainly linoleic acid), and monounsaturated fatty acids were close to average also. During the two intervention periods, fat intake fell to 26% of energy, and dietary cholesterol fell more modestly to about 210 mg daily; thus, the diet resembled a National Cholesterol Education Program Step 2 diet, especially since saturated fatty acids were mostly reduced.17
Supplemental experimental foods were provided during the test periods, comprising soft margarine and biscuits and muffins baked from the margarine. Purified deodorized flaxseed oil was the basic oil in one period (percent fatty acid composition of the margarine: palmitic 8.7, stearic 3.7, oleic 15, linoleic 10.5, ALA 36.7, and trans 7.4).
Sunola oil, an oleic acid–rich variant of sunflower oil, provided the basic oil for the other test period (percent fatty acid composition of the margarine: palmitic 9.2, stearic 4.4, oleic 52, linoleic 11, ALA 0.3, and trans 4.6).
Both margarines thus had minor additives of other oils or fats so that the palmitic and linoleic acid contents were similar; linoleic acid was restricted to maximize the effect of ALA. α-Tocopherol was added to equalize its concentration in both margarines and to provide antioxidant to the flaxseed products (final α-tocopherol was 463 mg/kg). The amounts consumed were proportional to the required fat content and were therefore higher for the men: from 20 to 30 g margarine, 1 to 3 biscuits, and 1 to 2 muffins daily. Both types of products were highly acceptable and indeed were difficult to distinguish from one another. Additional prepackaged meals of known composition were provided for evening meals to improve compliance.
The calculations on which Table 2⇑ data are based were derived from 3-day food records (weighed with electronic scales, including 2 weekdays and 1 weekend day). The records were analyzed by a program based on English food tables supplemented with relevant Australian food data.18
Plasma was separated from chilled blood samples and frozen at −80°C. Measurements were carried out in batches for plasma glucose, cholesterol, and triglycerides using enzymatic kits on a Cobas-Bio automated analyzer (Roche). HDL cholesterol was separated from plasma by selective precipitation of other lipoproteins.19 Plasma insulin was measured by enzyme immunoassay (Tosoh AIA-PACK IRI).
LDL Oxidation Studies
These were carried out only in the 12 subjects in whom the initial intervention was with flaxseed oil, and oxidation outcomes were compared in only the two dietary periods. LDL was separated from plasma (stored at −80°C) by rapid isolation using a Beckman Optima TLX bench-top ultracentrifuge (Beckman Instruments). LDL for oxidation experiments was dialyzed at 4°C against PBS (pH 7.4) that had been purged with N2 and sterilized by filtration (0.2 μm).
Oxidation of LDL was determined as the production of conjugated dienes by continuous monitoring of the change in absorbance at 234 nm according to the method of Esterbauer et al.20 Freshly prepared LDL (50 μg protein/mL) was incubated with 5 μm CuSO4 at 37°C in a Beckman DU65 spectrophotometer fitted with a peltier heater (Beckman Instruments). Absorbance at 234 nm was automatically recorded at 2-minute intervals for 120 minutes. Lag time and propagation rate were determined as previously described.20
MDA generated in oxidized LDL and in medium was measured by the TBARS method as described by Buege and Aust21 except that the sample volume was 0.1 mL, the reagent volume was 0.2 mL, and the sample absorbance was measured at 535 nm in a Cobas-Bio automated centrifugal analyzer. The concentration of MDA was calculated using the extinction coefficient for MDA (1.56×105 mol/L−1·cm−1) as previously described. LDL α-tocopherol was measured by high-performance liquid chromatography using the method of Yang and Lee.22 Total cholesterol content of isolated LDL was measured as described above for plasma cholesterol. Fatty acid methyl esters in LDL and in plasma were determined by gas chromatography as previously described.23
Systemic Arterial Compliance
Systemic arterial compliance (SAC) was estimated using the “area method” of Liu et al,24 which requires measurement of volumetric blood flow and associated driving pressure to derive an estimated compliance over the total arterial system according to the following formula: SAC=Ad/[R(Ps−Pd)], where Ad is the area under the blood pressure diastolic decay curve from end systole to end diastole, R is total peripheral resistance, Ps is end-systolic blood pressure, and Pd is end-diastolic blood pressure.
Volume flow was calculated as the product of average systolic flow and aortic root area measured by two-dimensional echocardiography (Hewlett-Packard model 77020A phased-array sector scanner). Continuous ascending aortic flow velocity was measured using a handheld Doppler flow velocimeter (MD1 Multi-Doplex, Huntleigh Technology) placed on the suprasternal notch. This device provides an analog signal proportional to the instantaneous frequency determined by the number of detected zero crossings per unit time of the backscattered Doppler signal, which can be related via the Doppler equation to flow in the ascending aorta. This technique represents an average (approximately root mean square) value for flow, differing from the method used for most clinical applications estimating maximum flow. Because the derived numerical value for flow determined using zero crossing analysis will be less than that obtained invasively, we have chosen to report our results in arbitrary compliance units (dimensionally equal to milliliter per millimeter of mercury).7
Aortic root driving pressure was estimated by applanation tonometry25 of the proximal right carotid artery using a noninvasive Millar Mikro-Tip pressure transducer (model SPT-301). The pressures obtained by this method were calibrated against mean and diastolic brachial pressure measurements made simultaneously using a Dinamap vital signs monitor (1846SX, Critikon). We have previously validated this method against invasively obtained pressure signals9 ; the waveforms generated in the carotid and in the aorta are similar.
Both flow and pressure signals were digitized at 200 Hz using a Data Translation DT 2801 analog-to-digital conversion board. Data were acquired and analyzed with purpose-written software (J.D.C.) using DAOS version 7.1 (Laboratory Software). The computation of compliance proceeds automatically; the observer is required only to ensure stable baselines and consistently reproducible pressure-flow traces and to define end-systolic and end-diastolic points.
The calculations were carried out by an independent expert who is not one of the authors and who was blinded to the extent that he was not aware of the nature of the study at the time. Furthermore, the compliance data are automatically recorded by the computer, the role of the operator being limited to ensuring the quality of the waveforms.
Differences between mean compliance values, as well as plasma lipids, glucose, and insulin concentrations, and calculated dietary intakes for the four treatments were determined by repeated measures ANOVA; if significantly different, contrasts in dietary period means were compared using the paired Student-Newman-Keuls method.
Differences between LDL oxidizability parameters for the Sunola and flaxseed treatments were determined by paired Student's t test. Multivariate analysis was carried out to clarify the interrelationships between lag times of LDL oxidation and the α-tocopherol and fatty acid contents of LDL.
There were four dietary periods during which dietary instructions were carefully explained and supervised by multiple clinic visits, telephone calls, and spot-checks in the subjects' homes. Dietary records that included weighing of eaten foods showed consistency with the overall design (Table 2⇑). The two intervention periods resulted in fat reduction from 35% to 26% of energy, predominantly through restricting saturated fatty acids from 50% to 26% of total fat. These reductions were highly significant. Polyunsaturated fatty acid intake as a percentage of total fat was raised substantially during the flaxseed oil period (mainly n-3) to provide about 13% of total energy. Monounsaturated fatty acid (mainly oleic) provided about 15% of total energy during the Sunola oil period. Dietary cholesterol was also significantly lower during the two intervention periods. Sodium intake was less but not significantly so.
Energy intake was on average higher during the flaxseed oil diet than during the Sunola oil diet (P=.004). This was not reflected in group average body weights, which varied no more than 1 kg between any two consecutive periods. Energy intakes are prone to substantial error by the 3-day recording technique, much more so than that for fat.26
Several individual exceptions to the group averages need mention. Subject 1 failed during the second control period to return to the diet of the first control period: he lost 2.7 kg in 4 weeks by reducing his fat intake further. During the second control period, subjects 3 and 13 ate more monounsaturated and polyunsaturated fatty acid (as proportions of total fat) than planned. However, the dietary records showed that the remainder complied satisfactorily.
Systemic Arterial Compliance
The most striking finding was the increase in arterial compliance with both edible oils, especially with flaxseed oil (Table 3⇓). This is shown for only 13 of the 15 subjects, since the first control periods were missing for subjects 7 (technically unsatisfactory) and 11 (not carried out). Subjects 1 (excess weight loss during the second control period) and 13 (unplanned high polyunsaturated fat intake in the second control period) have been included. Arterial compliance on both oil diets was very significantly higher than during the initial control period and was significantly higher with flaxseed oil than in the second control period or with Sunola. Individual data for the 13 subjects are shown in Fig 1⇓.
The behavior of the three subjects whose initial intervention was with flaxseed oil was similar to that of the 10 who began with Sunola. Because compliance improves (rises) if arterial pressure falls, the values for mean arterial pressure (shown in Table 3⇑) are those computed from the continual automatic measurements made during the compliance studies. Whereas arterial compliance was inversely correlated with mean arterial pressure, especially in the first control period (r=−.60, P<.03), the correlation was no longer significant during the flaxseed period. Furthermore, none of the correlations relating individuals' changes in blood pressure to changes in compliance between any two consecutive periods were significant. Mean pressure did fall significantly from the first control period, from 104 to 91, 91, and 93 mm Hg during the two oil and the second control periods, respectively. Importantly, those values were similar during the two oil periods and the second control period and therefore are not a confounding factor in interpreting those data. The mean arterial pressure for the group of 104±15 mm Hg reflected mild hypertension (mean arterial pressure of 102 to 126 mm Hg) in 10 subjects.
Systemic arterial compliance was not related significantly to any of the parameters describing plasma lipids, glucose, or insulin.
The group averages for plasma total cholesterol, triglyceride, LDL cholesterol, and HDL cholesterol are shown in Table 4⇓. Total and LDL cholesterol values were lower with both oils and more so with Sunola; however, only Sunola lowered both total cholesterol and LDL cholesterol significantly compared with values during the first control period, but only by paired t test and not by ANOVA. (The second control period was less reliable, with three subjects failing to raise saturated fat intake to original levels.)
HDL cholesterol levels were lowest with flaxseed oil, significantly lower than with Sunola or either control period. The fall in HDL cholesterol with flaxseed relative to control may have partly reflected the reduced fat intake. Plasma triglyceride concentrations were similar.
LDL Fatty Acid Profile and Oxidizability
The fatty acid profiles of the plasma lipids during the Sunola, flaxseed, and second control periods are shown in Table 5⇓. ALA rose 10-fold during the flaxseed period. Oleic acid rose proportionately much less with Sunola, which is commonly observed despite high consumption of oleic acid. EPA and DPA (22:5 n-3) more than doubled, but DHA content did not change significantly. As planned, the saturated fatty acid composition of LDL was similar with the two interventions, and the proportions of linoleic acid were not significantly different between Sunola and flaxseed oil. The composition of LDL fatty acids showed similar changes.
To determine whether a carryover of ALA and of EPA plus DPA occurred from the flaxseed oil diet (second intervention in 12 of 15 studies) into the second control period, plasma fatty acid concentrations were analyzed. There was no significant carryover; the respective ALA and EPA percentages with the Sunola, flaxseed, and second control periods were 0.44%, 3.54%, and 0.34% and 0.57%, 1.64%, and 0.82% (showing a trend for EPA to have remained elevated in the second control period but not significantly so).
LDL oxidizability was increased with flaxseed compared with Sunola (Table 6⇓). The lag time (ie, the period before oxidation could be detected) was significantly shorter with flaxseed oil. The significantly higher TBARS (MDA), a major final product of lipid oxidation, is consistent with greater oxidation. However, neither oxidation rate nor diene concentration increased more with flaxseed oil.
There was a significant inverse relationship between ALA content of LDL and lag time (r=−.62, P<.01) (Fig 2⇓). A similar inverse correlation was found between EPA content of LDL and lag time (r=−.53, P<.01), but this was no longer significant when the ALA data were entered into a multivariate analysis. Although the average α-tocopherol concentrations in LDL were not significantly different between the flaxseed and Sunola periods, α-tocopherol contents in LDL from individual subjects were positively correlated with lag times (r=.71, P<.001). Multivariate analysis showed that the effects of ALA and of α-tocopherol were independent of each other. The increased oxidizability with ALA was therefore not due to a lack of α-tocopherol.
Glucose Tolerance Test and Insulin Responses
These were carried out three times, during the first control and the two intervention periods. Fasting plasma glucose concentrations were <6 mmol/L in all individuals on each occasion (group averages: 4.9, 4.7, and 4.7 mmol/L for the three periods). Of the 45 values at 2 hours after glucose, only three were >7.8 mmol/L but <11 mmol/L, an accepted cutoff point for glucose intolerance. Importantly, the group means for the glucose AUC values were virtually identical during the control, Sunola oil, and flaxseed oil periods (808, 804, and 807 units, respectively). By contrast, the insulin response was greater with flaxseed oil than after either the control or Sunola periods. The insulin AUCs (Fig 3⇓) were 8514, 9594, and 10 820 units, respectively; P=.016 flaxseed versus control, P=.092 flaxseed versus Sunola. This was due to significantly higher plasma insulin values at 30 and 60 minutes with flaxseed. The fasting plasma insulin values during the 3 periods were 16.1, 12.4, and 12.0 mU/L on average, respectively, higher than average for healthy lean persons as measured in this laboratory (mean <10 mU/L).
Thus, all subjects showed one or more markers for the insulin resistance syndrome: obesity, low HDL cholesterol, raised triglyceride level, raised blood pressure, or glucose intolerance.
The intervention strategy in this study was two pronged: to reduce fat intake and modify the nature of the fatty acids. The dietary options for managing the metabolic derangements associated with abdominal adiposity have not been clearly defined, other than restricting energy. It is uncertain whether reducing fat, without changing energy, is beneficial or detrimental for insulin sensitivity, which is probably the key abnormality within the metabolic syndrome. High fat intake can induce insulin resistance in rats27 and possibly in both normal28 and diabetic subjects.29 Nevertheless, substituting carbohydrates for fat is generally recommended for treating diabetes. Substituting monounsaturated for saturated fatty acids has successfully improved diabetic management,30 and insulin sensitivity appears to be positively correlated with the higher unsaturated fatty acid concentrations in skeletal muscle,31 the major site for glucose removal. Whereas only three subjects in this study were glucose intolerant, the mean fasting plasma insulin concentration (16.1 mU/L) was about 50% higher than in a large number of lean men and women.
The characteristic dyslipidemia also requires energy restriction, but in the context of a eucaloric, stringently fat-reduced diet, both monounsaturated and polyunsaturated long-chain fatty acids may not prevent lowering of HDL cholesterol.32 In the present group, the mean HDL cholesterol concentration (1.06±0.29 mmol/L) was low, and plasma triglycerides were at the upper level of normal.
Because arterial compliance was a key measurement in the present study, we also took into consideration that highly unsaturated long-chain fatty acids have been reported to improve arterial compliance in diabetic subjects8 and to oppose the vasoconstrictive effects of nor-epinephrine10 ; ie, fish oils increase blood flow and lower vascular resistance.
We therefore chose for one intervention a reduced-fat oleic acid–rich diet, and for the second intervention we chose a similar reduction in fat but with ALA as the candidate fatty acid. The average consumption of ALA (20 g daily) is the largest reported for studies of this nature. We had previously found that 9 g did not influence plasma lipids or blood pressure in normal subjects, in whom half that amount of fish oil lowered triglycerides and blood pressure and raised HDL cholesterol.33 Others have also failed to observe any effect on plasma lipids with 14 g daily.34 The proportion of ALA converted to longer-chain highly unsaturated fatty acids is unknown; we observed a doubling of plasma EPA with 9 g ALA per day (and little change in DHA). The higher consumption of ALA in this study led to proportionately higher LDL (and plasma) EPA (Table 5⇑), with little change in DHA content.
The most impressive finding was a substantial rise in systemic arterial compliance with flaxseed oil, which was of the order observed by Cameron and Dart9 to have been achieved through exercise training. It was greater than that reported by McVeigh et al8 in diabetic subjects after 6 weeks of fish oil supplementation (3 g n-3 fatty acid). Although their method of determining arterial compliance was not identical to the one used in this study, it was also based on an analysis of the diastolic portion of the pressure-pulse waveform.35 The effect of Sunola on compliance is not as clear. Although higher than during the first control period, blood pressures were lower, which in itself would improve compliance.36 Blood pressures were then similar during the Sunola, flaxseed, and second control periods. Because arterial compliance was better with flaxseed oil than during any other period, those data are clear-cut. Compliance was not significantly higher with Sunola than during the second control period when blood pressures were equal. Although the second control period was slightly distorted because three subjects failed to fully resume their initial intake of saturated fat, this is unlikely to have altered the conclusion that oleic acid probably did not improve arterial compliance. For all individuals, compliance “tracked” well between the second intervention and second control period (r=.72, P<.003), making it unlikely that the behavior of these three subjects influenced unduly the interpretation of the final period. Blood pressure tends to fall during the initial periods of many trials, and we therefore do not ascribe the lower pressures during the three latter periods to dietary intervention. The critical point is that differences in pressures were not a confounding factor in the interpretation of the compliance results when flaxseed oil was the supplement because (1) there was no correlation among individuals between the changes in arterial pressure and changes in arterial compliance between any two consecutive periods, and (2) the correlation between blood pressure and compliance was significant with the first control diet but no longer so with the flaxseed oil diet. There was no significant carryover from the flaxseed oil into the second control period as shown by LDL and plasma ALA and EPA concentrations. Other measures of arterial compliance have suggested a benefit from eating fish in studies from Japan37 and Australia.38 By contrast, arterial stiffness appears to rise with age.7
Several aspects deserve discussion: the characteristics of arterial compliance, the mechanisms responsible for the change with at least flaxseed oil, and the rapidity (4 weeks) with which that change occurred. Arterial compliance represents the volume-pressure relationship within the arterial circulation or the distensibility or elastic properties of the proximal arterial system to changes in volume and pressure. Thus, stiffness increases and compliance decreases with clear structural changes, but arterial wall properties are also influenced by functional characteristics such as the tonicity of the artery or changes in pressure. The rapidity with which flaxseed oil improved compliance indicates an effect on the functional components that determine compliance. Because endothelial events influence the smooth muscle layer in the artery and because endothelial function is rapidly modifiable, we favor a mechanistic change based on endothelium-related arterial relaxation.
Fish oil n-3 fatty acids affect vascular relaxation almost certainly by changing endothelial function, although sympathetic nervous activity that can also be altered by dietary fish oil may contribute to the reduction in vascular resistance.39 We have previously shown that fish oil supplements inhibit the reduction in forearm arterial blood flow that occurs when norepinephrine or angiotensin are infused.10 Fish oil fatty acids also modify eicosanoid metabolism, which in turn might affect arterial compliance. Through which of these mechanisms the flaxseed oil–induced rise in compliance occurred was not studied in the present dietary trial. It is likely, however, that the conversion of ALA to EPA was partly responsible, although ALA may itself exert some of the properties of its longer-chain, more unsaturated product.
One further consideration is that the test subjects were all overweight. In parallel studies in most of these subjects, we have found significant correlations between body fatness and the vascular responsiveness to norepinephrine (an effect on smooth muscle cells) and inversely with postischemia-mediated dilation of the forearm arterial circulation, an index of endothelial function. Therefore, we do know that these subjects demonstrated endothelial dysfunction and excess vascular reactivity attributable to fatness. It is therefore possible that ALA may be particularly valuable in overweight persons with metabolic syndrome, in whom the risk of arterial disease may thus be lessened.
The beneficial effect of ALA on compliance occurred despite a small but significant fall in HDL cholesterol and increased LDL oxidizability in vitro. Both oppose endothelium-mediated dilation.39 40 A theoretical disadvantage of a high intake of ALA and polyunsaturated fatty acids in general is increased vulnerability to oxidation. At least in vitro, that possibility was confirmed. It is interesting that if this also applied in vivo, it did not apparently adversely affect arterial compliance. An end product of lipid oxidation, MDA, was significantly higher with LDL when the subjects consumed flaxseed oil (Table 6⇑). The lag time was significantly shortened despite not significantly different contents of α-tocopherol. Decreased lag time reflects less antioxidant, but it can also indicate higher utilization of an antioxidant, which is consistent with increased generation of MDA. That in vitro oxidizability did not translate into adverse arterial compliance raises the relevance of increased LDL oxidizability in vitro and the risk of vascular complications. We have also reported increased oxidizability and uptake by macrophages of LDL in persons taking fish oil supplements41 ; yet, high consumption of fish and fish oil opposes other biological factors that predispose to vascular disease.42 The apparent paradox of flaxseed oil markedly improving arterial compliance in vitro despite raising in vitro LDL oxidizability resembles that seen with fish oil.
In the present study, plasma total and LDL cholesterol were lowest with the Sunola/low fat diet, but with the flaxseed oil/low fat diet these lipids were not significantly different from control (Table 4⇑), although flaxseed flour has been reported to lower cholesterol.43 On the other hand, the flaxseed oil/low fat diet gave the lowest HDL cholesterol level; this may have been due to the reduction in fat that was apparently prevented by oleic acid but not by ALA.
Whereas glucose disposal, as shown by the virtually identical glucose AUC values, was not affected by either oil, there appeared to be some deterioration in insulin sensitivity (Fig 3⇑). This was statistically significant with flaxseed oil and has not been reported previously. On the other hand, fish oil supplementation has at times been found to impair insulin sensitivity. This issue is unresolved, with reports of both impaired and unaffected glucose tolerance after fish oil consumption.44 45 46
This study has demonstrated for the first time that a seed oil rich in n-3 fatty acid can benefit an important index of systemic arterial biology. It is an index that deteriorates with age and in diabetics. The rapidity of improvement focuses on the reversibility of vascular functions by dietary fatty acids. The present findings cannot distinguish between a direct effect of ALA or indirect influence through the generation of EPA. They do provide further support for increasing the consumption of n-3 fatty acids.
Selected Abbreviations and Acronyms
|AUC||=||area under the curve|
|TBARS||=||thiobarbituric acid–reactive substances|
Support was provided by the Grains Research and Development Corporations, Meadow Lea Foods, the National Heart Foundation, and the Manpei Suzuki Diabetes Foundation (Dr Sasahara). Foods were generously manufactured and provided by Meadow Lea Foods, Sydney, Australia (Drs R. Bowrey and C. Eyers). We thank E. Dewar for echocardiography measurements and Dr L. Liang for help with the compliance studies.
Hubert HB, Manning F, McNamara PM, Castelli WP. Obesity as an independent risk factor for cardiovascular disease: a 26-year follow-up of participants in the Framingham Heart Study. Circulation. 1983;67:968-977.
Larsson B, Bengtsson C, Bjorntorp P, Lapidus L, Sjostrom L, Svardsudd K, Tibblin G, Wedel H, Welin L, Wilhelmsen L. Is abdominal fat distribution a major explanation for the sex difference in the incidence of myocardial infarction? The study of men born in 1913 and the study of women, Göteborg, Sweden. Am J Epidemiol. 1992;135:266-273.
McVeigh GE, Brennan GM, Cohn JN, Finkelstein SM, Hayes RJ, Johnston GD. Fish oil improves arterial compliance in non–insulin-dependent diabetes mellitus. Arterioscler Thromb. 1994;14:1425-1429.
Cameron JD, Dart AM. Exercise training increases total systemic arterial compliance in humans. Am J Physiol. 1994;266:H693-H701.
Chin JFP, Gust AP, Nestel PJ, Dart AM. Marine oils dose-dependently inhibit vasoconstriction of forearm resistance vessels in humans. Hypertension. 1993;21:22-28.
Ershow AG, for the DELTA Investigation. Dietary effects on lipoproteins and thrombogenic activity in subjects with markers for insulin resistance (DELTA-2). FASEB J. 1996;10:A462. Abstract.
Egashira K, Hirooka Y, Kai H, Sugimachi M, Suzuki M, Inou T, Takeshita A. Reduction in serum cholesterol with pravastatin improves endothelium-dependent coronary vasomotion in patients with hypercholesterolemia. Circulation. 1994;89:2519-2524.
Anderson TJ, Meredith IT, Yeung AC, Frei B, Selwyn AP, Ganz P. The effect of cholesterol-lowering and antioxidant therapy on endothelium-dependent coronary vasomotion. N Engl J Med. 1995; 332:488-493.
Tremoli E, Maderna P, Marangoni F, Colli S, Eligini S, Catalano I, Angeli MT, Pazzucconi F, Gianfranceschi G, Davi G, Stragliotto E, Sirtori CR, Galli C. Prolonged inhibition of platelet aggregation after n-3 fatty acid ethyl ester ingestion by healthy volunteers. Am J Clin Nutr. 1995;61:607-613.
Composition of Foods Australia (COFA). Canberra, Australia: AGPS; 1989.
Warnick GR, Benderson J, Albers JJ. Dextran sulfate-Mg2+ precipitation procedure for quantitation of high-density-lipoprotein cholesterol. Clin Chem. 1982;28:1379-1388.
Esterbauer H, Dieber-Rotheneder M, Striegl G, Waeg G. Role of vitamin E in preventing the oxidation of low-density lipoprotein. Am J Clin Nutr. 1991;53:314S-321S.
Yang CS, Lee MJ. Methodology of plasma retinol, tocopherol and carotenoid assays in cancer prevention studies. J Nutr Growth Cancer. 1987;4:19.
Abbey M, Belling GB, Noakes M, Hirata F, Nestel PJ. Oxidation of low-density lipoproteins: intraindividual variability and the effect of dietary linoleate supplementation. Am J Clin Nutr. 1993;57:391-398.
Liu Z, Brin KP, Yin FC. Estimation of total arterial compliance: an improved method and evaluation of current methods. Am J Physiol. 1986;251:H588-H600.
Kelly R, Hayward C, Avolio A, O'Rourke M. Noninvasive determination of age-related changes in the human arterial pulse. Circulation. 1989;80:1652-1659.
Jonnalagadda SS, Mitchell DC, Smicklas-Wright H, Kris-Etherton PM, Van Heel N, Kamally W. Underestimation of energy intake: comparison of self-reported and actual intake. FASEB J. 1996;10:A724. Abstract.
Storlien LH, Jenkins AB, Chisholm DJ, Pascoe WS, Khouri S, Kraegen EW. Influence of dietary fat composition on development of insulin resistance in rats: relationship to muscle triglyceride and ω-3 fatty acids in muscle phospholipid. Diabetes. 1991;40:280-289.
Lichtenstein AH, Ausman LM, Carrasco W, Jenner JL, Gualtieri LJ, Goldin BR, Ordovas JM, Schaefer EJ. Effects of canola, corn, and olive oils on fasting and postprandial plasma lipoproteins in humans as part of a National Cholesterol Education Program Step 2 diet. Arterioscler Thromb. 1993;13:1533-1542.
Kestin M, Clifton P, Belling GB, Nestel PJ. n-3 fatty acids of marine origin lower systolic blood pressure and triglycerides but raise LDL cholesterol compared with n-3 and n-6 fatty acids from plants. Am J Clin Nutr. 1990;51:1028-1034.
Mantzioris E, James MJ, Gibson RA, Cleland LG. Dietary substitution with an α-linolenic acid-rich vegetable oil increases eicosapentaenoic acid concentrations in tissues. Am J Clin Nutr. 1994;59:1304-1309.
Finkelstein SM, Collins VR, Cohn JN. Arterial vascular compliance response to vasodilators by Fourier and pulse contour analysis. Hypertension. 1988;12:380-387.
Zeiher AM, Schachinger V, Hohnloser SH, Saubier B, Just H. Coronary atherosclerotic wall thickening and vascular reactivity in humans: elevated high-density lipoprotein levels ameliorate abnormal vasoconstriction in early atherosclerosis. Circulation. 1994; 89:2525-2532.
Suzukawa M, Abbey M, Howe PRC, Nestel PJ. Effects of fish oil fatty acids on low density lipoprotein size, oxidizability, and uptake by macrophages. J Lipid Res. 1995;36:473-484.
Fasching P, Ratheiser K, Waldhäusl W, Rohac M, Osterrode W, Mowotny P, Vierhapper H. Metabolic effects of fish-oil supplementation in patients with impaired glucose tolerance. Diabetes. 1991;40:583-589.
Eritsland J, Arnesen H, Seljeflot I, Høstmark AT. Long-term metabolic effects of n-3 polyunsaturated fatty acids in patients with coronary artery disease. Am J Clin Nutr. 1994;61:831-836.