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Effect of dietary fatty acids on the postprandial fatty acid composition of triacylglycerol-rich lipoproteins in healthy male subjects

Abstract

Objective: The aim of the present study was to investigate the effect of trans-18:1 isomers compared to other fatty acids, especially saturates, on the postprandial fatty acid composition of triacylglycerols (TAG) in chylomicrons and VLDL.

Design: A randomised crossover experiment where five interesterified test fats with equal amounts of palmitic acid (P fat), stearic acid (S fat), trans-18:1 isomers (T fat), oleic acid (O fat), or linoleic acid (L fat) were tested.

Subjects: A total of 16 healthy, normolipidaemic males (age 23±2 y) were recruited.

Interventions: The participants ingested fat-rich test meals (1 g fat per kg body weight) and the fatty acid profiles of chylomicron and VLDL TAG were followed for 8 h.

Results: The postprandial fatty acid composition of chylomicron TAG resembled that of the ingested fats. The fatty acids in chylomicron TAG were randomly distributed among the three positions in accordance with the distributions in test fats. Calculations of postprandial TAG concentrations from fatty acid data revealed increasing amounts up to 4 h but lower response curves (IAUC) for the two saturated fats in accordance with previous published data. The T fat gave results comparable to the O and L fats. The test fatty acids were much less reflected in VLDL TAG and there was no dietary influence on the response curves.

Conclusions: The fatty acid composition in the test fats as well as the positional distributions of these were maintained in the chylomicrons. No specific clearing of chylomicron TAG was observed in relation to time.

Sponsorship: Danish Research Development Program for Food Technology.

Introduction

The responses of plasma lipoproteins following absorption of dietary fats have gathered increasing interest due to the fact that human beings, by eating regular meals, spend most of the time in the postprandial phase. Consequently, this state is presumably more representative of the physiological status than the fasting situation. Plasma triacylglycerols (TAG) increase shortly after the first meal of the day and return to baseline concentrations several hours after the last meal. Intestine-derived chylomicrons as well as liver-derived very low density lipoproteins (VLDL), which both contribute to the postprandial responses (Cohn et al, 1988, 1989; Potts et al, 1991), compete for hydrolysis by lipoprotein lipase (LPL) as well as the hepatic receptor mediated removal of remnants from plasma (Brunzell et al, 1973; Chen & Reaven, 1991; Schneeman et al, 1993; Karpe & Hultin, 1995). Previous studies have shown that the magnitude of postprandial lipaemia, especially the presence of remnant particles, plays an important role in the pathogenesis and progression of atherosclerosis (Zilversmith, 1979; Simons et al, 1987; Groot et al, 1991; Patsch et al, 1992; Phillips et al, 1993; Karpe et al, 1994). Delayed clearance of triacylglycerol-rich lipoproteins (TRL) circulating in the blood is therefore associated with increased atherogenic risk.

The composition and positional distribution of fatty acids in dietary fats are important factors of fat digestion and absorption. Gastric and pancreatic lipases hydrolyse fatty acids from the sn-1 and sn-3 positions of dietary TAG yielding 2-monoacylglycerols (2-MAG) and free fatty acids (FFA) (Small, 1991; Carriere et al, 1993). Absorption of 2-MAG and FFA into the enterocytes is followed by re-esterification and incorporation of TAG in chylomicrons. The absorbed TAG molecules therefore retain the fatty acids in the sn-2 position as in the dietary TAG, whereas the fatty acids in the sn-1 and sn-3 positions are randomised and partly substituted by endogenous fatty acids (Mattson & Volpenhein, 1964).

The influence of trans-fatty acids in the diet has been discussed during recent years. Human experiments (Mensink et al, 1992; Nestel et al, 1992; Lichtenstein et al, 1993; Wood et al, 1993; Judd et al, 1994) have pointed to an unfavourable effect of trans-fatty acids on blood lipids. An epidemiological study performed by Willett et al (1993) showed a relation between intake of trans-fatty acids from partially hydrogenated vegetable oils and risk of coronary heart disease, whereas no correlation to butter consumption was seen. However, in the prospective, population-based Zutphen Elderly Study published by Oomen et al (2001), a high intake of all isomers of trans-fatty acids was strongly associated with risk of coronary heart disease.

The increased evidence for a correlation between postprandial lipaemia and atherogenicity makes a comparison of the absorption and clearing of TRL after ingestion of various fats interesting. The aim of the present study was to investigate the postprandial effect of various fatty acids, including trans-18:1 isomers, which were especially compared to saturated fatty acids (SFA), on the fatty acid profiles of TAG in chylomicrons and VLDL. This is important as trans-fatty acids, present in food products that contain partially hydrogenated oils and ruminant fats, are often substituted by the atherogenic SFA due to their similar physical properties. Five interesterified test fats with almost equal amounts of the target fatty acids: palmitic acid, stearic acid, trans-18:1 isomers, and for comparison also oleic acid and linoleic acid referred to as the P, S, T, O, and L fats, respectively, were studied. Among the other fatty acids present in the test fats, oleic acid was the major component representing almost the same amount as the target fatty acid. In addition, the positional distributions of the fatty acids in the test fats were similar for all fatty acids due to interesterification of the fats. In contrast to earlier experiments, the postprandial effects in this study can be ascribed solely to the content of target fatty acids in the test fats and not to different positions or other fatty acids present. Results of the effect on postprandial lipid profile (including TAG and cholesterol concentrations), plasma fatty acids, lipoprotein lipase, and cholesterol ester transfer activities have been published elsewhere (Tholstrup et al, 2001).

Methods

Subjects

A total of 16 healthy male subjects were recruited among students from The Royal Veterinary and Agricultural University at Copenhagen. They were selected and matched according to the following criteria: (1) age 21–30 y, (2) body mass index (BMI) <28.5 kg/m2, (3) fasting plasma TAG concentration <1.7 mmol/l, (4) fasting plasma cholesterol concentration <6.0 mmol/l, (5) smoking <10 cigarettes per day, and (6) vigorous exercise <3 h per day.

The 16 males recruited were between 21 and 28 y (23±2 y) and they had a mean BMI of 23±2 kg/m2 (range: 20–28 kg/m2). All subjects were in good health and none was taking medications that could affect plasma lipids. Fasting plasma TAG and cholesterol concentrations at the start of the study were 0.8±0.3 mmol/l (0.4–1.3 mmol/l) and 4.0±0.5 mmol/l (3.1–5.1 mmol/l), respectively (Tholstrup et al, 2001). One of the participants missed the T fat.

The study was carried out according to the Helsinki II Declaration and approved by the Ethics Committee of the County of Copenhagen, Denmark. All subjects gave written informed consent before participation in the trial.

Design

The human experiment was performed as a randomised crossover design with at least 3 weeks between the postprandial studies. The subjects received a controlled diet in 48 h before being tested to obtain a uniform starting level. Standardised food items consisting of ready-made dinners, bread, cakes, and margarine were delivered to the subjects. The foods matched a typical Danish diet with 44 E% total fat: 40% SFA, 41% monounsaturated (MUFA), and 19% polyunsaturated fatty acids (PUFA) (Tholstrup et al, 2001). When the participants arrived early in the morning, they had been fasting for 12 h and asked to refrain from smoking during the fasting period, from drinking alcohol during the preceding 48 h and from heavy exercise during the past 36 h. They were served a test meal containing 51 E% fat, 6 E% protein and 43 E% carbohydrate. Protein and carbohydrate content was derived from mashed potatoes and juice. High oleic sunflower oil, high linoleic sunflower oil, partially hydrogenated high oleic sunflower oil (all from Aarhus Oliefabrik A/S, Aarhus, Denmark), pure tripalmitin, tristearin, and triolein (Hüls, Marl, Germany) were used as ingredients in test fats. The fats were mixed to give the same amounts of the fatty acids to be tested and then interesterified. The high fat meals, prepared by incorporating the test fats in mashed potatoes (1 g fat and 1 g potato flakes per kg body weight resulting in 65–80 g per subject)), were eaten within 15 min. The postprandial fatty acid composition of chylomicron and VLDL TAG was followed by taking venous blood samples (EDTA Vacutainer) just before and 2, 4, 6, and 8 h after ingestion of the test meal. Water but no food or other beverages was allowed throughout the 8-h study period. The test meals were well tolerated by all subjects.

Analysis of lipids

TRL fractions (chylomicrons (Svedberg flotation unit (Sf)>400) and VLDL (density (d) <1.006 kg/l)). were isolated by ultracentrifugation of the EDTA plasma as described by Tholstrup et al (1998). Briefly, plasma samples were overlaid with saline solution (d=1.006 kg/l) in ultracentrifuge tubes and centrifuged at 100 000 × g for 23 min at 20°C. After tube-slicing, the top fractions (Sf>400) contained primarily chylomicrons, but contamination by small amounts of apo-B-100-containing lipoproteins cannot be excluded. The bottom fractions (Sf<400) were readjusted with saline solution to a d of 1.006 kg/l and centrifuged at 170 000 × g for 16 h at 4°C. The top fractions (d < 1.006 kg/l) contained VLDL and chylomicron remnants. Total lipids were extracted from chylomicron and VLDL fractions with chloroform:methanol 1:1 (vol/vol). TAG were separated from other lipids by thin-layer chromatography (TLC) on silica plates (DC-Fertigplatten Kieselgel 60, Merck, Germany) with the solvent mixture hexane:diethyl ether:glacial acetic acid 80:20:1 (vol/vol/vol). The lipids were visualised with 2′,7′-dichlorofluorescein in 96% ethanol and the TAG bands were scraped off, saponified with 0.5 M NaOH in methanol and subsequently methylated with 14% (wt/vol) BF3 in methanol in the presence of 0.02% (wt/vol) hydroquinone as an antioxidant. Heptadecanoic acid (17:0) was used as internal standard. The fatty acid methyl esters were extracted with heptane and analysed by gas–liquid chromatography (GLC) on a Hewlett-Packard 5880A instrument with flame ionisation detector (FID) and a SP2380 capillary column (30 m × 0.32 mm i.d., 0.2 μm film; Supelco, Bellefonte, PA, USA). The initial oven temperature was 140°C. The temperature was raised to 160°C at 2°C/min, then held constant for 2 min and finally raised 3°C/min to 200°C, where it was unchanged for 5 min. The inlet pressure of the carrier gas (helium) was 76 kPa. The injector (split mode) and the detector were maintained at 250 and 260°C, respectively.

The cis- and trans-isomers of 18:1 were determined by GLC on a Fison 8160 instrument equipped with FID and a CP-Sil 88 capillary column (100 m × 0.25 mm i.d., 0.2 μm film; Chrompack, Middelburg, The Netherlands). The initial oven temperature of 170°C was held for 50 min and then raised to 225°C at 4°C/min. The inlet pressure of the carrier gas (helium) was 210 kPa. The injector (split mode) and the detector were maintained at 275°C.

To obtain larger fractions of chylomicrons for determination of the fatty acid composition in the sn-2 position of TAG, the 4-h postprandial samples were prepared by a method slightly modified from Mackness and Durrington (1992) and described by Jensen et al (1999). The positional distributions were determined by a procedure modified from Christie (1982). In brief, Tris buffer (1 M tris(hydroxymethyl)methylamine pH 8.0), calcium chloride solution, and taurocholate solution were added to the extracts and equilibrated at 40°C for 1 min before pancreatin (Merck, Darmstadt, Germany) was added. After vigorous mixing for 5 min at 40°C, 6 M hydrochloric acid was applied to stop the hydrolysis and the solutions were extracted with diethyl ether. 2-MAG were separated from other lipids by TLC on silica plates with the solvent mixture hexane:diethyl ether:glacial acetic acid 50:50:1 (vol/vol/vol). The lipids were visualised with 2′,7′-dichlorofluorescein in 96% ethanol and the 2-MAG bands were scraped off and treated as described above for TAG bands.

Calculations

The levels of TAG in chylomicrons and VLDL were calculated from the fatty acid composition using 17:0 as internal standard for quantification. The amount of the glycerol residue was also taken into account. The total TAG content of the lipoproteins postprandially, corrected for fasting TAG concentration (incremental area under the TAG curve, IAUC), was calculated by the trapezoidal method (Matthews et al, 1990).

Statistical analysis

Results in the tables are expressed as mean±standard deviation (s.d.). The statistical evaluation was performed with the SAS program (6.12) for UNIX (SAS Institute, Inc., Cary, NC, USA) using the generalised linear models (GLM) or with the SigmaStat statistical software. Differences in the fasting fatty acid composition of chylomicrons and VLDL, respectively, were assessed by one-way repeated measures analysis of variance (ANOVA). Postprandial changes in the fatty acid profiles of the two TRL fractions, respectively, were tested by repeated measures ANOVA as well. Differences in chylomicron and VLDL IAUC for the five test fats were tested by two-way ANOVA, and repeated measures ANOVA were used to determine the statistical significance between measurements obtained from fasting and postprandial samples. If significance was obtained, the two-way or repeated measures ANOVA were followed by Student–Newmann–Keuls method (all values compared two by two) or Dunnett's method (postprandial vs fasting values). Differences were considered significant at P<0.05.

Results

Test fats

To obtain equal quantities of the target fatty acids to be compared, five different test fats were mixed from the selected fats and oils. After interesterification, the test fat TAG contained 43–47 wt% of the target fatty acid and 43–46 wt% cis-18:1 (Figure 1). The notation P, S, T, O, and L fats refers to the target fatty acid (palmitic acid, stearic acid, trans-18:1 isomers, oleic acid, and linoleic acid, respectively) present in high concentration in the test fat. Oleic acid was the major cis-18:1 isomer in all the test fats, including the T fat. The trans-18:1 isomers detected in the T fat were ranging from 5t- to 12t-18:1 with about 19% elaidic acid. The remaining fatty acids in the test fats were nearly the same. The P, T, O, and L fats contained 4.5–4.8 wt% 18:0 and the P, S, O, and L fats only trace amounts of trans-18:1. The levels of 16:0 varied from 2.6 to 5.2 wt% and the amounts of 18:2 n-6 from 1.8 to 6.3 wt%. The relatively low content of linoleic acid in the T fat was not considered critical since the test fat was only ingested once and not for a longer study period.

Figure 1
figure1

Major fatty acids (wt% of total) in test fat triacylglycerols (TAG) and the sn-2 position of test fat TAG (sn-2 TAG).

Comparison of the fatty acids in the sn-2 position of TAG (Figure 1) with the total distributions in TAG showed that the fatty acids were randomly distributed among the three positions in TAG.

Fatty acid composition of TAG in chylomicrons and VLDL

The major fatty acids in chylomicron and VLDL TAG are presented in Tables 1, 2, 3, 4 and 5. The fatty acid composition of chylomicron TAG in the fasting samples was quite similar for the five test days except for small variations in the contents of cis-18:1 (37.8±2.8–40.1±1.8 wt%). No differences in the fatty acid profiles of fasting VLDL TAG between the five test days were observed.

Table 1 Major fatty acids (wt% of total) in chylomicron and VLDL TAG 0–8 h after the Pa fat challenge
Table 2 Major fatty acids (wt% of total) in chylomicron and VLDL TAG 0–8 h after the Sa fat challenge
Table 3 Major fatty acids (wt% of total) in chylomicron and VLDL TAG 0–8 h after the Ta fat challenge
Table 4 Major fatty acids (wt% of total) in chylomicron and VLDL TAG 0–8 h after the Oa fat challenge
Table 5 Major fatty acids (wt% of total) in chylomicron and VLDL TAG 0–8 h after the La fat challenge

In chylomicron TAG, significant increases were found in the percentages of all target fatty acids in the postprandial samples indicating that already after 2 h the incorporation of test fats was playing a major role. Maximum percentages of target fatty acids were reached after 4 h. For the P, T, O, and L fats, the relative amounts of target fatty acids in chylomicron TAG were 87–93% of the percentages in the test fats, whereas the relative amount of stearic acid in chylomicron TAG only constituted 66% of the value in the S fat. In the 6- and 8-h postprandial samples, the fatty acid composition showed a tendency to return to baseline values, but they were still far from these levels. The fatty acid profiles after 8 h were more like those after 2 h. The distribution of the trans-18:1 isomers present in the T fat was reflected in chylomicron TAG in all postprandial samples (data not shown). Minor increases were observed in the proportions of the 11t-18:1 isomer, whereas the 12t-18:1 isomer was slightly discriminated against in chylomicron TAG compared to the presence in the T fat.

As for chylomicron TAG, the percentages of all target fatty acids in VLDL TAG were significantly increased in the postprandial samples. However, maximum influence of the test fat values was first observed after 6 h. For the P, O, and L fats, the relative amounts of target fatty acids in VLDL TAG were 67–70% of the percentages in the test fats, whereas the corresponding figures were 19% and 31% in the S and T fats, respectively. The fatty acid composition in the 8-h postprandial samples was quite similar to those after 6 h. The distribution of the trans-18:1 isomers in the T fat was not reflected to the same extent in VLDL TAG as in chylomicron TAG (data not shown). Relatively lower proportions of the 6t- to 8t-18:1 and 12t-18:1 isomers and correspondingly higher proportions of the 9t-18:1 and 11t-18:1 isomers were found in VLDL TAG than in the T fat.

The composition of chylomicron TAG at 2, 4, 6, and 8 h after the test meals thus resembled those of the ingested fats for all groups. The fatty acid distributions in test fats were also reflected in VLDL TAG but far from the extent seen in chylomicron TAG.

Positional distributions of TAG in chylomicrons

The major fatty acids (about 95 wt%) in TAG and the sn-2 position of TAG from chylomicrons 4 h after the test meals are shown in Figure 2. Besides, small amounts of 14:0, 16:1 n-7, 18:3 n-3, 20:4 n-6, and 22:6 n-3 were present, each less than 1 wt%. As observed for the test fats, the fatty acids in chylomicron TAG were randomly distributed among the sn-2 position and the outer positions. The chylomicron TAG formed had almost the same fatty acids in the sn-2 position as found in the test fats (Figure 1).

Figure 2
figure2

Major fatty acids in chylomicron triacylglycerols (TAG) and the sn-2 position of chylomicron TAG (sn-2 TAG) (4-h samples).

Calculation of total TAG responses in chylomicrons and VLDL from fatty acid data

The total amounts of TAG in the two lipoprotein fractions calculated from GLC results and quantified using 17:0 as internal standard were followed for 8 h (Table 6). The results agreed with the spectrophotometric data we previously reported (Tholstrup et al, 2001). No differences were observed in the fasting values of either chylomicron or VLDL TAG for the five test fats. The chylomicron TAG contents increased significantly after 2 h and remained with significantly higher values during the postprandial period tested except for the 8-h values for the O and L fats. The mean chylomicron TAG values peaked at 4 h corresponding to the maximum percentages of target fatty acids.

Table 6 Time course of total TAG in chylomicrons and VLDL as calculated from fatty acid data

For VLDL, the mean values for TAG were also highest after 4 h except for the O fat (max. at 2 h) even though maximum influence of the test fat values on the fatty acid profiles was first observed after 6 h. The VLDL TAG contents were significantly increased after 2 and 4 h compared to the fasting values for all five test fats, but only the P and S fat levels remained significantly higher after 6 h. Contrary to the P fat content returning to baseline values at 8 h, the TAG levels for the S, T, O, and L fats were significantly lower at the end of the study than the starting values.

As a measure of the postprandial response during 8 h, the incremental areas under the TAG curves (IAUC) were calculated and given as mean±s.d. for the five test meals (Table 7). The chylomicron data for the S fat were significantly lower than seen for the T, O, and L fats, and the P fat showed significantly lower values than the T and O fats. On the contrary, there were no significant differences in the effect of the test fats on the VLDL TAG values. In both cases, great individual variations were observed. The values calculated for chylomicrons were almost 10 times the ones for VLDL.

Table 7 Postprandial response to fat load in chylomicrons and VLDL

Discussion

The present study dealt with the postprandial effect of different dietary fatty acids on fatty acid composition of TAG in chylomicrons and VLDL. As expected, the fasting fatty acid composition of chylomicron and VLDL TAG, respectively (Tables 1, 2, 3, 4 and 5), was almost similar for all test days, which is due to the standardised diet consumed for 2 days before the postprandial studies. As seen from the fatty acid profiles of chylomicron TAG in the postprandial samples, all test fatty acids, including trans-18:1, were extensively incorporated into the chylomicron TAG. This is in agreement with earlier human studies of postprandial changes in the fatty acid composition of chylomicron TAG (Bonanome & Grundy, 1989; Gibney & Daly, 1994; Nestel et al, 1995; Shishehbor et al, 1998; Hunter et al, 2001). The highest degree of similarity between fatty acids present in test fat and chylomicron TAG was observed in the 4-h postprandial samples, where the chylomicron TAG levels also showed maxima (Table 6). The significantly lower amount of stearic acid in chylomicron TAG relative to the percentage in the S fat compared with the corresponding proportions of target fatty acids for the other test fats (66 vs 87–93%) supports a hypothesis of lower absorption of stearic acid (Tholstrup et al, 2001). Even 8 h after the fat loads, where the TAG contents were relatively low, the fatty acid composition of chylomicron TAG showed a great extent of similarity with the consumed test fatty acids.

The fatty acid profiles of VLDL TAG were less influenced by the test fatty acids than those of chylomicron TAG, although significant differences from fasting compositions were found for all postprandial samples. The maximum effect of test fats on the composition of VLDL TAG was observed after 6 h, that is, 2 h later than in chylomicron TAG due to the time course for uptake of chylomicron remnants and subsequent synthesis and release of VLDL particles.

The fatty acid composition of chylomicron TAG samples showed only small amounts of long-chain PUFA indicating that the contribution from endogenous fatty acids was of minor importance. The pool of liver fatty acids had on the contrary a diluting effect on the fatty acids present in VLDL TAG, resulting in a less marked reflection of the test fats in the fatty acid profiles especially for 18:0 and trans-18:1 isomers.

The fatty acid distributions in TAG and the sn-2 position of TAG from test fats and those from chylomicrons 4 h after the test meals (Figures 1 and 2) were almost similar as previously shown by Summers et al (1999) and Yli-Jokipii et al (2001). Consequently, discrimination against any of the target fatty acids present in either the sn-1 and sn-3 positions or the sn-2 position was not evident.

In an attempt to verify whether a specific clearing was taking place during the postprandial period, ratios between wt% target fatty acid and wt% oleic acid in chylomicron and VLDL TAG were calculated (Table 8). Oleic acid was chosen as reference since it was supplemented in all test fats in the same amount. It could be supposed that for example the PUFA might be retained in the remnants to go preferentially to the liver, whereas fatty acids meant for energy production might be cleared specifically. As described before, the fasting fatty acid composition of chylomicron TAG played only a minor role in the distributions at 2, 4, 6, and 8 h, respectively, because considerable amounts of all test fatty acids were already present in chylomicrons 2 h after the meal challenge. The relatively low ratios for 18:0 after the S fat were probably associated with less effective absorption, as mentioned earlier. Contrary, the ratios for 16:0 after the P fat were of the same magnitude as the ratios after the T and L fats. Since only small changes were observed from 4 to 8 h in the ratios for all five test fats, the clearing of chylomicron TAG was not specifically related to fatty acids. This means that LPL cleaved chylomicron TAG in a random manner and no selection of fatty acids for liver synthesis took place. The ratios after the T fat were close to unity indicating similar levels of cis-18:1 and trans-18:1 in chylomicron TAG from 4 to 8 h.

Table 8 Clearing ratio of target fatty acids in chylomicron and VLDL TAG as related to oleic acid

In VLDL TAG, the ratios for 16:0 and 18:0 after the P and S fat, respectively, showed only minor variations. Again, much lower ratios were seen after the S fat. On the contrary, the ratios for trans-18:1 after the T fat and 18:2 n-6 after the L fat increased due to increasing amounts of the target fatty acids in VLDL TAG with time. The considerable lower ratios for the S and T fats observed indicate that these fatty acids are oxidised in the liver and not regarded as important fatty acids to be distributed to the other tissues.

The total TAG concentrations in the postprandial samples (Table 6) were calculated from total fatty acid quantification (Tables 1, 2, 3, 4 and 5). As expected, significant TAG elevations induced by the oral fat loads were observed for both chylomicrons and VLDL. This is in agreement with the results previously reported by our group (Tholstrup et al, 2001) using a commercial, enzymatic TAG kit (based on triolein) for the same samples. The present method in which the fatty acid distribution is determined is, however, a more exact tool for studying how postprandial clearing proceeds.

The total fat load (Table 7) was calculated from the TAG data (Table 6) and corrected for baseline levels to reflect changes occurring after the test meals. For chylomicron TAG lower values were found for the P and S fats compared with the T, O, and L fats, which may suggest a less-sufficient absorption of the two saturated fats during the 8-h postprandial period or a distinct ongoing absorption after the 8 h. A tendency towards higher concentrations of chylomicron TAG for the two saturated test fats at 8 h was seen. The lower postprandial load may also indicate a quicker clearance of the P and S fats compared to the other test fats. However, this was not observed (Table 8). The possibility that ingestion of the S fat resulted in the production of smaller chylomicron particles which may be isolated with the VLDL fraction cannot be fully excluded. However, the higher VLDL TAG response for the S fat is nonsignificant compared to the other test fats. We therefore believe that the contamination is small.

Our results of the postprandial TAG responses are in accordance with a number of studies (de Bruin et al, 1993; Muesing et al, 1995; Gatto et al, 2003), partly in line with Jones et al (1999), Sanders et al (2000), and Hunter et al (2001), but different from other investigations (Salomaa et al, 1993; Mero et al, 1998; Roche et al, 1998; Zampelas et al, 1998). For further discussion of these responses see Tholstrup et al (2001). All these investigations used nonrandomised edible fats and oils, the deviating results might therefore be due to differences in the positional distributions of the fatty acids in the dietary fats; a possibility that was fully excluded in our study.

The importance of the positional distributions of the fatty acids in dietary TAG has also been investigated. Zampelas et al (1994) studied the influence of the positional distribution of palmitic acid on postprandial TAG responses in humans. They reported that consumption of liquid meals containing 30 wt% 16:0 with 6 or 73% in the sn-2 position of the TAG, respectively, resulted in similar effects on postprandial lipaemia. On the contrary, Yli-Jokipii et al (2001) found higher postprandial plasma TAG responses after ingestion of a palm oil meal with palmitic acid predominantly in the outer positions of TAG than after a transesterified palm oil diet with palmitic acid randomly distributed among the three positions. The same tendency was observed for chylomicron TAG. Summers et al (1998, 1999) have reported results from human studies of the postprandial metabolism of structured TAG containing primarily palmitic acid or stearic acid in the sn-2 position or the sn-1 and sn-3 positions, respectively instead of oleic acid. The fats were given with milk shakes and bread/corn flakes. No differences were seen in the handling of specific fatty acids in the circulation or in subcutaneous adipose tissue indicating that different fatty acids were cleared equally by LPL. In a human study performed by Sanders et al (2001), cacao butter, high oleic acid sunflower oil, and stearic acid-rich structured TAG (Salatrim) were incorporated in muffins and ingested together with milk shakes. The cacao butter and oleic acid diets increased the postprandial serum TAG responses more than the structured TAG meal probably due to the short-chain SFA present in Salatrim. Redgrave et al (1988) found a positional effect of stearic acid in rats. The TAG hydrolysis and chylomicron remnant clearance were retarded when stearic acid was present in the sn-2 position of the TAG fed compared with the sn-3 position. Furthermore, in accordance with our data, trans-fatty acids substituted in the sn-2 position of TAG instead of oleic acid had no effect on lipolysis but retarded chylomicron remnant clearance.

In contrast to the reported studies, where the fatty acids were present in different amounts and/or in different positions of the dietary TAG, our findings were a result of a direct effect of the specific fatty acids present in the test fats due to the use of balanced randomised fats.

In conclusion, the present study demonstrated that the fatty acids present in the randomised test fats were extensively incorporated into chylomicron TAG and to a much lesser degree into VLDL TAG. The positional distributions of the test fatty acids were maintained in chylomicron TAG to a great extent as measured by the fatty acid composition in TAG and the sn-2 position of TAG. No preferential clearing of chylomicron TAG, indicated by the conserved ratios between target fatty acids and oleic acid, was observed in relation to time. Postprandial VLDL TAG showed only minor changes in the ratios after ingestion of palmitic and stearic acid-rich test fats. On the contrary, trans-18:1 and linoleic acid test fats resulted in increased ratios.

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Acknowledgements

The present study was financed by the Danish Research Development Program for Food Technology (FØTEK 2). We thank Marianne Rosenberg, Lis Christensen, and Karen Rasmussen for their excellent technical assistance.

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Correspondence to A Bysted.

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Guarantors: A Bysted and G Hølmer.

Contributors: TT, BS, GH, and AB were involved in designing the study. AB analysed all blood samples and prepared chylomicron fractions for determination of the positional distributions. AB performed the statistical calculations and analyses, wrote the original manuscript and edited all subsequent versions. GH and PL supervised the analyses. GH contributed to the original and subsequent manuscripts. TT and BS performed the human trial, prepared lipoprotein fractions for fatty acid analyses, and revised the manuscript.

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Bysted, A., Hølmer, G., Lund, P. et al. Effect of dietary fatty acids on the postprandial fatty acid composition of triacylglycerol-rich lipoproteins in healthy male subjects. Eur J Clin Nutr 59, 24–34 (2005). https://doi.org/10.1038/sj.ejcn.1602028

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Keywords

  • trans-fatty acids
  • saturated fatty acids
  • postprandial
  • triacylglycerol-rich lipoproteins
  • absorption
  • clearing

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