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Biochemistry and Nutrition Group, and * Food Biotechnology and Engineering Group, BioCentrum-DTU, Technical University of Denmark, Lyngby, Denmark
2To whom correspondence should be addressed. E-mail: tpo{at}biocentrum.dtu.dk.
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ABSTRACT |
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 TOP ABSTRACT MATERIALS AND METHODSRESULTSDISCUSSIONLITERATURE CITED |
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KEY WORDS: • lipase activity • lymphatic absorption • (n-3) fatty acids • structured oils
The preduodenal lipases and pancreatic lipase catalyze the intestinal hydrolysis of ingested triacylglycerols (TAGs)3 with preference for the sn-1,3 positions, resulting in sn-2 monoacylglycerols (sn-2 MAGs) and FFAs (1,2). The nature of the fatty acids in the TAG molecule determines the hydrolysis rate; previously performed experiments showed that medium-chain fatty acids (MCFAs) were hydrolyzed more rapidly than the highly unsaturated long-chain fatty acids (LCFAs) (3–5). The major route of absorption for the LCFAs and sn-2 MAGs is through the enterocytes with high conservation of the fatty acids located in the sn-2 position of the dietary fat (6). In the enterocytes, the sn-2 MAGs are reesterified with fatty acids of exogenous and endogenous origin to form a new population of TAGs, which are packed into chylomicrons and secreted to the lymph. MCFAs are absorbed primarily via the portal vein for oxidation in the liver (7).
Medium-chain triacylglycerols (MCTs) are sources of rapidly available energy. The obvious problem with MCTs is that they supply no PUFAs, which may lead to essential fatty acid deficiency on a long-term basis. Structured TAGs combine the advantages of the easily digested and absorbed MCFAs with the delivery of various PUFAs with specific effects in the body. These could improve the absorption of essential fatty acids when malabsorption problems are present (8–12) or increase the uptake of PUFAs for tissue regeneration after surgery (13,14). Structured TAGs may be produced by chemical interesterification, resulting in random distribution of the fatty acids in the TAGs, or they may be produced by enzymatic interesterification, leading to more specific structures (15). The recent development of new and less costly enzymes (16) makes the enzymatic process more attractive for use in the future.
Marine oils contain (n-3) PUFAs, especially 20:5(n-3) and 22:6(n-3). The sn-2 position of the glycerol moiety in fish oil is enriched with PUFAs, especially 22:6(n-3), whereas in the oils of marine mammals, including whale and seal oil, this is reversed, i.e., there is less PUFA enrichment in the sn-2 position (17). A high intake of (n-3) PUFAs has been associated with a beneficial effect on the plasma lipid profile, leading to a low incidence of coronary heart disease (18). Furthermore, (n-3) fatty acids were shown to influence the brain and visual development during infancy (19) and to have immunomodulating effects (20). The interest in these effects led to studies investigating the effects of structured TAGs containing (n-3) PUFAs on the nitrogen balance after burns (21), on endotoxic shock (22), postsurgery (13,23), and on tumor growth (24,25).
The aim of the present study was to measure the in vitro pancreatic lipase activity and lymphatic transport in rats under normal absorption conditions of specific structured oils containing 10:0 and (n-3) LCFAs of marine origin to determine whether the positional distribution of fatty acids on the TAG molecule influenced these factors and thereby the potential availability of (n-3) PUFAs in the body. The absorption of MLM (M = medium-chain fatty acid, L = long-chain fatty acid) oils containing (n-3) fatty acids was investigated in previous studies (26,27), whereas the hydrolysis and the absorption of a comparable LML oil have not, to our knowledge, been investigated previously.
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MATERIALS AND METHODS |
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TOPABSTRACTMATERIALS AND METHODSRESULTSDISCUSSIONLITERATURE CITED |
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Pancreatic lipase activity. The oils were subjected to in vitro pancreatic lipase activity measurements using the method described by Martin et al. (30) with modifications. Briefly, 3 mL of 0.5 mol/L Tris:HCl buffer solution (pH 7.7) containing 0.01% sodium taurocholate (Sigma) and 0.18% CaCl2 (Sigma) was added to 10 mg of oil and sonicated for 2 min before digestion. Lipolysis was started by adding 3000 U of pancreatic lipase (E.3.1.1.3, type VI-S, Sigma) together with 6 µg colipase (Sigma) with vigorous stirring at 37°C. The reaction was stopped at selected time points (1, 5, 10, 20, and 40 min) by acidification.
Internal standards of TAG, diacylglycerol (DAG), MAG, and FFA were added. Total lipid was extracted with chloroform and subsequently separated into lipid classes by TLC. The plates (Silica gel 60, Merck) were developed with chloroform:isopropanol:acetic acid (95:5:1, by vol) and visualized with 2,7-dichloroflourescein (0.2% in ethanol). TAG, DAG, MAG, and FFA spots were quantitatively scraped off and methylated with BF3 according to a modified procedure originally established by Hamilton et al. (31). Briefly, lipids were saponified for 5 min with 1 mL of 1 mol/L NaOH in CH3OH, and converted to FAMEs with 1 mL BF3 (14% acidometric in CH3OH) for 2 min, both steps at 80°C in closed tubes. After being washed with 2 mL water, FAMEs were extracted with 2 x 0.5 mL heptane. The combined heptane phases were washed with 1 mL saturated NaCl solution. Using this procedure, it was possible to quantify FAMEs from free MCFAs due to the closed system and lack of solvent evaporation steps.
FAMEs were analyzed by GLC as described previously (29). Peak areas were calculated using a Hewlett-Packard computing integrator and used to calculate the amount of each lipid product using the internal standards.
Housing and surgery of rats. The experiment was approved by the Danish Committee for Animal Experiments. Male specific pathogen–free Wistar rats (Taconic M&B) were housed 2 per plastic cage in a temperature- (21°C) and humidity- (50%) controlled environment on a 12-h light:dark cycle and fed a standard nonpurified diet (Altromin No. 1324, Chr. Petersen A/S) as used previously (29); water was freely available. The rats were acclimated to the housing conditions for 10 d before surgery and weighed 290 ± 6 g at the time of the experiment. Rats were anesthetized i.m. with a Zoletil mixture (0.06 mL/100 g; The Royal Veterinary and Agricultural University) and were subjected to cannulation of the main mesenteric lymph duct (29). After surgery, the rats were placed in individual restraining cages (32) with tap water freely available, no food, but a steady infusion of saline (0.15 mol/L NaCl, 0.004 mol/L KCl, and 0.28 mol/L glucose) at 2 mL/h through the feeding tube. At 4–6 h after the operation the rats were administered 0.2 mL of butorphanol (Torbugesic, diluted 1:10 with sterile water, Fort Dodge Laboratories).
Administration of oil and collection of lymph. On d 1 postsurgery, the experiment was started by collection of a baseline fraction of lymph from –1 to 0 h. At time "0," a sonicated emulsion of 0.5 mL (455 mg) oil and 0.5 mL of a solution containing 20 mmol/L taurocholate (Sigma) and 10 g/L choline (Sigma) in distilled water was injected through the feeding tube followed by 0.5 mL saline; the infusion of saline was continued at 2 mL/h. Lymph was collected in tubes in 1-h fractions for the following 8 h and a combined fraction was obtained from 8 to 23 h followed by a 1-h fraction from 23 to 24 h. The tubes contained 100 µL (700 µL for the overnight fraction) of a 10% (wt/v) Na2-EDTA solution (E. Merck); they were frozen immediately after collection and kept at –20°C until analysis. After collection of the last fraction, the rats were killed by an overdose of sodium pentobarbital infused through the lymph catheter.
Fatty acid composition of lymph samples. After the addition of an internal standard (15:0 as methyl ester), total lipids were extracted from the lymph according to the method of Folch et al. (33). The fatty acid composition of total lipids was determined after transesterification catalyzed by KOH in methanol (34). FAMEs were analyzed by GLC. The fatty acids were identified by comparing the retention time with standards of known fatty acid composition (Nu-Chek-Prep).
Fatty acid composition of lipid classes in lymph. Lymph from all rats in each group at each time was pooled to reduce the analytical work and a 200 µL lymph pooled sample, together with internal standards of TAG, phosphatidylcholine, DAG, FFA, and cholesterol ester (CE), was extracted from the pooled samples as mentioned above, separated on TLC plates, and methylated with BF3 as described previously (35).
Calculations and statistical analysis. Results are expressed as means ± SEM (n = 3 for hydrolysis results, n = 7 for absorption results). Recoveries of fatty acids were calculated as the ratio between the accumulated amount of a fatty acid in the lymph at a specific time point and the administered amount of that fatty acid. Differences between in vitro pancreatic lipase activity at each time point were analyzed by one-way ANOVA (GraphPad PRISM version 3.02, GraphPad software). Differences between the lymphatic transport, lymph flow, and recoveries of fatty acids in rats administered different oils were tested by repeated-measures ANOVA. Tukey’s Multiple Comparison post test was used to determine which means differed. Differences were considered significant at P < 0.05.
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RESULTS |
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TOPABSTRACTMATERIALS AND METHODSRESULTSDISCUSSIONLITERATURE CITED |
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Lymphatic transport of fatty acids. The lymphatic transport of total fatty acids from 2 to 7 h (Fig. 2A) was higher in rats administered fish oil than in those given MLM or LML (P < 0.01). For all 3 groups, the peak transport occurred 2 h after oil administration, reaching values of 130 µmol/h for rats given fish oil and 80–90 µmol/h for those administered MLM and LML; although the transport after administration of fish oil decreased toward the baseline value during the absorptive phase, the transport after administration of MLM and LML did not change during the period from 2 to 8 h. After 24 h, the transport of total fatty acids in all 3 groups had returned to baseline. At 24 h, the accumulated transport of total fatty acids was greater in rats administered fish oil than in those administered MLM (P < 0.05) (Table 2).
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The accumulated lymphatic transport of other fatty acids reflected the content of the fatty acids in the experimental oils. The transport of 14:0, 16:0, 16:1(n-7), 18:1(n-9), 18:2(n-6), 18:3(n-3), 20:1(n-9), and 22:1(n-11) was higher after fish oil administration than after MLM and LML administration [P < 0.05; results shown only for 18:2(n-6)]. The accumulated transport of 22:5(n-3) was higher after LML administration compared with fish oil and MLM administration (P < 0.001).
Recovery of administered fatty acids. Recoveries of total fatty acids in lymph of rats administered MLM and LML (Table 3) were lower compared with rats administered fish oil at 8 h (P < 0.05), but at 24 h, only the recovery after MLM administration was lower (P < 0.05). The recovery of 10:0 in lymph at 24 h was higher after LML administration compared with MLM administration (P < 0.05). Higher recoveries of 20:5(n-3) and 22:6(n-3) were observed at 8 h after fish oil administration compared with LML administration (P < 0.05), but this difference had vanished at 24 h with recoveries close to 50%. In comparing the recoveries of 10:0, 20:5(n-3), 22:6(n-3), and total fatty acids at 8 and 24 h, they were significantly higher at 24 h (P < 0.05), except for the recoveries of 10:0 after administration of MLM, and the recoveries of 20:5(n-3) after administration of fish oil and MLM, indicating that the absorption of fatty acids continued beyond 8 h. The recovery of 18:2(n-6) was much higher than 100%, indicating extensive transport of endogenous 18:2(n-6). The recovery of 18:2(n-6) after MLM and LML administration was higher than after fish oil administration (P < 0.05).
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DISCUSSION |
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TOPABSTRACTMATERIALS AND METHODSRESULTSDISCUSSIONLITERATURE CITED |
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The lymphatic transport of the almost purely exogenous fatty acids 10:0 and 20:5(n-3), as well as other fatty acids in the experimental oils, reflected the content of the fatty acids in the oils and their position on the TAG molecule. Experiments by Åkesson et al. (6) showed that 
75% of LCFAs were conserved in the sn-2 position of dietary TAGs during absorption. From structural analysis of lymph lipids, Jensen et al. (36) estimated that <40% of MCFAs were conserved during absorption when they were located in the sn-2 position, indicating a higher tendency for acyl migration during hydrolysis and absorption of MCFAs compared with LCFAs. Several studies emphasized that the location of a fatty acid in the sn-2 position of dietary fat, even though it is an MCFA, will increase the absorption of this fatty acid (9,26,36,37). Ikeda et al. (37) compared the lymphatic absorption of oils such as LLL, MLM, and LML containing 18:2(n-6) and found that the absorption of 18:2(n-6) was favored by location in the sn-2 position combined with MCFA in the sn-1,3 positions and that MCFAs were absorbed more efficiently from the LML oil. In the present study, we observed a higher transport of 10:0 from LML compared with MLM, a higher transport of 22:6(n-3) from MLM compared with LML, and a tendency for higher transport of 20:5(n-3) from MLM compared to LML. The conservation of fatty acids in the sn-2 position is an important issue when considering the possible advantages in customizing fats with particular TAG-structures and maintaining the location of fatty acids in specific positions after absorption.
The absorption of the 2 structured oils probably influenced the resynthesis of TAG in the enterocytes differently compared with the absorption of a dietary fat without MCFAs. The very high recoveries of 18:2(n-6) observed 24 h after MLM and LML administration (1162 ± 94 and 1035 ± 46% in contrast to 323 ± 16% observed after fish oil administration) indicated that the endogenous stores of fatty acids had to be mobilized for TAG resynthesis to a much higher degree after intake of the structured oils compared with fish oil. We hypothesize that the low absorption of MLM resulted in a shortage of FFA for TAG resynthesis because 85% of the administered 10:0 was not found in lymph and probably was absorbed through the portal vein for oxidation in the liver. This would delay the resynthesis and thus the entire absorption. Bernard and Carlier (7) found that 49% of infused radiolabeled 10:0 was recovered in portal blood in contrast to 6–11% for infused LCFAs. For resynthesis of TAG after LML administration, there could be a shortage of sn-2 MAGs. As described above, there is a higher tendency for acyl migration during absorption when MCFAs are present in the sn-2 position. A shortage of sn-2 MAG would probably result in a shift from the 2-MAG pathway for TAG resynthesis to the glycero-3-phosphate pathway, normally accounting for only 20–30% of TAG resynthesized during absorption (38). A shift to another pathway requires that the enzymes involved in the pathway be upregulated and this would then again delay the resynthesis and thereby the entire absorption. The hypothesized effects of fatty acid composition and TAG structure on TAG resynthesis in the enterocytes require confirmation in future studies.
We observed that the 24-h recoveries of 20:5(n-3) and 22:6(n-3) were far from 100% (close to 50% after administration of the 3 oils), indicating that LCFAs may be transported via the portal vein as well. Other explanations could be that the fatty acids were oxidized, remained in the enterocytes, or were drained into other intestinal lymph ducts. A relatively low recovery of LCFAs [50% of 18:2(n-6)] was also observed by Vistisen et al. (39) using 13C-labeled TAG for administration to lymph-cannulated rats.
Although different absorption profiles were observed in the present study, similar proportions of TAG, PL, DAG, FFA, and CE were measured in lymph samples from the 3 experimental groups at maximum absorption. The dietary influence on fatty acid composition of the different lipid classes was evident primarily in the TAG fraction, as expected, but fatty acids of dietary origin were also incorporated into the other lipid classes, indicating that newly synthesized PL and CE were incorporated into the chylomicrons released to lymph from the enterocytes. In agreement, Vistisen et al. (39) determined that 2–3% of 13C-labeled 18:2(n-6) was recovered in the PL fraction of lymph lipids.
Our experiment demonstrated that the intramolecular TAG structure affected absorption during the first hours after oil administration, but did not affect the overall absorption of (n-3) PUFAs and thereby the overall digestibility of the oils because recoveries of 20:5n(n-3) and 22:6(n-3) 24 h after MLM and LML administration were similar. Under normal absorption conditions, different TAG structures had similar effects, but the potential of structured lipids to influence the absorption in situations of malabsorption or when high-energy demands are present certainly exists. Administration of MLM oils containing LCFAs either from soybean oil (8) or from rapeseed oil (9) to malabsorbing rats with diverted bile and pancreatic ducts led to higher absorption of the LCFAs from structured oils than from other oils. In patients with cystic fibrosis, structured oils were absorbed more quickly than safflower oil and led to a greater increase in plasma 18:2(n-6) compared with controls (11,40). In burned rats, Swenson et al. (24) observed a protein-sparing effect of structured lipid made from fish oil compared with safflower oil. The effects of MLM and LML containing (n-3) PUFAs in the presence of malabsorption still have to be compared to examine whether the structured oils have the potential to increase the absorption of LCFAs in such situations.
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ACKNOWLEDGMENTS |
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FOOTNOTES |
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3 Abbreviations used: CE, cholesterol ester; DAG, diacylglycerol; LCFA, long-chain fatty acid; LML, long-chain/medium-chain/long-chain structured oil; MAG, monoacylglycerol; MCFA, medium-chain fatty acid; MCT, medium-chain triacylglycerol; MLM, medium-chain/long-chain/medium-chain structured oil; PL, phospholipid; TAG, triacylglycerol.
Manuscript received 8 December 2004. Initial review completed 27 January 2005. Revision accepted 12 April 2005.
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