Specific molecular and colloidal structures of milk fat affecting lipolysis,absorption and postprandial lipemia

In milk fat, fatty acids are located at specific positions on the triacylglycerol backbone. The sn‐2 position contains most saturated long‐chain fatty acids, while the sn‐3 position contains short‐chain fatty acids. Moreover, these triacylglycerols are structured as milk fat globules surrounded by their native membrane containing phospholipids. This native structure can be modified by the dairy processes to generate various possible colloidal structures with milk fat. The structure of triacylglycerols and the milk fat ultrastructure can impact on fatty acid digestion and absorption, which has a potential effect on cardiovascular risk factors linked to postprandial hypertriglyceridemia. The review points out the impact of the triacylglycerol structure and the ultrastructure of milk fat on these risk factors.     

Positional distribution of fatty acids in the triacylglycerols of developing oil palm fruit

The pattern of accumulation of triacylglycerols, their fatty acid compositions and the positional distribution of the fatty acids at thesn-2- andsn-1,3-positions of the triacylglycerol molecules at progressive stages of oil palm fruit development were determined. There was an exponential rate of increase of triacylglycerols and their fatty acids toward the end of fruit development. The fatty acid composition of the triacylglycerols in the early stages of development, prior to active accumulation, was more or less similar, but differed appreciably from the later stages, and the transition of fatty acid composition toward that of normal palm oil occurred at around 16 wk after anthesis (WAA) and stabilized at 20 WAA. All fatty acids increased in terms of absolute quantity. There was an overall consistency in fatty acid positional distribution, irrespective of development stage. More saturated fatty acids were found to be esterified at thesn-1,3-positions and more unsaturated fatty acids at thesn-2-position of triacylglycerol. Higher rate of incorporation of 16:0 at the 1,3-positions during the active phase of triacylglycerol synthesis was observed, while 18:1 acid exhibited a reverse trend.     

Evidence that palmitic acid is absorbed assn-2 monoacylglycerol from human milk by breast-fed infants

Milk fatty acids consist of about 20–25% palmitic acid (16∶0), with about 70% of 16∶0 esterified to thesn-2 position of the milk triacylglycerols. Hydrolysis of dietary triacylglycerols by endogenous lipases producessn-2 monoacylglycerols and free fatty acids, which are absorbed, reesterified, and then secreted into plasma. Unesterified 16∶0 is not well absorbed and readily forms soaps with calcium in the intestine. The positioning of 16∶0 at thesn-2 position of milk triacylglycerols could explain the high coefficient of absorption of milk fat. However, the milk lipase, bile salt-stimulated lipase, has been suggested to complete the hydrolysis of milk fat to free fatty acids and glycerol. These studies determined whether 16∶0 is absorbed from human milk assn-2 monopalmitin by comparison of the plasma triacylglycerol total andsn-2 position fatty acid composition between breast-fed and formula-fed term gestation infants. The human milk and formula had 21.0 and 22.3% of 16∶0, respectively, with 54.2 and 4.8% 16∶0 in the fatty acids esterified to the 2 position. The plasma triacylglycerol total fatty acids had 26.0±0.6 and 26.2±0.6% of 16∶0, and thesn-2 position fatty acids had 23.3±3.3 and 7.4±0.7% of 16∶0 in the three-month-old exclusively breast-fed (n=17) and formula-fed (n=18) infants, respectively. Marked differences were found in the plasma total and the 2 position phospholipid percentage of 20∶4ω6, i.e., 11.6±0.3 and 6.9±0.6 (total), 17.7±1.4 and 9.7±0.6 (sn-2 position) and percentage of 22∶6ω3, 4.6±0.3 and 2.1±0.3 (total), 5.6±0.6 and 2.0±0.2 (sn-2 position) for the breast-fed and formula-fed infants, respectively. These studies provide convincing evidence that 16∶0 is absorbed from human milk assn-2 monoacyl-glycerol. The metabolic significance of the differences in positional distribution of fatty acids in the plasma lipids of breast-fed and formula-fed infants is not known.     

Production of Human Milk Fat Substitutes by Interesterification of Tripalmitin with Ethyl Oleate Catalyzed by Candida parapsilosis Lipase/Acyltransferase

In human milk fat, the saturated fatty acids, namely palmitic acid, are located at the sn-2 position of triacylglycerols (TAG) while unsaturated fatty acids (e.g. oleic acid) are esterified at position sn-1,3. Thus, sn-1,3-dioleoyl-2-palmitoylglycerol (OPO) is the target TAG to be used as human milk fat substitutes (HMFS) in infant formulas. In this study, the noncommercial recombinant lipase/acyltransferase from Candida parapsilosis (CpLIP2) was immobilized in Accurel MP1000, and used as a biocatalyst for the interesterification of tripalmitin with ethyl oleate in a solvent-free medium, to obtain structured lipids used as HMFS. Different molar ratios (MR) of ethyl oleate to tripalmitin (2:1–8:1) were used. After 4 h reaction at 60°C, about 30 mol% of oleic acid incorporation was already observed for all tested MR. An apparent equilibrium was reached after 8–24 h, with 32–51 mol% final incorporation, increasing with the MR. The incorporation of oleic acid into TAG was compared with the maximum predicted values when a random or a sn-1,3-regioselective biocatalyst was used. The obtained values are consistent with the maximum incorporation expected for a sn-1,3-regioselective enzyme. In fact, the amount of oleic acid at position sn-2 was approximately 15% for all the MR tested, which is explained by the acyl migration phenomenon. CpLIP2 exhibited higher activity than most commercial immobilized lipases (e.g. faster reaction in solvent-free media, low enzyme load, and low MR needed), and showed a recognized sn-1,3 regioselective behavior.     

Interesterification of Lard and Soybean Oil Blends Catalyzed by Immobilized Lipase in a Continuous Packed Bed Reactor

Structured lipids (SL), formulated by blends of lard and soybean oil in different ratios, were subjected to continuous enzymatic interesterification catalyzed by an immobilized lipase from Thermomyces lanuginosus (Lipozyme TL IM) in a continuous packed bed reactor. The original and interesterified blends were examined for fatty acid and triacylglycerol composition, regiospecific distribution, and solid fat content. Blends of lard and soybean oil in the proportions 80:20 and 70:30 (w/w), respectively, demonstrated a fatty acid composition, and proportions of polyunsaturated/saturated fatty acids (PUFA/SFA) and monounsaturated/polyunsaturated fatty acids (MUFA/PUFA), that are appropriate for the formulation of pediatric products. These same blends were suited for this purpose after interesterification because their sn-2 positions were occupied by saturated fatty acids (52.5 and 45.4%, respectively), while unsaturated fatty acids predominantly occupied sn-1,3 positions, akin to human milk fat. Interesterification caused rearrangement of triacylglycerol species.     

Absorption of synthetic,stereochemically defined acylglycerols in the rat

The stereochemistry of fat digestion and absorption was investigated in rats with thoracic duct fistulas, after feeding synthetic triacylglycerol or alkyldiacylglycerol. After feeding 1,2-dilauroyl-3-oleoyl-sn-glycerol, dilauroyloleoylglycerol and lauroyldioleoylglycerol were the most abundant chyle triacylglycerols. Positional analysis of the fatty acid distribution and the absence of optical activity indicated that the following structures dominated:rac-1,2-dilauroyl-3-oleoylglycerol andrac-1,3-dioleoyl-2-lauroylglycerol. Therefore, the triacylglycerol resynthesized from 2-lauroylglycerol (precursor to 60% of chyle triacylglycerol) and other precursors was essentially racemic. Chyle phospholipids contained largely endogenous fatty acids, and the proportion of lauric acid was very low. A racemic mixture of 1,2-di[³H] oleoyl-3-tetradecyl-sn-glycerol and 1-tetradecyl-2,3-di[¹⁴C] oleoyl-sn-glycerol was absorbed to a lower degree than triacylglycerol. The appearance of oleic acid with different labels in chyle and intestinal lipids did not differ, indicating the absence of stereospecificity in fat digestion. Possible explanations for the low absorption are discussed.     

Stereospecific analysis of glycerolipids of egg yolk of Japanese quail (Coturnix coturnix japonica)

Phosphatidylcholine, phosphatidylethanolamine and triacylglycerol were isolated from egg yolk of the Japanese quail. Fatty acid compositions at the two and three positions of glycerol in the glycerolipids were determined by stereospecific analysis employing phospholipase A₂. The distribution of the total number of carbon atoms in the fatty acid moieties of triacylglycerol was also quantitated by high temperature gas liquid chromatography. The distribution of acyl groups in each of the positions of the phosphatidylcholine, phosphatidylethanolamine and triacylglycerol was not random, and each position has a characteristic composition. The phosphatidylcholine and phosphatidylethanolamine had distinctive fatty acid distributions for positionsn-2 of the triacylglycerol had a predominance of unsaturated fatty acids of which 18∶1 (69.9%) was the major component. Positionsn-3 contained 49.3% saturated fatty acids and was more saturated than positionsn-1 by 8.1%. The experimentally determined distribution of the carbon numbers in triacyl glycerol deviated significantly from the distribution predicted by 1-random-2-random-3-random association of the fatty acids. The data suggest that in Japanese quail there is marked preferencial synthesis of some triacylglycerols.     

Positional analyses of triacylglycerol fatty acids in the milk fat of the antarctic fur seal (Arctocephalus gazella)

The positional distribution of fatty acids has been determined for the milk triacylglycerols of the Antarctic fur seal,Arctocephalus gazella. Of particular interest was the positional distribution of the polyunsaturated n−3 fatty acids in milk triacylglycerols (TG). In adipocytes of pinnipeds, TG are synthesized with the n−3 fatty acids primarily in thesn-1,3 positions. To determine the positional distribution, extracts of enzymatically digested lipids were separated by thin-layer chromatography, and the constituent fatty acids were separated and quantified by gas-liquid chromatography. Monoenoic and saturated fatty acids comprised over 75% of the total, the ratio of monoenoic to saturated fatty acids being 2∶1. The percent content of the long-chain n−3 fatty acids, 20∶5, 22∶5 and 22∶6, ranged between 15–20%. The positional analyses revealed that at thesn-2 position of milk TG, saturated fatty acids were in excess (57%), and the content of n−3 fatty acids was less than 5%. More than 80% of the n−3 fatty acids in milk were located in thesn-1,3 positions. The data indicate that in pinnipeds TG are synthesized in the mammary gland and adipose tissue with fatty acids having similar positional distributions.     

Effects of different medium-chain fatty acids on intestinal absorption of structured triacylglycerols

To study the effect of the chain length of medium-chain fatty acids on the intestinal absorption of long-chain fatty acids, we examined the lymphatic transport of fat following administration of five purified structured triacylglycerols (STAG) containing different medium-chain fatty acids in the sn-1, 3 positions and long-chain fatty acids in the sn-2 position in a rat model. Significant amounts of medium-chain fatty acids were found in lymph samples after intragastric administration of 1,3-dioctanoyl-2-linoleyl-sn-glycerol (8∶0/18∶2/8∶0), 1,3-didecanoyl-2-linoleyl-sn-glycerol, and 1,3-didodecanoyl-2-linoleyl-sn-glycerol. The accumulated lymphatic transport of medium-chain fatty acids increased with increasing carbon chain length. The recoveries of caprylic acid (8∶0), capric acid (10∶0), and lauric acid (12∶0) were 7.3±0.9, 26.3±2.4, and 81.7±6.9%, respectively. No significant differences were observed for the maximal intestinal absorption of linoleic acid (18∶2n−6) when the chain length of medium-chain fatty acids at the primary positions was varied, and the absorption of 18∶2 and oleic acid (18∶1) from 8∶0/18∶2/8∶0 and 1,3-dioctanoyl-2-oleyl-sn-glycerol was similar. We conclude that the chain length of the medium-chain fatty acids in the primary positions of STAG does not affect the maximal intestinal absorption of long-chain fatty acids in the sn-2 position in the applied rat model, whereas the distribution of fatty acids between the lymphatics and the portal vein reflects the chain length of the fatty acids. Presented in part at the 3rd ISSFAL Conference, Lyon, France, June 1–5, 1998.     

Peanut triacylglycerols: Effect of season and production location

Stereospecific analysis of triacylglycerols from 4 peanut varieties grown for up to 3 years at 4 locations showed diversity in percentage fatty acid placement. Distribution of oleic and linoleic acids at each position was significantly correlated to the amount in the total triacylglycerol for varieties grown at one location. However, correlations for thesn-3 position were not significant when data from more than one location were analyzed together. Generally, higher percentages of oleic or linoleic acid in the triacylglycerol resulted in a greater proportion of the particular fatty acid in thesn-2 position. Apparently, fatty acid concentrations as influenced by growing location have a significant influence on peanut triacylglycerol structure.     

Varietal differences in peanut triacylglycerol structure

Stereospecific analysis of triacylglycerols from six peanut varieties showed diversity in percent fatty acid placement. Distribution of the fatty acids among thesn-1,-2 and-3 positions was clearly nonrandom. The percentages of palmitic and stearic acids, generally very low at thesn-2 position, were more predominant at thesn-1 than thesn-3 position. Long chain fatty acids were located almost exclusively at thesn-3 position. Thesn-2 position of all varieties was high in unsaturated fatty acids. Triacyglycerols were sufficiently different to suggest that concentrations of specific triacylglycerol species may vary with variety. Mention of firm names or trade products does not imply that they are endorsed or recommended by the Department of Agriculture over other firms or similar products not mentioned.     

Determination of positional distribution of short-chain fatty acids in bovine milk fat on chiral columns

The positional distribution of acetic and butyric acids in bovine milk fat triacylglycerols was determined by chiral-phase high-performance liquid chromatography (HPLC) of the derived diacylglycerols. Enriched fractions of acetic and butyric acid-containing triacylglycerols were isolated by normal-phase thin-layer chromatography (TLC) from a molecular distillate of butter oil, and they were fully hydrogenated. Mixedsn-1,2(2,3)- andX-1,3-diacylglycerols of short- and long-chainlength, which were generated by partial Grignard degradation of the hydrogenated triacylglycerols, were isolated by borate-TLC. The enantiomericsn-1,2-andsn-2,3-diacylglycerols and theX-1,3-diacylglycerols as their 3,5-dinitrophenylurethanes were resolved by HPLC on chiral columns. Both acetic and butyric acids were exclusively associated with thesn- 2,3- andX-1,3-diacylglycerols of short and long chainlength. These results establish the presence of acetic and butyric acids in thesn-3-position of bovine milk fat triacylglycerols. Other short-and medium-chainlength acids were found in progressively increasing proportions also in thesn-1- andsn-2-positions.     

Triacylglycerol structure of human colostrum and mature milk

Because triacylglycerol (TAG) structure influences the metabolic fate of its component fatty acids, we have examined human colostrum and mature milk TAG with particular attention to the location of the very long chain polyunsaturated fatty acid on the glycerol backbone. The analysis was based on the formation of various diacylglycerol species from human milk TAG upon chemical (Grignard degradation) or enzymatic degradation. The structure of the TAG was subsequently deduced from data obtained by gas chromatographic analysis of the fatty acid methyl esters in the diacylglycerol subfractions. The highly specific TAG structure observed was identical in mature milk and colostrum. The three major fatty acids (oleic, palmitic and linoleic acids) each showed a specific preference for a particular position within milk TAG: oleic acid for thesn-1 position, palmitic acid for thesn-2 position and linoleic acid for thesn-3 position. Linoleic and α-linolenic acids exhibited the same pattern of distribution and they were both found primarily in thesn-3 (50%) andsn-1 (30%) positions. Their longer chain analogs, arachidonic and docosahexaenoic acids, were located in thesn-2 andsn-3 positions. These results show that polyunsaturated fatty acids are distributed within the TAG molecule of human milk in a highly specific fashion, and that in the first month of lactation the maturation of the mammary gland does not affect the milk TAG structure.     

Stereospecific analysis of triacyl-sn-glycerolsvia resolution of diastereomeric diacylglycerol derivatives by high-performance liquid chromatography on silica

The compositions of positionssn-1, 2 and 3 of triacylglycerols can be determined by partial hydrolysis with ethyl magnesium bromide, derivatization of the total products with (S)-(+)-1-(1-naphthyl)ethyl isocyanate and isolation of the diacyl-sn-glycerol urethane derivatives by chromatography on solid-phase extraction columns containing an octadecylsilyl phase. The diastereomericsn-1,2-and 2,3-diacylglycerol derivatives are separated by high-performance liquid chromatography on silica for determination of their fatty acids by gas chromatography. Each step in the process has been evaluated rigorously. The compositions of all three positions can be calculated with good accuracy from the analyses of these compounds and that of the total triacylglycerols. Although the 1,3-sn-diacylglycerol derivatives can also be isolated easily, they do not give reliable results for the composition of positionsn-2 because acyl migration occurs during their generation. The stereospecific analysis procedure has been applied to some plant and animal triacyl-sn-glycerols of commercial and scientific interest, containing predominantly C₁₆ and C₁₈ fatty acids,i.e. safflower, sunflower, olive and palm oils, tallow, egg and rat adipose tissue. The method is not at present suited to the analysis of more complex triacylglycerols, such as milk fat or fish oils, and problems associated with these are discussed.     

Aspects of Positional Distribution of Fatty Acids in Triacylglycerols of Skin,White and Dark Muscle of Mackerel Scomber scombrus in Relation to Hypertension

The triacylglycerols from skin, white and dark muscle of Atlantic mackerel Scomber scombrus were isolated and the distribution of fatty acids on the glycerol examined with pancreatic lipase. The dark muscle, which is the smallest dietary fat source of the three tissues, had less EPA and more DHA than the other two tissues. The EPA deficiency was located in the 2-position. The DHA was similar in both the 2-and 1,3-positions. The white muscle triacylglycerol showed a much higher proportion of DHA in the 2-position than in the 1,3-positions. The skin triacylglycerol was relatively undifferentiated in respect to distribution of EPA and DHA. Saturated acids were least in the 2-position of the white muscle triacylglycerol and highest in the 1,3-position of the same fat. Not unexpectedly, the monoethylenic fatty acids of exogenous origin were located in the 1,3-positions and the highest level was in the dark muscle triacylglycerol. The distribution of the whole of the EPA and DHA is not dissimiliar from that projected from literature of fish triacylglycerols. It is concluded that benefits attributed to supplement of dietary mackerel, relative to herring, in certain types of hypertension follow from the total ω-3 fatty acid intake and not from any special distribution on glycerol of mackerel fatty acids.     

Identification of triacylglycerols in the enzymatic transesterification of rapeseed and butter oil

A blend of rapeseed and butter oil was transesterified using immobilized Thermomyces lanuginosus lipase (Lipozyme® TL IM) as catalyst. The reaction was followed by reversed-phase HPLC and the triacylglycerol peaks were tentatively identified from their elution times by calculating equivalent carbon numbers. Further identification was made using HPLC-electrospray tandem mass spectrometry. A few of the triacylglycerols detected were typical combinations of fatty acids originating from rapeseed oil, such as α-linolenic acid, and short-chain fatty acids from butter oil. Due to the regioselectivity of the lipase, the transesterification reaction involved mainly fatty acids in the sn-1 and sn-3 positions. However, significant changes in the fatty acid composition in the sn-2 position were detected after 6 h.     

Acylglycerol structure of genetic varieties of peanut oils of varying atherogenic potential

Detailed investigation was made of the triacylglycerol structure of three varieties of peanut oils of varying atherogenic activity. By means of chromatographic and stereospecific analyses, it was shown that all the oils had markedly nonrandom enantiomeric structures with the long chain saturated fatty acids (C₂₀−C₂₄) confined exclusively to thesn-3-position, whereas the palmitic and oleic acids were distributed about equally between thesn-1-andsn-3-positions, with the linoleic acid being found preferentially in thesn-2-position. On the basis of detailed studies of the molecular species of the separatesn-1,2-,sn-2,3- andsn-1,3-diacylglycerol moieties, it was concluded that the fatty acids in the three positions of the glycerol molecule are combined with each other solely on the basis of their relative molar concentrations. As a result, it was possible to calculate the composition of the molecular species of the peanut oil triacylglycerols (including the content of the enantiomers and the reverse isomers) by means of the 1-random 2-random 3-random distribution. In general, the three peanut oils possessed triacylglycerol structures which where closely similar to that derived earlier for a commercial peanut oil of North American origin. Since their oil has exhibited a high degree of atherogenic potential, it was anticipated that the present oils would likewise be atherogenic, which has been confirmed by biological testing. However, there are certain differences in the triacylglycerol structures among these oils, which can be correlated with the variations in their atherogenic activity. The major differences reside in the linoleic/oleic acid ratios in the triacylglycerols, especially in thesn-2-position, and in the proportions in which these acids are combined with the long chain fatty acids. On the basis of the characteristic structures identified in the earlier analyzed atherogenic peanut oil, the peanut oil of South American origin would be judged to possess the greatest atherogenic potential and this has been borne out by biological testing.     

Regioselective analysis of the fatty acid composition of triacylglycerols with conventional high-performance liquid chromatography

A new method for regioselective analysis of triacyglycerols via conventional high-performance liquid chromatography (HPLC) has been developed. The method is simple and avoids the time-consuming purification processes normally characteristics of regioselective analyses. The procedure utilizes an sn-1,3-specific lipase from Rhizopus arrhizus to deacylate the fatty acid residues located at the sn-1 and sn-3 positions of triacylglycerols. The fatty acid residues esterified at the sn-2 position are determined by subtraction of the results of the sn-1,3 analysis from an overall composition analysis based on complete saponification of the original sample. The fatty acid mixtures are converted to p-bromophenacyl esters and analyzed using conventional HPLC techniques. The analytical procedure has been verified using a standard structured triacylglycerol. The analytical results for three edible vegetable oils are compared with those obtained via the method proposed by P.J. Williams and co-workers.     

Evaluation of <Emphasis Type="Italic">Rhizopus oryzae</Emphasis> Lipase for the Determination of Regiodistribution in Triacylglycerols with Medium Chain Fatty Acids

The nutritional profile and rheological behaviors of lipids is both due to fatty acid composition and regiodistribution on external and internal positions of triacylglycerol. Actual methods for regiodistribution analysis having some restrictions, there is still a need for investigating a safe, simple and environmentally friendly method for the sn-2 position analysis that could especially be used for the analysis of fats containing medium and short chain fatty acids. The objective of this study was to evaluate the 1,3-selectivity and typoselectivity of Rhizopus oryzae lipase in the presence of short/medium chain fatty acids in partial hydrolysis conditions used for regiodistribution analysis. Structured triacylglycerols containing eight-carbon-chain length fatty acids in the sn -2 position were chemically synthesized using DCC/DMAP coupling agent and purification steps by flash-chromatography. The final product showed very high purity and was used as the substrate for 1,3-selectivity evaluation. Typoselectivity was assessed by investigating partial hydrolysis of equimolar blends of homogeneous TAG. This study confirmed the 1,3-selectivity of Rhizopus oryzae lipase in the hydrolysis conditions used, and revealed that this lipase was less influenced by fatty acids chain length than pancreatic lipase. Considering this, Rhizopus oryzae lipase appeared to be a good candidate for regiodistribution analysis of fats containing medium and short chain fatty acids.     

Influence of triacylglycerol structure and fatty acid profile of dietary fats on milk triacylglycerols in the rat. A two-generation study

We investigated the influence of dietary fatty acid profile and triacylglycerol structure on the fatty acid profile and triacylglycerol structure of milk lipids in two generations of rats. Three groups of rats received diets containing 20% fat of which approximately 20% was n-3 fatty acids located in different positions of the triacylglycerol: a fish oil-based diet [docosahexaenoic acid (22:6n-3) predominantly in thesn-2 position], a seal oil-based diet (22:6n-3) predominantly in thesn-1/sn-3 position or a plant oil-based diet [α-linolenic acid (18:3n-3) distributed evenly between the three positions]. This design allowed us to investigate (i) the effect of the triacylglycerol structure of the dietary fat; (ii) the effect of receiving the n-3 fatty acids as long-chain derivatives or as the precursor, 18:3n-3; and (iii) the long-term effects over two generations. The fatty acid profiles of the milk lipids largely reflected the diets, but in the second generation, the level of medium-chain fatty acids was higher (P<0.05) in the milk from rats fed the fish oil diet (24%) compared with the other dietary groups (15 and 18%). This suggests an increased endogenous synthesis of fatty acids in the mammary glands of the fish oil-fed rats. The levels of long-chain n-3 fatty acids in milk were higher (P<0.05) in rats fed maire n-3 fatty acids in milk were higher (P<0.05) in rats fed marie oils (8–12%) compared with rats fed vegetable oil (1%) in both generations. The level of long-chain n-3 fatty acids was significantly higher in the milk from the fish oil-fed rats (12.3%) compared to the seal-oil fed rats (8.0%) in the first generation, but not in the second generation (8.9 vs. 9.1%). The general structure of milk triacylglycerols was maintained in the three experimental groups with 16:0 acylated in thesn-2 position and 18:1 in thesn-1/sn-3 positions. The triacylglycerol structure of mammalian milk appears to be conserved even during extreme dietary manipulation over two generations and an extensive enrichment with long-chain n-3 polyunsaturated fatty acids requires their presence in the diet.