Use of structured triacylglycerols containing predominantly stearic and oleic acids to probe early events in metabolic processing of dietary fat

Pre-duodenal lipid digestion of emulsions: Relevance, colloidal aspects and mechanistic insight

The digestion of lipids in the human body has several health and nutritional implications. Lipid digestion is an interfacial phenomenon meaning that water-soluble lipases need to first adsorb to the oil-water interface before enzymatic conversions can start. The digestion of lipids mainly occurs on colloidal structures dispersed in water, such as oil-in-water (o/w) emulsions, which can be designed during food formulation/processing or structured during digestion. From a food design perspective, different in vitro studies have demonstrated that the kinetics of lipid digestion can be influenced by emulsion properties. However, most of these studies have been performed with pancreatic enzymes to simulate lipolysis in the small intestine. Only few studies have dealt with lipid digestion in the gastric phase and its subsequent impact on intestinal lipolysis. In this aspect, this review compiles information on the physiological aspects of gastric lipid digestion. In addition, it deals with colloidal and interfacial aspects starting from emulsion design factors and how they evolve during in vitro digestion. Finally, molecular mechanisms describing gastric lipolysis are discussed.

The impact of the complexing agent on the sensitivity of collision-induced dissociation spectra to fatty acid position for a set of XYZ-type triglycerides

RATIONALE

The development of an automated platform for the positional analysis of triglycerides (TAGs) based on electrospray ionization tandem mass spectrometry (ESI-MS/MS) continues to be pursued. This work evaluates the positional sensitivities of the collision-induced dissociation (CID) spectra of a representative set of XYZ triglycerides using sodium, lithium, and ammonium salts as complexing agents.

METHODS

A set of triglycerides were synthesized and analyzed via ESI-MS/MS using an ion trap mass spectrometer. Using three different complexing agents, the product ion spectra of the corresponding precursor ions for twelve XYZ TAGs were collected, where X, Y, and Z represent C_16:0 , C_18:1(c-9) , C_{18:2(cc-9,12)} , and C_{20:4(cccc-5,8,11,14)} fatty acid chains. These data were then used to prepare ternary plots for four positional isomer systems to evaluate the positional sensitivity differences among the three different complexing agents.

RESULTS

The positional sensitivities for each of the four positional isomer systems were robust for the sodium and lithium adducts. The CID data for the sodium and lithium TAGs demonstrated an unfavorable loss of the fatty acid in the center position and showed a higher sensitivity to fatty acid position, when compared with the CID data for ammonium adducts, especially for the arachidonic acid containing triglycerides.

CONCLUSIONS

The data shows that that the relative abundances of the DAG product ions for the XYZ-type TAGs when using sodium and lithium complexing agent adducts are sensitive to fatty acid position and are consistent for the diverse array of TAGs studied in this work. This suggests that using sodium or lithium as the complexing agent may be advantageous for the development of an automated platform for the positional analysis of complex TAG mixtures based on ESI-MS/MS.

Circulating Non-Esterified Fatty Acids as Biomarkers for Fat Content and Composition in Pigs

Simple Summary

Circulating non-esterified fatty acids (NEFA) may be valuable as biomarkers for intramuscular fat content and fatty acid composition, as well as for other meat quality traits, in finishing heavy Duroc pigs. However, circulating NEFA composition may be affected by other factors such as age, fasting duration, and genetic variants related with adipogenesis and fatty acid metabolism pathways (e.g., SCD and LEPR). This study revealed that the circulating NEFA composition, especially the oleic acid content, reflects the metabolic status of an animal at a given time but has limited value as biomarker of intramuscular fat content and fatty acid composition.

Abstract

Circulating non-esterified fatty acids (NEFA) can reflect the composition of dietary fat or adipose tissues depending on the fasting conditions. Therefore, circulating NEFA may be valuable as biomarkers for meat quality traits, such as intramuscular fat content and fatty acid composition in finishing pigs. Genetic variants that regulate lipid metabolism can also modulate the circulating NEFA. We conducted an experiment with 150 heavy Duroc pigs to evaluate fluctuations in the circulating NEFA composition due to age, fasting duration and two genetic polymorphisms, one in the leptin receptor (LEPR; rs709596309) and one in the stearoyl-CoA desaturase (SCD; rs80912566) gene. Circulating NEFA were more saturated and less monounsaturated than the subcutaneous and intramuscular adipose tissues. Absolute circulating NEFA content was more influenced by fasting duration than age. The SCD polymorphism did not impact NEFA content or composition. The LEPR polymorphism affected the content but not the fatty acid composition. Circulating oleic acid NEFA content after a short fasting was positively correlated with intramuscular fat content and, after a long fasting, with intramuscular oleic acid content. We conclude that circulating NEFA reflect environmental and genetic metabolic changes but are of limited value as biomarkers for intramuscular fat content and fatty acid composition.

Interesterified palm oil impairs glucose homeostasis and induces deleterious effects in liver of Swiss mice

BACKGROUND

Interesterified fats have largely replaced the partially hydrogenated oils which are the main dietary source of trans fat in industrialized food. This process promotes a random rearrangement of the native fatty acids and the results are different triacylglycerol (TAG) molecules without generating trans isomers. The role of interesterified fats in metabolism remains unclear. We evaluated metabolic parameters, glucose homeostasis and inflammatory markers in mice fed with normocaloric and normolipidic diets or hypercaloric and high-fat diet enriched with interesterified palm oil.

METHODS

Male Swiss mice were randomly divided into four experimental groups and submitted to either normolipidic palm oil diet (PO), normolipidic interesterified palm oil diet (IPO), palm oil high-fat diet (POHF) or interesterified palm oil high-fat diet (IPOHF) during an 8 weeks period.

RESULTS

When compared to the PO group, IPO group presented higher body mass, hyperglycemia, impaired glucose tolerance, evidence of insulin resistance and greater production of glucose in basal state during pyruvate in situ assay. We also observed higher protein content of hepatic PEPCK and increased cytokine mRNA expression in the IPO group when compared to PO. Interestingly, IPO group showed similar parameters to POHF and IPOHF groups.

CONCLUSION

The results indicate that substitution of palm oil for interesterified palm oil even on normocaloric and normolipidic diet could negatively modulate metabolic parameters and glucose homeostasis as well as cytokine gene expression in the liver and white adipose tissue. This data support concerns about the effects of interesterified fats on health and could promote further discussions about the safety of the utilization of this unnatural fat by food industry.

Differential postprandial incorporation of 20:5n-3 and 22:6n-3 into individual plasma triacylglycerol and phosphatidylcholine molecular species in humans

The mechanisms by which digested fat is absorbed and transported in the circulation are well documented. However, it is uncertain whether the molecular species composition of dietary fats influences the molecular species composition of meal-derived lipids in blood. This may be important because enzymes that remove meal-derived fatty acids from the circulation exhibit differential activities towards individual lipid molecular species. To determine the effect of consuming oils with different molecular compositions on the incorporation of 20:5n-3 and 22:6n-3 into plasma lipid molecular species. Men and women (18-30 years) consumed standardised meals containing 20:5n-5 and 22:6n-3 (total 450 mg) provided by an oil from transgenic Camelina sativa (CSO) or a blended fish oil (BFO) which differed in the composition of 20:5n-3 and 22:6n-3 - containing molecular species. Blood was collected during the subsequent 8 h. Samples were analysed by liquid chromatography-mass spectrometry. The molecular species composition of the test oils was distinct from the composition of plasma triacylglycerol (TG) or phosphatidylcholine (PC) molecular species at baseline and at 1.5 or 6 h after the meal. The rank order by concentration of both plasma PC and TG molecular species at baseline was maintained during the postprandial period. 20:5n-3 and 22:6n-3 were incorporated preferentially into plasma PC compared to plasma TG. Together these findings suggest that the composition of dietary lipids undergoes extensive rearrangement after absorption, such that plasma TG and PC maintain their molecular species composition, which may facilitate lipase activities in blood and/or influence lipoprotein structural stability and function.

Palmitic Acid Versus Stearic Acid: Effects of Interesterification and Intakes on Cardiometabolic Risk Markers - A Systematic Review

Fats that are rich in palmitic or stearic acids can be interesterified to increase their applicability for the production of certain foods. When compared with palmitic acid, stearic acid lowers low-density lipoprotein (LDL)-cholesterol, which is a well-known risk factor for coronary heart disease (CHD), but its effects on other cardiometabolic risk markers have been studied less extensively. In addition, the positional distribution of these two fatty acids within the triacylglycerol molecule may affect their metabolic effects. The objective was to compare the longer-term and postprandial effects of (interesterified) fats that are rich in either palmitic or stearic acids on cardiometabolic risk markers in humans. Two searches in PubMed/Medline, Embase (OVID) and Cochrane Library were performed; one to identify articles that studied effects of the position of palmitic or stearic acids within the triacylglycerol molecule and one to identify articles that compared side-by-side effects of palmitic acid with those of stearic acid. The interesterification of palmitic or stearic acid-rich fats does not seem to affect fasting serum lipids and (apo) lipoproteins. However, substituting palmitic acid with stearic acid lowers LDL-cholesterol concentrations. Postprandial lipemia is attenuated if the solid fat content of a fat blend at body temperature is increased. How (the interesterification of) palmitic or stearic acid-rich fats affects other cardiometabolic risk markers needs further investigation.

Probiotic Enterococcus faecalis AG5 effectively assimilates cholesterol and produces fatty acids including propionate

Probiotics are known for their health-promoting abilities through the production of various metabolites, and also benefits are membrane associated. Probiotic Enterococcus faecalis AG5 has been previously assessed for its probiotic properties and this study further strengthens that claim. Strain AG5 was inoculated with 0.3% sodium glychocholate (glycine conjugated) and bile salt mixture for different time periods. The percentage assimilation of cholesterol ranged from 14% to 33.1% in the bile salt mixture, but was slightly higher, ranging from 16.55% to 53.1%, in sodium thioglycocholate alone. Similarly, the bile salt hydrolysis activity was measured in terms of cholic acid release at different intervals. The overall range of cholic acid released ranged from 70.4 to 479 µg/mL. Fatty acid production is also an important criterion for a probiotic selection and it was found that the strain produced several long-chain fatty acids (LCFA) such as tetradecanoic acid, hexadecanoic acid, octadecanoic acid and bis(6-methylheptyl) phthalate. Besides LCFA, AG5 produced short-chain fatty acids (SCFA) such as propionic acid, which was estimated comparing liquid chromatography and mass spectrometry with standard SCFA. Thus the ability of deconjugation of bile salt and SCFA production is a value-added property of probiotic AG5, with the promise of being beneficial for human health.

The Impact of the Complexing Cation on the Sensitivity of the Collisional-Induced Dissociation Spectra to Fatty Acid Position for a Set of YXY/YYX-type Triglycerides

RATIONAL

The development of an automated platform for the positional analysis of triglycerides based on electrospray tandem mass spectrometry continues to be pursued. This work compares the positional sensitivities of the collisional-induced dissociation spectra for a representative set of YXY/YYX triglycerides using ammonium, silver, sodium and lithium as complexing agents.

METHODS

A set of triglycerides were synthesized and analyzed by electrospray tandem mass spectrometry using an ion trap mass spectrometer. Using different salt additives, the product ion spectra of the corresponding parent ions for twelve systems of the form YXY/YYX, where Y and X represent C16:0 , C18:1(c-9), C18:2(cc-9,12) and C20:4(cccc-5,8,11,14) , were collected. The data was used to prepare two-point calibration plots for each of the twelve positional isomer systems using each of the four complexing agents.

RESULTS

The positional sensitivities for all twelve positional isomer systems were robust for both the sodium and lithium TAG adducts. The CID data for both the sodium and lithium TAG adducts are much less sensitive to the degree of unsaturation and double bond position of the fatty acids constituents than the CID data for the ammonium adducts.

CONCLUSION

Using sodium or lithium TAG adducts may be advantageous for the development of an accurate predictive model for performing positional analysis of complex TAG mixtures based on electrospray tandem mass spectrometry. Ammonium adducts are likely complicated by the ability of the ammonium ion to provide extra stability to some parent ions through hydrogen bond-like interactions.

A simple and economical strategy for obtaining calibration plots for relative quantification of positional isomers of YYX/YXY triglycerides using high-performance liquid chromatography/tandem mass spectrometry

RATIONALE

Positional analysis of intact triglycerides could provide greater insights into the link between fatty acid position and lipotoxic diseases. However, this methodology has been impeded by lack of commercial availability of positionally pure triglycerides. This work reports on a strategy for defining calibration plots for YXY/YYX triglyceride systems based on the product ion intensities in the collision-induced dissociation spectra of ammoniated precursor ions.

METHODS

A set of triglycerides were synthesized and analyzed by electrospray ionization tandem mass spectrometry using an ion trap mass spectrometer. The product ion spectra of the ammoniated precursor ions were collected for 42 triglyceride systems of the form YXY/YYX, where Y represents C_16:0 , C_18:1(c-9) and C_{20:4(cccc-5,8,11,14)} . Three-point calibration plots were prepared by plotting the relative abundance of the YY⁺ product ion vs. the relative abundance of the YYX positional isomer.

RESULTS

The calibration plots were shown to give relative abundances of positional isomers accurate to within ±0.02 for most systems. Using an ion trap, under a controlled set of collision parameters, the slopes of the calibration plots can be used to compare the sensitivities of the product ion intensities to fatty acid position for various triglyceride systems. The average slopes of the calibration plots for the C_16:0 , C_18:1(c-9) and C_{20:4(cccc-5,8,11,14)} systems were 0.29 ± 0.05, 0.21 ± 0.05 and 0.045 ± 0.005, respectively.

CONCLUSIONS

While the presence of multiple unsaturated fatty acids tends to slightly decrease the slopes of the calibration plots, the data suggest that the sensitivities are sufficient for performing positional analysis of most triglyceride systems. However, the presence of unsaturated fatty acids that contain double bonds close to the carbonyl group, such as arachidonic acid, tends to dramatically decrease positional sensitivity.

What are interesterified fats and should we be worried about them in our diet?

Interesterified (IE) fats are used in a wide range of food products and were introduced as a replacement for trans fats, which are known to be detrimental to cardiovascular health. However, the effects of interesterification on metabolism and subsequent effects on cardiovascular health are not understood and previous studies have seldom investigated industrially-relevant IE fats. No legislation currently exists regarding the labelling of IE fats in food products and therefore estimates of average consumption rates in the UK population are currently unavailable. In order to meet the urgent need for a systematic investigation of the health effects of consumer-relevant IE fats, it is essential to estimate current IE fat intakes and to investigate biological mechanisms that might mediate acute and chronic cardiometabolic effects of commercially relevant IE fats.

Molecular implications of adenosine in obesity

Adenosine has broad activities in organisms due to the existence of multiple receptors, the differential adenosine concentrations necessary to activate these receptors and the presence of proteins able to synthetize, degrade or transport this nucleoside. All adenosine receptors have been reported to be involved in glucose homeostasis, inflammation, adipogenesis, insulin resistance, and thermogenesis, indicating that adenosine could participate in the process of obesity. Since adenosine seems to be associated with several effects, it is plausible that adenosine participates in the initiation and development of obesity or may function to prevent it. Thus, the purpose of this review was to explore the involvement of adenosine in adipogenesis, insulin resistance and thermogenesis, with the aim of understanding how adenosine could be used to avoid, treat or improve the metabolic state of obesity. Treatment with specific agonists and/or antagonists of adenosine receptors could reverse the obesity state, since adenosine receptors normalizes several mechanisms involved in obesity, such as lipolysis, insulin sensitivity and thermogenesis. Furthermore, obesity is a preventable state, and the specific activation of adenosine receptors could aid in the prevention of obesity. Nevertheless, for the treatment of obesity and its consequences, more studies and therapeutic strategies in addition to adenosine are necessary.

Obesity: An overview of possible role(s) of gut hormones, lipid sensing and gut microbiota

Obesity is one of the major challenges for public health in 21st century, with 1.9 billion people being considered as overweight and 600 million as obese. There are certain diseases such as type 2 diabetes, hypertension, cardiovascular disease, and several forms of cancer which were found to be associated with obesity. Therefore, understanding the key molecular mechanisms involved in the pathogenesis of obesity could be beneficial for the development of a therapeutic approach. Hormones such as ghrelin, glucagon like peptide 1 (GLP-1) peptide YY (PYY), pancreatic polypeptide (PP), cholecystokinin (CCK) secreted by an endocrine organ gut, have an intense impact on energy balance and maintenance of homeostasis by inducing satiety and meal termination. Glucose and energy homeostasis are also affected by lipid sensing in which different organs respond in different ways. However, there is one common mechanism i.e. formation of esterified lipids (long chain fatty acyl CoAs) and the activation of protein kinase C δ (PKC δ) involved in all these organs. The possible role of gut microbiota and obesity has been addressed by several researchers in recent years, indicating the possible therapeutic approach toward the management of obesity by the introduction of an external living system such as a probiotic. The proposed mechanism behind this activity is attributed by metabolites produced by gut microbial organisms. Thus, this review summarizes the role of various physiological factors such as gut hormone and lipid sensing involved in various tissues and organ and most important by the role of gut microbiota in weight management.

Evaluation of the immediate vascular stability of lipoprotein lipase-generated 2-monoacylglycerol in mice

2-Monoacylglycerols are gaining increasing interest as signaling lipids, beyond endocannabinoids, for example, as ligands for the receptor GPR119 and as mediators of insulin secretion. In the vascular system, they are formed by the action of lipoprotein lipase (LPL); however, their further disposition is unclear. Assuming similar affinity for uptake and incorporation into tissues of 2-oleoylglycerol and 2-oleylglyceryl ether, we have synthesized a (3)H-labeled 2-ether analog of triolein (labeled in alkyl group) and compared its disposition with (14)C-labeled triolein (labeled in glycerol) 20 min after intravenous coadministration in a ratio of 1:1 to mice. We found that peripheral tissues and the liver in particular are able to take up 2-monoacylglycerols as seen from (3)H uptake. In muscle and adipose tissue, 2-monoacylglycerols are probably further hydrolyzed as seen by an increased (3)H/(14)C ratio, whereas in the liver and the heart, data suggest that they are also subjected to re-esterification to triacylglycerol, as seen by an unchanged (3)H/(14)C ratio in the lipid fraction of the tissues. Our findings suggest that LPL-generated 2-monoacylglycerol is likely to be stable in the vascular system and thus have a potential to circulate or at least exert effects in tissues where it may be locally produced.

Re-esterified palm oils, compared to native palm oil, do not alter fat absorption, postprandial lipemia or growth performance in broiler chicks

Re-esterified palm oils are obtained from the chemical esterification of palm acid oils (rich in free fatty acids) with glycerol, both economically interesting by-products from oil refining and biodiesel industries, respectively. Thus, re-esterified palm oils could be an economically interesting alternative to native palm oil in broiler chick diets. However, because they may have different physicochemical properties than have their corresponding native oil, we assessed the effect of fatty acid (FA) positional distribution within acylglycerol molecules and the effect of acylglycerol composition on FA apparent absorption, and their possible consequences on the evolution of postprandial lipemia and growth performance in broiler chicks. Seventy-two 1-day-old female broiler chicks were randomly distributed into 18 cages. The three treatments used were the result of a basal diet supplemented with 6 wt% of native palm oil (N-TAG), re-esterified palm oil (E-TAG), or re-esterified palm oil high in mono- and diacylglycerols (E-MDAG). Chemical esterification raised the fraction of palmitic acid at the sn-2 position from 9.63 mol% in N-TAG oil to 17.9 mol% in E-TAG oil. Furthermore, E-MDAG oil presented a high proportion of mono- (23.1 wt%) and diacylglycerols (51.2 wt%), with FA mainly located at the sn-1,3 positions, which resulted in a lower gross-energy content and an increased solid-fat index at the chicken’s body temperature. However, re-esterified palm oils did not alter fat absorption, postprandial lipemia, or growth performance, compared to native palm oil, so they can be used as alternative fat sources in broiler chick diets.

Regulation of energy metabolism by long-chain fatty acids

In mammals, excess energy is stored primarily as triglycerides, which are mobilized when energy demands arise. This review mainly focuses on the role of long chain fatty acids (LCFAs) in regulating energy metabolism as ligands of peroxisome proliferator-activated receptors (PPARs). PPAR-alpha expressed primarily in liver is essential for metabolic adaptation to starvation by inducing genes for beta-oxidation and ketogenesis and by downregulating energy expenditure through fibroblast growth factor 21. PPAR-delta is highly expressed in skeletal muscle and induces genes for LCFA oxidation during fasting and endurance exercise. PPAR-delta also regulates glucose metabolism and mitochondrial biogenesis by inducing FOXO1 and PGC1-alpha. Genes targeted by PPAR-gamma in adipocytes suggest that PPAR-gamma senses incoming non-esterified LCFAs and induces the pathways to store LCFAs as triglycerides. Adiponectin, another important target of PPAR-gamma may act as a spacer between adipocytes to maintain their metabolic activity and insulin sensitivity. Another topic of this review is effects of skin LCFAs on energy metabolism. Specific LCFAs are required for the synthesis of skin lipids, which are essential for water barrier and thermal insulation functions of the skin. Disturbance of skin lipid metabolism often causes apparent resistance to developing obesity at the expense of normal skin function.

Multiscale structures of lipids in foods as parameters affecting fatty acid bioavailability and lipid metabolism

On a nutritional standpoint, lipids are now being studied beyond their energy content and fatty acid (FA) profiles. Dietary FA are building blocks of a huge diversity of more complex molecules such as triacylglycerols (TAG) and phospholipids (PL), themselves organised in supramolecular structures presenting different thermal behaviours. They are generally embedded in complex food matrixes. Recent reports have revealed that molecular and supramolecular structures of lipids and their liquid or solid state at the body temperature influence both the digestibility and metabolism of dietary FA. The aim of the present review is to highlight recent knowledge on the impact on FA digestion, absorption and metabolism of: (i) the intramolecular structure of TAG; (ii) the nature of the lipid molecules carrying FA; (iii) the supramolecular organization and physical state of lipids in native and formulated food products and (iv) the food matrix. Further work should be accomplished now to obtain a more reliable body of evidence and integrate these data in future dietary recommendations. Additionally, innovative lipid formulations in which the health beneficial effects of either native or recomposed structures of lipids will be taken into account can be foreseen.

Metabolic characteristics of human subcutaneous abdominal adipose tissue after overnight fast

Subcutaneous abdominal adipose tissue is one of the largest fat depots and contributes the major proportion of circulating nonesterified fatty acids (NEFA). Little is known about aspects of human adipose tissue metabolism in vivo other than lipolysis. Here we collated data from 331 experiments in 255 healthy volunteers over a 23-year period, in which subcutaneous abdominal adipose tissue metabolism was studied by measurements of arterio-venous differences after an overnight fast. NEFA and glycerol were released in a ratio of 2.7:1, different (P < 0.001) from the value of 3.0 that would indicate no fatty acid re-esterification. Fatty acid re-esterification was 10.2 ± 1.4%. Extraction of triacylglycerol (TG) (fractional extraction 5.7 ± 0.4%) indicated intravascular lipolysis by lipoprotein lipase, and this contributed 21 ± 3% of the glycerol released. Glucose uptake (fractional extraction 2.6 ± 0.3%) was partitioned around 20-25% for provision of glycerol 3-phosphate and 30% into lactate production. There was release of lactate and pyruvate, with extraction of the ketone bodies 3-hydroxybutyrate and acetoacetate, although these were small numerically compared with TG and glucose uptake. NEFA release (expressed per 100 g tissue) correlated inversely with measures of fat mass (e.g., with BMI, r(s) = -0.24, P < 0.001). We examined within-person variability. Systemic NEFA concentrations, NEFA release, fatty acid re-esterification, and adipose tissue blood flow were all more consistent within than between individuals. This picture of human adipose tissue metabolism in the fasted state should contribute to a greater understanding of adipose tissue physiology and pathophysiology.

Palmitic acid in the sn-2 position of triacylglycerols acutely influences postprandial lipid metabolism

BACKGROUND

The triacylglycerol structure of saturated fats may influence postprandial lipemia.

OBJECTIVE

We tested the hypothesis that high-fat meals rich in palmitic acid (16:0) in the sn-2 position decrease lipemia.

DESIGN

Postprandial changes in plasma lipids, apolipoprotein B48, and cytokines were compared in healthy men (n = 25) and women (n = 25) by using a randomized crossover design after meals that provided 50 g fat supplied as high-oleic sunflower oil (control), palm olein (PO), interesterified palm olein (IPO), and lard containing 0.6, 9.2, 39.1, and 70.5 mol% 16:0, respectively, at sn-2.

RESULTS

The sn-2-rich meals elicited different postprandial responses in plasma concentrations of nonesterified fatty acid (meal × time, P = 0.00014), triacylglycerol (meal × time, P = 0.002), and apolipoprotein B48 (meal × time × sex, P = 0.008). Nonesterified fatty acid concentrations were lower up to 3 h after lard and IPO meals than after control or PO meals. Triacylglycerol increased less steeply after lard and IPO meals than after control and PO meals; the incremental AUCs (iAUCs) were 34% (95% CI: 7%, 124%; P < 0.05) and 26% (95% CI: 16%, 132%; P < 0.05) lower after lard than after control and PO meals, respectively. In men, the maximal increment in apolipoprotein B48 was 14% (95% CI: 3%, 25%; P < 0.05) and 16% (95% CI: 2%, 30%; P < 0.05) lower for lard and IPO, respectively, compared with control. The postprandial iAUC in triacylglycerol was 51% lower in women (P = 0.001) than in men. Plasma IL-6 increased postprandially, but IL-8, TNF-α, and E-selectin decreased after all meals.

CONCLUSION

Fats with a higher proportion of palmitic acid in the sn-2 position decrease postprandial lipemia in healthy subjects. This trial was registered at controlled-trials.com as ISRCTN20774126.

Triacylglycerol structure and interesterification of palmitic and stearic acid-rich fats: an overview and implications for cardiovascular disease

The position of fatty acids in the TAG molecule (sn-1, sn-2 and sn-3) determines the physical properties of the fat, which affects its absorption, metabolism and distribution into tissues, which may have implications for the risk of CHD. The TAG structure of fats can be manipulated by the process of interesterification, which is of increasing commercial importance, as it can be used to change the physical characteristics of a fat without the generation of trans-fatty acids. Interesterified fats rich in long-chain SFA are commercially important, but few studies have investigated their health effects. Evidence from animal and human infant studies suggests that TAG structure and interesterification affect digestibility, atherogenicity and fasting lipid levels, with fats containing palmitic and stearic acid in the sn-2 position being better digested and considered to be more atherogenic. However, chronic studies in human adults suggest that TAG structure has no effect on digestibility or fasting lipids. The postprandial effects of fats with differing TAG structure are better characterised but the evidence is inconclusive; it is probable that differences in the physical characteristics of fats resulting from interesterification and changes in TAG structure are key determinants of the level of postprandial lipaemia, rather than the position of fatty acids in the TAG. The present review gives an overview of TAG structure and interesterified palmitic and stearic acid-rich fats, their physical properties and their acute and chronic effects in human adults in relation to CHD.

The solid fat content of stearic acid-rich fats determines their postprandial effects

BACKGROUND

The process of randomization is used commercially to harden fats as an alternative to partial hydrogenation, but its effects on cardiovascular disease risk factors are uncertain.

OBJECTIVE

The objective was to compare the chronic and acute effects of randomization of a fat rich in 1,3-distearyl, 2-oleyl glycerol on fasting and postprandial lipids, glucose, insulin, and activated clotting factor VII (FVIIa) concentrations.

DESIGN

A crossover design study in 16 men compared fasting and postprandial lipid, glucose, insulin, and FVIIa concentrations at baseline and after a 3-wk diet providing 30 g unrandomized or randomized shea butter and sunflower oil blends (SSOBs), both of which contained approximately 50% stearic acid. Fecal fat excretion was measured during each dietary period. Postprandial changes were assessed after the consumption of meals providing 50 g test fat. A subsequent study compared postprandial changes after the consumption of an oleic acid-rich sunflower oil meal and an unrandomized SSOB meal.

RESULTS

Both SSOBs were well digested and absorbed. Randomization did not affect fasting or postprandial lipid, glucose, insulin, or FVIIa concentrations. Compared with the oleic acid-rich meal, the unrandomized SSOB resulted in 53% lower postprandial lipemia, 23% higher hepatic lipase activity, and a 25% lower postprandial increase in FVIIa concentration. The solid fat contents at 37 degrees C were 22%, 41%, and 0% with the unrandomized SSOB, randomized SSOB, and oleic acid-rich meals, respectively.

CONCLUSIONS

Stearic acid-rich triacylglycerol in both unrandomized and randomized forms does not adversely affect lipid risk factors for cardiovascular disease. The high proportion of solid fat at 37 degrees C may explain the decreased postprandial lipemic response.

Acute postprandial effect of hydrogenated fish oil, palm oil and lard on plasma cholesterol, triacylglycerol and non-esterified fatty acid metabolism in normocholesterolaemic males

The majority of research has focused on the association betweentransunsaturated fatty acids (TUFA) from hydrogenated vegetable oils and heart disease even though TUFA are also produced from hydrogenated fish oil. We compared the acute effect of three solid fats on postprandial cholesterol, triacylglycerol (TAG) and NEFA concentrations in normocholesterolaemic males. Eight healthy male volunteers consumed each of the three 40g fat meals (partially hydrogenated fish oil (PHFO), palm oil and lard) in random order and blood samples were drawn at 2, 4, 6 and 8h thereafter for lipid analysis. The postprandial response in plasma TAG, TAG-rich lipoprotein-TAG (TRL-TAG), total cholesterol and plasma NEFA, measured as the area under the postprandial curve, was not significantly different between the three meals (p>0·05), which varied in MUFA, PUFA and TUFA content. There was no marked elevation of longer-chain fatty acids (C20–22,cisortransisomers) into the TRL-TAG fraction following the PHFO meal even though they provided 40% of the total fatty acids in the PHFO meal. The postprandial TRL-TAG response to PHFO was expected to be higher, as it is higher in TUFA, lower in PUFA and similar in saturated fatty acid composition compared with the lard and palm oil test meals. The absence of a higher postprandial response following ingestion of PHFO could be as a result of reduced absorption and increased oxidation of long-chain fatty acids (bothcisandtransisomers)

Influence of triacylglycerol structure of stearic acid-rich fats on postprandial lipaemia

Exaggerated postprandial lipaemia may increase the risk of CHD by contributing to both thrombotic and atherogenic processes. Previous research has focused on the quantity and composition of dietary fat, whereas the effect of triacylglycerol (TAG) structure on postprandial lipaemia and clotting factor VII activity has received little attention. TAG with similar fatty acid composition may have different biochemical and physical properties that are dependent on their TAG structure, and these differences may affect lipid metabolism. Recent findings suggest that differences in the physical properties of stearic acid-rich fats are associated with differences in postprandial lipaemia, and may play an important role in determining their rates of digestion and absorption.

Effect of interesterification of palmitic acid-rich triacylglycerol on postprandial lipid and factor VII response

The process of interesterification results in changes in triacylglycerol (TAG) structure and is used to increase the melting point of dietary fats. The acute health effects of this process on palmitic acid-rich fats are uncertain with regard to postprandial lipemia, insulin and factor VII activated (FVIIa) concentrations. Two randomized crossover trials in healthy male subjects compared the effects of meals containing 50 g fat [interesterified palm oil (IPO) versus native palm oil (NPO); n=20, and IPO versus high-oleic sunflower oil (HOS); n=18], on postprandial changes in lipids, glucose, insulin, chylomicron composition and FVIIa. Compared with NPO, IPO decreased postprandial TAG and insulin concentrations. Both NPO and IPO increased FVIIa concentrations postprandially; mean increases at 6 h were 21 and 19%, respectively. Compared with HOS, IPO decreased postprandial TAG (47% lower incremental area under the curve) and reduced the postprandial increase in FVIIa concentration by 64% at 6 h; no significant differences in hepatic and total lipase activities or insulin concentrations were noted. All three test meals increased postprandial leukocyte counts (average 26% at 6 h). The fatty acid composition of the chylomicron TAG was similar to the test fats following all test meals. It is concluded that interesterification of palm oil does not result in adverse changes in postprandial lipids, insulin or FVIIa compared to high oleate and native palm oils.

Adipose tissue metabolism, diabetes and vascular disease--lessons from in vivo studies

There is an epidemic of obesity, insulin resistance and cardiovascular disease. Adipose tissue plays a major metabolic role and produces hormones with important physiological effects. In vitro studies remove regulatory factors, such as blood flow, making results difficult to interpret, and animal studies cannot necessarily be extrapolated to humans. Fortunately, adipose tissue can be studied in vivo with microdialysis, adipose tissue vein cannulation, measurement of blood flow using 133Xenon washout, stable isotope tracers and biopsies. In vivo studies have shown that adipose tissue is an efficient buffer against the postprandial flux of non-esterified fatty acids (NEFA) in the circulation, protecting other tissues. When there is excess adipose tissue, this buffering effect may be impaired. The postprandial blood flow response is also reduced, potentially causing an atherogenic lipid profile and atheroma. A systems biology approach, combining in vivo techniques with genomics, proteomics and metabolomics, will clarify links between adipose tissue and vascular disease.

Influence of stearic acid on postprandial lipemia and hemostatic function

It has been suggested that fats rich in stearic acid may result in exaggerated postprandial lipemia and have adverse effects on hemostatic function. The effects of test meals containing different saturated and monounsaturated FA were compared in healthy subjects in a series of studies to investigate this hypothesis. Stearic acid, when present as cocoa butter, resulted in similar postprandial lipemia and factor VII activation compared with a meal containing high-oleic sunflower oil. Stearic acid when presented as shea butter or as randomized stearate-rich TAG resulted in decreased postprandial lipemia and decreased postprandial activation of factor VII. Stearic acid-rich test meals did not result in impaired fibrinolytic activity compared with either a low-fat meal or a meal high in oleate. The difference in responses between the different stearic acid-rich fats appears to be due to varying solid fat contents of the fats at 37 degrees C.

The metabolism of structured triacylglycerols

The triacylglycerol (TAG) structure in addition to the overall fatty acid profile is of importance when considering the nutritional effect of a dietary fat. This review aims at summarizing our current knowledge of the digestion, absorption, uptake, and transport of structured TAGs, with particular emphasis on the following aspects: gastric emptying, specificity of pancreatic lipase, lymphatic transport and clearance of chylomicrons, effects of lipid structure on tissue lipid compositions and the fecal loss of fats. So an overview will be provided for how the structure and fatty acid composition of TAGs affect their absorption and the distribution of the fatty acids in the body following digestion and absorption.

Obesity and metabolic disease: is adipose tissue the culprit?

Obesity is a risk factor for the development of type 2 diabetes and CVD. Is adipose tissue the culprit in the relationship between obesity and metabolic disease? It is certainly possible to argue that adipose tissue function is disturbed in obesity in such a way that adverse consequences may follow. For instance, lipolysis is down regulated, the sensitivity of lipolysis to insulin is reduced and there are disturbances in the regulation of adipose tissue blood flow. However, when examined critically these changes can be seen as adaptations to the increased adipose tissue mass, making the situation better rather than worse. In terms of the many peptide and other factors now known to be secreted from adipose tissue, it is easier to argue that adipose tissue is the culprit. However, for no single 'adipokine' is there as yet unequivocal evidence of a link between adipose tissue secretion and adverse metabolic events in other tissues. The best documented of these adipokines in relation to insulin resistance is adiponectin. Here, unusually, adiponectin confers insulin sensitivity, and its secretion is down regulated in obesity. It could be again that adipose tissue has down regulated its function in an attempt to compensate for its increased mass, although certainly that down-regulation is too extreme. On balance, it is clear that adipose tissue is a link in the chain of events leading to metabolic disease, but in many respects it is an innocent intermediary trying to deal with the consequences of positive energy balance, the real culprit.

Determining the relative amounts of positional isomers in complex mixtures of triglycerides using reversed-phase high-performance liquid chromatography-tandem mass spectrometry

A reversed-phase HPLC-tandem mass spectrometry (RP-HPLC-MS-MS) method was refined for the positional analysis of complex mixtures of TAG. This method has the advantages of speed, ease of automation, and specificity over traditional digestion-based methods for the positional analysis of TAG. Collision-induced dissociation (CID) of ammoniated TAG in an ion-trap mass spectrometer produced spectra that were dependent on the FA position. Dominant DAG fragments were formed from the loss of a FA moiety from the ammoniated TAG species. The loss of FA in the outer positions was favored over their loss in the central position. The combination of RP-HPLC and CID produced spectra that were free of the isotope effects that can complicate spectral interpretation in existing methods. The combination also provided selectivity based on the chromatographic fractionation of TAG, in addition to the selectivity inherent in the CID process. Proof-of-concept experiments were performed with binary mixtures of TAG from the SOS/SSO, OSO/OOS, and the PSO/POS/SPO positional isomer systems (where S is 18:0, stearic acid; O is 18:1 (cis-9), oleic acid; and P is 16:0, palmitic acid). Plots of fractional DAG fragment intensities vs. fractional composition of the binary mixtures were linear. These plots were used to determine the fractional composition of each of these isomeric systems in a variety of vegetable oils and animal fats. Current limitations, future developments, and applications of this method are discussed.

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

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.

Chylomicron and VLDL TAG structures and postprandial lipid response induced by lard and modified lard

Alterations in chylomicron and VLDL TAG and the magnitude of postprandial lipemia were studied in healthy volunteers after two meals of equal FA composition but different TAG FA positional distribution. Molecular level information of individual lipoprotein TAG regioisomers was obtained with a tandem MS method. The incremental area under the response curve of VLDL TAG was larger (P = 0.021) after modified lard than after lard. In plasma TAG, the difference did not quite reach statistical significance (P = 0.086). In general, there were less TAG with palmitic acid in the sn-2 position and more TAG with oleic acid in the sn-2 position in chylomicrons than in fat ingested. From 1.5 to 8 h postprandially, the proportion of individual chylomicron TAG was constant or influenced by TAG M.W. VLDL TAG regioisomerism was similar regardless of the positional distribution of fat ingested. Significant alterations were seen in VLDL TAG FA, in M.W. fractions, and in individual regioisomers with respect to time. The TAG sn-14:0-18:1-18:1 + sn-18:1-18:1-14:0, sn-16:0-16:1-18:1 + sn-18:1-16:1-16:0, and sn-16:1-18:1-18:1 + sn-18:1-18:1-16:1 decreased (P < 0.05); and sn-16:0-16:0-18:2 + sn-18:2-16:0-16:0, sn-16:0-16:0-18:1 + sn-18:1-16:0-16:0, sn-16:0-18:1-16:0, and sn-16:0-18:1-18:2 + sn-18:2-18:1-16:0 increased (P < 0.05) after both meals. In conclusion, positional distribution of TAG FA was found to affect postprandial lipid metabolism in healthy normolipidemic subjects.

Effect of triacylglycerol structure on absorption and metabolism of isotope-labeled palmitic and linoleic acids by humans

The effect of dietary TAG structure and fatty acid acyl TAG position on palmitic and linoleic acid metabolism was investigated in four middle-aged male subjects. The study design consisted of feeding diets containing 61 g/d of native lard (NL) or randomized lard (RL) for 28 d. Subjects then received an oral dose of either 1,3-tetradeuteriopalmitoyl-2-dideuteriolinoleoyl-rac-glycerol or a mixture of 1,3-dideuteriolinoleoyl-2-tetradeuteriopalmitoyl-rac-glycerol and 1,3-hexadeuteriopalmitoyl-2-tetradeuteriolinoleoyl-rac-glycerol. Methyl esters of plasma lipids isolated from blood samples drawn over a 2-d period were analyzed by GC-MS. Results showed that absorption of the 2H-fatty acids (2H-FA) was not influenced by TAG position. The 2H-FA at the 2-acyl TAG position were 85+/-4.6% retained after absorption. Substantial migration of 2H-16:0 (31.2+/-8.6%) from the sn-2 TAG position to the sn-1,3 position and 2H-18:2n-6 (52.8+/-6.4%) from the sn-1,3 position to the sn-2 position of chylomicron TAG occurred after initial absorption and indicates the presence of a previously unrecognized isomerization mechanism. Incorporation and turnover of the 2H-FA in chylomicron TAG, plasma TAG, and plasma cholesterol esters were not influenced by TAG acyl position. Accretion of 2H-16:0 from the sn-2 TAG position in 1-acylphosphatidylcholine was 1.7 times higher than 2H-16:0 from the sn-1,3 TAG positions. Acyl TAG position did not influence 2H-18:2n-6 incorporation in PC. The concentration of 2H-18:2n-6-derived 2H-20:4n-6 in plasma PC from subjects fed the RL diet was 1.5 times higher than for subjects fed the NL diet, and this result suggests that diets containing 16:0 located at the sn-2 TAG position may inhibit 20:4n-6 synthesis. The overall conclusion is that selective rearrangement of chylomicron TAG structures diminishes but does not totally eliminate the metabolic and physiological effects of dietary TAG structure.

The Effect of Eicosapentaenoic Acid on Rat Lymphocyte Proliferation Depends Upon Its Position in Dietary Triacylglycerols

Animal and human studies have shown that greatly increasing the amount of fish oil [rich in long-chain (n-3) PUFA] in the diet can decrease lymphocyte functions. The effects of a more modest provision of long-chain (n-3) PUFA and whether eicosapentaenoic acid (20:5) and docosahexaenoic acid (22:6) have the same effects as one another are unclear. Whether the position of 20:5 or 22:6 in dietary triacylglycerols (TAG) influences their incorporation into immune cells and their subsequent functional effects is not known. In this study, male weanling rats were fed for 6 wk one of 9 diets that contained 178 g lipid/kg and that differed in the type of (n-3) PUFA and in the position of these in dietary TAG. The control diet contained 4.4 g alpha-linolenic acid (18:3)/100 g total fatty acids. In the other diets, 20:5 or 22:6 replaced a portion (50 or 100%) of 18:3, and were in the sn-2 or the sn-1(3) position of dietary TAG. There were significant dose-dependent increases in the proportion of 20:5 or 22:6 in spleen mononuclear cell phospholipids when 20:5 or 22:6 was fed. These increases were at the expense of arachidonic acid and were largely independent of the position of 20:5 or 22:6 in dietary TAG. Spleen lymphocyte proliferation increased dose dependently when 20:5 was fed in the sn-1(3) position of dietary TAG. There were no significant differences in interleukin-2, interferon-gamma or interleukin-10 production among spleen cells from rats fed the different diets. Prostaglandin E(2) production by spleen mononuclear cells was decreased by inclusion of either 20:5 or 22:6 in the diet in the sn-1(3) position. Thus, incorporation of 20:5 or 22:6 into spleen mononuclear cell phospholipids is not influenced by the position in dietary TAG. However, the pattern of incorporation may be influenced, and there are some differential functional effects of the position of long-chain (n-3) PUFA in dietary TAG. A moderate increase in the intake of 20:5 at the sn-1(3) position of dietary TAG increases lymphocyte proliferation.

Influence of triacylglycerol structure on the postprandial response of factor VII to stearic acid-rich fats

BACKGROUND

The consumption of a synthetic, randomized, stearic acid-rich triacylglycerol results in decreased postprandial lipemia and activated factor VII (FVII:a) compared with cocoa butter (a nonrandomized, symmetrical, stearic acid-rich triacylglycerol). It was hypothesized that this difference is a consequence of the differences in structure between the 2 triacylglycerols.

OBJECTIVE

The objective was to test whether the consumption of randomized cocoa butter decreases postprandial lipemia and FVII:a.

DESIGN

A randomized crossover trial with 17 male subjects compared the effects of meals containing 50 g fat provided as a symmetrical (cocoa butter) or an asymmetrical (randomized cocoa butter) triacylglycerol on postprandial changes in lipids, chylomicron composition, and FVII:a.

RESULTS

After randomization, the postprandial area under the curve for plasma triacylglycerol decreased by 41% (P < 0.01). At 3 h the plasma concentrations of triacylglycerol, palmitic acid, stearic acid, and oleic acid were 26%, 18%, 34%, and 19% lower, respectively. The proportion of oleic acid in the sn-2 position of the chylomicron triacylglycerol was reduced from 67.4 mol% to 35.9 mol% and resulted in an increase in the proportion of stearic acid in the sn-2 position from 9.2 mol% to 25.4 mol%. FVII:a did not increase 6 h after consumption of the randomized cocoa butter (: 1.2; 95% CI: -2.7, 4.6 U/L) but increased significantly (: 7.7; 95% CI: 2.5,12.9 U/L) 6 h after consumption of the unrandomized cocoa butter.

CONCLUSIONS

Symmetrical stearic acid-rich triacylglycerol with oleic acid in the sn-2 position appears to be absorbed more rapidly than is asymmetrical triacylglycerols with long-chain saturated fatty acids in the sn-2 position, which leads to activation of FVII.

Regulation of dietary fatty acid entrapment in subcutaneous adipose tissue and skeletal muscle

Using stable isotopic labeling of dietary fatty acids in conjunction with arteriovenous difference measurements, we have assessed the regulation of lipoprotein lipase-derived fatty acid entrapment in subcutaneous adipose tissue and forearm muscle in healthy subjects in the postprandial state. Eight volunteers fasted overnight and were then given a mixed meal containing [ 1-(13)C]palmitic acid and [1-(13)C]oleic acid. At baseline and for 6 h after the meal, blood samples were obtained from an arterialized hand vein and veins draining subcutaneous abdominal adipose tissue and forearm muscle, and arteriovenous differences were calculated. Entrapment of labeled fatty acids released by circulating triacylglycerol hydrolysis was close to 100% at 60 min, decreasing to 10-30% by 360 min. Entrapment of labeled fatty acids in forearm muscle was >100% and did not change with time. This study shows that entrapment of dietary fatty acids in adipose tissue in the postprandial period is a highly regulated process (varying with time) and that this can be studied in humans using stable isotope- labeled fatty acids in combination with measurement of appropriate arteriovenous differences. Also, fatty acid trapping in skeletal muscle is fundamentally different from that in adipose tissue, in that all the fatty acids released by lipoprotein lipase in skeletal muscle are taken up by the tissue.

Impaired postprandial adipose tissue blood flow response is related to aspects of insulin sensitivity

Obesity has been associated with dysfunctional postprandial adipose tissue blood flow (ATBF), but it has also been recognized that the interindividual response is highly variable. The present work aimed at characterizing this variability. Fifteen subjects were given 75 g oral glucose, and abdominal subcutaneous ATBF was monitored by the (133)Xe washout method. Determinants of insulin sensitivity based on nonesterified fatty acid (NEFA) suppression after oral glucose administration [ISI(NEFA)] were higher in the top tertile ATBF response group (1.29 +/- 0.09 vs. 0.90 +/- 0.08 in the lower tertiles, P = 0.01). ISI(NEFA) was related to ATBF response (r(s) = 0.73, P < 0.002) as well as insulin sensitivity based on postprandial glycemia [ISI(gly), r(s) = 0.58, P < 0.05], whereas the homeostasis model assessment (HOMA) index (r(s) = -0.39, P = 0.16) was not. The relationship between increase in ATBF and ISI(NEFA) was independent of BMI (P = 0.015) in multivariate analysis. Subjects with a high ATBF response had significantly higher increase of plasma norepinephrine (P < 0.05), indicating a link between postprandial insulinemia, sympathetic activation, and ATBF response. There is a close relationship between insulin sensitivity and the regulation of postprandial ATBF, independent of adiposity. Impaired regulation of ATBF seems to be another facet of the insulin resistance syndrome.

Studies on effects of dietary fatty acids as related to their position on triglycerides

This article reviews published literature on how the stereospecific structure of dietary triglycerides may affect lipid metabolism in humans. Animal studies have shown enhanced absorption of fatty acids in the sn-2 position of dietary triglycerides. Increasing the level of the saturated fatty acid palmitic acid in the sn-2 position (e.g., by interesterification of the fat to randomize the positions of the fatty acids along the glycerol backbone) has been shown in rabbits to increase the atherogenic potential of the fat without impacting levels of blood lipids and lipoproteins. In contrast, enhancing the level of stearic acid in the sn-2 position has not been found to affect either atherogenic potential or levels of blood lipids and lipoproteins in rabbits. Fatty acids other than palmitic and stearic have not been studied systematically with respect to possible positional effects. A limited number of human studies have shown no significant effects of interesterified fats on blood lipid parameters. However, it is unknown whether modifying the stereospecific structure of dietary triglycerides would affect atherogenicity or other long-term health conditions in humans. It is possible that incorporation of palmitic acid into the sn-2 position of milk fat is beneficial to the human infant (as a source of energy for growth and development) but not to human adults. Additional research is needed to determine whether processes like interesterification, which can be used to alter physical parameters of dietary fats (e.g., melting characteristics), may result in favorable or unfavorable long-term effects in humans.

Synthetic and modified glycerides: effects on plasma lipids

It has been suggested that the molecular species or structure of the triglyceride, i.e. not only what fatty acids are present but also their relative order in the sn1, 2, or 3 position on the triglyceride, can influence the metabolism of the triglyceride and its fatty acids, including lipoprotein metabolism. One rationale for this possibility assumes that the fatty acid in the sn2 position can be absorbed intact, i.e. as the sn2 monoglyceride, whereas the sn1,3 fatty acids are absorbed as free fatty acids that metabolize independently. Some sn2 monoglyceride might ultimately serve as the backbone for gut or liver phospholipids, exerting downstream influence on lipid metabolism. Experiments that test this hypothesis directly by feeding triglycerides with modified structure during carefully controlled fat intake are few, particularly in humans, but their results tend to support the paradigm.

The metabolic availability of dietary triacylglycerols from two high oleic oils during the postprandial period does not depend on the amount of oleic acid ingested by healthy men

Monounsaturated oils, virgin olive oil (VOO) and high oleic sunflower oil (HOSO) are suggested to have selective physiologic effects on humans in the fasting state. The aim of the study was to evaluate whether two oils with equal amounts of oleic acid but with different compositions of minor fatty acids and triacylglycerol molecular species (TAG) could produce different triacylglycerol-rich lipoprotein (TRL)-TAG responses in the postprandial state. Eight normolipidemic men consumed the following three meals in random order on separate occasions with 2 wk between meals: control meal, control meal plus VOO and control meal plus HOSO. Plasma total TAG and TRL-TAG were measured hourly for 7 h after ingestion. TAG and sn-2 positional fatty acids within TAG were analyzed in the TRL fraction. Plasma total TAG concentrations in response to the dietary oils did not differ. However, TRL triglyceridemia was significantly lower after VOO intake (P < 0.05). The molecular species in the TRL fraction returned toward basal levels more quickly (P < 0.05) after VOO than HOSO intake. 2-Positional fatty acid analysis demonstrated higher proportions of stearic and palmitic acids and a lower proportion of oleic acid (P < 0.05) in TRL-TAG derived from HOSO. This study shows that VOO intake results in attenuated postprandial TAG concentration and faster TRL-TAG disappearance from blood compared with HOSO, suggesting that the oleic acid content may not be the main factor affecting TAG metabolism. Minor fatty acids such as linoleic acid and the 2-positional distribution of saturated stearic and palmitic acids into the TAG molecule may be important determinants of postprandial lipemia in normolipidemic men.

Uptake of individual fatty acids into adipose tissue in relation to their presence in the diet

BACKGROUND

The fatty acid composition of adipose tissue triacylglycerol reflects, but is not identical to, the fatty acid composition of the habitual diet.

OBJECTIVE

We investigated whether the fatty acid composition of adipose tissue is explained by differences between fatty acids in early storage in adipose tissue after a meal.

DESIGN

Nine healthy men ate a meal containing several fatty acids. Blood samples were taken for 6 h after the meal from an arterialized hand vein and a vein draining the anterior abdominal subcutaneous adipose tissue.

RESULTS

Net storage of fatty acids in adipose tissue occurred between 1 and 4 h after the meal. In relation to the amount fed, storage of fatty acids differed (P < 0. 01) between classes (n-3 polyunsaturated < saturated < n-6 polyunsaturated < monounsaturated); oleic acid was stored in the greatest amounts. These differences agreed closely with published data, except for n-3 polyunsaturated fatty acids. The only individual metabolic step at which significant differences between fatty acids was shown was incorporation of fatty acids into chylomicron triacylglycerol. Differences between fatty acids in rate of extraction from chylomicron triacylglycerol and net uptake into adipose tissue in the postprandial period were significant (P < 0. 01), but not when expressed in relation to proportions in chylomicron triacylglycerol.

CONCLUSIONS

The characteristic fatty acid pattern of adipose tissue may predominantly reflect the early metabolic handling of different fatty acids. Adipose tissue uptake of n-3 polyunsaturated fatty acids is slow in relation to that of other fatty acids.