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

Palmitic acid in the sn-2 position of dietary triacylglycerols does not affect insulin secretion or glucose homeostasis in healthy men and women

Background/objectives:

Dietary triacylglycerols containing palmitic acid in the sn-2 position might impair insulin release and increase plasma glucose.

Subjects/Methods:

We used a cross-over designed feeding trial in 53 healthy Asian men and women (20–50 years) to test this hypothesis by exchanging 20% energy of palm olein (PO; control) with randomly interesterified PO (IPO) or high oleic acid sunflower oil (HOS). After a 2-week run-in period on PO, participants were fed PO, IPO and HOS for 6 week consecutively in randomly allocated sequences. Fasting (midpoint and endpoint) and postprandial blood at the endpoint following a test meal (3.54 MJ, 14 g protein, 85 g carbohydrate and 50 g fat as PO) were collected for the measurement of C-peptide, insulin, glucose, plasma glucose-dependent insulinotropic polypeptide and glucagon-like peptide-1, lipids and apolipoproteins; pre-specified primary and secondary outcomes were postprandial changes in C-peptide and plasma glucose.

Results:

Low density lipoprotein cholesterol was 0.3 mmol/l (95% confidence interval (95% CI)) 0.1, 0.5; P<0.001) lower on HOS than on PO or IPO as predicted, indicating good compliance to the dietary intervention. There were no significant differences (P=0.58) between diets among the 10 male and 31 female completers in the incremental area under the curve (0–2 h) for C-peptide in nmol.120 min/l: GM (95% CI) were PO 220 (196, 245), IPO 212 (190, 235) and HOS 224 (204, 244). Plasma glucose was 8% lower at 2 h on IPO vs PO and HOS (both P<0.05).

Conclusion:

Palmitic acid in the sn-2 position does not adversely impair insulin secretion and glucose homeostasis.

Classical endocannabinoid-like compounds and their regulation by nutrients

Endocannabinoid-like compounds are structurally related to the true endocannabinoids but do not contain highly unsaturated fatty acids, and they do not bind the cannabinoid receptors. The classical endocannabinoid-like compounds include N-acylethanolamines and 2-monoacylglycerols, and their structural resemblance to the endocannabinoids makes them players in the endocannabinoid system, where they can interfere with the actions of the true endocannabinoids, because they in several cases engage the same synthesizing and degrading enzymes. In addition they have pharmacological actions of their own, which are particularly interesting in a nutritional and metabolic context. Exogenously supplied oleoylethanolamide, palmitoylethanolamide, and linoleoylethanolamide have anorexic effects, and the endogenous formation of these N-acylethanolamines in the small intestine may serve an important role in regulating food intake, through signaling via PPARα and the vagus nerve to the brain appetite center. A chronic high-fat diet will decrease intestinal levels of these anorectic N-acylethanolamines and this may contribute to the hyperphagic effect of high-fat diet; 2-monoacylglycerols mediate endocrine responses in the small intestine; probably trough activation of GPR119 on enteroendocrine cells, and diet-derived 2-monoacylglycerols, for example, 2-oleoylglycerol and 2-palmitoylglycerol might be important for intestinal fat sensing. Whether these 2-monoacylglycerols have signaling functions in other tissues is unclear at present.

Palmitic acid in the sn-2 position decreases glucose-dependent insulinotropic polypeptide secretion in healthy adults

BACKGROUND/OBJECTIVES

Dietary triacylglycerols (TAGs) containing palmitic acid in the sn-2 position might impair insulin release and increase plasma glucose. We tested this hypothesis by comparing postprandial responses to fats with varying proportions of palmitic acid in the sn-2 position.

SUBJECTS/METHODS

Using a crossover-designed randomized controlled trial in healthy men (n=25) and women (n=25), we compared four meals on postprandial changes in glucose (primary outcome), insulin, C-peptide, glucose, glucose-dependent insulinotropic polypeptide (GIP) and polypeptide YY (PYY) concentrations. The meals provided 14 g protein, 85 g carbohydrate and 50 g test fat, supplied as high oleic sunflower (HOS) oil (control), palm olein (PO), interesterified palm olein (IPO) and lard containing 0.6, 9.2, 39.1 and 70.5 mol% palmitic acid at sn-2, respectively.

RESULTS

No differences in plasma glucose, insulin and C-peptide response between meals were found. GIP release was lower (P<0.001) for IPO and lard compared with HOS and PO meals; the maximal increments (geometric mean and 95% confidence interval) for HOS, PO, IPO and lard were 515 (468, 569), 492 (448, 540), 398 (350, 452) and 395 (364, 429) ng/l, respectively. There was a trend for the postprandial increase in PYY to be lower in women on the IPO and lard meals than those on the HOS and PO meals.

CONCLUSIONS

Dietary TAGs with an increased proportion of palmitic acid in the sn-2 position do not have acute adverse effects on the insulin and glucose response to meals in healthy men and women, but they decrease GIP release.

Modulation of obesity-induced inflammation by dietary fats: mechanisms and clinical evidence

Obesity plays a pivotal role in the development of low-grade inflammation. Dietary fatty acids are important modulators of inflammatory responses. Saturated fatty acids (SFA) and n-6 polyunsaturated fatty acids (PUFA) have been reported to exert pro-inflammatory effects. n-3 PUFA in particular, possess anti-inflammatory properties. Numerous clinical studies have been conducted over decades to investigate the impact of dietary fatty acids on inflammatory response in obese individuals, however the findings remained uncertain. High fat meals have been reported to increase pro-inflammatory responses, however there is limited evidence to support the role of individual dietary fatty acids in a postprandial state. Evidence in chronic studies is contradictory, the effects of individual dietary fatty acids deserves further attention. Weight loss rather than n-3 PUFA supplementation may play a more prominent role in alleviating low grade inflammation. In this context, the present review provides an update on the mechanistic insight and the influence of dietary fats on low grade inflammation, based on clinical evidence from acute and chronic clinical studies in obese and overweight individuals.

Reappraisal of SFA and cardiovascular risk

This review reappraises dietary advice to reduce and replace SFA for the prevention of CVD. In the 1970s, SFA accounted for about 18% UK food energy, by 2001 it had fallen to 13% and continues to be above the <11% target. Compared with carbohydrates, C12–C16 SFA raise serum total cholesterol (TC), LDL-cholesterol (LDL-C) and HDL-cholesterol (HDL-C) without affecting the TC:HDL-C ratio; other SFA have neutral effects on the fasting lipid profile. Replacing 3% dietary SFA with MUFA or PUFA lowers LDL-C by 2% and TC:HDL-C ratio by 0·03. No other specific adverse effects of SFA compared with MUFA on risk CVD factors have been proven. Meta-analyses of prospective cohort studies report the relative risks (95% CI) of high v. low intakes of SFA to be 1·07 (0·96, 1·19) for CHD, 0·81 (0·62, 1·05) for stroke and 1·00 (0·89, 1·11) for CVD mortality and were not statistically significant. Exchanging 5% energy SFA for PUFA or carbohydrates found hazard ratios (95% CI) for CHD death to be 26% (−23, −3) and 4% (−18, 12; NS) lower, respectively. Meta-analysis of randomised controlled trials with clinical endpoints reports mean reductions (95% CI) of 14% (4, 23) in CHD incidence and 6% (−25, 4; NS) in mortality in trials, where SFA was lowered by decreasing and/or modifying dietary fat. In conclusion, SFA intakes are now close to guideline amounts and further reductions may only have a minor impact on CVD.

Dietary triacyglycerols rich in sn-2 palmitate alter post-prandial lipoprotein and unesterified fatty acids in term infants

Human milk TAG contain 20-25% 16:0, with over 70% of the 16:0 at the TAG sn-2 position. The benefits of TAG sn-2 16:0 have been ascribed to reducing 16:0 excretion as insoluble fatty acid soaps. This study builds on knowledge that infants conserve milk TG sn-2 16:0 post-absorption. Comparison of plasma lipids from 120 day old infants fed formula containing 25-27% 16:0 with 29% 16:0 or 5% 16:0 at the TAG sn-2 position showed higher formula sn-2 16:0 led to lower 18:1n-9, but higher 18:2n-6 and 22:6n-3 in the infant plasma unesterified fatty acids, higher 18:0 in LDL TAG, and higher apo B and lower apo A-1. TAG-sn-2 16:0 may provide 16:0 in remnant particles for hepatic elongation to 18:0, needed for plasma and tissue phospholipids. We suggest attention to the plasma unesterified fatty acids as possible sources of fatty acids for membrane phospholipid synthesis.

High-fat meal induced postprandial inflammation

Raised levels of circulating inflammatory markers are associated with coronary artery disease, obesity and type II diabetes. It has been proposed that the ingestion of high-fat meals may serve as a stimulus to raise systemic inflammatory tone, although interventional studies have yielded conflicting results. We here review 57 studies of high-fat meal induced acute postprandial inflammation to identify the most frequently reported markers of postprandial inflammation and to compare these results with the highly consistent low-grade endotoxaemia model in man. Most plasma borne markers of inflammation, such as cytokines and soluble adhesion molecules, were not consistently raised after a high-fat meal. However, pro-inflammatory leukocyte surface markers, mRNA and proteins were elevated in almost all studies in which they were measured. These markers followed kinetics similar to those observed following intravenous injection of low doses of endotoxin in man, were positively associated with likelihood of contamination of test meals with pro-inflammatory bacterial molecules and were reduced in several studies examining parallel meals supplemented with foodstuffs containing anti-inflammatory phytochemicals. Future studies of postprandial inflammation may yield more consistent evidence by focusing on leukocyte, rather than plasma-borne, markers of inflammation and by considering the test meal content of pro- and anti-inflammatory dietary constituents.

High beta-palmitate fat controls the intestinal inflammatory response and limits intestinal damage in mucin Muc2 deficient mice

BACKGROUND

Palmitic-acid esterified to the sn-1,3 positions of the glycerol backbone (alpha, alpha'-palmitate), the predominant palmitate conformation in regular infant formula fat, is poorly absorbed and might cause abdominal discomfort. In contrast, palmitic-acid esterified to the sn-2 position (beta-palmitate), the main palmitate conformation in human milk fat, is well absorbed. The aim of the present study was to examine the influence of high alpha, alpha'-palmitate fat (HAPF) diet and high beta-palmitate fat (HBPF) diet on colitis development in Muc2 deficient (Muc2(-/-)) mice, a well-described animal model for spontaneous enterocolitis due to the lack of a protective mucus layer.

METHODS

Muc2(-/-) mice received AIN-93G reference diet, HAPF diet or HBPF diet for 5 weeks after weaning. Clinical symptoms, intestinal morphology and inflammation in the distal colon were analyzed.

RESULTS

Both HBPF diet and AIN-93G diet limited the extent of intestinal erosions and morphological damage in Muc2(-/-) mice compared with HAPF diet. In addition, the immunosuppressive regulatory T (Treg) cell response as demonstrated by the up-regulation of Foxp3, Tgfb1 and Ebi3 gene expression levels was enhanced by HBPF diet compared with AIN-93G and HAPF diets. HBPF diet also increased the gene expression of Pparg and enzymatic antioxidants (Sod1, Sod3 and Gpx1), genes all reported to be involved in promoting an immunosuppressive Treg cell response and to protect against colitis.

CONCLUSIONS

This study shows for the first time that HBPF diet limits the intestinal mucosal damage and controls the inflammatory response in Muc2(-/-) mice by inducing an immunosuppressive Treg cell response.

Dietary fatty acids and lipoprotein metabolism: new insights and updates

PURPOSE OF REVIEW

Dyslipidemia is a powerful risk factor for cardiovascular disease (CVD). Dietary fatty acid composition regulates lipids and lipoprotein metabolism and may confer CVD benefit. This review updates understanding of the effect of dietary fatty acids on lipoprotein metabolism in humans.

RECENT FINDINGS

High dietary fish-derived n-3 polyunsaturated fatty acid (PUFA) consumption diminished hepatic triglyceride-rich lipoprotein (TRL) secretion and enhanced TRL to LDL conversion. n-3 PUFA also decreased TRL-apoB-48 concentration by decreasing TRL-apoB-48 secretion. High n-6 PUFA intake decreased liver fat, and plasma proprotein convertase subtilisin/kexin type 9, triglycerides, total-cholesterol and LDL-cholesterol concentrations. Intake of saturated fatty acids with increased palmitic acid at the sn-2 position was associated with decreased postprandial lipemia, which might be due to decreased triglyceride absorption. Replacing carbohydrate with monounsaturated fatty acids increased TRL catabolism. Ruminant trans-fatty acid decreased HDL cholesterol, but the mechanisms are unknown. A new role for APOE genotype in regulating lipid responses was also described.

SUMMARY

The major advances in understanding the effect of dietary fatty acids on lipoprotein metabolism have focused on n-3 PUFA. This knowledge provides insights into the importance of regulating lipoprotein metabolism as a mode to improve plasma lipids and potential CVD risk. Further studies are required to better understand the cardiometabolic effects of other dietary fatty acids.