Role of triglyceride-rich lipoproteins from the liver and intestine in the etiology of postprandial peaks in plasma triglyceride concentration

An Updated Perspective on the Dual-Track Model of Enterocyte Fat Metabolism

The small intestinal epithelium has classically been envisioned as a conduit for nutrient absorption, but appreciation is growing for a larger and more dynamic role for enterocytes in lipid metabolism. Considerable gaps remain in our knowledge of this physiology, but it appears that the enterocyte's structural polarization dictates its behavior in fat partitioning, treating fat differently based on its absorption across the apical versus the basolateral membrane. In this review, we synthesize existing data and thought on this dual-track model of enterocyte fat metabolism through the lens of human integrative physiology. The apical track includes the canonical pathway of dietary lipid absorption across the apical brush-border membrane, leading to packaging and secretion of those lipids as chylomicrons. However, this track also reserves a portion of dietary lipid within cytoplasmic lipid droplets for later uses, including the "second-meal effect," which remains poorly understood. At the same time, the enterocyte takes up circulating fats across the basolateral membrane by mechanisms that may include receptor-mediated import of triglyceride-rich lipoproteins or their remnants, local hydrolysis and internalization of free fatty acids, or enterocyte de novo lipogenesis using basolaterally absorbed substrates. The ultimate destinations of basolateral-track fat may include fatty acid oxidation, structural lipid synthesis, storage in cytoplasmic lipid droplets, or ultimate resecretion, although the regulation and purposes of this basolateral track remain mysterious. We propose that the enterocyte integrates lipid flux along both of these tracks in order to calibrate its overall program of lipid metabolism.

Burmese pythons exhibit a transient adaptation to nutrient overload that prevents liver damage

As an opportunistic predator, the Burmese python (Python molurus bivittatus) consumes large and infrequent meals, fasting for up to a year. Upon consuming a large meal, the Burmese python exhibits extreme metabolic responses. To define the pathways that regulate these postprandial metabolic responses, we performed a comprehensive profile of plasma metabolites throughout the digestive process. Following ingestion of a meal equivalent to 25% of its body mass, plasma lipoproteins and metabolites, such as chylomicra and bile acids, reach levels observed only in mammalian models of extreme dyslipidemia. Here, we provide evidence for an adaptive response to postprandial nutrient overload by the python liver, a critical site of metabolic homeostasis. The python liver undergoes a substantial increase in mass through proliferative processes, exhibits hepatic steatosis, hyperlipidemia-induced insulin resistance indicated by PEPCK activation and pAKT deactivation, and de novo fatty acid synthesis via FASN activation. This postprandial state is completely reversible. We posit that Burmese pythons evade the permanent hepatic damage associated with these metabolic states in mammals using evolved protective measures to inactivate these pathways. These include a transient activation of hepatic nuclear receptors induced by fatty acids and bile acids, including PPAR and FXR, respectively. The stress-induced p38 MAPK pathway is also transiently activated during the early stages of digestion. Taken together, these data identify a reversible metabolic response to hyperlipidemia by the python liver, only achieved in mammals by pharmacologic intervention. The factors involved in these processes may be relevant to or leveraged for remediating human hepatic pathology.

High-throughput LC-MS method to investigate postprandial lipemia: considerations for future precision nutrition research

Elevated postprandial lipemia is an independent risk factor for cardiovascular disease, yet methods to quantitate postmeal handling of dietary lipids in humans are limited. This study tested a new method to track dietary lipid appearance using a stable isotope tracer (²H₁₁-oleate) in liquid meals containing three levels of fat [low fat (LF), 15 g; moderate fat (MF), 30 g; high fat (HF), 60 g]. Meals were fed to 12 healthy men [means ± SD, age 31.3 ± 9.2 yr, body mass index (BMI) 24.5 ± 1.9 kg/m²] during four randomized study visits; the HF meal was administered twice for reproducibility. Blood was collected over 8 h postprandially, triglyceride (TG)-rich lipoproteins (TRL), and particles with a Svedberg flotation rate >400 (S_f > 400, n = 8) were isolated by ultracentrifugation, and labeling of two TG species (54:3 and 52:2) was quantified by LC-MS. Total plasma TRL-TG concentrations were threefold greater than S_f > 400-TG. Both S_f > 400- and TRL-TG 54:3 were present at higher concentrations than 52:2, and singly labeled TG concentrations were higher than doubly labeled. Furthermore, TG 54:3 and the singly labeled molecules demonstrated higher plasma absolute entry rates differing significantly across fat levels within a single TG species (P < 0.01). Calculation of fractional entry showed no significant differences in label handling supporting the utility of either TG species for appearance rate calculations. These data demonstrate the utility of labeling research meals with stable isotopes to investigate human postprandial lipemia while simultaneously highlighting the importance of examining individual responses. Meal type and timing, control of prestudy activities, and effects of sex on outcomes should match the research goals. The method, optimized here, will be beneficial to conduct basic science research in precision nutrition and clinical drug development.NEW & NOTEWORTHY A novel method to test human intestinal lipid handling using stable isotope labeling is presented and, for the first time, plasma appearance and lipid turnover were quantified in 12 healthy men following meals with varying amounts of fat. The method can be applied to studies in precision nutrition characterizing individual response to support basic science research or drug development. This report discusses key questions for consideration in precision nutrition that were highlighted by the data.

Medium-chain fatty acids lower postprandial lipemia: A randomized crossover trial

Epidemiological and interventional studies have linked saturated fatty acids (SFA) with elevated levels of low-density lipoprotein cholesterol (LDL-C) and increased CVD risk. However, the effects of the SFA chain length on postprandial lipemia in humans are not well elucidated. The aim of this study was to investigate the impact of short, medium and long-chain SFA on postprandial blood lipids in healthy volunteers. Sixteen healthy volunteers consumed test biscuits containing 40 g of either butter (BB), coconut oil (CB) or lard (LB) in a single-blinded, randomized crossover design. Blood samples were collected fasting and 2, 3, 4, and 6 hours postprandially and assessed for blood lipids (total cholesterol, TC; high-density lipoprotein cholesterol, HDL-C; LDL-C and triglyceride, TG). The postprandial TG response following CB was 59.8% lower than following BB (p < 0.01) and 58.8% lower than LB (p < 0.01), although no difference was observed between the BB and the LB responses. The net area under the LDL-C concentration curve was significantly larger after consumption of the CB compared to the BB, despite no significant differences in postprandial net area under the TC and HDL-C concentration curves. Consumption of medium-chain SFA as CB resulted in lower postprandial TG excursions compared to short-chain SFA as BB and long-chain SFA as LB, despite their identical fat and caloric content. These results suggest that SFA differ in their potential to elevate postprandial lipid levels, and that coconut oil, a rich source of medium-chain SFA may not be as hyperlipidemic as animal fats rich in long chain SFA. ANZCTR IDENTIFIER: 12617000903381. CLINICAL TRIAL REGISTRY NUMBER: The study was registered with the Australia New Zealand Trial registry as ACTRN12617000903381.

Oral Glucose Mobilizes Triglyceride Stores From the Human Intestine

BACKGROUND & AIMS

The small intestine regulates plasma triglyceride (TG) concentration. Within enterocytes, dietary TGs are packaged into chylomicrons (CMs) for secretion or stored temporarily in cytoplasmic lipid droplets (CLDs) until further mobilization. We and others have shown that oral and intravenous glucose enhances CM particle secretion in human beings, however, the mechanisms through which this occurs are incompletely understood.

METHODS

Two separate cohorts of participants ingested a high-fat liquid meal and, 5 hours later, were assigned randomly to ingest either a glucose solution or an equivalent volume of water. In 1 group (N = 6), plasma and lipoprotein TG responses were assessed in a randomized cross-over study. In a separate group (N = 24), duodenal biopsy specimens were obtained 1 hour after ingestion of glucose or water. Ultrastructural and proteomic analyses were performed on duodenal biopsy specimens.

RESULTS

Compared with water, glucose ingestion increased circulating TGs within 30 minutes, mainly in the CM fraction. It decreased the total number of CLDs and the proportion of large-sized CLDs within enterocytes. We identified 2919 proteins in human duodenal tissue, 270 of which are related to lipid metabolism and 134 of which were differentially present in response to glucose compared with water ingestion.

CONCLUSIONS

Oral glucose mobilizes TGs stored within enterocyte CLDs to provide substrate for CM synthesis and secretion. Future studies elucidating the underlying signaling pathways may provide mechanistic insights that lead to the development of novel therapeutics for the treatment of hypertriglyceridemia.

Recent Advances in Triacylglycerol Mobilization by the Gut

Dietary lipid absorption and lipoprotein secretion by the gut are important in maintaining whole-body energy homeostasis and have significant implications for health and disease. The processing of dietary lipids, including storage within and subsequent mobilization and transport from enterocyte cytoplasmic lipid droplets or other intestinal lipid storage pools (including the secretary pathway, lamina propria and lymphatics) and secretion of chylomicrons, involves coordinated steps that are subject to various controls. This review summarizes recent advances in our understanding of the mechanisms that underlie lipid storage and mobilization by small intestinal enterocytes and the intestinal lymphatic vasculature. Therapeutic targeting of lipid processing by the gut may provide opportunities for the treatment and prevention of dyslipidemia, and for improving health status.

An additional bolus of rapid-acting insulin to normalise postprandial cardiovascular risk factors following a high-carbohydrate high-fat meal in patients with type 1 diabetes: A randomised controlled trial

AIM

To evaluate an additional rapid-acting insulin bolus on postprandial lipaemia, inflammation and pro-coagulation following high-carbohydrate high-fat feeding in people with type 1 diabetes.

METHODS

A total of 10 males with type 1 diabetes [HbA_1c 52.5 ± 5.9 mmol/mol (7.0% ± 0.5%)] underwent three conditions: (1) a low-fat (LF) meal with normal bolus insulin, (2), a high-fat (HF) meal with normal bolus insulin and (3) a high-fat meal with normal bolus insulin with an additional 30% insulin bolus administered 3-h post-meal (HFA). Meals had identical carbohydrate and protein content and bolus insulin dose determined by carbohydrate-counting. Blood was sampled periodically for 6-h post-meal and analysed for triglyceride, non-esterified-fatty acids, apolipoprotein B48, glucagon, tumour necrosis factor alpha, fibrinogen, human tissue factor activity and plasminogen activator inhibitor-1. Continuous glucose monitoring captured interstitial glucose responses.

RESULTS

Triglyceride concentrations following LF remained similar to baseline, whereas triglyceride levels following HF were significantly greater throughout the 6-h observation period. The additional insulin bolus (HFA) normalised triglyceride similarly to low fat 3-6 h following the meal. HF was associated with late postprandial elevations in tumour necrosis factor alpha, whereas LF and HFA was not. Fibrinogen, plasminogen activator inhibitor-1 and tissue factor pathway levels were similar between conditions.

CONCLUSION

Additional bolus insulin 3 h following a high-carbohydrate high-fat meal prevents late rises in postprandial triglycerides and tumour necrosis factor alpha, thus improving cardiovascular risk profile.

Modulation of human postprandial lipemia by changing ratios of polyunsaturated to saturated (P/S) fatty acid content of blended dietary fats: a cross-over design with repeated measures

BACKGROUND

Postprandial lipemia (PL) contributes to coronary artery disease. The fatty acid composition of dietary fats is potentially a modifiable factor in modulating PL response.

METHODS

This human postprandial study evaluated 3 edible fat blends with differing polyunsaturated to saturated fatty acids (P/S) ratios (POL = 0.27, AHA = 1.00, PCAN = 1.32). A cross-over design included mildly hypercholestrolemic subjects (9 men and 6 women) preconditioned on test diets fats at 31% energy for 7 days prior to the postprandial challenge on the 8th day with 50 g test fat. Plasma lipids and lipoproteins were monitored at 0, 1.5, 3.5, 5.5 and 7 hr.

RESULTS

Plasma triacylglycerol (TAG) concentrations in response to POL, AHA or PCAN meals were not significant for time x test meal interactions (P > 0.05) despite an observed trend (POL > AHA > PCAN). TAG area-under-the-curve (AUC) increased by 22.58% after POL and 7.63% after PCAN compared to AHA treatments (P > 0.05). Plasma total cholesterol (TC) response was not significant between meals (P > 0.05). Varying P/S ratios of test meals significantly altered prandial high density lipoprotein-cholesterol (HDL-C) concentrations (P < 0.001) which increased with decreasing P/S ratio (POL > AHA > PCAN). Paired comparisons was significant between POL vs PCAN (P = 0.009) but not with AHA or between AHA vs PCAN (P > 0.05). A significantly higher HDL-C AUC for POL vs AHA (P = 0.015) and PCAN (P = 0.001) was observed. HDL-C AUC increased for POL by 25.38% and 16.0% compared to PCAN and AHA respectively. Plasma low density lipoprotein-cholesterol (LDL-C) concentrations was significant (P = 0.005) between meals and significantly lowest after POL meal compared to PCAN (P = 0.004) and AHA (P > 0.05) but not between AHA vs PCAN (P > 0.05). AUC for LDL-C was not significant between diets (P > 0.05). Palmitic (C16:0), oleic (C18:1), linoleic (C18:2) and linolenic (C18:3) acids in TAGs and cholesteryl esters were significantly modulated by meal source (P < 0.05).

CONCLUSIONS

P/S ratio of dietary fats significantly affected prandial HDL-C levels without affecting lipemia.

Postprandial lipaemia suppresses endothelium-dependent arterial dilation in patients with hypothyroidism

Endothelial dysfunction represents an early step in the development of atherosclerosis. The purpose of this study was to investigate the relationship between postprandial lipaemia and endothelial dysfunction in patients with overt hypothyroidism (oHT) and subclinical hypothyroidism (sHT). Female subjects with oHT and sHT, as well as female healthy subjects with euthyroid state were enrolled (10 cases in each group). The examination of flow-mediated dilation (FMD) was performed before and after an oral fat-loading by high resolution ultrasound. Endothelial dysfunction after an oral fat challenge was related to the extent of hypertriglyceridemia and free radicals. FMD decreased significantly at 4-h point in 3 groups, (p < 0.05) and then FMD in control and sHT restored to baseline at 8-h point, it was lower than baseline in sHT group at 6-h point (p = 0.042). However, FMD continued to decrease at 6-h point (p < 0.001), and then increased toward to baseline at 8-h point, which was still lower than baseline (p = 0.039) in oHT. Spearman's analysis showed a negative correlation between FMD and triglyceride, a negative correlation between FMD and thiobarbituric acid reactive substances (TBARS), and a positive correlation between triglyceride and TBARS levels during oral lipid-loading test in hypothyroid patients (p < 0.001) and controls (p < 0.05). In hypothyroid subjects including oHT and sHT, even in healthy individuals, FMD was impaired after an oral fat challenge. The endothelial dysfunction observed after an oral fat challenge was related to the extent of hypertriglyceridemia and oxygen-derived free radicals.

Fasting and postprandial apolipoprotein B-48 levels in healthy, obese, and hyperlipidemic subjects

Apolipoprotein (apo) B-48 is the only specific marker of intestinal lipoproteins. We evaluated a novel enzyme-linked immunosorbent assay (ELISA) standardized with recombinant apo B-48 to measure apo B-48 in plasma and triglyceride-rich lipoproteins (TRLs, density <1.006 g/mL). Coefficients of variation were less than 2.5%. Assay values correlated well (r = 0.82, P < .001) with values obtained by gel scanning of TRLs (n = 75 samples); however, the gel scanning method yielded values that were about 50% lower than ELISA values. About 60% to 70% of apo B-48 was found in TRLs. In 12 healthy subjects, median fasting plasma apo B-48 levels were 0.51 mg/dL and were increased by 121% to 147% in the fed state. In 63 obese subjects, median fasting apo B-48 values were 0.82 mg/dL; and feeding resulted in almost no change in total cholesterol, non-high-density lipoprotein cholesterol, or total apo B values, whereas triglyceride, remnant lipoprotein cholesterol, and apo B-48 levels were significantly higher (P < .05; by +73%, +58%, and +106%), and direct low-density lipoprotein cholesterol and direct high-density lipoprotein cholesterol were significantly lower (P < .001, by -13% and -20%) than fasting values. Relative to controls, 270 hyperlipidemic subjects had significantly higher (P < .001, +115%) fasting total apo B and higher apo B-48 values (P = .06, +37%). Our data indicate that the apo B-48 ELISA tested provides highly reproducible results and is excellent for research studies. Median apo B-48 values in healthy subjects are about 0.5 mg/dL and increase more than 100% in the fed state. Elevated levels are observed in obese and hyperlipidemic subjects.

Assessing individual metabolic responsiveness to a lipid challenge using a targeted metabolomic approach

The development of assessment techniques with immediate clinical applicability is a priority for resolving the growing epidemic in metabolic disease. Many imbalances in diet-dependent metabolism are not detectable in the fasted state. Resolving the high inter-individual variability in response to diet requires the development of techniques that can detect metabolic dysfunction at the level of the individual. The intra- and inter-individual variation in lipid metabolism in response to a standardized test meal was determined. Following an overnight fast on three different days, three healthy subjects consumed a test meal containing 40% of their daily calories. Plasma samples were collected at fasting, and 1, 3, 6, and 8 h after the test meal. Plasma fatty acid (FA) concentrations within separated lipid classes and lipoprotein fractions were measured at each time point. The intra-individual variation within each subject across three days was lower than the inter-individual differences among the three subjects for over 50% of metabolites in the triacylglycerol (TG), FA, and phosphatidylcholine (PC) lipid classes at 6 h, and for 25-50% of metabolites across lipid classes at 0, 1, 3, and 8 h. The consistency of response within individuals was visualized by principal component analysis (PCA) and confirmed by ANOVA. Three representative metabolites that discriminated among the three individuals in the apolipoprotein B (ApoB) fraction, TG16:1n7, TG18:2n6, and PC18:3n3, are discussed in detail. The postprandial responses of individuals were unique within metabolites that were individual discriminators (ID) of metabolic phenotype. This study shows that the targeted metabolomic measurement of individual metabolic phenotype in response to a specially formulated lipid challenge is possible even without lead-in periods, dietary and lifestyle control, or intervention over a 3-month period in healthy free-living individuals.

The influence of sleep and sleep loss upon food intake and metabolism

The present review investigates the role of sleep and its alteration in triggering metabolic disorders. The reduction of the amount of time sleeping has become an endemic condition in modern society and the current literature has found important associations between sleep loss and alterations in nutritional and metabolic aspects. Studies suggest that individuals who sleep less have a higher probability of becoming obese. It can be related to the increase of ghrelin and decrease of leptin levels, generating an increase of appetite and hunger. Sleep loss has been closely associated with problems in glucose metabolism and a higher risk for the development of insulin resistance and diabetes, and this disturbance may reflect decreased efficacy of the negative-feedback regulation of the hypothalamic–pituitary–adrenal axis. The period of sleep is also associated with an increase of blood lipid concentrations, which can be intensified under conditions of reduced sleep time, leading to disorders in fat metabolism. Based on a review of the literature, we conclude that sleep loss represents an important risk factor for weight gain, insulin resistance, type 2 diabetes and dyslipidaemia. Therefore, an adequate sleep pattern is fundamental for the nutritional balance of the body and should be encouraged by professionals in the area.

Dietary fatty acids make a rapid and substantial contribution to VLDL-triacylglycerol in the fed state

Exaggerated postprandial lipemia is associated with coronary heart disease and type II diabetes, yet few studies have examined the effect of sequential meals on lipoprotein metabolism. We have used 13C-labeled fatty acids to trace the incorporation of fatty acid derived from a meal into apolipoprotein B-100 (apoB-100)-containing lipoproteins and plasma nonesterified fatty acids (NEFA) following two consecutive meals. Healthy volunteers (n=8) were given breakfast labeled with [1-(13)C]palmitic acid, eicosapentaenoic acid, and docosahexaenoic acid, followed 5 h later by lunch containing [1-(13)C]oleic acid. Blood samples were taken over a 9-h period. ApoB-100-containing lipoproteins were isolated by immunoaffinity chromatography. Chylomicron-triacylglycerol (TG) concentrations peaked at 195 min following breakfast but at 75 min following lunch (P<0.001). VLDL-TG concentrations, in contrast, rose to a broad peak after breakfast and then fell steadily after lunch. Breakfast markers followed chylomicron-TG concentrations and appeared in plasma NEFA with a similar profile, whereas [1-(13)C]oleic acid peaked 2 h after lunch in plasma TG and NEFA. Breakfast markers appeared steadily in VLDL, peaking 1-3 h after lunch, whereas [1-(13)C]oleic acid was still accumulating in VLDL at 9 h. Around 17% of VLDL-TG originated from recent dietary fat 5 h after breakfast, and around 40% at the end of the experiment. We conclude that there is rapid flux of fatty acids from the diet into endogenous pools. Further study of these processes may open up new targets for intervention to reduce VLDL-TG concentrations and postprandial lipemia.

Effect of weight loss on the postprandial response to high-fat and high-carbohydrate meals in obese women

OBJECTIVE

To assess the effect of weight loss on the plasma lipid and remnant-like lipoprotein cholesterol (RLPc) response to a high-fat or a high-carbohydrate meal in a population of obese women.

DESIGN

Nutritional intervention study.

SUBJECTS

Sixteen obese women (mean body mass index (BMI): 37.6+/-5 kg/m(2)).

METHODS

Subjects were asked to follow an energy-restricted diet (800 kcal/day) for 7 weeks, followed by a 1-week maintenance diet. Before and after weight loss, each participant was given (in random order) two iso-energetic meals containing either 80% fat and 20% protein (the high-fat meal) or 80% carbohydrate and 20% protein (the high-carbohydrate meal). Blood samples were collected over the following 10-h period. A two-way analysis of variance with repeated measures was used to assess the effect of the meal and postprandial time on biological variables and postprandial responses (notably RLPc levels).

RESULTS

Weight loss was associated with a significant decrease in fasting triglyceride (P=0.0102), cholesterol (P<0.0001), low-density lipoprotein cholesterol (P=0.0003), high-density lipoprotein-cholesterol (P=0.0009) and RLPc (P=0.0015) levels. The triglyceride response to the high-fat meal was less intense after weight reduction than before (interaction P<0.002). This effect persisted after adjustment on baseline triglyceride levels. The triglyceride response to the high-carbohydrate meal was biphasic (i.e. with two peaks, 1 and 6 h after carbohydrate intake). After adjustment on baseline values, weight reduction was associated with a trend towards a reduction in the magnitude of the second triglyceride peak (interaction P<0.054). In contrast, there was no difference in postprandial RLPc responses before and after weight loss, again after adjustment on baseline levels.

CONCLUSION

Our data suggest that weight loss preferentially affects postprandial triglyceride metabolism.

Vildagliptin therapy reduces postprandial intestinal triglyceride-rich lipoprotein particles in patients with type 2 diabetes

AIMS/HYPOTHESIS

We assessed the effects of vildagliptin, a novel dipeptidyl peptidase IV inhibitor, on postprandial lipid and lipoprotein metabolism in patients with type 2 diabetes.

SUBJECTS, MATERIALS AND METHODS

This was a single-centre, randomised, double-blind study in drug-naive patients with type 2 diabetes. Patients received vildagliptin (50 mg twice daily, n=15) or placebo (n=16) for 4 weeks. Triglyceride, cholesterol, lipoprotein, glucose, insulin, glucagon and glucagon-like peptide-1 (GLP-1) responses to a fat-rich mixed meal were determined for 8 h postprandially before and after 4 weeks of treatment.

RESULTS

Relative to placebo, 4 weeks of treatment with vildagliptin decreased the AUC(0-8h) for total trigyceride by 22+/-11% (p=0.037), the incremental AUC(0-8h) (IAUC(0-8h)) for total triglyceride by 85+/-47% (p=0.065), the AUC(0-8h) for chylomicron triglyceride by 65+/-19% (p=0.001) and the IAUC(0-8h) for chylomicron triglyceride by 91+/-28% (p=0.002). This was associated with a decrease in chylomicron apolipoprotein B-48 (AUC(0-8h), -1.0+/-0.5 mg l(-1) h, p=0.037) and chylomicron cholesterol (AUC(0-8h), -0.14+/-0.07 mmol l(-1) h, p=0.046). Consistent with previous studies, 4 weeks of treatment with vildagliptin also increased intact GLP-1, suppressed inappropriate glucagon secretion, decreased fasting and postprandial glucose, and decreased HbA(1c) from a baseline of 6.7% (change, -0.4+/-0.1%, p<0.001), all relative to placebo.

CONCLUSIONS/INTERPRETATION

Treatment with vildagliptin for 4 weeks improves postprandial plasma triglyceride and apolipoprotein B-48-containing triglyceride-rich lipoprotein particle metabolism after a fat-rich meal. The mechanisms underlying the effects of this dipeptidyl peptidase IV inhibitor on postprandial lipid metabolism remain to be explored.

Postprandial lipemia as a risk factor for cardiovascular disease in patients with hypothyroidism

Postprandial lipoprotein metabolism is suggested to play a role in the pathogenesis of atherosclerosis. In this study, we investigated postprandial lipemia and its relationship to cardiovascular risk factors in patients with overt and subclinical hypothyroidism. Twenty-nine female patients with TSH levels greater than 5 microIU/mL and 12 euthyroid control female subjects were included in the study. Fifteen patients had subclinical hypothyroidism and 14 had overt hypothyroidism. All subjects underwent an oral lipid tolerance test. If triglyceride levels increased by 80% or more, subjects were considered postprandial lipemia positive. Control, overt hypothyroid, and subclinical hypothyroid groups were not statistically different with respect to anthropometric measurements, fasting blood C-reactive protein, uric acid, homocysteine, glucose, insulin, lipoprotein (a), apolipoprotein B levels, and homeostasis model assessment index. Fasting triglyceride levels correlated positively with TSH levels. Postprandial lipemia frequency was higher in overt hypothyroid subjects than in the control group. The subclinical hypothyroid group did not differ from the hypothyroid group with respect to postprandial lipemia frequency. In subjects with TSH levels higher than 5 microIU/mL, PPL risk was increased sevenfold. The results of this study show that postprandial triglyceride metabolism is affected in hypothyroidism.

Prescribing Aerobic Exercise for the Regulation of Postprandial Lipid Metabolism

Prolonged presence of elevated plasma triglycerides (TGs) during the postprandial period has been suggested to increase the risk for coronary artery disease. Aerobic exercise attenuates postprandial lipaemia and this has generally been described as a short-term effect of the exercise. Effects of exercise on postprandial lipaemia have mostly been investigated, and documented, with large exercise-induced energy expenditures (i.e. 1000 kcal). The exact mechanisms involved in the attenuation of postprandial lipaemia with exercise are not completely understood, but it appears that at least two mechanisms are involved: a decrease in TG secretion by the liver and an increase in plasma TG clearance by the muscle. Changes in the metabolism of other lipids, such as those in high-density lipoprotein cholesterol, have been documented only when the exercise is performed some hours before the fat meal. Although factors such as the physical fitness and percentage body fat of an individual are likely to also be involved, the most important factors determining the magnitude of the attenuation in postprandial lipaemia appear to be the magnitude of the exercise-induced energy expenditure and the intensity of exercise. To date, the evidence suggests that healthy individuals can generally induce favourable changes in postprandial lipaemia with aerobic exercise that: (i) is completed during the period extending from 16 hours before a meal through 1.5 hours after a meal; (ii) is of moderate intensity; and (iii) results in an energy expenditure of approximately 500 kcal (or more).

Changes in plasma triacylglycerol concentrations after sequential lunch and dinner in healthy subjects

OBJECTIVES

The present study examines the kinetic of plasma triacylglycerol (TAG) after sequential ingestion of lunch and dinner as well as the contribution of dietary fat ingested at lunch to subsequent post-dinner TAG composition.

METHOD

Six healthy subjects were included. After standardized breakfast (7: 30AM), 2 mixed meals with fat loads composed of 44 g olive oil (rich in oleic acid) at lunch (12PM) and 44 g sunflower oil (rich in linoleic acid) at dinner (7PM) were ingested. [1-13C] palmitate was added in lunch only. Plasma TAG and chylomicron-TAG (CMTAG) levels were measured sequentially after meals. [1-13C] palmitate enrichment and concentrations of oleic acid and linoleic acid were measured in all lipid fractions.

RESULT

Post-dinner plasma TAG peak was delayed as compared to lunch (3 hours vs 1 hour, p=0.002) whereas the magnitude of the postprandial peaks was not significantly different between lunch and dinner (2.4+/-0.3 vs 2.0+/-0.4 mmol/L, p=0.85). [1-13C] palmitate enrichment was maximal 5 hours after lunch in all lipid fractions and decreased slowly thereafter. After dinner ingestion, the rate of decline of [1-13C] palmitate enrichment plateaued during the first 60 minutes. Oleic acid increased slightly and immediately after dinner and remained the predominant fatty acid in all lipid fractions during the first hour after dinner. A delayed peak of plasma and CM-TAG was observed after dinner as compared to lunch without difference in the magnitude of peaks.

CONCLUSION

The contribution of dietary fat ingested at lunch to post-dinner lipemia is confirmed despite the relatively long lasting interval between the 2 meals (7 h) and the absence of any early peak of plasma TAG after dinner.

Meal ingestion provokes entry of lipoproteins containing fat from the previous meal: possible metabolic implications

BACKGROUND

Prolonged and exaggerated postprandial plasma triacylglycerol (TAG) concentrations are considered as an independent risk factor for coronary artery disease. Western populations eat many meals at regular intervals, and can be in a postprandial state for at least 17 h of a 24 h period. After consuming 2 meals an early plasma TAG peak has been observed after the second meal, the origin of which is unclear.

AIM OF THE STUDY

To test the hypothesis that the early TAG peak observed following sequential meals was of intestinal origin and represented fat derived from the previous meal.

METHODS

Postprandial plasma lipaemic responses of 17 healthy postmenopausal women were studied by giving a test breakfast followed by a lunch. Watermiscible retinyl palmitate (RP) was added to the breakfast, but not the lunch test meal. Plasma TAG, retinyl esters (RE) and apo B-48 were determined for a 10 h period following breakfast.

RESULTS

In response to the test meals, RE, apo B-48 and TAG showed multiple peaks. Despite omission of RP from the lunch, RE showed an early peak response after ingestion of lunch in 15 of 17 subjects. The peak response after lunch of all three markers appeared significantly earlier compared with their respective peak responses after the breakfast (P < 0.0001). The area of RE response after lunch was significantly correlated with the RE lipaemic response to the breakfast (r = 0.67; P < 0.004) and to the fasting TAG concentration (r = 0.48; P < 0.05).

CONCLUSIONS

Since the lunch did not contain RP, the distinctive second influx of RE after lunch was believed to have originated from the breakfast. This, together with the fact that all three markers showed an earlier response to the lunch than the breakfast, supports the view that ingestion of a second meal provokes entry of fat from the previous meal, from an as yet unidentified site (gut, enterocytes, lymph). The results indicate that the degree of TAG "storage" from previous meals might be a function of TAG tolerance and provide a possible site of regulation of the entry of fat into the systemic circulation.

Interrelationships Between Human Apolipoprotein A-I and Apolipoproteins B-48 and B-100 Kinetics Using Stable Isotopes

OBJECTIVE

Our purpose was to determine the relationship between apolipoprotein (apo) A-I and apoB-48 and apoB-100 metabolism in moderately hypercholesterolemic humans.

METHODS AND RESULTS

The kinetics of apoA-I within high-density lipoprotein (HDL), apoB-48 and apoB-100 within triglyceride-rich lipoproteins, and apoB-100 within intermediate-density lipoprotein and low density-lipoprotein (LDL) were examined with a primed constant infusion of [5,5,5-(2)H(3)] leucine in the fed state (hourly feeding) in 23 subjects after consumption of a 36% total fat diet. Lipoproteins were isolated by ultracentrifugation; apolipoproteins by SDS-PAGE gels; and isotope enrichment assessed by gas chromatograph/mass spectrometry. Kinetic parameters were calculated by multicompartmental modeling of the data with SAAM II. ApoA-I production rate (PR) was correlated with LDL apoB-100 pool size (PS; r=0.49; P=0.017) and LDL cholesterol (r=0.61; P=0.002), whereas apoA-I fractional catabolic rate (FCR) was inversely correlated with apoB-48 FCR (r=-0.40; P=0.05) but not with very low-density lipoprotein apoB-100 FCR.

CONCLUSIONS

Two links exist between apoA-I and apoB kinetics: 1) when LDL apoB-100 PS is high, there is increased apoA-I PR; and 2) delayed chylomicron remnant clearance (represented by apoB-48 FCR) is associated with enhanced apoA-I FCR, a finding indicating that alterations in intestinal lipoproteins may be more important in determining HDL cholesterol levels than changes in liver lipoproteins. Using stable isotopes in humans, 2 links were observed between apoA-I and apoB kinetics: (1) when LDL apoB-100 PS is high, there is increased apoA-I PR; and (2) delayed chylomicron remnant clearance is associated with enhanced apoA-I FCR, indicating that alterations in intestinal lipoproteins may be more important in determining HDL-C levels than changes in liver lipoprotein particles.

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.

Differential regulation of fatty acid trapping in mouse adipose tissue and muscle by ASP

Acylation-stimulating protein (ASP) is a lipogenic hormone secreted by white adipose tissue (WAT). Male C3 knockout (KO; C3(-/-)) ASP-deficient mice have delayed postprandial triglyceride (TG) clearance and reduced WAT mass. The objective of this study was to examine the mechanism(s) by which ASP deficiency induces differences in postprandial TG clearance and body composition in male KO mice. Except for increased (3)H-labeled nonesterified fatty acid (NEFA) trapping in brown adipose tissue (BAT) of KO mice (P = 0.02), there were no intrinsic tissue differences between wild-type (WT) and KO mice in (3)H-NEFA or [(14)C]glucose oxidation, TG synthesis or lipolysis in WAT, muscle, or liver. There were no differences in WAT or skeletal muscle hydrolysis, uptake, and storage of [(3)H]triolein substrate [in situ lipoprotein lipase (LPL) activity]. ASP, however, increased in situ LPL activity in WAT (+64.8%, P = 0.02) but decreased it in muscle (-35.0%, P = 0.0002). In addition, after prelabeling WAT with [(3)H]oleate and [(14)C]glucose, ASP increased (3)H-lipid retention, [(3)H]TG synthesis, and [(3)H]TG-to-[(14)C]TG ratio, whereas it decreased (3)H-NEFA release, indicating increased NEFA trapping in WAT. Conversely, in muscle, ASP induced effects opposite to those in WAT and increased lipolysis, indicating reduced NEFA trapping within muscle by ASP (P < 0.05 for all parameters). In conclusion, novel data in this study suggest that 1) there is little intrinsic difference between KO and WT tissue in the parameters examined and 2) ASP differentially regulates in situ LPL activity and NEFA trapping in WAT and skeletal muscle, which may promote optimal insulin sensitivity in vivo.

Human immunodeficiency virus type 1-related lipoatrophy and lipohypertrophy are associated with serum concentrations of leptin

The relationship between the adipocyte-derived hormone leptin, insulin resistance, and fat redistribution in patients with human immunodeficiency virus (HIV) infection has not been established. We classified a cohort of HIV type 1 (HIV-1)-infected patients with >or=6 months of antiretroviral exposure as having no lipodystrophy (51 patients [43% of the cohort]), lipoatrophy (23 patients [19% of the cohort]), mixed lipodystrophy (29 patients [24% of the cohort]), or lipohypertrophy (17 patients [14% of the cohort]), on the basis of physical examination, anthropometric measurements, and the findings of dual-emission x-ray absorptiometry and computed tomography. Measurements of insulin resistance were higher for patients with each category of lipodystrophy, compared with those observed for patients with no lipodystrophy (P<.001). Mean leptin levels (+/- standard deviation) were lowest in patients with lipoatrophy (1.76+/-1.20 ng/mL), highest in patients with lipohypertrophy (9.10+/-6.86 ng/mL), and significantly different from those in patients without lipodystrophy (3.14+/-2.30 ng/mL; both P<.01). In this cohort of antiretroviral-experienced HIV-infected patients, a low serum level of leptin was independently associated with insulin resistance in patients with lipoatrophy, after controlling for total and regional body fat.

Oral fat exposure increases the first phase triacylglycerol concentration due to release of stored lipid in humans

Oral exposure to dietary fat (through modified sham feeding, which entails mastication and expectoration of foods) augments the postprandial triacylglycerol (TAG) concentration, in part, though augmented lipid absorption. This study was designed to characterize early events in this process. At 2200 h, 25 healthy adults (13 men, 12 women) consumed 80 g of almonds (high oleic acid content) and fasted until approximately 0700 h. After placement of a catheter in a hand vein and 4 blood draws at 10-min intervals, 50 1-g safflower oil (high linoleic acid content) capsules were consumed. After another blood draw, modified sham feeding was initiated with a cracker only or cracker with cream cheese in random order with 1 wk between trials. Oral exposures occurred at 5-min intervals for 60 min then at 15-min intervals from min 60 to 120. Additional blood draws occurred at 2, 4, 6, 8, 10, 12, 14, 30, 60, 120, 240, 360 and 480 min. Oral stimulation, especially by fat, prompted the rapid (mean approximately 23 min) release of lipid stored from the previous meal (almonds) in all participants. This resulted in multimodal postprandial triacylglycerol (TAG) peaks generally occurring at 0-30 min, 60-120 min and 240-480 min after loading and initiation of oral stimulation. TAG magnitudes during these times were correlated (r = 0.40-0.89, P < 0.001-P = 0.053). It is proposed that the sensory-enhanced release of lipid from the residual pool initiates an early TAG rise, which augments the peak attributable to absorption of meal lipid; this in turn supplements a later peak associated with release of endogenously synthesized TAG because lipid from all three sources competed for a common clearance mechanism. If substantiated, additional understanding of the behavioral factors (e.g., eating patterns) that initiate this cascade will be warranted.

Resistance of adipose tissue lipoprotein lipase to insulin action in rats fed an obesity-promoting diet

This study aimed to assess whether adipose lipoprotein lipase (LPL) becomes resistant to insulin in a nutritional model of resistance of glucose metabolism to insulin. Sprague-Dawley rats were fed for 4 wk chow or a purified high-sucrose, high-fat (HSHF) diet that induced overt insulin resistance. Rats were fasted for 24 h and then refed chow for 1, 3, or 6 h. The postprandial rise in insulinemia was similar in both dietary cohorts, whereas glycemia was higher in HSHF-fed than in chow-fed animals, indicating glucose intolerance and insulin resistance. In chow-fed rats, adipose LPL activity increased two- to fourfold postprandially, but only minimally (30%) in HSHF-fed rats. Muscle LPL decreased postprandially in HSHF-fed rats, suggesting intact sensitivity to insulin, but it increased in chow-fed animals. Peak postprandial triglyceridemia was higher (+70%) in insulin-resistant than in control rats. The postprandial rate of appearance of triglycerides in the circulation was similar in control and insulin-resistant rats, indicating that hypertriglyceridemia of the latter was the result of impaired clearance. These results demonstrate that adipose LPL becomes resistant to insulin in diet-induced IR and further suggest that, under certain nutritional conditions, modifications in adipose LPL modulation associated with insulin resistance, along with low muscle LPL, heightens postprandial hypertriglyceridemia through attenuated triglyceride clearance.

Use of water-miscible retinyl palmitate as markers of chylomicrons gives earlier peak response of plasma retinyl esters compared with oil-soluble retinyl palmitate

Delayed peak response of plasma retinyl esters (RE) relative to plasma triacylglycerols (TAG) and apolipoprotein (Apo) B-48 responses following a fat load supplemented with vitamin A raised doubts about the use of vitamin A to label dietary-derived lipids and lipoproteins. The present study compared the use of water-miscible and oil-soluble retinyl palmitate (RP) as markers of dietary-derived lipoproteins in healthy subjects along with the measurements of postprandial plasma TAG and ApoB-48 responses to investigate whether the delayed peak response observed was due to delayed intestinal output of RE from oil-based solutions. Nine healthy female subjects were given a standard test meal containing a dose (112 mg) of RP in either water-miscible or oil-soluble form in random order, on two separate occasions after a 12 h overnight fast. The results showed that the mean plasma RE concentrations reached a peak significantly later than mean plasma TAG and ApoB-48 concentrations when oil-soluble RP was consumed, whereas plasma RE peaked earlier relative to plasma TAG and ApoB-48 responses when water-miscible RP was used. The results suggested a more rapid absorption with a significantly higher and earlier peak response of plasma RE when water-miscible RP was consumed. This was in contrast to the delayed initial appearance and later sustained higher concentrations of plasma RE during the late postprandial period when oil-soluble RP was consumed. The RE response to the water-miscible RP showed better concordance with plasma TAG response than that of oil-soluble RP.

Differentiating the effects of raising low levels of high-density lipoprotein cholesterol versus lowering normal triglycerides: further insights from the veterans affairs high-density lipoprotein intervention trial

In the Veterans Affairs High-Density Lipoprotein Intervention Trial (VA-HIT), a 6% increase in high-density lipoprotein cholesterol (HDL-C) was associated with a 22% reduction in the incidence of fatal and nonfatal myocardial infarction and death from coronary artery disease. A curvilinear correlation was demonstrated between incremental changes in HDL-C and outcome. However, triglyceride levels, which decreased by 31%, were not predictive of clinical events. Some design limitations may be implicated in this lack of statistical correlation between triglycerides and outcome. Evidence from other studies, notably the Framingham Heart Study, the Prospective Cardiovascular Munster Heart Study (PROCAM), and the Baltimore Coronary Observational Long-Term Study (COLTS) suggest that the triglyceride cutpoint of 200 mg/dL is too high. The Bezafibrate Infarction Prevention (BIP) trial found a significant correlation between reduction in death from coronary artery disease and nonfatal MI and reduced triglycerides in a subset of patients with baseline triglycerides >200 mg/dL.

Human apolipoprotein (Apo) B-48 and ApoB-100 kinetics with stable isotopes

The kinetics of apolipoprotein (apo) B-100 and apoB-48 within triglyceride-rich lipoproteins (TRLs) and of apoB-100 within IDL and LDL were examined with a primed-constant infusion of (5,5,5-(2)H(3)) leucine in the fed state (hourly feeding) in 19 subjects after consumption of an average American diet (36% fat). Lipoproteins were isolated by ultracentrifugation and apolipoproteins by SDS gels, and isotope enrichment was assessed by gas chromatography/mass spectrometry. Kinetic parameters were calculated by multicompartmental modeling of the data with SAAM II. The pool sizes (PS) of TRL apoB-48, VLDL apoB-100, and LDL apoB-100 were 17+/-10, 273+/-167, and 3325+/-1146 mg, respectively. There was a trend toward a faster fractional catabolic rate (FCR) for VLDL apoB-100 than for TRL apoB-48 (6.73+/-3.48 versus 5.02+/-2.07 pools/d, respectively, P=0.06). The mean FCRs for IDL and LDL apoB-100 were 10.07+/-7.28 and 0.27+/-0.08 pools/d, respectively. The mean production rate (PR) of TRL apoB-48 was 6.5% of VLDL apoB-100 (1. 3+/-0.90 versus 20.06+/-6.53 mg. kg(-1). d(-1), P<0.0001). TRL apoB-48 PS was correlated with apoB-48 PR (r=0.780, P<0.0001) but not FCR (r=-0.1810, P=0.458). VLDL apoB-100 PS was correlated with both PR (r=0.713, P=0.0006) and FCR (r=-0.692, P=0.001) of VLDL apoB-100 and by apoB-48 PR (r=0.728, P=0.0004). LDL apoB-100 PS was correlated with FCR (r=-0.549, P=0.015). These data indicate that (1) the FCRs of TRL apoB-48 and VLDL apoB-100 are similar in the fed state, (2) TRL apoB-48 PS is correlated with TRL apoB-48 PR, (3) VLDL apoB-100 PS is correlated with both PR and FCR of VLDL apoB-100 and PR of TRL apoB-48, and (4) LDL apoB-100 PS is correlated with LDL FCR.

Postprandial triacylglycerols from dietary virgin olive oil are selectively cleared in humans

The aims of the present study were to evaluate the effect of a meal rich in virgin olive oil on triacylglycerol composition of human postprandial triacylglycerol-rich lipoproteins (fraction Sf > 400), and to assess the role of the triacylglycerol molecular species concentration and polarity on lipoprotein clearance. Fasting (0 h) and postprandial blood samples were collected hourly for 7 h from eight healthy normolipidemic subjects after the ingestion of the meal. Plasma and lipoprotein triacylglycerol concentrations increased quickly over fasting values and peaked twice at 2 and 6 h during the 7-h postprandial period. The triacylglycerols in the lipoprotein fraction at 2 h generally reflected the composition of the olive oil, however, the proportions of the individualmolecular species were altered by the processes leading to their formation. Among the major triacylglycerols, the proportion of triolein (OOO; 43.6%) decreased (P < 0.05), palmitoyl-dioleoyl-glycerol (POO; 31. 1%) and stearoyl-dioleoyl-glycerol (SOO; 2.1%) were maintained and linoleoyl-dioleoyl-glycerol (LOO; 11.4%) and palmitoyl-oleoyl-linoleoyl-glycerol (POL; 4.6%) significantly increased (P < 0.05) compared with the composition of the triacylglycerols in the olive oil. Smaller amounts of endogenous triacylglycerol (0.8%), mainly constituted of the saturated myristic (14:0)and palmitic (16:0) fatty acids, were also identified. Analysis of total fatty acids suggested the presence of molecular species composed of long-chain polyunsaturated fatty acids of the (n-3) family, docosapentaenoic acid, [22:5(n-3)] and docosahexaenoic acid (DHA), [22:6(n-3)] and of the (n-6) family [arachidonic acid, [20:4(n-6)]. The fastest conversion of lipoproteins to remnants occurred from 2 to 4 h and was directly related to the concentration of the triacylglycerols in the lipoprotein particle (r = 0.9969, P < 0.05) and not with its polarity (r = 0.1769, P > 0.05). The rates of clearance were significantly different among the major triacylglycerols (OOO, POO, OOL and POL) (P < 0.05) and among the latter ones and PLL (palmitoyl-dilinoleoyl-glycerol, POS (palmitoyl-oleoyl-stearoyl-glycerol) and OLL (oleoyl-dilinoleoyl-glycerol) (P < 0.01). OOO was removed faster and was followed by POO, OOL, POL, PPO (dipalmitoyl-oleoyl-glycerol), SOO, PLL, POS and OLL.

Delayed changes in postprandial lipid in young normolipidemic men after a nocturnal vitamin A oral fat load test

The oral fat load tests (OFLT) used to study postprandial lipemia are generally conducted during the day. A nocturnal fat load test could be convenient and physiologically more appropriate. We have therefore compared the lipemic responses of 9 normolipidemic young men to OFLT given at 2200 h (nocturnal) and at 0700 h (diurnal). Triglyceride and retinyl palmitate concentrations were measured for 10 h. Peak plasma concentrations or areas under curves (AUC) for triglyceride after the diurnal and nocturnal tests were not significantly different [2.17 +/- 0.78 (diurnal) vs. 2.04 +/- 0.87 mmol/L (nocturnal) and 13.12 +/- 4.45 (diurnal) vs. 13.74 +/- 5.79 mmol/(L. h) (nocturnal)]. Peak plasma concentrations and AUC retinyl palmitate for the two tests were not different [1.71 +/- 0.69 (diurnal) vs. 1.42 +/- 0.66 mg/L (nocturnal) and 7.17 +/- 3.98 (diurnal) vs. 6.63 +/- 4.23 mg/(L. h) (nocturnal)]. The diurnal triglyceride peak occurred significantly earlier (4.3 +/- 1.2 h) than the nocturnal peak (5.8 +/- 1.7 h, P < 0.05). We have developed a model using only three sample time points to predict AUC [triglyceride at 0 h, triglyceride at average peak-time (4 h for diurnal and 6 h for nocturnal tests), and triglyceride at 10 h], thus reducing the number of blood samples. The predicted AUC was well correlated with the total AUC after nocturnal OFLT (r = 0.98, P < 0.0001). The nocturnal test appeared to be well tolerated by the subjects. The three-point simplified protocol may well be suitable for studies on large groups of subjects.

A successful dietary treatment fails to normalize plasma triglyceride postprandial response in type IV patients

The role of plasma triglycerides as a risk factor for cardiovascular disease is still under scrutiny. While recent studies have shown that postprandial triglyceridemia is an independent risk factor, normalization of fasting plasma triglycerides through modification of nutritional habits remains the primary approach in the treatment of hypertriglyceridemia. To address the issue of whether a satisfactory dietary regimen results in the control of postprandial lipemia, 53 type IV hypertriglyceridemic patients underwent an hypolipidemic diet for 3 months. All patients had a reduction of fasting lipid parameters (average TG: from 516+/-208 to 229+/-99 mg/dl; total cholesterol (Chol): from 261+/-42 to 213+/-40 mg/dl and HDL Chol: from 33+/-9 to 38+/-8 mg/dl). Taking plasma TG < or =200 mg/dl as the target for dietary intervention 26 patients were classified as 'responders' while the remaining 27 were 'non responders'. Even if fasting total TG, total Chol, HDL and LDL Chol were normal, both responders and non responders (P<0.0001) showed an exaggerated postprandial response to an oral fat load as compared to controls (20 normolipidemic subjects). Also when 10 responders and 10 controls, all male, were matched for plasma TG (129+/-43 versus 121+/-41 mg/dl) and other lipid parameters, a statistically significant difference between the two groups was observed at the time of each of the postprandial tests (P<0.0001) and for the area under the curve. The fact that the post prandial response is poorly modified by a dietary regimen, that effectively reduces plasma fasting TG, suggests that commonly used dietary regimens fail to restore a normal postprandial metabolism. Whether the cardiovascular risk for these patients is reduced after diet remains, therefore, to be addressed.

Differential effects of saturated and monounsaturated fatty acids on postprandial lipemia and incretin responses in healthy subjects

BACKGROUND

Elevations of postprandial triacylglycerol-rich plasma lipoproteins and suppressions of HDL-cholesterol concentrations are considered potentially atherogenic. Long-term studies have shown beneficial effects of monounsaturated fatty acids (eg, oleic acid) on fasting lipid and lipoprotein concentrations in humans. A direct stimulatory effect of oleic acid on the secretion of glucagon-like peptide 1 (GLP-1) was shown in animal studies.

OBJECTIVE

We compared the postprandial responses of glucose, insulin, fatty acids, triacylglycerol, gastric inhibitory polypeptide (GIP), and GLP-1 to test meals rich in saturated and monounsaturated fatty acids.

DESIGN

Ten young, lean, healthy persons ingested 3 meals: an energy-free soup consumed with 50 g carbohydrate (control meal), the control meal plus 100 g butter, and the control meal plus 80 g olive oil. Triacylglycerol and retinyl palmitate responses were measured in total plasma, in a chylomicron-rich fraction, and in a chylomicron-poor fraction.

RESULTS

No significant differences in glucose, insulin, or fatty acid responses to the 2 fat-rich meals were seen. Plasma triacylglycerol responses were highest after the butter meal, with chylomicron triacylglycerol rising 2.5-5-fold. Retinyl palmitate responses were higher and more prolonged after the butter meal than after the control and olive oil meals, whereas both postprandial HDL-cholesterol concentrations and GLP-1 and GIP responses were higher after the olive oil meal than after the butter meal.

CONCLUSIONS

Olive oil induced lower triacylglycerol concentrations and higher HDL-cholesterol concentrations than butter, without eliciting differences in concentrations of glucose, insulin, or fatty acids. Furthermore, olive oil induced higher concentrations of GLP-1 and GIP than did butter, which may point to a relation between fatty acid composition, incretin responses, and triacylglycerol metabolism in the postprandial phase.

The HMG-CoA reductase inhibitor atorvastatin increases the fractional clearance rate of postprandial triglyceride-rich lipoproteins in miniature pigs

We have previously shown in vivo that the 3-hydroxy-3-methylglutaryl coenzyme A reductase inhibitor atorvastatin decreases hepatic apolipoprotein B (apoB) secretion into plasma. To test the hypothesis that atorvastatin modulates exogenous triglyceride-rich lipoprotein (TRL) metabolism in vivo, an oral fat load (2 g fat/kg body wt) containing retinol (50 000 IU) was given to 6 control miniature pigs and to 6 animals after 28 days of treatment with atorvastatin 3 mg. kg-1. d-1. A multicompartmental model was developed by use of SAAM II and kinetic analysis performed on the plasma retinyl palmitate (RP) data. Peak TRL (d<1.006 g/mL; Sf>20) triglyceride concentrations were decreased 29% by atorvastatin, and the time to achieve this peak was delayed (5.2 versus 2.3 hours; P<0.01). The TRL triglyceride 0- to 12-hour area under the curve was decreased by 24%. In contrast, atorvastatin treatment had no effect on peak TRL RP concentrations, time to peak, or its rate of appearance into plasma; however, the TRL RP 0- to 12-hour area under the curve was decreased by 20%. Analysis of the RP kinetic parameters revealed that the TRL fractional clearance rate was increased significantly, 1.4-fold (3.093 versus 2.276 pools/h; P=0.012), with atorvastatin treatment. The percent conversion of TRL RP from the rapid-turnover to the slow-turnover compartment was decreased by 47% with atorvastatin treatment. The TRL RP fractional clearance rate was negatively correlated with very low density lipoprotein apoB production rate measured in the fasting state (r=-0.49). Thus, although atorvastatin had no effect on intestinal TRL assembly and secretion, plasma TRL clearance was significantly increased, an effect that may relate to a decreased competition for removal processes by hepatic very low density lipoprotein.

Remnant lipoprotein cholesterol and triglyceride reference ranges from the Framingham Heart Study

Remnants of triglyceride-rich lipoproteins of both intestinal and liver origin are considered atherogenic, but they have been difficult to isolate and measure. An assay has been developed that allows the measurement of remnant-like particle cholesterol (RLP-C) and triglyceride (RLP-TG). RLP-C and RLP-TG concentrations were measured in &gt;3000 fasting plasma samples obtained from participants in exam cycle 4 of the Framingham Offspring Study and stored at −80 °C. After exclusions, comparisons were made for 2821 samples (1385 women, 1436 men; mean age, 52 years). For women, the mean RLP-C and RLP-TG values were 0.176 ± 0.058 mmol/L (6.8 ± 2.3 mg/dL) and 0.204 ± 0.159 mmol/L (18.1 ± 14.1 mg/dL), respectively; for men, the mean values were 0.208 ± 0.096 mmol/L (8.0 ± 3.7 mg/dL) and 0.301 ± 0.261 mmol/L (26.7 ± 23.1 mg/dL), respectively. Women had significantly lower RLP-C and RLP-TG values (P &lt;0.0001) than men; premenopausal women had significantly lower values than postmenopausal women (P &lt;0.0001); and younger subjects (&lt;50 years) had significantly lower values than older individuals (P &lt;0.0001). The 75th percentile values for RLP-C and RLP-TG were 0.186 mmol/L (7.2 mg/dL) and 0.225 mmol/L (19.9 mg/dL), respectively, for women, and 0.225 mmol/L (8.7 mg/dL) and 0.346 mmol/L (30.6 mg/dL) for men. These data provide reference ranges for use in the evaluation of RLP-C and RLP-TG as potential indicators of risk for coronary heart disease.

Alimentary lipemia is enhanced in fiber-fed rats

The effect of dietary fiber on the pattern of postprandial lipemia was examined in two studies with male Wistar rats. In the first study, groups of rats were killed after food deprivation (0 h) or 1, 4.5 or 8.5 h after a high fat test meal containing either cellulose (CL) or oat bran (OB). Plasma triglycerides (TG) were higher in the OB group at 4.5 h compared with both the 0-h and the CL-groups at 4. 5 h. In both groups, LDL and TG-rich lipoprotein cholesterol (TRL-C) concentrations were higher at 8.5 h than at 0 h; HDL cholesterol was significantly lower at 8.5 h than at 0 h for the OB group only. The enhanced lipemia when OB was fed may stimulate cholesterol movement from HDL to LDL and TRL. To examine whether TRL secretion rates were responsible for the enhanced lipemia, a second study was conducted. Rats were fitted with jugular catheters and allowed to recover. Two groups were fed either CL or OB and infused with Triton-1339 (400 mg/kg). Two control groups were not fed and were infused with either Triton or saline. Rats were killed 2.5 h after infusion. Plasma TG was 10-fold higher in the Triton group than in the saline group, but did not differ between the OB and CL groups. The relative contribution of TRL-C to total cholesterol was significantly greater in the Triton control than in the OB and CL groups. Enhanced secretion of TRL was not responsible for the lipemia observed in the first study. Rather, alterations in clearance rate were responsible.

Comparison of three fatty meals in healthy normolipidaemic men: high post-prandial retinyl ester response to soybean oil

BACKGROUND

Oral fat tolerance tests (FTTs) have been widely used as a tool to investigate post-prandial lipaemia. However, there is no consensus regarding the type and amount of fat used in the tests.

METHODS

We compared three commonly used FTTs, each containing 63 g of fat: a mixed meal, a liquid cream meal and a liquid soybean oil meal. The study group consisted of 10 healthy normolipidaemic men. We measured triglycerides (TGs), retinyl esters (REs), apolipoprotein E (apoE), apolipoprotein B-48 (apoB-48) and apolipoprotein B-100 (apoB-100) in plasma and in triglyceride-rich lipoprotein (TRL) fractions separated by density-gradient ultracentrifugation at baseline and 3, 4, 6, and 8 h after the FTTs.

RESULTS

We observed similar TGs, apoE, apoB-48 and apoB-100 responses after all three FTTs, despite the different fatty acid composition of the meals. In contrast, the commonly used marker for exogenous particles, RE, differed clearly when polyunsaturated (soybean oil) and saturated fat (cream or mixed meal) were used. The RE response in plasma (P < 0.005, repeated measures ANOVA), in chylomicrons (P < 0.013) and in very low-density lipoprotein (VLDL) 1 (P < 0.017), as well as the RE area under the incremental curve in plasma and chylomicron fractions, were markedly increased after the soybean oil meal compared with the mixed meal and cream meal tests. The peak of RE response occurred parallel to the responses of other markers (i.e. TG or apoB-48) of post-prandial TRL during soybean oil meal. In contrast, RE peak concentration was delayed after saturated fat-containing meals. After soybean oil, FTT plasma cholesterol concentration was lower and the chylomicron cholesterol concentration was higher compared with mixed or cream meals, but no differences were detected in post-prandial high-density lipoprotein (HDL)-cholesterol concentration.

CONCLUSION

When the amount of fat is similar, post-prandial responses of TG, apoE, apoB-48, apoB-100 and HDL-cholesterol were comparable after different FTTs.

Rapid chylomicron appearance following sequential meals: effects of second meal composition

Previous studies have noted the presence of an early postprandial peak in plasma triacylglycerol concentrations following successive fat-rich meals. An earlier study has shown that the triacylglycerol in this early peak originates from a previous meal. The present study was performed to investigate the effects of different second meals on the plasma triacylglycerol response. Six healthy subjects were studied on four occasions each. At 5 h following a fat-rich breakfast they ingested one of the following in a balanced design: a fat-rich meal, a low-fat meal, water or nothing by mouth. Blood samples were taken for 2.5 h following the second meal. An early peak in chylomicron and plasma triacylglycerol concentrations was seen following both low-fat and fat-rich second meals but not following water. During studies investigating postprandial lipaemia, further meals must be avoided, even if they contain no fat, although water may be allowed.

An NCEP II diet reduces postprandial triacylglycerol in normocholesterolemic adults

We compared the fasting and postprandial response to a National Cholesterol Education Program (NCEP) II diet with that of a diet high in total (40% of energy) and saturated fat. In free-living conditions, 17 healthy normolipidemic, normoglycemic men and women consumed a high-fat diet, a maintenance diet and the NCEP II diet, for 1 mo each. At the completion of each dietary period, an oral fat load (70 g/m2 body surface area) was administered and plasma triacylglycerol (TAG) determined every 2 h for 8 h. Compared with the high-fat phase, the NCEP II diet was associated with significantly lower energy intake (12.1 +/- 1.1 vs. 7 +/- 0.7 MJ/d) and final body weight (78 +/- 3.8 vs. 76.3 +/- 3.5 kg) (P < 0.01). Plasma high density lipoprotein cholesterol, apolipoprotein (apo) A-I and ApoB concentrations were also significantly lower when subjects consumed the NCEP II diet rather than the high-fat diet (P </= 0.004). There were no significant differences in subjects fasting TAG, glucose or insulin concentration between the high fat and NCEP II diet periods. However, the postprandial plasma TAG response to the fat load was lower after completing the NCEP II than after the high-fat diet period (P = 0.045). Under free-living conditions, a NCEP II diet was associated with weight loss and a decrease in postprandial but not fasting TAG. Because dietary alteration may not affect fasting TAG levels, thorough assessment of a dietary intervention should include measurements of postprandial TAG.

Dietary fat clearance in normal subjects is modulated by genetic variation at the apolipoprotein B gene locus

Apolipoprotein B (apo B) plays a dominant role in cholesterol homeostasis. Several polymorphic sites within or adjacent to the gene locus for apo B have been detected. The X+ allele (XbaI restriction site present) of the XbaI restriction fragment polymorphism on the apo B gene has been found in some studies to be associated with higher serum cholesterol and/or triglyceride levels and with greater dietary response. The present study was designed to evaluate whether the apo B XbaI polymorphism was associated with the interindividual variability observed during postprandial lipemia. Fifty-one healthy young male volunteers [20 X-/X- (X-), and 31 X+/X- or X+/X+ (X+)], homozygotes for the apo E3 allele, were subjected to a vitamin A-fat load test. Subjects with the X- genotype had significantly greater retinyl palmitate (RP) and apo B-48 postprandial responses on both the large and the small TRL lipoprotein fractions compared with X+ subjects. In summary, subjects with the X-/X- genotype at the apo B locus have a greater postprandial response than X+ subjects. These differences observed in postprandial lipoprotein metabolism could explain some of the reported associations of this polymorphism to coronary heart disease risk.

Normal intestinal dietary fat and cholesterol absorption, intestinal apolipoprotein B (ApoB) mRNA levels, and ApoB-48 synthesis in a hypobetalipoproteinemic kindred without any ApoB truncation

The purpose of this study was to characterize intestinal apolipoprotein B (apoB) metabolism in subjects with familial hypobetalipoproteinemia (FHBL), where segregation analysis supports linkage to the apoB gene but no apoB truncations are present. We investigated cholesterol and fat absorption, intestinal apoB mRNA synthesis and editing, as well as apoB-48 synthesis. Plasma triglycerides (TG) and retinyl palmitate in the chylomicron fractions were analyzed after 12 hours of fasting and then repeatedly for 14 hours after ingestion of a vitamin A-containing high-fat meal. Cholesterol absorption was assessed using a dual stable-isotope method. Mean peak times and concentrations and areas under the curve (AUCs) for fat absorption and mean percentages of cholesterol absorption were comparable in affected and nonaffected family members. Intestinal biopsies were extracted for total RNA and also incubated with 35S-methionine for measurements of apoB synthesis. Similar quantities of apoB mRNA were found to be expressed in the intestine in affected and control subjects by RNase protection assay. ApoB mRNA editing assay showed that the majority of apoB-100 mRNA was edited to the apoB-48 form to a similar extent in both groups. Virtually no apoB-100 protein was synthesized by the intestine in any subject, and apoB-48 protein synthesis was not significantly different in the affected individuals. These data are consistent with in vivo metabolism data that show normal production rates for liver-derived apoB-100 but increased apoB-100 fractional catabolic rates in affected members of this family. Thus, the molecular defect probably does not affect transcription, translation, or secretion of apoB-containing lipoproteins, but may instead affect their clearance.

Decreased production and increased catabolism of apolipoprotein B-100 in apolipoprotein B-67/B-100 heterozygotes

Apolipoprotein (apo) B-67 is a truncated form of apoB-100 due to deletion of an adenine at cDNA 9327. Heterozygotes have one allele making apoB-100; therefore, plasma apoB levels would be predicted to be at least 50% of normal. However, apoB-67 heterozygotes have total plasma apoB levels that are 24% of normal. To determine the mechanisms responsible for the lower-than-expected levels of apoB, in vivo kinetics of apoB-100 were performed in three apoB-67/apoB-100 heterozygotes and compared with those of six control subjects by using a primed-constant infusion of [5,5,5-2H3]leucine in the fed state. Kinetic parameters were calculated by multicompartmental modeling of the data. The mean total apoB plasma concentration of the apoB-67 subjects was 21.8+/-6.1 mg/dL, or 24% of that of control subjects (89.6+/-24.1 mg/dL, P=.002). ApoB-67 subjects had lower mean VLDL apoB-100 production rates (3.6+/-1.2 versus 13.9+/-3.5 mg x kg(-1) x d(-1), P=.002) and lower mean transport rates of apoB-100 into LDL (3.5+/-1.4 versus 12.6+/-4.1 mg x kg(-1) x d(-1), P=.008) compared with control subjects. The transport rate into IDL was not significantly different (1.2+/-0.5 versus 6.2+/-4.0 mg x kg(-1) x d(-1), P=.07). The fractional catabolic rate of VLDL apoB-100 was significantly higher in apoB-67 subjects than in control subjects (18.1+/-8.6 versus 7.6+/-1.6 mg x kg(-1) x d(-1), P=.017). ApoB-100 IDL and LDL fractional catabolic rates were not significantly different. VLDL apoB-100 pool size in apoB-67 subjects was 11% of that of control subjects (15.8+/-7.7 versus 141.6+/-33.7 mg, P=.0004) due to a 74% lower production rate (26% of control values) and a 2.4-fold higher fractional catabolic rate. LDL apoB-100 pool size in apoB-67 subjects was 22% of that of control subjects (665.3+/-192.4 versus 2968.3+/-765.2 mg, P=.002) due primarily to a lower production rate (27% of control values). Thus, both decreased production of VLDL and LDL apoB-100 and increased catabolism of VLDL apoB-100 are responsible for the low levels of apoB-100 in apoB-67 subjects.

Kinetics of retinyl esters during postprandial lipemia in man: a compartmental model

Orally ingested vitamin A (retinol) is incorporated into intestinal chylomicrons (CHYLO) in the form of retinyl esters (RE) along with newly absorbed dietary triglycerides (TG). As the intestinal lipoproteins undergo hydrolysis in the circulation, the majority of the RE remain with the secreted intestinal particles and have been used as a marker for intestinally derived lipoproteins during the early phase of the postprandial state. A multicompartmental model was developed for the kinetics of RE during postprandial lipemia in individuals with normal lipid levels (n = 16) and in patients with hyperlipidemia (n = 44). The assumptions used in the development of the model are presented in this report. Some of the key findings include (1) as much as 50% of the newly synthesized RE may be secreted by the intestine as very-low-density lipoprotein (VLDL)-sized particles of S(f) 20 to 400 following consumption of a test meal containing a moderate amount of fat (20 to 30 g); (2) in most individuals, approximately 50% of the RE secreted in S(f) greater than 400 are converted to smaller, less buoyant fractions, and 50% are irreversibly removed directly from the plasma; (3) as much as 5% to 20% of the ingested retinol may be secreted as small intestinal lipoproteins with the buoyance of low-density lipoprotein (LDL) in some individuals; and (4) less than 5% of RE flux through S(f) 20 to 400 is converted to S(f) less than 20, and the primary catabolic pathway for RE in this fraction is direct uptake. Comparable estimates can be obtained for the kinetic parameters when repeat studies are made in the same subjects under comparable conditions.