Kinetics of triglyceride turnover of very low density lipoproteins of human plasma

Triglyceride-rich lipoproteins and their remnants: metabolic insights, role in atherosclerotic cardiovascular disease, and emerging therapeutic strategies-a consensus statement from the European Atherosclerosis Society

Recent advances in human genetics, together with a large body of epidemiologic, preclinical, and clinical trial results, provide strong support for a causal association between triglycerides (TG), TG-rich lipoproteins (TRL), and TRL remnants, and increased risk of myocardial infarction, ischaemic stroke, and aortic valve stenosis. These data also indicate that TRL and their remnants may contribute significantly to residual cardiovascular risk in patients on optimized low-density lipoprotein (LDL)-lowering therapy. This statement critically appraises current understanding of the structure, function, and metabolism of TRL, and their pathophysiological role in atherosclerotic cardiovascular disease (ASCVD). Key points are (i) a working definition of normo- and hypertriglyceridaemic states and their relation to risk of ASCVD, (ii) a conceptual framework for the generation of remnants due to dysregulation of TRL production, lipolysis, and remodelling, as well as clearance of remnant lipoproteins from the circulation, (iii) the pleiotropic proatherogenic actions of TRL and remnants at the arterial wall, (iv) challenges in defining, quantitating, and assessing the atherogenic properties of remnant particles, and (v) exploration of the relative atherogenicity of TRL and remnants compared to LDL. Assessment of these issues provides a foundation for evaluating approaches to effectively reduce levels of TRL and remnants by targeting either production, lipolysis, or hepatic clearance, or a combination of these mechanisms. This consensus statement updates current understanding in an integrated manner, thereby providing a platform for new therapeutic paradigms targeting TRL and their remnants, with the aim of reducing the risk of ASCVD.

Hypertriglyceridemia-Causes, Significance, and Approaches to Therapy

Hypertriglyceridemia (HTG) is a common metabolic disorder with both genetic and lifestyle factors playing significant roles in its pathophysiology. HTG poses a risk for the development of cardiovascular disease (CVD) in the population at large and for pancreatitis in about two percent of individuals with extremely high levels of triglycerides (TG). This manuscript summarizes the mechanisms underlying the development of HTG as well as its management, including emerging therapies targeted at specific molecular pathways.

Exercise and dietary-mediated reductions in postprandial lipemia

Postprandial hyperlipemia produces long-term derangements in lipid/lipoprotein metabolism, vascular endothelial dysfunction, hypercoagulability, and sympathetic hyperactivity which are strongly linked to atherogenesis. The purpose of this review is to (1) provide a qualitative analysis of the available literature examining the dysregulation of postprandial lipid metabolism in the presence of obesity, (2) inspect the role of adiposity distribution and sex on postprandial lipid metabolism, and (3) examine the role of energy deficit (exercise- and/or energy restriction-mediated), isoenergetic low-carbohydrate diets, and omega-3 (n-3) fatty acid supplementation on postprandial lipid metabolism. We conclude from the literature that central adiposity primarily accounts for sex-related differences in postprandial lipemia and that aerobic exercise attenuates this response in obese or lean men and women to a similar extent through potentially unique mechanisms. In contrast, energy restriction produces only mild reductions in postprandial lipemia suggesting that exercise may be superior to energy restriction alone as a strategy for lowering postprandial lipemia. However, isoenergetic very low-carbohydrate diets and n-3 fatty acid supplementation reduce postprandial lipemia indicating that macronutrient manipulations reduce postprandial lipemia in the absence of energy restriction. Therefore, interactions between exercise/energy restriction and alterations in macronutrient content remain top priorities for the field to identify optimal behavioral treatments to reduce postprandial lipemia.

Stable isotope-labeled tracers for the investigation of fatty acid and triglyceride metabolism in humans in vivo

Understanding lipid metabolism and its regulation requires information on the rates at which lipids are produced within the body, absorbed (dietary lipids) into the body, transported within the body, and utilized by various tissues. This article focuses on the use of stable isotope-labeled tracers for the quantitative evaluation of major pathways of fatty acid and triglyceride metabolism in humans in vivo. Adipose tissue lipolysis and free fatty acid appearance in plasma, fatty acid tissue uptake and oxidation, and hepatic very low-density lipoprotein triglyceride secretion are among the metabolic pathways that can be studied by using stable isotope labeled tracers, and will be discussed in detail. The methodology has been in use for many years and is constantly being refined. A variety of tracers and analytical approaches are available and can be used; knowing the advantages, assumptions, and limitations of each is essential for the planning of studies and the interpretation of data, which can provide unique insights into human lipid metabolism.

A single bout of whole-body resistance exercise augments basal VLDL-triacylglycerol removal from plasma in healthy untrained men

A single bout of prolonged aerobic exercise lowers plasma TAG (triacylglycerol) concentrations the next day by increasing the efficiency of VLDL (very-low-density lipoprotein)-TAG removal from the circulation. The effect of resistance exercise on VLDL-TAG metabolism is not known. Therefore we evaluated VLDL-TAG kinetics by using stable isotope-labelled tracers in eight healthy untrained men (age, 25.3+/-0.8 years; body mass index, 24.5+/-0.6 kg/m(2)) in the post-absorptive state in the morning on two separate occasions: once after performing a single 90-min bout of strenuous isokinetic resistance exercise (three sets x ten repetitions, 12 exercises at 80% of maximum peak torque production, with a 2-min rest interval between exercises) on the preceding afternoon and once after an equivalent period of rest. Fasting plasma VLDL-TAG concentrations in the morning after exercise were significantly lower than in the morning after rest (0.23+/-0.04 compared with 0.33+/-0.06 mmol/l respectively; P=0.001). Hepatic VLDL-TAG secretion rate was not different (P=0.31), but plasma clearance rate of VLDL-TAG was significantly higher (by 26+/-8%) after exercise than rest (31+/-3 compared with 25+/-3 ml/min respectively; P=0.004), and the mean residence time of VLDL-TAG in the circulation was significantly shorter (113+/-10 compared with 144+/-18 min respectively; P=0.02). Fasting plasma NEFA (non-esterified fatty acid; 'free' fatty acid) and serum beta-hydroxybutyrate concentrations were both significantly higher after exercise than rest (P<0.05), whereas plasma glucose and serum insulin concentrations were not different (P>0.30). We conclude that, in healthy untrained men, a single bout of whole-body resistance exercise lowers fasting plasma VLDL-TAG concentrations by augmenting VLDL-TAG removal from plasma. The effect appears to be qualitatively and quantitatively similar to that reported previously for aerobic exercise.

Dietary monounsaturated fat activates metabolic pathways for triglyceride-rich lipoproteins that involve apolipoproteins E and C-III

BACKGROUND

Dietary monounsaturated fat (MUFA) and complex carbohydrates have different effects on triglyceride-rich lipoprotein (TRL) metabolism.

OBJECTIVE

We hypothesized that apolipoprotein (apo) E and apo C-III might be involved in these dietary effects because of their crucial role in TRL metabolism.

DESIGN

Twelve adults consumed, for 3 wk each, 2 isocaloric diets: first a carbohydrate-rich diet (48% complex carbohydrate, 8% MUFAs) and then a MUFA-rich diet (31% complex carbohydrate, 24% MUFAs) 12 mo later. The dietary composition of other macronutrients in the 2 diets was similar. Body weight was kept constant. Postprandial apo B kinetic studies using stable-isotope tracers were performed after each dietary intervention. Multiple VLDL, intermediate-density lipoprotein (IDL), and LDL fractions were prepared on the basis of apo E and apo C-III contents.

RESULTS

The MUFA diet increased by approximately 4-6-fold, the secretion of VLDLs and IDLs containing both apo E and apo C-III (E+CIII+) (P < 0.05). These are TRLs that mostly cleared from the circulation and are minor precursors of LDL. The MUFA diet also decreased by 60% (P < 0.05) the secretion of the TRLs without apo E or apo C-III (major precursors of LDL in plasma) and decreased their flux to LDLs. Total LDL flux did not change because the MUFA diet increased the flux to LDL from E-CIII+ TRLs, a process that requires the removal of apo C-III. In addition, the MUFA diet significantly increased the TRL fractional catabolic rate by 50% and doubled the percentage of TRLs that were cleared rather than being converted to LDLs.

CONCLUSION

MUFA intake activates synthetic and rapid catabolic pathways for TRL metabolism that involve apo E and apo C-III and suppresses the metabolism of more slowly metabolized VLDLs and IDLs, which do not contain these apolipoproteins.

Measurement of endogenous and exogenous triacylglycerol kinetics in the fed and fasted states

Emerging evidence has shown that an abnormal postprandial accumulation of dietary fat is atherogenic. However, there is a lack of data describing the mechanisms for accumulation of triacylglycerol (TAG) in the postprandial period. There is therefore a need to establish a specific measure of the kinetics of endogenous and exogenous TAG in the postprandial period.

Energy expenditure, insulin, and VLDL-triglyceride production in humans

Hypertriglyceridemia is considered a cardiovascular risk factor in diabetic and nondiabetic subjects. In this study, we aimed to determine potential regulators of very low density lipoprotein-triglyceride (TG) production. VLDL-TG kinetics were measured in 13 men and 12 women [body mass index [mean (range)]: 24.8 (20.2-35.6) kg/m(2)]. VLDL-TG production was assessed from the plasma decay of a bolus injection of ex vivo labeled VLDL particles ([1-(14)C]triolein-VLDL-TG). Similar VLDL-TG production (micromol/min) was found in men and women. VLDL-TG production was not significantly correlated with palmitate flux ([9,10-(3)H]palmitate) (r = 0.09, P = 0.67) or palmitate concentration (r = -0.29, P = 0.2) but was correlated significantly with fasting insulin concentration (r = 0.46, P < 0.05) and resting energy expenditure (REE) (r = 0.45, P < 0.05). The latter correlation improved when adjusted for sex. The best multivariate model with VLDL-TG production as the dependent variable and REE, body composition, hormones, and substrate levels as independent variables included fasting insulin (P = 0.02) and REE (P = 0.02) (r(2) = 0.32, P < 0.001). We conclude that VLDL kinetics are similar in men and women and that REE and plasma insulin are significant independent predictors of VLDL-TG production. FFA availability and body fat distribution are unrelated to VLDL production. We suggest that REE plays a greater role in VLDL-TG production than previously anticipated. REE and insulin should be taken into account when VLDL-TG production comparisons between groups are made.

Sources of fatty acids stored in liver and secreted via lipoproteins in patients with nonalcoholic fatty liver disease

Nonalcoholic fatty liver disease (NAFLD) is characterized by the accumulation of excess liver triacylglycerol (TAG), inflammation, and liver damage. The goal of the present study was to directly quantify the biological sources of hepatic and plasma lipoprotein TAG in NAFLD. Patients (5 male and 4 female; 44 +/- 10 years of age) scheduled for a medically indicated liver biopsy were infused with and orally fed stable isotopes for 4 days to label and track serum nonesterified fatty acids (NEFAs), dietary fatty acids, and those derived from the de novo lipogenesis (DNL) pathway, present in liver tissue and lipoprotein TAG. Hepatic and lipoprotein TAG fatty acids were analyzed by gas chromatography/mass spectrometry. NAFLD patients were obese, with fasting hypertriglyceridemia and hyperinsulinemia. Of the TAG accounted for in liver, 59.0% +/- 9.9% of TAG arose from NEFAs; 26.1% +/- 6.7%, from DNL; and 14.9% +/- 7.0%, from the diet. The pattern of labeling in VLDL was similar to that in liver, and throughout the 4 days of labeling, the liver demonstrated reciprocal use of adipose and dietary fatty acids. DNL was elevated in the fasting state and demonstrated no diurnal variation. These quantitative metabolic data document that both elevated peripheral fatty acids and DNL contribute to the accumulation of hepatic and lipoprotein fat in NAFLD.

Sources of fatty acids stored in liver and secreted via lipoproteins in patients with nonalcoholic fatty liver disease

Nonalcoholic fatty liver disease (NAFLD) is characterized by the accumulation of excess liver triacylglycerol (TAG), inflammation, and liver damage. The goal of the present study was to directly quantify the biological sources of hepatic and plasma lipoprotein TAG in NAFLD. Patients (5 male and 4 female; 44 +/- 10 years of age) scheduled for a medically indicated liver biopsy were infused with and orally fed stable isotopes for 4 days to label and track serum nonesterified fatty acids (NEFAs), dietary fatty acids, and those derived from the de novo lipogenesis (DNL) pathway, present in liver tissue and lipoprotein TAG. Hepatic and lipoprotein TAG fatty acids were analyzed by gas chromatography/mass spectrometry. NAFLD patients were obese, with fasting hypertriglyceridemia and hyperinsulinemia. Of the TAG accounted for in liver, 59.0% +/- 9.9% of TAG arose from NEFAs; 26.1% +/- 6.7%, from DNL; and 14.9% +/- 7.0%, from the diet. The pattern of labeling in VLDL was similar to that in liver, and throughout the 4 days of labeling, the liver demonstrated reciprocal use of adipose and dietary fatty acids. DNL was elevated in the fasting state and demonstrated no diurnal variation. These quantitative metabolic data document that both elevated peripheral fatty acids and DNL contribute to the accumulation of hepatic and lipoprotein fat in NAFLD.

Why Syndrome X? From Harold Himsworth to the insulin resistance syndrome

Although the concept of Syndrome X was introduced in the Banting Medal address of 1988 (Reaven, 1988), the notion that led to its genesis had started approximately 50 years earlier. In this short history, an attempt will be made to trace the two paths of scientific discovery that were formally merged in New Orleans in 1988 to form the scientific foundation of Syndrome X. In addition, the developments in the last 16 years that have led from the notion of Syndrome X to the broader concept of an Insulin Resistance Syndrome (IRS) will be briefly summarized.

Measuring very low density lipoprotein-triglyceride kinetics in man in vivo: how different the various methods really are

PURPOSE OF REVIEW

The purpose of this article is to briefly outline the methods that are currently available for the determination of very low density lipoprotein-triglyceride (VLDL-TG) kinetics in man in vivo.

RECENT FINDINGS

A number of novel methodologies have been developed over the years for quantifying VLDL-TG production, clearance, and turnover rates. Besides the splanchnic arteriovenous balance technique, tracer methods with radioactive and, more recently, stable isotopes have been widely used. Most of the latter approaches utilize an isotopically labelled substrate, such as glycerol, fatty acid or acetate, which is eventually incorporated into a VLDL-TG moiety, and monitor the time course of change in specific activity or enrichment. A procedure of in vivo labelling of VLDL-TG with stable isotopes and use of the labelled VLDL-TG as a tracer has also been described in man. There is, however, considerable variability in estimates of VLDL-TG kinetics obtained by the various techniques, which cannot be readily attributed to normal physiological variation. Still, a large part of this discrepancy may be related to differences in VLDL-TG pool size within the normal range, which seem to account for approximately 40-50% of the total variance in VLDL-TG kinetics in both men and women.

SUMMARY

Several methods are available for quantifying VLDL-TG kinetics in man in vivo, varying in the selection of tracer, mode of administration and sampling, and data analysis. These inherent features, along with different pool sizes, result in multifold variable estimations of VLDL-TG kinetic parameters.

The influence of diet upon liver function tests and serum lipids in healthy male volunteers resident in a Phase I unit

AIM

To investigate the effect of diet upon liver function tests and serum lipids within the restricted environment of a Phase I unit.

METHODS

An open randomized three-way crossover study was designed with subjects consuming three types of diet. The diets comprised, a balanced normal calorie diet, a high-carbohydrate high-calorie diet and a high-fat high-calorie diet. Each diet was consumed in a randomized sequence over 8 days with a recovery period of 14 days between periods. The blood concentrations of various laboratory parameters were measured at intervals throughout each dietary period and during the recovery periods.

RESULTS

Blood transaminase activity and triglyceride concentrations increased significantly whilst subjects consumed a high-carbohydrate high-calorie diet but not when fed either a high-fat high-calorie diet or a balanced normal calorie diet.

CONCLUSIONS

The rises in transaminases and triglycerides were caused by the carbohydrate content of the diet rather than its calorific value. Sucrose rather than starch was the carbohydrate which caused the rise in transaminases and triglycerides. The importance of controlling diet in Phase I studies is stressed.

Mechanism for the increase in plasma triacylglycerol concentrations after consumption of short-term, high-carbohydrate diets

BACKGROUND

High-carbohydrate (HC) diets are recommended for lowering the risk of coronary heart disease because they decrease plasma LDL-cholesterol concentrations. However, an unfavorable effect of HC diets is an increase in plasma triacylglycerol concentrations. The underlying mechanisms of this effect are still unclear.

OBJECTIVE

We examined the effect of diet composition on VLDL-triacylglycerol metabolism using in vivo isotopically labeled VLDL-triacylglycerol tracers.

DESIGN

Six healthy subjects were studied on 2 occasions: after 2 wk of an HC diet (75% carbohydrates, 10% fat, and 15% protein) and after 2 wk of an isoenergetic high-fat (HF) diet (30% carbohydrates, 55% fat, and 15% protein).

RESULTS

The plasma VLDL-triacylglycerol concentration was higher after the HC diet than after the HF diet (690 +/- 186 compared with 287 +/- 104 micromol/L; P < 0.05) because of higher rates of VLDL-triacylglycerol production (0.76 +/- 0.12 compared with 0.45 +/- 0.15 micromol x kg(-1) x min(-1); P < 0.05) rather than diminished VLDL-triacylglycerol clearance (1.5 +/- 0.5 compared with 1.7 +/- 0.5 mL x kg(-1) x min(-1) after the HC diet than after the HF diet, respectively). The increase in VLDL-triacylglycerol production was probably mediated by a decrease in hepatic fatty acid oxidation after the HC diet (0.13 +/- 0.02 compared with 0.69 +/- 0.24 micromol x kg(-1) x min(-1); P < 0.05), which presumably increased hepatic fatty acid availability for triacylglycerol synthesis.

CONCLUSIONS

The increase in fasting plasma triacylglycerol concentrations in response to short-term HC diets is due to accelerated VLDL-triacylglycerol secretion. Increased hepatic fatty acid availability, resulting from reduced hepatic fatty acid oxidation, is most likely responsible for the observed increase in VLDL-triacylglycerol secretion.

Effect of high-carbohydrate feeding on triglyceride and saturated fatty acid synthesis

It has been known for decades that low-fat, high-carbohydrate diets can increase plasma triglyceride levels, but the mechanism for this effect has been uncertain. Recently, new isotopic and nonisotopic methods have been used to determine in vivo whether low-fat, high-carbohydrate diets increase triglyceride levels by stimulating fatty acid synthesis. The results of a series of studies in lean and obese weight-stable volunteers showed that very-low-fat (10%), high-carbohydrate diets enriched in simple sugars increased the fraction of newly synthesized fatty acids, along with a proportionate increase in the concentration of plasma triglyceride. Furthermore, the concentration of the saturated fatty acid, palmitate, increased and the concentration of the essential polyunsaturated fatty acid, linoleate, decreased in triglyceride and VLDL triglyceride. The magnitude of the increase in triglyceride varied considerably among subjects, was unrelated to sex, body mass index, or insulin levels, and was higher when fatty acid synthesis was constantly elevated rather than having a diurnal variation. It was notable that minimal stimulation of fatty acid synthesis occurred with higher fat diets (>30%) or with 10% fat diets enriched in complex carbohydrate. Public health recommendations to reduce dietary fat must take into account the distinct effects of different types of carbohydrate that may increase plasma triglycerides and fatty acid synthesis in a highly variable manner. The mediators and health consequences of this dietary effect deserve further study.

Carbohydrate-induced hypertriacylglycerolemia: historical perspective and review of biological mechanisms

Current trends in health promotion emphasize the importance of reducing dietary fat intake. However, as dietary fat is reduced, the dietary carbohydrate content typically rises and the desired reduction in plasma cholesterol concentrations is frequently accompanied by an elevation of plasma triacylglycerol. We review the phenomenon of carbohydrate-induced hypertriacylglycerolemia, the health effects of which are among the most controversial and important issues in public health nutrition today. We first focus on how seminal observations made in the late 1950s and early 1960s became the basis for subsequent important research questions and areas of scientific study. The second focus of this paper is on the current knowledge of biological mechanisms that contribute to carbohydrate-induced hypertriacylglycerolemia. The clinical rationale behind mechanistic studies is this: if carbohydrate-induced hypertriacylglycerolemia shares a metabolic basis with endogenous hypertriacylglycerolemia (that observed in subjects consuming high-fat diets), then a similar atherogenic risk may be more likely than if the underlying metabolic mechanisms differ. The third focus of the paper is on both the positive metabolic changes that occur when high-carbohydrate diets are consumed and the potentially negative health effects of such diets. The review concludes with a summary of some important research questions that remain to be addressed. These issues include the level of dietary carbohydrate that induces carbohydrate-induced hypertriacylglycerolemia, whether the phenomenon is transient or can be avoided, whether de novo lipogenesis contributes to the phenomenon, and what magnitude of triacylglycerol elevation represents an increase in disease risk.

Effects of a low-fat, high-carbohydrate diet on VLDL-triglyceride assembly, production, and clearance

Low-fat, high-carbohydrate (LF/HC) diets commonly elevate plasma triglyceride (TG) concentrations, but the kinetic mechanisms responsible for this effect remain uncertain. Subjects with low TG (normolipidemic [NL]) and those with moderately elevated TG (hypertriglyceridemic [HTG]) were studied on both a control and an LF/HC diet. We measured VLDL particle and TG transport rates, plasma nonesterified fatty acid (NEFA) flux, and sources of fatty acids used for the assembly of VLDL-TG. The LF/HC diet resulted in a 60% elevation in TG, a 37% reduction in VLDL-TG clearance, and an 18% reduction in whole-body fat oxidation, but no significant change in VLDL-apo B or VLDL-TG secretion rates. Significant elevations in fasting apo B-48 concentrations were observed on the LF/HC in HTG subjects. In both groups, fasting de novo lipogenesis was low regardless of diet. The NEFA pool contributed the great majority of fatty acids to VLDL-TG in NL subjects on both diets, whereas in HTG subjects, the contribution of NEFA was somewhat lower overall and was reduced further in individuals on the LF/HC diet. Between 13% and 29% of VLDL-TG fatty acids remained unaccounted for by the sum of de novo lipogenesis and plasma NEFA input in HTG subjects. We conclude that (a) whole-food LF/HC diets reduce VLDL-TG clearance and do not increase VLDL-TG secretion or de novo lipogenesis; (b) sources of fatty acids for assembly of VLDL-TG differ between HTG and NL subjects and are further affected by diet composition; (c) the presence of chylomicron remnants in the fasting state on LF/HC diets may contribute to elevated TG levels by competing for VLDL-TG lipolysis and by providing a source of fatty acids for hepatic VLDL-TG synthesis; and (d) the assembly, production, and clearance of elevated plasma VLDL-TG in response to LF/HC diets therefore differ from those for elevated TG on higher-fat diets.

Insulin resistance and human disease: a short history

The notion that tissue resistance to insulin might play an important role in certain disease states is approximately 60 years old. However, recognition of its central role in this regard is a relatively recent phenomenon. In this review an effort has been made to trace a brief history of insulin resistance from its inception to its current position as the fundamental abnormality in both type 2 diabetes and Syndrome X.

Fractional removal rate of fat emulsion (K2) remains to be low in APOE3/3 phenotype subjects with serum triglyceride level above 180mg/dl

The relation between fractional catabolic rate (K2) of an intravenously injected fat emulsion, Intralipid, and the level of serum triglyceride (TG) was evaluated to cast light on TG-rich lipoprotein metabolism in 182 subjects who were homozygotes for the most common form of apoE3/3. Both normolipidemic individuals and primary hyperlipidemic patients were included. To assess the influence of variation in the apoE phenotype on fat emulsion metabolism, 25 subjects with the apoE 4/3 phenotype and 21 with apoE 3/2 phenotype were also evaluated. In the apoE 3/3 subjects, K2 decreased with increasing TG level up to 180mg/dl, but above 180mg/dl, K2 remained at a constant level. This TG value was therefore concluded to be a cut off beyond which the TG-dependent decrease in K2 disappeared. No apparent correlation between K2 and serum TG was observed in subjects with apoE 4/3 or 3/2 phenotypes. In subjects with TG above 180mg/dl, presumably both accelerated synthesis and limited removal are involved in the development of their hypertriglyceridemia. Since K2 demonstrated little change with increase of TG over 180mg/dl in apoE 3/3 subjects, we concluded that the capacity to catabolize fat emulsion reaches a kinetic saturation. The TG value of 180mg/dl may be a physiological significance. The relation between K2 and TG was specific in apoE 3/3, as it was not observed in subjects having apoE 4/3 or 3/2 phenotypes.

Role of human liver lipogenesis and reesterification in triglycerides secretion and in FFA reesterification

To measure 1) the contribution of hepatic de novo lipogenesis (DNL) and plasma free fatty acid (FFA) reesterification to plasma triglyceride (TG) secretion and 2) the role of oxidation and hepatic and extrahepatic reesterification in FFA utilization, five normal subjects drank deuterate water and were infused (postabsorptive state) with [1-13C]palmitate and [1,2,3-2H5]glycerol. Total lipid oxidation (Lox) was measured by indirect calorimetry. FFA oxidation (2.76 +/- 0.65 mumol.kg.-1.min-1) accounted for 45% of FFA turnover rate (Rt) (1.04 mumol.kg-1.min-1) and 91% of Lox; FFA reesterification was 3.27 +/- 0.54 mumol.kg-1.min-1. Fractional and absolute TG Rt were 0.21 +/- 0.02 h-1 and 0.11 +/- 0.05 mumol.kg-1.min-1. DNL accounted for 3.9 +/- 0.9% of TG secretion, and hepatic FFA reesterification accounted for 49.4 +/- 5.7%; this last process represented a utilization of FFA of 0.16 +/- 0.02 mumol.kg-1.min-1. We conclude that, in the postabsorptive state, 1) DNL and FFA reesterification account for only 50-55% of TG secretion, the remaining presumably being provided by stored lipids or lipoproteins taken up by liver, 2) most reesterification occurs in extrahepatic tissues, and 3) oxidation and reesterification each contribute about one-half to FFA utilization; FFA oxidation accounts for almost all Lox.

Energy and macronutrient metabolism

In general, obesity is a state of high energy stores, high energy intake, and high energy expenditure. The high energy expenditure is largely due to the increased fat-free mass. The failure to find a positive relationship between reported energy intake and body size reflects a greater under-reporting of calorie intake among obese individuals. Obesity, therefore, develops as a consequence of a chronic imbalance between intake and expenditure, although the cause of this is not apparent from the energy balance equation. However, this equation can be dissected into its component nutrient balance equations because net de novo lipogenesis is negligible in free-living humans. Fat calories are handled very differently from non-fat calories. Non-fat nutrient oxidation rates rise and fall to match the fluctuations in non-fat intake so that non-fat calorie balance is actively maintained. In contrast, changes in fat intake do not acutely affect fat oxidation but are matched by changes in storage. Therefore, within the fat balance equation there is ample scope for a chronic imbalance between fat intake and oxidation. Also, there is some evidence that carbohydrate balance may be an important signal for hunger and satiety. These concepts imply that, under free-living, ad libitum eating conditions, changes in nutrient intake composition (e.g. an increased proportion of fat in the diet) or changes in nutrient oxidation composition (e.g. a decrease in the proportion of fat oxidized) will lead to body weight change (in these cases, to weight gain). Considering obesity as a consequence of normal physiology (with its normal variation between individuals) in a 'pathological' environment (high fat diet, low exercise) offers an important perspective for explaining the interpopulation and interindividual differences in obesity and for formulating treatment and prevention options. Low energy expenditure (relative to body size), high respiratory quotient and insulin sensitivity have been shown to be predictors of weight gain, although upon gaining weight these metabolic factors tend to 'normalize'. Metabolic responses to underfeeding or overfeeding are largely predictable from the changes in calorie intake and changes in body composition, but some adaptive changes may occur.

Cholesterol secretion from hepatocytes induced by triacylglycerol and apolipoprotein E

The mechanism for the increase in plasma cholesterol levels in cholesterol-fed rats following chylomicron transport was investigated in intact animals, in isolated perfused liver, and in hepatocytes in monolayer cultures. Intravenous administration of egg phosphatidylcholine in amounts greater than those required to cause a plasma cholesterol response when given as chylomicrons was without effect. This makes it unlikely that increased plasma cholesterol levels resulted from the recruitment of tissue cholesterol by the plasma chylomicron phospholipids that persisted in the plasma after triacylglycerol clearance. The hepatic origin of the increased plasma cholesterol levels was directly confirmed by two hepatic perfusion experiments. When cholesterol-fed rats received intravenous chylomicrons prior to isolated hepatic perfusion, more cholesterol was secreted by the liver than when the rats were injected intravenously with buffer. Perfusion of apolipoprotein E (apo E)-rich triacylglycerol emulsions through the livers also enhanced cholesterol secretion. The increase in hepatocyte cholesterol secretion seen with cholesterol-fed rats was also noted in monolayer cultures following incubation with apo E rich-triacylglycerol emulsions. The apolipoprotein or the emulsion alone, or apo E-rich phosphatidylcholine liposomes, had no effect. The data confirm previous indirect observations that the liver is the source of cholesterol that appears in plasma following transport of chylomicrons or following a lipid-rich meal in cholesterol-fed rats. The data also re-emphasize the importance of providing apo E with triacylglycerol emulsions to initiate secretion of lower density lipoproteins by the liver.

Insulin resistance, glucose intolerance, and hyperinsulinemia. Hypertriglyceridemia versus hypercholesterolemia

Plasma glucose and insulin responses to oral glucose and mixed meals and the ability of insulin to stimulate glucose disposal were quantified in normal volunteer subjects and patients with types IIA, IIB, and IV hyperlipoproteinemia (HLP). The results indicated that patients with either type IIB or IV HLP had higher plasma glucose (p < 0.05-< 0.001) and insulin (p < 0.001) responses to both oral glucose and mixed meals compared with the normal subjects and patients with type IIA HLP. Steady-state plasma glucose concentrations (mmol/L) were also higher (p < 0.001) in patients with types IIB (13.3 +/- 0.6) and IV (12.8 +/- 1.2) HLP during a continuous infusion of somatostatin, glucose, and insulin than either the control group (volunteer subjects) (6.2 +/- 0.9) or patients with type IIA HLP (5.6 +/- 1.0). Because the steady-state plasma insulin concentrations were similar in all four groups, patients with either type IIB or IV HLP were resistant to insulin-mediated glucose uptake. These data indicate that patients with hypertriglyceridemia are insulin resistant, glucose intolerant, and hyperinsulinemic, irrespective of the plasma cholesterol concentration. The results further demonstrate that hypercholesterolemic patients with normal triglyceride concentrations do not have any abnormalities of glucose and insulin metabolism.

Determinants of fasting plasma triglyceride levels: metabolism of hepatic and intestinal lipoproteins

The contribution of the kinetics of exogeneous and endogenous lipoproteins in determining the level of triglyceride in fasting plasma was assessed in a group of 19 normolipidaemic and hypertriglyceridaemic subjects. From data derived during a 9-h infusion of [15N]-glycine, we have assessed very low density lipoprotein apolipoprotein B production, and from data analysed by kinetic modelling obtained following ingestion of retinol and triolein, we have assessed chylomicron and chylomicron remnant clearance in a group of 19 normolipidaemic and hypertriglyceridaemic subjects. A strong positive correlation was observed between the fasting plasma triglyceride level and the reciprocal of the apolipoprotein B fractional synthetic rate (r = 0.83, P less than 0.01). A positive correlation was also found with the rate of clearance of chylomicron remnants (Sf 20-400; r = 0.87, P less than 0.01) and of chylomicrons (Sf greater than 400; r = 0.69, P less than 0.01). No correlation was found between the fasting plasma triglyceride level and either of the plasma post-heparin lipolytic activities. Multivariate analysis revealed that 95% of the variance in triglyceride levels could be explained by the apolipoprotein B fractional synthetic rate and the chylomicron remnant clearance rate. The strong correlation between chylomicron remnant clearance, a measure of exogenous lipid metabolism, and fasting (hence, endogenous) plasma triglyceride levels suggests that remnants of chylomicrons and very low density lipoproteins share some common components of the removal process.

Phenotypic expression of heterozygous lipoprotein lipase deficiency in the extended pedigree of a proband homozygous for a missense mutation

Familial lipoprotein lipase (LPL) deficiency is a rare genetic disorder accompanied by well-characterized manifestations. The phenotypic expression of heterozygous LPL deficiency has not been so clearly defined. We studied the pedigree of a proband known to be homozygous for a mutation resulting in nonfunctional LPL. Hybridization of DNA from 126 members with allele-specific probes detected 29 carriers of the mutant allele. Adipose tissue LPL activity, measured previously, was reduced by 50% in carriers, but did not reliably distinguish them from noncarriers. Carriers were prone to the expression of a form of familial hypertriglyceridemia characterized by increased plasma triglyceride, VLDL cholesterol and apolipoprotein B, and decreased LDL and HDL cholesterol concentrations. These manifestations were age modulated, with conspicuous differences between carriers and noncarriers observed only after age 40. Several noncarriers exhibited similar lipid abnormalities, but without the inverse relationship between VLDL cholesterol and LDL cholesterol noted among carriers. In addition to age and carrier status, the potentially reversible conditions, obesity, hyperinsulinemia and lipid-raising drug use were contributory. Thus heterozygous lipoprotein lipase deficiency, together with age-related influences, may account for a form of familial hypertriglyceridemia.

Effect of a high-carbohydrate, low-saturated-fat diet on apolipoprotein B and triglyceride metabolism in Pima Indians

The mechanisms by which high-carbohydrate, low-saturated-fat diets lower LDL cholesterol (LDLC) concentrations are unknown. In this study, kinetics of VLDL, intermediate density lipoprotein (IDL), and LDL apoprotein B and VLDL triglyceride were determined in seven nondiabetic (ND) and seven non-insulin-dependent diabetic (NIDDM) Pima Indian subjects on high-fat and high-carbohydrate (HICHO) diets. Metabolic changes were similar in ND and NIDDM. On the HICHO diet, LDLC decreased (131 +/- 8 vs. 110 +/- 7 mg/dl, P less than 0.0001) in all subjects. Mean fasting and 24-h triglyceride (TG) concentrations were unchanged, as were mean production rates and fractional clearance rates (FCR) of VLDL apoB and VLDL TG. The mean VLDL apoB pool size (303 +/- 20 vs. 371 +/- 38 mg, P = 0.01) increased owing to a decrease in the mean transport rate (10.7 +/- 1.1 vs. 8.4 +/- 0.9 mg/kg fat-free mass (ffm) per day, P less than 0.0001) and the mean rate constant (2.3 +/- 0.2 vs. 1.5 +/- 0.2, P less than 0.001) for the VLDL apoB to IDL apoB conversion pathway. The mean transport rate of VLDL apoB to LDL apoB via IDL (10.2 +/- 0.9 vs. 8.0 +/- 0.8 mg/kg ffm per day, P less than 0.001) decreased. Mean LDL apoB concentrations decreased (70 +/- 5 vs. 61 +/- 5 mg/dl, P less than 0.001) on the HICHO diet. Means for total LDL apoB transport rate, LDL apoB FCR, and LDLC/apoB ratios were unchanged. In summary, the HICHO diet decreased the activity of mechanisms that convert VLDL to LDL, which contributed to the decrease in LDLC in all subjects. There was also evidence in some subjects for increased activity of LDL apoB clearance mechanisms, and a decrease in the LDLC to apoB ratio.

Impaired triacylglycerol catabolism in hypertriglyceridemia of the diabetic, cholesterol-fed rabbit: a possible mechanism for protection from atherosclerosis

The etiology of the hypertriglyceridemia in alloxan-diabetic rabbits was studied by two independent methods. Production and removal rates of VLDL triacylglycerol were measured in diabetic rabbits by injection of [3H]palmitate-labelled VLDL. Similarly, triacylglycerol total removal rates were determined in non-diabetic rabbits which were infused with Intralipid to mimic the plasma triacylglycerol concentrations of diabetic rabbits. Compared to nondiabetic rabbits, triacylglycerol removal rats were decreased in diabetic rabbits, particularly at higher levels of plasma triacylglycerol. During cholesterol and triacylglycerol supplementation of the diet, post-heparin plasma lipoprotein lipase activity of diabetic rabbits with severe hypertriglyceridemia averaged 36% of that of nondiabetics, suggesting an impaired triacylglycerol removal capacity. Furthermore, plasma triacylglycerol was inversely related to post-heparin plasma lipoprotein lipase activity among diabetic rabbits. VLDL triacylglycerol production increased with increasing plasma triacylglycerol concentration among diabetic cholesterol-fed rabbits with moderately severe hypertriglyceridemia, but reached an apparent plateau among rabbits with plasma triacylglycerol concentrations from approx. 2000-9000 mg/dl. Thus, severe hypertriglyceridemia in this model of insulin deficiency can be attributed only partially to VLDL hypersecretion, whereas a removal defect, resulting in saturation of the triacylglycerol removal mechanism, appears to be largely responsible. The impaired removal of plasma triacylglycerol is also related to the presence of cholesterol predominantly in lipoproteins of increased size. The data support the hypothesis that protection against atherosclerosis in cholesterol-fed diabetic rabbits results from exclusion of very large cholesterol-containing lipoproteins from the arterial wall.

Very low density lipoprotein triglyceride metabolism in relatives of hypertriglyceridemic probands. Evidence for genetic control of triglyceride removal

The production and catabolism of very low density lipoprotein triglycerides (VLDL-TG) were determined in 11 index patients with primary hypertriglyceridemia and in their 70 first-degree relatives. In the probands the mean value for VLDL-TG production rate was twice normal, and the mean fractional catabolic rate (FCR) was reduced to 50% from normal. A similar kinetic pattern was also observed in most hypertriglyceridemic relatives. In the normotriglyceridemic relatives the mean values of both kinetic parameters were comparable to those of controls. No kinetic differences were observed between families with familial hypertriglyceridemia, familial combined hyperlipidemia, or genetically unclassified hypertriglyceridemia (all diagnosed by lipoprotein phenotypes). Thus, no explanation for the phenotypic differences between the two forms of familial hyperlipoproteinemia was found in plasma VLDL-TG metabolism. When the families were grouped according to the VLDL-TG production rate of the proband, there was no significant difference between the VLDL-TG production rates of relatives of "overproducer" probands and relatives of the probands with normal VLDL-TG production rate. In contrast, relatives of low FCR probands had significantly lower mean FCR than the relatives of probands with a normal FCR. This difference in FCR was present both in hypertriglyceridemic and normotriglyceridemic relatives. These results suggest that the catabolism (lipolysis) of VLDL-TG is under genetic control, whereas the VLDL-TG production rate is mainly related to obesity. It is likely that hypertriglyceridemia often develops on the basis of VLDL overproduction in individuals who have a genetically low VLDL triglyceride removal (lipolytic) capacity.

Sex, plasma lipoproteins, and atherosclerosis: prevailing assumptions and outstanding questions

We review the hypothesis that the incidence of coronary heart disease (CHD) is higher in men than in women due to differences in plasma lipoprotein risk factors between the sexes. Men and women appear to be equally susceptible to the effects of lipoprotein risk factors for CHD, and the difference between the sexes in lipoprotein risk factors for CHD appears to be consistent with their being, at least in part, responsible for the sex difference in CHD. This is apparent both when men and women of equal age are compared, and when age-related variations in the sex differences in plasma lipoproteins and CHD are considered. Differences between the sexes in lipoprotein concentrations are still present when sex differences in adiposity, cigarette smoking, physical activity, and diet are taken into account. Evidence relating these sex differences in CHD and lipoproteins to the effects of sex hormones is critically examined. It is commonly accepted that androgens induce changes in lipoprotein concentrations that would predispose towards CHD, whereas estrogens are held to have opposite effects. However, much of the evidence for this comes from studies of changes associated with administration of synthetic gonadal steroids or with changes in gonadal function. Studies of differences in lipoprotein metabolism in normal men and women are extremely limited. In males high-density lipoprotein (HDL) cholesterol levels fall at puberty, correlating with the rise in plasma testosterone concentrations. In females, HDL levels do not change at puberty, despite the rise in estrogen concentrations. Evidence for lipoprotein changes during the menopause, when estrogen levels decline, is equivocal. Similarly, the evidence for an increase in CHD incidence at the menopause is inconclusive. National mortality data indicate that the decreasing sex difference in CHD after 50 years of age is due to a declining rate of increase in men rather than to an acceleration in CHD incidence in women. In men the age-related increase in low-density lipoprotein (LDL) concentrations diminishes beyond 50 years of age, whereas in women the rate of increase remains unchanged. Studies of the effects of gonadectomy are of doubtful relevance in assessing the roles of sex hormones in CHD, and have not been performed with sufficient rigor to provide definitive conclusions.(ABSTRACT TRUNCATED AT 400 WORDS)

The use of isotopic tracers in studying lipid metabolism in human subjects

We have attempted to evaluate some of the tracer methodologies involved in studying lipid metabolism in humans. The magnitude of this subject prohibits a comprehensive review of all areas. Since the major function of adipose tissue appears to be to supply the body with energy, we have particularly emphasized the approaches used to study the mobilization and oxidation of fat. The importance of these issues, as well as the increasing availability of non-radioactive tracers, suggest an optimistic future for this area of research.

Abnormal lipoprotein metabolism in non-insulin-dependent diabetes mellitus. Pathogenesis and treatment

Although diabetes mellitus is by definition a state of abnormal carbohydrate metabolism, defects in lipoprotein metabolism are a prominent feature of the diabetic syndrome. This is particularly true of patients with non-insulin-dependent diabetes mellitus (NIDDM), and it is quite likely that the abnormal lipid metabolism contributes significantly to the increased morbidity and mortality from coronary artery disease that characterizes NIDDM. In this presentation, an attempt is made to develop a coherent picture as to the pathogenesis of the defects in lipoprotein metabolism seen in patients with NIDDM, as well as to discuss therapeutic approaches to the treatment of these abnormalities.

Very low density lipoprotein metabolism in diabetes mellitus

The concentration of VLDL and their major lipid, triglyceride, are regulated at many levels from the initial availability of the substrates needed for their synthesis all the way to the function of the enzymes and receptors involved in their removal from plasma. It should be clear from this review that in diabetes mellitus metabolic derangements resulting from the absolute lack of insulin or from resistance to the actions of insulin can affect VLDL triglyceride metabolism at any or all of these regulatory points. The outcome of this interplay between diabetes and VLDL metabolism is the common occurrence of elevated plasma VLDL and triglyceride concentrations in individuals with both Type 1 and Type 2 diabetes mellitus. Mildly elevated plasma levels of triglycerides are nearly universal in diabetics; more significant hypertriglyceridemia can be the consequence of either metabolic decompensation or the concomitant inheritance of a familial pattern of hyperlipoproteinemia. The combination of the latter two situations usually presents with as severe hypertriglyceridemia. Although deregulation can occur at many points, the most common abnormality associated with hypertriglyceridemia in human diabetes appears to be overproduction of VLDL triglycerides. Increased rates of synthesis of VLDL apoB may also be a common consequence of diabetes. The basis for this belief is the accumulated data from kinetic studies in humans and in experimental models of diabetes in rats. Although the latter may also demonstrate defects in catabolism when insulin deficiency is severe, catabolic abnormalities appear to be uncommon as the primary force in the development of hypertriglyceridemia in humans. Finally, despite the complexity of the systems regulating VLDL metabolism and the many metabolic abnormalities that may be present in diabetic subjects, it appears that reduction of the hyperglycemia by means of dietary or pharmacologic interventions is associated with normalization of the rates of synthesis and catabolism of the VLDL and their triglycerides. In view of the probable atherogenecity of VLDL, particularly in individuals with diabetes, such intervention must be aimed at both plasma glucose and lipid concentrations.

Non-insulin-dependent diabetes mellitus, abnormal lipoprotein metabolism, and atherosclerosis

Although diabetes mellitus is, by definition, a state of abnormal carbohydrate metabolism, defects in lipoprotein metabolism are a prominent feature of the diabetic syndrome. This is particularly true of patients with non-insulin-dependent diabetes mellitus (NIDDM), and it is quite likely that the abnormal lipid metabolism contributes significantly to the increased morbidity and mortality from macrovascular diseases that characterize NIDDM. In this presentation, an attempt will be made to develop a coherent picture as to the pathogenesis of the defects in lipoprotein metabolism seen in patients with NIDDM, as well as to discuss some new information as to possible links between abnormal lipoprotein metabolism and the development of coronary artery disease (CAD).

Defective metabolism of hypertriglyceridemic low density lipoprotein in cultured human skin fibroblasts. Normalization with bezafibrate therapy

The metabolism of hypertriglyceridemic low density lipoprotein (HTG-LDL) was investigated in upregulated cultured human skin fibroblasts. Low density lipoprotein (LDL) was isolated by zonal centrifugation from the plasma of seven HTG subjects, before and 2 wk after the initiation of bezafibrate (BZ) therapy. HTG-LDL is a cholesterol-poor, triglyceride-rich lipoprotein of smaller diameter than BZ-LDL or normal LDL (N-LDL). Binding, cell association, and proteolytic degradation of HTG-LDL were compared with that of BZ-LDL and N-LDL and were found to be significantly lower by a paired t test analysis (P less than 0.001). After 6 h preincubation with unlabeled HTG-LDL, the incorporation of [14C]acetate to sterols was significantly higher than with BZ-LDL or N-LDL (577 +/- 43.7; 330 +/- 41.5; 262 +/- 47, mean +/- SE, picomoles sterols per milligram cell protein per 2 h, respectively; P less than 0.001 by paired t test). To determine the effectiveness of HTG-LDL and BZ-LDL on the down-regulation of LDL receptor activity, up-regulated cells were incubated for 48 h with HTG-LDL and BZ-LDL. LDL receptor activity was significantly higher after preincubation with HTG-LDL compared with BZ-LDL, and the rates of sterol synthesis were similarly increased. These results demonstrate that HTG-LDL does not down-regulate the LDL receptor activity as efficiently as BZ-LDL and that its cholesterol content is not enough to adequately suppress cellular sterol synthesis. Significant correlation between LDL composition and cholesterol synthesis by cultured cells was found with all LDL preparations over a wide range of cholesteryl ester to protein ratio (0.8-2.2). This correlation indicates that the compositional and structural abnormalities of HTG-LDL, and especially the low cholesterol content of the lipoprotein, alter LDL metabolism and cellular cholesterol formation.

Effect of sepsis on VLDL kinetics: responses in basal state and during glucose infusion

The effect of gram-negative sepsis on the kinetics and oxidation of very low-density lipoprotein (VLDL) fatty acids was assessed in conscious dogs in the normal state and 24 h after infusion of live Escherichia coli. VLDL, labeled with [2-3H]glycerol and [1-14C]palmitic acid, was used to trace VLDL kinetics and oxidation, and [1-13C]palmitic acid bound to albumin was infused simultaneously to quantify kinetics and oxidation of free fatty acid (FFA) in plasma. Sepsis caused a fivefold increase in the rate of VLDL production (RaVLDL). In the control dogs, the direct oxidation of VLDL-fatty acids was not an important contributor to their overall energy metabolism, but in dogs with sepsis, 17% of the total rate of CO2 production could be accounted for by VLDL-fatty acid oxidation. When glucose was infused into dogs with insulin and glucagon levels clamped at basal levels (by means of infusion of somatostatin and replacement of the hormones), RaVLDL increased significantly in the control dogs, but it did not increase further in dogs with sepsis. We conclude that the increase in triglyceride concentration in fasting dogs with gram-negative sepsis is the result of an increase in VLDL production and that the fatty acids in VLDL can serve as an important source of energy in sepsis.