Aims/hypothesis: We determined whether hepatic fat content and plasma adiponectin concentration regulate VLDL 1 production. Methods: A multicompartment model was used to simultaneously determine the kinetic parameters of triglycerides (TGs) and apolipoprotein B (ApoB) in VLDL 1 and VLDL 2 after a bolus of [ 2 H 3 ]leucine and [ 2 H 5 ]glycerol in ten men with type 2 diabetes and in 18 non-diabetic men. Liver fat content was determined by proton spectroscopy and intra-abdominal fat content by MRI. Results: Univariate regression analysis showed that liver fat content, intra-abdominal fat volume, plasma glucose, insulin and HOMA-IR (homeostasis model assessment of insulin resistance) correlated with VLDL 1 TG and ApoB production. However, only liver fat and plasma glucose were significant in multiple regression models, emphasising the critical role of substrate fluxes and lipid availability in the liver as the driving force for overproduction of VLDL 1 in subjects with type 2 diabetes. Despite negative correlations with fasting TG levels, liver fat content, and VLDL 1 TG and ApoB pool sizes, adiponectin was not linked to VLDL 1 TG or ApoB production and thus was not a predictor of VLDL 1 production. However, adiponectin correlated negatively with the removal rates of VLDL 1 TG and ApoB. Conclusions/interpretation: We propose that the metabolic effect of insulin resistance, partly mediated by depressed plasma adiponectin levels, increases fatty acid flux from adipose tissue to the liver and induces the accumulation of fat in the liver. Elevated plasma glucose can further increase hepatic fat content through multiple pathways, resulting in overproduction of VLDL 1 particles and leading to the characteristic dyslipidaemia associated with type 2 diabetes.

Insulin resistance is a key feature of the metabolic syndrome and often progresses to type 2 diabetes. Both insulin resistance and type 2 diabetes are characterized by dyslipidemia, which is an important and common risk factor for cardiovascular disease. Diabetic dyslipidemia is a cluster of potentially atherogenic lipid and lipoprotein abnormalities that are metabolically interrelated. Recent evidence suggests that a fundamental defect is an overproduction of large very low–density lipoprotein (VLDL) particles, which initiates a sequence of lipoprotein changes, resulting in higher levels of remnant particles, smaller LDL, and lower levels of high-density liporotein (HDL) cholesterol. These atherogenic lipid abnormalities precede the diagnosis of type 2 diabetes by several years, and it is thus important to elucidate the mechanisms involved in the overproduction of large VLDL particles. Here, we review the pathophysiology of VLDL biosynthesis and metabolism in the metabolic syndrome. We also review recent research investigating the relation between hepatic accumulation of lipids and insulin resistance, and sources of fatty acids for liver fat and VLDL biosynthesis. Finally, we briefly discuss current treatments for lipid management of dyslipidemia and potential future therapeutic targets.

A carbohydrate-restricted diet is a widely recommended intervention for non-alcoholic fatty liver disease (NAFLD), but a systematic perspective on the multiple benefits of this diet is lacking. Here, we performed a short-term intervention with an isocaloric low-carbohydrate diet with increased protein content in obese subjects with NAFLD and characterized the resulting alterations in metabolism and the gut microbiota using a multi-omics approach. We observed rapid and dramatic reductions of liver fat and other cardiometabolic risk factors paralleled by (1) marked decreases in hepatic de novo lipogenesis; (2) large increases in serum β-hydroxybutyrate concentrations, reflecting increased mitochondrial β-oxidation; and (3) rapid increases in folate-producing Streptococcus and serum folate concentrations. Liver transcriptomic analysis on biopsy samples from a second cohort revealed downregulation of the fatty acid synthesis pathway and upregulation of folate-mediated one-carbon metabolism and fatty acid oxidation pathways. Our results highlight the potential of exploring diet-microbiota interactions for treating NAFLD.

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Objective-We sought to compare the synthesis and metabolism of VLDL 1 and VLDL 2 in patients with type 2 diabetes mellitus (DM2) and nondiabetic subjects. Methods and Results-We used a novel multicompartmental model to simultaneously determine the kinetics of apolipoprotein (apo) B and triglyceride (TG) in VLDL 1 and VLDL 2 after a bolus injection of [ 2 H 3 ]leucine and [ 2 H 5 ]glycerol and to follow the catabolism and transfer of the lipoprotein particles. Our results show that the overproduction of VLDL particles in DM2 is explained by enhanced secretion of VLDL 1 apoB and TG. Direct production of VLDL 2 apoB and TG was not influenced by diabetes per se. The production rates of VLDL 1 apoB and TG were closely related, as were the corresponding pool sizes. VLDL 1 and VLDL 2 compositions did not differ in subjects with DM2 and controls, and the TG to apoB ratio of newly synthesized particles was very similar in the 2 groups. Plasma glucose, insulin, and free fatty acids together explained 55% of the variation in VLDL 1 TG production rate. Conclusion-Insulin resistance and DM2 are associated with excess hepatic production of VLDL 1 particles similar in size and composition to those in nondiabetic subjects. We propose that hyperglycemia is the driving force that aggravates overproduction of VLDL 1 in DM2. Key Words: diabetes Ⅲ dyslipidemia Ⅲ VLDL Ⅲ apolipoprotein B Ⅲ triglycerides Ⅲ compartmental modeling Ⅲ kinetics Ⅲ stable isotope B y 2025, Ͼ300 million people worldwide will have type 2 diabetes mellitus (DM2). Because atherosclerosis is an important complication of DM2, this will contribute significantly to an expected increase in cardiovascular disease worldwide. 1 One important cardiovascular risk factor associated with DM2 is a dyslipidemia characterized by high levels of triglyceride (TG)-rich VLDL, low levels of HDL cholesterol, small, dense LDL, and impaired and prolonged postprandial hyperlipidemia. 2 These abnormalities are present for years before DM2 is diagnosed clinically.The discovery of heterogeneity within the major lipoprotein classes (VLDL, LDL, and HDL) has opened new avenues to identify specific perturbations of diabetic dyslipidemia. 3 VLDL particles secreted from the liver vary in size and composition and can be classified by their density (0.94 to 1.06 g/mL), diameter (20 to 75 nm), and flotation [Svedberg flotation rate (Sf) 20 to 400]. VLDL can be separated into 2 main classes: large, buoyant VLDL 1 particles (Sf 60 to 400) and small, dense VLDL 2 particles (Sf 20 to 60). VLDL 1 particles contain more TG than VLDL 2 particles and are rich in apolipoprotein (apo) CIII and apoE. 4 Large VLDL 1 particles are the major subclass of endogenous TG-rich lipoproteins and seem to be the major determinant of the plasma TG concentration in normolipidemic subjects. 5 Although elevation of plasma TG is a consistent feature of diabetic dyslipidemia, little attention has focused on the VLDL subclass distribution in DM2. However, emerging data indicate a higher increase of VLDL 1 particles than of VL...

To elucidate the molecular mechanisms underlying non‐alcoholic fatty liver disease (NAFLD), we recruited 86 subjects with varying degrees of hepatic steatosis (HS). We obtained experimental data on lipoprotein fluxes and used these individual measurements as personalized constraints of a hepatocyte genome‐scale metabolic model to investigate metabolic differences in liver, taking into account its interactions with other tissues. Our systems level analysis predicted an altered demand for NAD + and glutathione (GSH) in subjects with high HS. Our analysis and metabolomic measurements showed that plasma levels of glycine, serine, and associated metabolites are negatively correlated with HS, suggesting that these GSH metabolism precursors might be limiting. Quantification of the hepatic expression levels of the associated enzymes further pointed to altered de novo GSH synthesis. To assess the effect of GSH and NAD+ repletion on the development of NAFLD, we added precursors for GSH and NAD + biosynthesis to the Western diet and demonstrated that supplementation prevents HS in mice. In a proof‐of‐concept human study, we found improved liver function and decreased HS after supplementation with serine (a precursor to glycine) and hereby propose a strategy for NAFLD treatment.

The use of stable isotopes in conjunction with compartmental modeling analysis has greatly facilitated studies of the metabolism of the apolipoprotein B (apoB)-containing lipoproteins in humans. The aim of this study was to develop a multicompartment model that allows us to simultaneously determine the kinetics of apoB and triglyceride (TG) in VLDL 1 and VLDL 2 after a bolus injection of [ 2 H 3 ]leucine and [ 2 H 5 ]glycerol and to follow the catabolism and transfer of the lipoprotein particles. Here, we describe the model and present the results of its application in a fasting steadystate situation in 17 subjects with lipid values representative of a Western population. Analysis of the correlations showed that plasma TG was determined by the VLDL 1 and VLDL 2 apoB and TG fractional catabolic rate. Furthermore, the model showed a linear correlation between VLDL 1 TG and apoB production. A novel observation was that VLDL TG entered the circulation within 21 min after its synthesis, whereas VLDL apoB entered the circulation after 33 min. These observations are consistent with a sequential assembly model of VLDL and suggest that the TG is added to a primordial apoB-containing particle in the liver. Regulation of the metabolism of VLDL subfractions has been an area of active interest that received fresh impetus from the introduction of stable isotope-based techniques in the late 1980s (1, 2). The use of tracer models has generated direct information on lipoprotein synthetic rates, which previously could only be inferred from the turnover of radiolabeled lipoproteins. One common approach is to inject a bolus of radioactive tracer, such as [ 3 H, 14 C]glycerol, and determine the subsequent monoexponential slope of the decline in plasma VLDL-specific radioactivity. A disadvantage of this approach is that it can underestimate the true VLDL turnover rate because it does not account for recycling of the injected bolus of tracer (3). Multicompartmental modeling improves the accuracy by attempting to account for tracer recycling (3-8). Such studies have revealed that VLDL 1 apolipoprotein B-100 (apoB-100) production and VLDL 2 apoB-100 production are independently regulated (9-11), indicating that regulatory steps in the assembly of VLDL govern the lipid content of the secreted particles. However, it is still unclear how the liver regulates the triglyceride (TG) content of VLDL particles to produce large VLDL 1 or small VLDL 2 . VLDL assembly is thought to involve at least two steps in which nascent VLDL particles are formed and then TG is added, resulting in larger particles (12,13).Several studies have analyzed VLDL TG turnover kinetics using stable isotopically labeled glycerol or palmitate tracers and mathematical modeling. However, VLDL subclasses were not analyzed in those studies, and VLDL apoB was not included in the models (3,14,15). To enhance our understanding of the pathways leading to VLDL 1 and VLDL 2 and of the metabolic fate of these particles, we developed for the first time a multicompartmental m...

Insulin downregulates VLDL(1) secretion and increases VLDL(2) secretion in participants with low liver fat but fails to suppress VLDL(1) secretion in participants with high liver fat, resulting in overproduction of VLDL(1). Thus, liver fat is associated with lack of VLDL(1) suppression in response to insulin.