Effects of insulin resistance and type 2 diabetes on lipoprotein subclass particle size and concentration determined by nuclear magnetic resonance

Predominance of Small Dense Low-Density Lipoproteins and Abnormal Glucose Regulation in Patients With Acute Coronary Syndrome

BACKGROUND

Although small dense low-density lipoprotein (sd-LDL) has an established association with diabetic dyslipidemia, previous studies have failed to show an association between sd-LDL and diabetes among coronary heart disease patients. This study investigated the prevalence of sd-LDL and abnormal glucose regulation in acute coronary syndrome (ACS).

METHODS AND RESULTS

LDL size at the onset of ACS was measured by nondenatured gradient gel electrophoresis in 314 of 429 consecutive patients. Sd-LDL was prevalent in 54% of the patients, irrespective of the presence of previously known diabetes (50% vs 60% in nondiabetes and diabetes, respectively). Diabetes was present in 122 (28%) of the patients, and 110 patients without diabetes underwent an oral glucose tolerance test. Impaired glucose tolerance (IGT) and newly detected diabetes were found in as many as 44% and 22% of the patients tested, even though their hemoglobinA1c levels were in the normal range (5.3+/-0.5%). The prevalence of sd-LDL was significantly higher in patients with glucose intolerance than in those with normal glucose tolerance (61% vs 42%).

CONCLUSION

IGT and diabetes were far more common than normal glucose regulation in ACS patients, and the abnormal glycometabolism was closely associated with highly atherogenic sd-LDL.

Increased small low-density lipoprotein particle number: a prominent feature of the metabolic syndrome in the Framingham Heart Study

BACKGROUND

Levels of LDL cholesterol (LDL-C) are frequently not elevated in individuals with the metabolic syndrome (MetSyn). However, the atherogenic potential of LDL may depend on the number and size of LDL particles in addition to the cholesterol content of LDL.

METHODS AND RESULTS

We examined the sex-specific cross-sectional relations of small LDL particle number (determined by nuclear magnetic resonance spectroscopy) to the presence of MetSyn and its components in 2993 Framingham Heart Study participants (mean age, 51 years; 53% women) without cardiovascular disease (CVD) and the relations of small LDL particle number to CVD incidence in people with MetSyn. The MetSyn (> or =3 of 5 traits as defined by the National Cholesterol Education Adult Treatment Panel III) was present in 27% of men and 17% of women. In both sexes, small LDL particle number increased from 0 to 5 MetSyn traits, a pattern partly accounted for by strong correlations between small LDL particle number and serum triglycerides (r=0.61, P<0.0001) and HDL-C (r=-0.55, P<0.0001). Compared with participants without the MetSyn, those with the MetSyn had a higher CVD event rate. However, among participants with the MetSyn, CVD rates were similar for groups with an elevated versus a lower number of small LDL particles (defined by the sex-specific median).

CONCLUSIONS

Small LDL particle number is elevated in the MetSyn, increases with the number of MetSyn components, and most prominently is correlated with triglycerides and HDL-C. Whereas increased small LDL particle number identified the MetSyn with high sensitivity, a higher small LDL particle number was not associated with greater CVD event rates in people with the MetSyn.

Serum 1H-nuclear magnetic spectroscopy followed by principal component analysis and hierarchical cluster analysis to demonstrate effects of statins on hyperlipidemic patients

Use of statins for prevention of coronary heart disease is based on the decrease of serum cholesterol and LDL cholesterol. To better investigate the changes in lipid profile after statin treatment, we propose here to use an analysis of serum by proton nuclear magnetic resonance (NMR) spectroscopy associated with a multivariate analysis of the main spectral components. Sera were obtained from 60 male patients treated for 6 weeks with simvastatin (30 patients) or atorvastatin (30 patients) for who LDL cholesterol decreased by over 45% in all selected patients. Proton nuclear magnetic resonance spectra were obtained and the region of methyl resonance from lipids was separated into six consecutive lines attributed to lipids which were analyzed by principal component analysis (PCA) and clustering by hierarchical cluster analysis (HCA) based on Euclidian distance coupled with the Ward's minimum variance method. PCA and HCA gave a map discriminating the 120 samples into five clusters, three clusters containing samples obtained at baseline and two others containing samples obtained after treatment. Both statins produced a decrease in lower-density lipoprotein components and an increase in higher density lipoprotein components. Patients with a coronary heart disease history could be discriminated after treatment by the increase in the component containing the highest proportion of HDL. Proton NMR spectroscopy of sera coupled with a PCA and an HCA was able to detect variations in the metabolism of lipids resulting from statin treatments.

Analysis of quantitative lipid traits in the genetics of NIDDM (GENNID) study

Coronary heart disease (CHD) is the leading cause of death among individuals with type 2 diabetes. Dyslipidemia contributes significantly to CHD in diabetic patients, in whom lipid abnormalities include hypertriglyceridemia, low HDL cholesterol, and increased levels of small, dense LDL particles. To identify genes for lipid-related traits, we performed genome-wide linkage analyses for levels of triglycerides and HDL, LDL, and total cholesterol in Caucasian, Hispanic, and African-American families from the Genetics of NIDDM (GENNID) study. Most lipid traits showed significant estimates of heritability (P < 0.001) with the exception of triglycerides and the triglyceride/HDL ratio in African Americans. Variance components analysis identified linkage on chromosome 3p12.1-3q13.31 for the triglyceride/HDL ratio (logarithm of odds [LOD] = 3.36) and triglyceride (LOD = 3.27) in Caucasian families. Statistically significant evidence for linkage was identified for the triglyceride/HDL ratio (LOD = 2.45) on 11p in Hispanic families in a region that showed suggestive evidence for linkage (LOD = 2.26) for triglycerides in this population. In African Americans, the strongest evidence for linkage (LOD = 2.26) was found on 19p13.2-19q13.42 for total cholesterol. Our findings provide strong support for previous reports of linkage for lipid-related traits, suggesting the presence of genes on 3p12.1-3q13.31, 11p15.4-11p11.3, and 19p13.2-19q13.42 that may influence traits underlying lipid abnormalities associated with type 2 diabetes.

Assessing risk across the spectrum of patients with the metabolic syndrome

The metabolic syndrome represents a constellation of interrelated risk factors that identify individuals at increased risk for the development of type 2 diabetes mellitus and cardiovascular events. Currently, the major cardiovascular risk factors and validated risk-assessment tools do not adequately account for the increased cardiovascular risk that accompanies the metabolic syndrome. In prospective population studies, cardiovascular risk assessment in individuals with the metabolic syndrome is improved by measures of low-density lipoprotein (LDL) particle number, C-reactive protein, and plasminogen activator inhibitor-1 levels. Although adiponectin and soluble tumor necrosis factor receptor-2 may be more integrally involved in insulin resistance, the studies with these biomarkers are less extensive. Risk assessment models for patients with the metabolic syndrome should consider inclusion of LDL particle number, inflammatory markers, and levels of plasminogen activator inhibitor-1.

Overproduction of VLDL 1 Driven by Hyperglycemia Is a Dominant Feature of Diabetic Dyslipidemia

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.

Pioglitazone increases the fractional catabolic and production rates of high-density lipoproteins apo AI in the New Zealand White Rabbit

Pioglitazone is an agonist of the peroxisome proliferator-activated receptor gamma (PPARgamma) that raises HDL-cholesterol plasma in humans. Whether pioglitazone-mediated modifications in HDL-apolipoprotein AI (apo AI) turnover in vivo contribute to this effect has not been completely elucidated. Therefore, we performed kinetic studies of HDL-apo AI radiolabeled with 125I in male New Zealand White rabbits after 6 weeks of 0.6 (n = 8), 1.75 (n = 8), and 2.6 mg/kg/day (n = 7) pioglitazone and vehicle (n = 12) treatment. Fractional catabolic rate (FCR) of HDL-apo AI was significantly higher in 1.75 and 2.6 mg/kg pioglitazone-treated animals, as compared with control rabbits (0.057+/-0.014 and 0.049+/-0.01 versus 0.025+/-0.005 pools/h, respectively); these changes were associated to a similar increase in apo AI production rates (PR) (1.24+/-0.62 and 1.14+/-0.40 versus 0.53+/-0.17 mg/kg/h, p < 0.01). Consequently, apo AI plasma levels in pioglitazone-treated animals were similar to those of controls. The apo AI-FRC and -PR correlated with the relative proportion of the HDL3c subclass, as determined by polyacrylamide gradient electrophoresis. Our data demonstrate that pioglitazone markedly modifies apo AI kinetics and enhances the proportion of small HDL3c particles, despite the unchanged apo AI concentration. Whether or not the pioglitazone-induced structural changes of HDL contribute to the anti-atherosclerotic effects of the drug remains to be determined.

A comparison of lipid and glycemic effects of pioglitazone and rosiglitazone in patients with type 2 diabetes and dyslipidemia

OBJECTIVE

Published reports suggest that pioglitazone and rosiglitazone have different effects on lipids in patients with type 2 diabetes. However, these previous studies were either retrospective chart reviews or clinical trials not rigorously controlled for concomitant glucose- and lipid-lowering therapies. This study examines the lipid and glycemic effects of pioglitazone and rosiglitazone.

RESEARCH DESIGN AND METHODS

We enrolled subjects with a diagnosis of type 2 diabetes (treated with diet alone or oral monotherapy) and dyslipidemia (not treated with any lipid-lowering agents). After a 4-week placebo washout period, subjects randomly assigned to the pioglitazone arm (n = 400) were treated with 30 mg once daily for 12 weeks followed by 45 mg once daily for an additional 12 weeks, whereas subjects randomly assigned to rosiglitazone (n = 402) were treated with 4 mg once daily followed by 4 mg twice daily for the same intervals.

RESULTS

Triglyceride levels were reduced by 51.9 +/- 7.8 mg/dl with pioglitazone, but were increased by 13.1 +/- 7.8 mg/dl with rosiglitazone (P < 0.001 between treatments). Additionally, the increase in HDL cholesterol was greater (5.2 +/- 0.5 vs. 2.4 +/- 0.5 mg/dl; P < 0.001) and the increase in LDL cholesterol was less (12.3 +/- 1.6 vs. 21.3 +/- 1.6 mg/dl; P < 0.001) for pioglitazone compared with rosiglitazone, respectively. LDL particle concentration was reduced with pioglitazone and increased with rosiglitazone (P < 0.001). LDL particle size increased more with pioglitazone (P = 0.005).

CONCLUSIONS

Pioglitazone and rosiglitazone have significantly different effects on plasma lipids independent of glycemic control or concomitant lipid-lowering or other antihyperglycemic therapy. Pioglitazone compared with rosiglitazone is associated with significant improvements in triglycerides, HDL cholesterol, LDL particle concentration, and LDL particle size.

Predictors of low-density lipoprotein particle size in a high-risk African-American population

A predominance of small, dense, low-density lipoprotein particles (pattern B) has been associated with increased cardiovascular risk independent of absolute cholesterol levels in primarily white populations. Because of the putative association of pattern B with increased risk, some investigators have proposed that routine measurement of low-density lipoprotein particle size may be beneficial for cardiovascular risk assessment. Because no studies have specifically examined this possibility in African-Americans, it remains unclear whether measurement of low-density lipoprotein particle size adds information beyond that of traditional lipid risk factors. We compared standard lipid profile measurements with extended measurements concurrently in an apparently healthy, high-risk population of African-American siblings of patients who had premature cardiovascular disease. We determined the extent to which patients who had pattern B would be identifiable from the usual lipid profile. A high triglyceride level alone was a strong independent correlate of pattern B. In subjects whose triglyceride level was >/=150 mg/dl, 67% had pattern B, whereas only 17% of subjects whose triglyceride level was <150 mg/dl had pattern B. The area under the receiver-operating characteristic curve was 0.77. Our data suggest that the standard lipid profile, primarily fasting triglyceride measurement, appears to be a useful surrogate for direct measurement of particle size in a high-risk African-American population.

Alterations of lipids and apolipoprotein CIII in very low density lipoprotein subspecies in type 2 diabetes

AIMS/HYPOTHESIS

Very low density lipoprotein (VLDL) particles are heterogeneous, comprising two main subspecies, VLDL 1 (Sf 60-400) and VLDL 2 (Sf 20-60). The aim of the study was to examine the distribution and composition of VLDL subspecies in type 2 diabetes.

SUBJECTS, MATERIALS AND METHODS

We studied the composition and concentration of triglyceride-rich lipoproteins (TRLs) in 217 type 2 diabetic patients and 93 control subjects between 50 and 75 years of age. Lipoprotein subspecies were separated by density-gradient ultracentrifugation. Apolipoprotein (apo) CIII and apo E in plasma and apo CIII in TRL subspecies were measured by nephelometry and apo CII in serum by a commercial kit using a single radial immunodiffusion method.

RESULTS

The concentrations of VLDL 1, VLDL 2 and intermediate density lipoprotein were significantly increased in type 2 diabetes subjects, the change being most marked for VLDL 1. There was a strong linear correlation between VLDL 1 triglycerides and plasma triglycerides in both groups (r = 0.879, p < 0.001 and r = 0.899, p < 0.001). Diabetic subjects had markedly higher plasma ratios of apo CII:apo CIII and apo CIII:apo E. Despite elevated plasma apo CIII, type 2 diabetic subjects had a relative deficiency of apo CIII in all TRL subspecies, suggesting profound disturbances of apo CIII metabolism.

CONCLUSIONS/INTERPRETATION

The elevation of VLDL 1 triglycerides is the major determinant of plasma triglyceride concentration in normal subjects and in type 2 diabetic individuals. Both apo CIII and apo E metabolism are disturbed in type 2 diabetes.

HDL-C and the diabetic patient: target for therapeutic intervention?

A low level of high-density lipoprotein cholesterol (HDL-C) is a key feature of the metabolic syndrome and type 2 diabetes. HDL particles exert an anti-atherogenic effect, and low HDL-C levels are associated with increased cardiovascular disease risk. The profile of lipoprotein sub-classes may also be abnormal in patients with the metabolic syndrome or type 2 diabetes, with an excess of atherogenic small low-density lipoprotein (LDL) particles. Statins are first-line lipid-modifying drugs that, in addition to varying in their effects on LDL-C, differ in their effects on HDL-C. Rosuvastatin has been shown to be at least as effective at increasing HDL-C compared with atorvastatin, pravastatin or simvastatin. Selecting an agent that will increase HDL-C levels, as well as lowering LDL-C levels, may be particularly beneficial in the treatment of patients with the metabolic syndrome and type 2 diabetes.

Management of diabetic dyslipidemia

Identification and management of dyslipidemia is an important element in the multi-factorial approach to prevent coronary heart disease. Diabetic dyslipidemia typically consists of elevated triglyceride, low high-density lipoprotein cholesterol, predominance of small, dense low-density lipoprotein (LDL) particles, and average LDL cholesterol (LDL-C). Lipid-lowering therapy has a beneficial effect on cardiovascular outcomes. Statin treatment is beneficial in patients who are older than 40 years of age, irrespective of the LDL-C value. To achieve lipid targets, attention should be directed first toward nonpharmacologic therapeutic interventions, such as diet, exercise, smoking cessation, weight loss, and improving glycemic control. Although statin therapy is recommended for most subjects, judicious use of combination therapy should be considered in the highest risk subjects.

High-density lipoprotein subfractions and risk of coronary artery disease

Numerous studies have shown that levels of high-density lipoprotein (HDL) cholesterol are inversely related to coronary artery disease risk. The HDL subfractions, however, seem to differ in their capacity to confer protection, with the large HDL2 subfraction appearing to be more important than the small HDL3 subfraction. Lipid-modifying drugs differ in their HDL-raising efficacy, and they also differ in how they affect HDL subfractions. Clinical trials show that raising total HDL cholesterol improves clinical and angiographic outcomes. It remains to be determined, however, whether a shift in distribution of HDL particles provides greater benefit than just an increase in total HDL.

Role of exercise training on cardiovascular disease in persons who have type 2 diabetes and hypertension

Exercise training is an essential component in the medical management of patients who have type 2 diabetes and hypertension. Regular exercise improves the cardiovascular health of individuals who have these conditions through multiple mechanisms (Fig. 1). These mechanisms include improvements in endothelial vasodilator function,left ventricular diastolic function, arterial stiffness.systematic inflammation, and reducing left ventricular mass. Exercise training also reduces total and abdominal fat, which mediate improvements in insulin sensitivity and blood pressure, and possibly, endothelial function. Persons who are in a prediabetic stage or those who have the metabolic syndrome may be able to prevent or delay the progression to overt diabetes by adopting a healthier lifestyle, of which increasing habitual levels of physical activity isa vital component. Most persons who have diabetes and hypertension or are at risk for these conditions should be able to initiate an exercise program safely after appropriate medical screen-ing and the establishment of an individualized exercise prescription. Despite the increasing amount of evidence that shows the benefits of exercise training, this modality of prevention and treatment continues to be underused. Although patients' lack of knowledge of the benefits of exercise or lack of motivation contributes to this underuse, a lack of clear and specific guidelines from health care professionals also is an important factor. Clinicians need to educate patients about the benefits of exercise for managing their type 2 diabetes and assist in formulating specific advice for increasing physical activity. Specific instructions should be given to patients, rather than general advice, such as "you should exercise more often." Many cardiac re-habilitation and clinical exercise programs can accommodate patients who have type 2 diabetes and hypertension. Such programs can establish individualized exercise prescriptions and provide an environment that is conducive for "lifestyle change" that underlies long-term compliance to exercise and risk factor modification.

The role of lipid management in diabetes

This article discusses the role of lipid management in diabetes.

Low-density lipoprotein size and subclasses are markers of clinically apparent and non-apparent atherosclerosis in type 2 diabetes

The atherogenic lipoprotein phenotype is characterized by an increase in plasma triglycerides, a decrease in high-density lipoprotein (HDL), and the prevalence of small, dense low-density lipoprotein (LDL) particles. The present study investigated the clinical significance of LDL size and subclasses as markers of atherosclerosis in diabetes type 2. Thirty-eight patients with type 2 diabetes, total cholesterol of less than 6.5 mmol/L, and hemoglobin A1c (HbA1c) of less than 9% were studied. Median age was 61 years, mean (+/-SD) body mass index 29 +/- 4.3 kg/m2 , and mean HbA1c 7.1 +/- 0.9 %. Laboratory parameters included plasma lipids and lipoproteins, lipoprotein (a), apolipoprotein (apo) A-I, apo B-100, apo C-III, and high-sensitivity C-reactive protein. Low-density lipoprotein size and subclasses were measured by gradient gel electrophoresis and carotideal intima media thickness (IMT) by duplex ultrasound. By factor analysis, 10 out of 21 risk parameters were selected: age, body mass index, systolic blood pressure, smoking (in pack-years), HbA1c, high-sensitivity C-reactive protein, lipoprotein (a), LDL cholesterol, HDL cholesterol, and LDL particle size. Multivariate analysis of variance of these 10 risk parameters identified LDL particle size as the best risk predictor for the presence of coronary heart disease (P = .002). Smaller LDL particle size was associated with an increase in IMT (P = .03; cut-off >1 mm). Within the different lipid parameters (total cholesterol, LDL cholesterol, HDL cholesterol, triglycerides, apo B, apo A-I, apo C-III, LDL particle size), LDL particle size was most strongly associated with the presence of coronary heart disease (P = .002) and IMT (P = .03). It is concluded that LDL size is the strongest marker for clinically apparent as well as non-apparent atherosclerosis in diabetes type 2.

Insulin resistance and adiposity influence lipoprotein size and subclass concentrations. Results from the Insulin Resistance Atherosclerosis Study

BACKGROUND

Insulin resistance and obesity are associated with a dyslipidemia composed of high levels of triglycerides (TG), low levels of high-density lipoprotein cholesterol (HDL-C), and no change in level of low-density lipoprotein cholesterol (LDL-C). We examined the association of insulin resistance and adiposity with lipoprotein particle size, concentration, and subclass concentrations.

METHODS

The Insulin Resistance Atherosclerosis Study is a multicenter cohort study of middle-aged men and women. Lipoprotein lipid concentrations were determined using standard methods. Lipoprotein size, particle concentration, and subclass concentrations were determined using nuclear magnetic resonance technology. Insulin resistance (SI) was determined based on the frequently sampled intravenous glucose tolerance test and the MINMOD program. A higher SI represents less insulin resistance. Fasting insulin, body mass index, waist circumference, and waist/hip ratio were assessed.

RESULTS

Among the 1371 participants were 754 women and 617 men; 459 Hispanics, 383 African Americans, and 529 non-Hispanic whites; 437 with type 2 diabetes, 301 with impaired glucose tolerance, and 633 with normal glucose tolerance. The mean (SD) age was 55.5 (8.5) years, body mass index was 29.3 (5.8) kg/m2 , and SI was 1.6 (1.8) units. Adjusted for age, sex, and ethnicity, SI was not associated with LDL-C (r = 0.01); however, S I was associated with LDL size (r = 0.34, P < .001), LDL particle concentration (r = -0.28, P < .001), small LDL (r = -0.34, P < .001), intermediate LDL (r = -0.37, P < .001), and large LDL (r = 0.21, P < .001). In addition, S I was associated with TG (r = -0.36, P < .001), VLDL particles (r = -0.08, P < .01), large VLDL (r = -0.32, P < .001), VLDL size (r = -0.38, P < .001), HDL-C (r = 0.37, P < .001), HDL particles (r = 0.09, P < .001), large HDL (r = 0.31, P < .001), and HDL size (r = 0.33, P < .001). A factor analysis revealed a factor that accounted for 41.4% of the variance across the lipoprotein measures and that was correlated with SI (r = -0.33, P < .001). Similar results of opposing direction were observed for analyses of lipoprotein measures with fasting insulin and adiposity.

CONCLUSIONS

The dyslipidemia associated with insulin resistance and obesity includes effects on lipoprotein metabolism that are missed when traditional lipoprotein cholesterol and total TG are examined. Lipoprotein size and subclasses should be examined in studies investigating the roles of insulin resistance and obesity in the pathogenesis and prevention of atherosclerosis.

Low-density lipoprotein particle number and risk for cardiovascular disease

The key role played by low-density lipoprotein (LDL) particles in the pathogenesis of coronary heart disease (CHD) is well accepted, as is the benefit of lowering LDL in high-risk patients. What remains controversial is whether we are using the best measure(s) of LDL to identify all individuals who would benefit from therapy. Many studies have shown that, at a given level of LDL cholesterol, individuals with predominantly small LDL particles (pattern B) experience greater CHD risk than those with larger-size LDL. However, it is not clear from this observation that small LDL particles are inherently more atherogenic than large ones because, at a given level of LDL cholesterol, individuals with small LDL have more LDL particles in total. The phenotype of small LDL particle size co-segregates with a cluster of metabolic factors, including elevated triglycerides and reduced HDL cholesterol, and in multivariate analyses has generally been found not to be independently associated with CHD risk. In contrast, LDL particle number measured by nuclear magnetic resonance has consistently been shown to be a strong, independent predictor of CHD.

Both subcutaneous and visceral adipose tissue correlate highly with insulin resistance in african americans

OBJECTIVE

The contribution of visceral adipose tissue (VAT) to insulin resistance is well-established; however, the role of subcutaneous abdominal adipose tissue (SAT) in insulin resistance remains controversial. Sex may determine which of these two components of abdominal obesity is more strongly related to insulin resistance and its consequences. The aim of this study was to determine whether both VAT and SAT contribute to insulin resistance in African Americans and to examine the effects of sex on this relationship.

RESEARCH METHODS AND PROCEDURES

This was a cross-sectional study of 78 nondiabetic African-American volunteers (44 men, 35 women; age 33.8 +/- 7.3 years; BMI 30.9 +/- 7.4 kg/m2). VAT and SAT volumes were measured using serial computerized tomography slices from the dome of the diaphragm to the iliac crest. The insulin sensitivity index (SI) was determined from the minimal model using data obtained from the frequently sampled intravenous glucose tolerance test.

RESULTS

In men, both VAT and SAT were negatively correlated with SI (r for both correlations = -0.57; p < 0.01). In women, the correlation coefficient between VAT and SI was -0.50 (p < 0.01) and between SAT and SI was -0.67 (p < 0.01). In women, the correlation coefficient for SI with SAT was significantly greater than the correlation coefficient with VAT (p = 0.02).

DISCUSSION

Both SAT and VAT are strongly correlated with insulin resistance in African Americans. For African-American women, SAT may have a greater effect than VAT on insulin resistance.

Dyslipidemia in type 2 diabetes

Type 2 diabetes mellitus is associated with a cluster of lipid abnormalities:elevated plasma triglycerides, reduced high-density lipoprotein cholesterol, and smaller and denser low-density lipoproteins,which have been associated with an increased risk of cardiovascular disease. Insulin resistance may contribute to dyslipidemia associated with type 2 diabetes by increasing hepatic secretion of large,triglyceride-rich very low-density lipoprotein particles and by impairing the clearance of lipoprotein particles from plasma. Lifestyle interventions may be effective in improving the diabetic dyslipidemia syndrome. For patients who do not respond to lifestyle changes, pharmacologic therapies (lipid-lowering medications and anti-diabetic agents) are available. Clinical trials demonstrate that the use of such pharmaceutics to treat diabetic dyslipidemia concomitantly reduces the risk of coronary artery disease.

Atherogenic Lipoprotein Particles in Atherosclerosis

The importance of low-density lipoprotein (LDL) cholesterol in the development of atherosclerosis has long been recognized, and LDL cholesterol remains the primary target of therapy for the prevention of coronary heart disease. Nevertheless, increasing research attention over the past decade has been devoted to the heterogeneity of LDL particles and the atherogenicity of lipids and lipoproteins other than LDL. Particularly atherogenic forms of LDL include small, dense LDL particles and oxidized LDL. All lipoproteins that contain apolipoprotein B, such as LDL, very-low-density lipoprotein, and intermediate-density lipoprotein, tend to promote atherosclerosis; however, these particles differ in their apolipoprotein and triglyceride content. High levels of plasma triglycerides increase the risk of acute coronary events. Lipoprotein(a) is now considered an independent risk factor in both men and women. Ultimately, better understanding of the roles of these lipid particles and subfractions in the initiation and progression of atherosclerosis may affect treatment decisions.

Lipids and lipoproteins in patients with type 2 diabetes

Insulin resistance and type 2 diabetes are associated with a clustering of interrelated plasma lipid and lipoprotein abnormalities, which include reduced HDL cholesterol, a predominance of small dense LDL particles, and elevated triglyceride levels. Each of these dyslipidemic features is associated with an increased risk of cardiovascular disease. Increased hepatic secretion of large triglyceride-rich VLDL and impaired clearance of VLDL appears to be of central importance in the pathophysiology of this dyslipidemia. Small dense LDL particles arise from the intravascular processing of specific larger VLDL precursors. Typically, reduced plasma HDL levels in type 2 diabetes are manifest as reductions in the HDL(2b) subspecies and relative or absolute increases in smaller denser HDL(3b) and HDL(3c). Although behavioral interventions such as diet and exercise can improve diabetic dyslipidemia, for most patients, pharmacological therapy is needed to reach treatment goals. There are several classes of medications that can be used to treat lipid and lipoprotein abnormalities associated with insulin resistance and type 2 diabetes, including statins, fibrates, niacin, and thiazolidinediones. Clinical trials have shown significant improvement in coronary artery disease after diabetic dyslipidemia treatment.

Metabolic abnormalities: triglyceride and low-density lipoprotein

Increased plasma triglyceride and reduced high-density lipoprotein cholesterol are key features of the metabolic syndrome. Although elevated low-density lipoprotein cholesterol is not an integral characteristic of this syndrome, there is commonly an increase in the proportion of small, dense low-density lipoprotein particles. Together, these abnormalities constitute the atherogenic dyslipidemia of the metabolic syndrome. This article reviews the pathophysiology of altered triglyceride and low-density lipoprotein metabolism in the metabolic syndrome, outlines the relationship of these lipoprotein abnormalities to increased risk of coronary heart disease,and highlights the application of this information to clinical practice. The role of reduced high-density lipoprotein in the metabolic syndrome is discussed elsewhere in this issue.

Pioglitazone plus a sulphonylurea or metformin is associated with increased lipoprotein particle size in patients with type 2 diabetes

OBJECTIVE

To determine the effect of pioglitazone plus metformin or a sulphonylurea on lipoprotein particle size and subclass distribution in patients with type 2 diabetes.

METHODS

Lipid profiles were determined for blood samples from patients participating in two randomised, double-blind, 24-week studies of pioglitazone 30 mg or 45 mg daily plus either metformin or a sulphonylurea.

RESULTS

Samples from 177 patients were evaluated; 96 of these patients received a sulphonylurea, and 81 received metformin. Pioglitazone combination treatment produced significant increases from baseline for average and peak low-density lipoprotein (LDL) particle size at weeks 12 and 24 (p<0.0001 for each; range 0.29-0.39 nm for average and 0.36-0.55 nm for peak particle size, respectively). Significant shifts in high-density lipoprotein (HDL) and LDL distribution showed an increase in large particles and a decrease in small particles. For pioglitazone plus metformin, significant increases in levels of apolipoprotein (Apo) Al, Apo Al/All-containing HDL, and lipoprotein(a) also were noted, whereas Apo B levels decreased.

CONCLUSIONS

These observed changes are thought to affect the atherogenic profile positively. Therefore, pioglitazone combination treatment may lead to decreased cardiovascular risk in patients with type 2 diabetes.

Sex and age differences in lipoprotein subclasses measured by nuclear magnetic resonance spectroscopy: the Framingham Study

BACKGROUND

The sex differential in coronary heart disease (CHD) risk, which is not explained by male/female differences in lipid and lipoprotein concentrations, narrows with age. We examined whether this differential CHD risk might, in part, be attributable to the sizes of lipoprotein particles or concentrations of lipoprotein subclasses.

METHODS

We analyzed frozen plasma samples from 1574 men and 1692 women from exam cycle 4 (1988-1990) of the Framingham Offspring Study. Nuclear magnetic resonance (NMR) spectroscopy was used to determine the subclass concentrations and mean sizes of VLDL, LDL, and HDL particles. Concentrations of lipids and apolipoproteins were measured by standard chemical methods.

RESULTS

In addition to the expected sex differences in concentrations of triglycerides, LDL-cholesterol, and HDL-cholesterol, women also had a lower-risk subclass profile consisting of larger LDL (0.4 nm) and HDL (0.5 nm) particles. The sex difference was most pronounced for HDL, with women having a twofold higher (8 vs 4 micromol/L) concentration of large HDL particles than men. Furthermore, similar to the narrowing of the sex difference in CHD risk with age, the observed male/female difference in HDL particle size also decreased with age. Although lipoprotein particle sizes were highly correlated with lipid and lipoprotein concentrations, the sex differences in the mean sizes of lipoprotein particles persisted (P <0.001) even after adjustment for lipid and lipoprotein concentrations.

CONCLUSIONS

Women have a less atherogenic subclass profile than men, even after accounting for differences in lipid concentrations.

Critical evaluation of adult treatment panel III criteria in identifying insulin resistance with dyslipidemia

OBJECTIVE

The goal of this study was to evaluate the efficacy of the Third Report of the National Cholesterol Education Program Expert Panel on Detection, Evaluation, and Treatment of High Blood Cholesterol in Adults (ATP III) in identifying insulin resistance.

RESEARCH DESIGN AND METHODS

This study included 74 nondiabetic Caucasians who were evaluated for insulin resistance and risk factors associated with the metabolic syndrome. Glucose disposal rate (GDR) was measured by hyperinsulinemic-euglycemic clamp and was used to quantify insulin resistance. Sensitivity and specificity of ATP III criteria in detecting insulin resistance were calculated for various cutoffs of GDR.

RESULTS

Insulin resistance was associated with increased waist circumference, fasting glucose, blood pressure, triglycerides, and decreased levels of HDL cholesterol. Only 12.2% of study subjects met ATP III criteria for metabolic syndrome, and ATP III criteria exhibited low sensitivity for detecting insulin resistance. Although high in specificities (>90%), the sensitivities of ATP III criteria ranged only between 20 and 50% when insulin resistance was defined as various GDR cutoff values below 10 to 12 mg.kg(-1).min(-1). The larger number of subjects who were insulin resistant but did not meet ATP III criteria were found to have an adverse cardiovascular disease risk profile, including higher BMI, waist circumference, fasting glucose, triglycerides, and an unfavorable lipoprotein subclass profile determined by nuclear magnetic resonance compared with insulin-sensitive individuals (i.e., increased large VLDL, increased small LDL, and decreased large HDL particle concentrations).

CONCLUSIONS

ATP III criteria have low sensitivity for identifying insulin resistance with dyslipidemia in nondiabetic individuals who are at increased risk for cardiovascular disease and diabetes. More sensitive criteria should be developed for clinical assessment of metabolic and cardiovascular disease risk relevant to the metabolic syndrome.

Peroxisome Proliferator-Activated Receptor-γ Agonist Increases Both Low-Density Lipoprotein Cholesterol Particle Size and Small High-Density Lipoprotein Cholesterol in Patients with Type 2 Diabetes Independent of Diabetic Control

OBJECTIVE

To ascertain whether troglitazone, independent of control of diabetes, increases low-density lipoprotein (LDL) particle size.

METHODS

We administered 600 mg of troglitazone (a peroxisome proliferator-activated receptor-gamma agonist) daily for 8 weeks to 10 patients with type 2 diabetes (8 of whom completed the study). Then troglitazone therapy was discontinued, and alternative medication for diabetic control was used for another 4 weeks. The LDL, very-low-density lipoprotein (VLDL), and high-density lipoprotein (HDL) concentrations and subpopulations, as well as blood glucose and hemoglobin A1c (HbA1c), were determined at weeks 0, 4, 8, and 12 and analyzed statistically.

RESULTS

Small, dense LDL cholesterol is commonly seen in patients with diabetes and is thought to be associated with an increased risk for coronary artery disease. After both 4 and 8 weeks of troglitazone therapy, control of diabetes was significantly improved (mean HbA1c values at baseline, week 4, and week 8 were 8.0 +/- 0.7%, 7.4 +/- 0.5%, and 7.0 +/- 0.7%, respectively; P<0.05). HbA1c (6.5 +/- 0.6% at 12 weeks) and blood glucose levels (126 +/- 19 mg/dL at 8 weeks versus 145 +/- 9 mg/dL at 12 weeks) were not significantly different 4 weeks after troglitazone therapy was discontinued. Troglitazone treatment increased the large LDL particle at 4 and 8 weeks, a change that significantly (P<0.05) enlarged the LDL particle size (20.5 +/- 0.3 nm, 21.2 +/- 0.3 nm, and 21.3 +/- 0.2 nm at baseline, week 4, and week 8, respectively). After 8 weeks of troglitazone therapy, VLDL triglycerides were reduced (195 +/- 37 mg/dL versus 136 +/- 28 mg/dL; P<0.05) and HDL was increased (31.6 +/- 2.4 mg/dL versus 35.5 +/- 2.9 mg/dL; P<0.05). This greater HDL value was due to an increase in the small HDL particles. A decrease in the larger VLDL particles (V5 and V6) resulted in a reduction in the mean VLDL particle size (59 +/- 3 nm versus 46 +/- 2 nm; P<0.05). Despite the fact that control of diabetes remained significantly improved after troglitazone therapy was discontinued, the LDL particle size decreased to the baseline value. This change was due to a reduction in the large LDL cholesterol particle (L3).

CONCLUSION

This study shows that troglitazone therapy increases LDL particle size, reduces VLDL particle size, and increases small HDL particles. These changes may lower the risk for coronary artery disease.

Lipoproteins in the DCCT/EDIC cohort: associations with diabetic nephropathy

BACKGROUND

Lipoproteins may contribute to diabetic nephropathy. Nuclear magnetic resonance (NMR) can quantify subclasses and mean particle size of very low density lipoprotein (VLDL), low density lipoprotein (LDL), and high density lipoprotein (HDL), and LDL particle concentration. The relationship between detailed lipoprotein analyses and diabetic nephropathy is of interest.

METHODS

In a cross-sectional study, lipoproteins from 428 women and 540 men from the Diabetes Control and Complications Trial/Epidemiology of Diabetes Interventions and Complications (DCCT/EDIC) cohort were characterized by conventional lipid enzymology, NMR, apolipoprotein levels, and LDL oxidizibility. Linear regression was performed for each lipoprotein parameter versus log albumin excretion rate (AER), with and without covariates for age, diabetes duration, HbA1c, hypertension, body mass index, waist-hip ratio, and DCCT treatment group. Significance was taken at P < 0.05.

RESULTS

By multivariate analysis, conventional profile, total triglycerides, total- and LDL cholesterol, but not HDL cholesterol, were associated with AER. NMR-determined large, medium, and small VLDL were associated with AER in both genders (except large VLDL in women), and intermediate density lipoprotein (IDL) was associated with AER (men only). LDL particle concentration and ApoB were positively associated with AER (in men and in the total cohort), and there was a borderline inverse association between LDL diameter and AER in men. Small HDL was positively associated with AER and a borderline negative association was found for large HDL. No associations were found with ApoA1, Lp(a), or LDL oxidizibility.

CONCLUSION

Potentially atherogenic lipoprotein profiles are associated with renal dysfunction in type 1 diabetes and further details are gained from NMR analysis. Longitudinal studies are needed to determine if dyslipoproteinemia can predict patients at risk of nephropathy, or if lipoprotein-related interventions retard nephropathy.