Impaired plasma nonesterified fatty acid tolerance is an early defect in the natural history of type 2 diabetes

Nicotinic acid increases adipose tissue dietary fatty acid trapping and reduces postprandial hepatic and cardiac fatty acid uptake in prediabetes

Increased adipose tissue (AT) dietary fatty acids (DFA) trapping limits fatty acid exposure to lean organs in the face of elevated postprandial nonesterified fatty acid (NEFA) flux from excess AT intracellular lipolysis in prediabetes. We hypothesized that pharmacological inhibition of postprandial AT intracellular lipolysis using short-acting nicotinic acid (NA) would increase AT DFA trapping and limit AT NEFA spillover to lean organs in subjects with prediabetes. Twenty subjects with impaired glucose tolerance and 19 individuals with normal glucose tolerance underwent four postprandial studies with positron emission tomography/computed tomography with radio-labeled fatty acid tracers and stable isotopic palmitate tracers. Over the 6-h postprandial period, NA increased AT DFA partitioning with reciprocal reduction in liver and in muscle. NA also robustly reduced cardiac and liver total (DFA + NEFA) postprandial fatty acid uptake. Short-acting NA administered postprandially thus enhances AT DFA trapping and markedly reduces postprandial hepatic and cardiac fatty acid uptake. (clinicaltrials.gov NCT02808182).

Alterations in nonesterified free fatty acid trafficking rather than hyperandrogenism contribute to metabolic health in obese women with polycystic ovary syndrome

OBJECTIVE

To determine whether alterations in nonesterified fatty acid (NEFA) dynamics or degree of hyperandrogenism (HA) contribute to the difference in insulin sensitivity between women with metabolically healthy obese polycystic ovary syndrome (PCOS) (MHO-PCOS) and women with metabolically unhealthy obese PCOS (MUO-PCOS).

DESIGN

Prospective cross-sectional study.

SETTING

Tertiary-care academic center.

PATIENTS

One hundred twenty-five obese women with PCOS.

INTERVENTION

Consecutive obese (body mass index [BMI] ≥ 30 kg/m2) oligo-ovulatory women (n = 125) with PCOS underwent an oral glucose tolerance test and a subgroup of 16 participants underwent a modified frequently sampled intravenous glucose tolerance test to determine insulin-glucose and -NEFA dynamics.

MAIN OUTCOME MEASURES

Degree of insulin resistance (IR) in adipose tissue (AT) basally (Adipo-IR) and dynamically (the nadir in NEFA levels observed [NEFAnadir], the time it took for NEFA levels to reach nadir [TIMEnadir], and the percent suppression in plasma NEFA levels from baseline to nadir [%NEFAsupp]); peak lipolysis rate (SNEFA) and peak rate of NEFA disposal from plasma pool (KNEFA); whole-body insulin-glucose interaction (acute response of insulin to glucose [AIRg], insulin sensitivity index [Si], glucose effectiveness [Sg], and disposition index [Di]); and HA (hirsutism score, total and free testosterone levels, and dehydroepiandrosterone sulfate levels).

RESULTS

A total of 85 (68%) women were MUO-PCOS and 40 (32%) were MHO-PCOS using the homeostasis model of assessment of IR. Subjects with MUO-PCOS and MHO-PCOS did not differ in mean age, BMI, waist-to-hip ratio, HA, and lipoprotein levels. By a modified frequently sampled intravenous glucose tolerance test, eight women with MUO-PCOS had lesser Si, KNEFA, and the percent suppression in plasma NEFA levels from baseline to nadir (%NEFAsupp) and greater TIMEnadir, NEFAnadir, and baseline adipose tissue IR index (Adipo-IR) than eight subjects with MHO-PCOS, but similar fasting NEFA levels and SNEFA. Women with MUO-PCOS had a higher homeostasis model of assessment-β% and fasting insulin levels than women with MHO-PCOS. In bivalent analysis, Si correlated strongly and negatively with Adipo-IR and NEFAnadir, weakly and negatively with TIMEnadir, and positively with KNEFA and %NEFAsupp, in women with MUO-PCOS only.

CONCLUSION

Independent of age and BMI, women with MUO-PCOS have reduced NEFA uptake and altered insulin-mediated NEFA suppression, but no difference in HA, compared with women with MHO-PCOS. Altered insulin-mediated NEFA suppression, rather than HA or lipolysis rate, contributes to variations in insulin sensitivity among obese women with PCOS.

Adipocyte hypertrophy associates with in vivo postprandial fatty acid metabolism and adipose single-cell transcriptional dynamics

Adipocyte hypertrophy is associated with metabolic complications independent of obesity. We aimed to determine: 1) the association between adipocyte size and postprandial fatty acid metabolism; 2) the potential mechanisms driving the obesity-independent, hypertrophy-associated dysmetabolism in vivo and at a single-cell resolution. Tracers with positron emission tomography were used to measure fatty acid metabolism in 40 men and women with normal or impaired glucose tolerance (NCT02808182), and single nuclei RNA-sequencing (snRNA-seq) to determine transcriptional dynamics of subcutaneous adipose tissue (AT) between individuals with AT hypertrophy vs. hyperplasia matched for sex, ethnicity, glucose-tolerance status, BMI, total and percent body fat, and waist circumference. Adipocyte size was associated with high postprandial total cardiac fatty acid uptake and higher visceral AT dietary fatty acid uptake, but lower lean tissue dietary fatty acid uptake. We found major shifts in cell transcriptomal dynamics with AT hypertrophy that were consistent with in vivo metabolic changes.

An insulin-regulated arrestin domain protein controls hepatic glucagon action

Glucagon signaling is essential for maintaining normoglycemia in mammals. The arrestin fold superfamily of proteins controls the trafficking, turnover, and signaling of transmembrane receptors as well as other intracellular signaling functions. Further investigation is needed to understand the in vivo functions of the arrestin domain-containing 4 (ARRDC4) protein family member and whether it is involved in mammalian glucose metabolism. Here, we show that mice with a global deletion of the ARRDC4 protein have impaired glucagon responses and gluconeogenesis at a systemic and molecular level. Mice lacking ARRDC4 exhibited lower glucose levels after fasting and could not suppress gluconeogenesis at the refed state. We also show that ARRDC4 coimmunoprecipitates with the glucagon receptor, and ARRDC4 expression is suppressed by insulin. These results define ARRDC4 as a critical regulator of glucagon signaling and glucose homeostasis and reveal a novel intersection of insulin and glucagon pathways in the liver.

Multiple micronutrient deficiencies alter energy metabolism in host and gut microbiome in an early-life murine model

Introduction

Micronutrients perform a wide range of physiological functions essential for growth and development. However, most people still need to meet the estimated average requirement worldwide. Globally, 2 billion people suffer from micronutrient deficiency, most of which are co-occurring deficiencies in children under age five. Despite decades of research, animal models studying multiple micronutrient deficiencies within the early-life period are lacking, which hinders our complete understanding of the long-term health implications and may contribute to the inefficacy of some nutritional interventions. Evidence supporting the Developmental Origins of Health and Disease (DOHaD) theory demonstrates that early-life nutritional deficiencies carry life-long consequences mediated through various mechanisms such as abnormal metabolic programming, stunting, altered body composition, and the gut microbiome. However, this is largely unexplored in the multiple micronutrient deficient host.

Methods

we developed a preclinical model to examine undernutrition's metabolic and functional impact on the host and gut microbiome early in life. Three-week-old weanling C57BL/6N male mice were fed a low-micronutrient diet deficient in zinc, folate, iron, vitamin A, and vitamin B12 or a control diet for 4-weeks.

Results

Our results showed that early-life multiple micronutrient deficiencies induced stunting, altered body composition, impaired glucose and insulin tolerance, and altered the levels of other micronutrients not depleted in the diet within the host. In addition, functional metagenomics profiling and a carbohydrate fermentation assay showed an increased microbial preference for simple sugars rather than complex ones, suggestive of a less developed microbiome in the low-micronutrient-fed mice. Moreover, we found that a zinc-only deficient diet was not sufficient to induce these phenotypes, further supporting the importance of studying co-occurring deficiencies.

Discussion

Together, these findings highlight a previously unappreciated role of early-life multiple micronutrient deficiencies in shaping the metabolic phenome of the host and gut microbiome through altered glucose energy metabolism, which may have implications for metabolic disease later in life in micronutrient-deficient survivors.

Serum Fibroblast Growth Factor 21 Level After an Oral Fat Tolerance Test is Related to Postprandial Free Fatty Acid Level

Purpose

The relationship between blood lipids and fibroblast growth factor (FGF) 21 in the postprandial period remains unclear. To investigate this, we observed the changes in blood lipid levels after an oral fat tolerance test (OFTT) and examined the short-term effects on FGF21.

Patients and Methods

A total of 158 non-diabetic adult volunteers who underwent OFTT were randomly recruited from the Hebei General Hospital. Participants were stratified into three groups according to fasting and 4-h postprandial triglyceride levels: normal fat tolerance (NFT), impaired fat tolerance (IFT), and hypertriglyceridemia (HTG). Blood samples were collected at 2-h intervals for 6 h. Circulating total cholesterol levels, triglycerides, high-density lipoprotein-cholesterol, low-density lipoprotein-cholesterol, free fatty acids (FFA), and FGF21 were assessed.

Results

Fasting FGF21 levels increased progressively in the NFT, IFT, and HTG groups and were strongly correlated with FFA levels (r = 0.531, P < 0.001). During the OFTT, the FFA and FGF21 levels decreased and then increased after reaching a nadir at 2 and 4 h, respectively. After adjusting for potential risk factors, the FFA incremental area under the curve (iAUC) was an independent influencing factor of FGF21 iAUC (P = 0.005).

Conclusion

Fasting FGF21 levels showed a strong positive correlation with FFA. During OFTT, changes in FGF21 levels were closely associated with alterations in FFA exogenously changed by OFTT. Moreover, they were linearly related to each other. Therefore, the serum FGF21 level is positively correlated to the FFA level in the postprandial period.

Total Postprandial Hepatic Nonesterified and Dietary Fatty Acid Uptake Is Increased and Insufficiently Curbed by Adipose Tissue Fatty Acid Trapping in Prediabetes With Overweight

Excessive lean tissue uptake of fatty acids (FAs) is important in the development of insulin resistance and may be caused by impaired dietary FA (DFA) storage and/or increased nonesterified FA (NEFA) flux from adipose tissue intracellular lipolysis. Cardiac and hepatic total postprandial FA uptake of NEFA+DFA has, however, never been reported in prediabetes with overweight. In this study, 20 individuals with impaired glucose tolerance (IGT) and 19 participants with normal glucose tolerance (NGT) and normal fasting glucose underwent postprandial studies with whole-body positron emission tomography/computed tomography (PET/CT) with oral [18F]fluoro-thia-heptadecanoic acid and dynamic PET/CT with intravenous [11C]palmitate. Hepatic (97 [range 36-215] mmol/6 h vs. 68 [23-132] mmol/6 h, P = 0.03) but not cardiac (11 [range 4-24] mmol/6 h vs. 8 [3-20] mmol/6 h, P = 0.09) uptake of most sources of postprandial FA (NEFA + DFA uptake) integrated over 6 h was higher in IGT versus NGT. DFA accounted for lower fractions of total cardiac (21% [5-47] vs. 25% [9-39], P = 0.08) and hepatic (19% [6-52] vs. 28% [14-50], P = 0.04) uptake in IGT versus NGT. Increased adipose tissue DFA trapping predicted lower hepatic DFA uptake and was associated with higher total cardiac FA uptake. Hence, enhanced adipose tissue DFA trapping in the face of increased postprandial NEFA flux is insufficient to fully curb increased postprandial lean organ FA uptake in prediabetes with overweight (ClinicalTrials.gov; NCT02808182).

Adipose Tissue Insulin Resistance Is Positively Associated With Serum Uric Acid Levels and Hyperuricemia in Northern Chinese Adults

OBJECTIVE

Adipose tissue plays a crucial role in serum uric acid (UA) metabolism, but the relative contribution of adipose tissue insulin resistance (IR) to serum UA levels and hyperuricemia have not explicitly been illustrated. Herein, we aimed to investigate the association between the adipose tissue insulin resistance index (Adipo-IR) and hyperuricemia in this cross-sectional study. The homeostasis model assessment of insulin resistance (HOMA-IR) index, another widely applied marker to determine systemic IR, was also explored.

METHODS

A total of 5821 adults were included in this study. The relationship between Adipo-IR or HOMA-IR and serum UA levels was assessed by multivariate linear regression. Binary logistic regression analyses were applied to determine the sex-specific association of the Adipo-IR tertiles and HOMA-IR tertiles with hyperuricemia. Participants were then divided into normal BMI (18.5 ≤ BMI < 24) and elevated BMI (BMI ≥ 24) groups for further analysis.

RESULTS

Both Adipo-IR and HOMA-IR were positively correlated with serum UA (P < 0.001). Compared with the lowest tertile, the risks of hyperuricemia increased across Adipo-IR tertiles (middle tertile: OR 1.52, 95%CI 1.24-1.88; highest tertile: OR 2.10, 95%CI 1.67-2.63) in men after full adjustment (P for trend < 0.001). In women, only the highest tertile (OR 2.09, 95%CI 1.52-2.87) was significantly associated with hyperuricemia. Those associations remained significant in participants with normal BMI status. As for HOMA-IR, only the highest tertile showed positive relationships with hyperuricemia in both genders after full adjustment (P for trend < 0.001). The association between HOMA-IR and hyperuricemia disappeared in men with normal BMI status.

CONCLUSIONS

Adipo-IR was strongly associated with serum UA and hyperuricemia regardless of BMI classification. In men with normal BMI, Adipo-IR, rather than HOMA-IR, was closely associated with hyperuricemia. Altogether, our finding highlights a critical role of adipose tissue IR on serum UA metabolism and hyperuricemia.

Changes in Cardiac Metabolism in Prediabetes

In type 2 diabetes mellitus (T2DM), there is an increased prevalence of cardiovascular disease (CVD), even when corrected for atherosclerosis and other CVD risk factors. Diastolic dysfunction is one of the early changes in cardiac function that precedes the onset of cardiac failure, and it occurs already in the prediabetic state. It is clear that these changes are closely linked to alterations in cardiac metabolism; however, the exact etiology is unknown. In this narrative review, we provide an overview of the early cardiac changes in fatty acid and glucose metabolism in prediabetes and its consequences on cardiac function. A better understanding of the relationship between metabolism, mitochondrial function, and cardiac function will lead to insights into the etiology of the declined cardiac function in prediabetes.

100th anniversary of the discovery of insulin perspective: insulin and adipose tissue fatty acid metabolism

Insulin inhibits systemic nonesterified fatty acid (NEFA) flux to a greater degree than glucose or any other metabolite. This remarkable effect is mainly due to insulin-mediated inhibition of intracellular triglyceride (TG) lipolysis in adipose tissues and is essential to prevent diabetic ketoacidosis, but also to limit the potential lipotoxic effects of NEFA in lean tissues that contribute to the development of diabetes complications. Insulin also regulates adipose tissue fatty acid esterification, glycerol and TG synthesis, lipogenesis, and possibly oxidation, contributing to the trapping of dietary fatty acids in the postprandial state. Excess NEFA flux at a given insulin level has been used to define in vivo adipose tissue insulin resistance. Adipose tissue insulin resistance defined in this fashion has been associated with several dysmetabolic features and complications of diabetes, but the mechanistic significance of this concept is not fully understood. This review focusses on the in vivo regulation of adipose tissue fatty acid metabolism by insulin and the mechanistic significance of the current definition of adipose tissue insulin resistance. One hundred years after the discovery of insulin and despite decades of investigations, much is still to be understood about the multifaceted in vivo actions of this hormone on adipose tissue fatty acid metabolism.

Seven-day overfeeding enhances adipose tissue dietary fatty acid storage and decreases myocardial and skeletal muscle dietary fatty acid partitioning in healthy subjects

Increased myocardial partitioning of dietary fatty acids (DFA) and decreased left ventricular (LV) function is associated with insulin resistance in prediabetes. We hypothesized that enhanced myocardial DFA partitioning and reduced LV function might be induced concomitantly with reduced insulin sensitivity upon a 7-day hypercaloric (+50% in caloric intake), high-saturated fat (~11%energy), and simple carbohydrates (~54%energy) diet (HIGHCAL) versus an isocaloric diet (ISOCAL) with a moderate amount of saturated fat (~8%energy) and carbohydrates (~50%energy). Thirteen healthy subjects (7 men/6 women) underwent HIGHCAL versus ISOCAL in a randomized crossover design, with organ-specific DFA partitioning and LV function measured using the oral 14(R,S)-[¹⁸F]fluoro-6-thia-heptadecanoic acid and [¹¹C]acetate positron emission tomography methods at the end of both interventions. HIGHCAL induced a decrease in insulin sensitivity indexes with no significant change in body composition. HIGHCAL led to increased subcutaneous abdominal (+4.2 ± 1.6%, P < 0.04) and thigh (+2.4 ± 1.2%, P < 0.08) adipose tissue storage and reduced cardiac (-0.31 ± 0.11 mean standard uptake value [(SUV), P < 0.03] and skeletal muscle (-0.17 ± 0.08 SUV, P < 0.05) DFA partitioning without change in LV function. We conclude that early increase in adipose tissue DFA storage protects the heart and skeletal muscles from potential deleterious effects of DFA.

Postprandial Saturated Fatty Acids Increase the Risk of Type 2 Diabetes: A Cohort Study in a Chinese Population

CONTEXT

Experimental evidence suggests saturated fatty acids (SFAs) are associated with insulin resistance, but results from epidemiological studies on fasting SFAs-diabetes risk are inconsistent.

OBJECTIVE

We investigated SFA (fasting and 2-hour postprandial) profiles and diabetes risk.

DESIGN SETTING

A total of 8940 participants were recruited for the Harbin People's Health Study in 2008. Serum SFAs (fasting and 2-hour postprandial) at baseline in Chinese men and women without diabetes were profiled, and type 2 diabetes was ascertained using World Health Organization criteria after 4 to 7 years of follow-up.

OUTCOME

Associations between 2-hour postprandial SFA (2h-SFA) and diabetes.

RESULTS

At baseline, incident cases of diabetes were older with a higher body mass index and waist circumference. After a mean follow-up of 6.7 years, 658 incident cases of diabetes occurred. After propensity score computation and inverse probability of treatment weighting (IPTW) estimation, fasting SFAs were unrelated to diabetes risk but IPTW-adjusted odds ratios (ORs) and 95% confidence intervals (CIs) for the highest tertile of 2-hour postprandial stearic acid (2h-SA), 2-hour postprandial palmitic acid (2h-PA), and 2h-SFA for diabetes risk were 2.50 (2.08 to 3.16), 1.56 (1.23 to 2.02), and 1.70 (1.34 to 2.17), respectively (P-trend < 0.0001). Similarly, 2h-SA/fasting SA, 2h-PA/fasting PA, and 2h-SFA/fasting SFA ratios [IPTW-adjusted OR (95% CI): 2.94 (2.39 to 3.58), 2.31 (1.80 to 2.93), and 2.42 (1.91 to 3.11), respectively; P-trend < 0.0001] predicted the diabetes risk.

CONCLUSIONS

Higher serum 2h-SFA (but not fasting SFA) independently predicted diabetes risk.

Abnormal Myocardial Dietary Fatty Acid Metabolism and Diabetic Cardiomyopathy

Patients with diabetes are at very high risk of hospitalization and death from heart failure. Increased prevalence of coronary heart disease, hypertension, autonomic neuropathy, and kidney failure all play a role in this increased risk. However, cardiac metabolic abnormalities are now recognized to play a role in this increased risk. Increased reliance on fatty acids to produce energy might predispose the diabetic heart to oxidative stress and ischemic damage. Intramyocellular accumulation of toxic lipid metabolites leads to a number of cellular abnormalities that might also contribute to cardiac remodelling and cardiac dysfunction. However, fatty acid availability from circulation and from intracellular lipid droplets to fuel the heart is critical to maintain its function. Fatty acids delivery to the heart is very complex and includes plasma nonesterified fatty acid flux as well as triglyceride-rich lipoprotein-mediated transport. Although many studies have shown a cross-sectional association between enhanced fatty acid delivery to the heart and reduction in left ventricular function in subjects with prediabetes and diabetes, these mechanisms change very rapidly during type 2 diabetes treatment. The present review focuses on the role of fatty acids in cardiac function, with particular emphasis on the possible role of early abnormalities of dietary fatty acid metabolism in the development of diabetic cardiomyopathy.

Combination of high-fat/high-fructose diet and low-dose streptozotocin to model long-term type-2 diabetes complications

The epidemic of type 2 diabetes mellitus (T2DM) is fueled by added fructose consumption. Here, we thus combined high-fat/high-fructose diet, with multiple low-dose injections of streptozotocin (HF/HF/Stz) to emulate the long-term complications of T2DM. HF/HF/Stz rats, monitored over 56 weeks, exhibited metabolic dysfunctions associated with the different stages of the T2DM disease progression in humans: an early prediabetic phase characterized by an hyperinsulinemic period with modest dysglycemia, followed by a late stage of T2DM with frank hyperglycemia, normalization of insulinemia, marked dyslipidemia, hepatic fibrosis and pancreatic β-cell failure. Histopathological analyses combined to [¹⁸F]-FDG PET imaging further demonstrated the presence of several end-organ long-term complications, including reduction in myocardial glucose utilization, renal dysfunction as well as microvascular neuropathy and retinopathy. We also provide for the first time a comprehensive µ-PET whole brain imaging of the changes in glucose metabolic activity within discrete cerebral regions in HF/HF/Stz diabetic rats. Altogether, we developed and characterized a unique non-genetic preclinical model of T2DM adapted to the current diet and lifestyle that recapitulates the major metabolic features of the disease progression, from insulin resistance to pancreatic β-cell dysfunction, and closely mimicking the target-organ damage occurring in type 2 diabetic patients at advanced stages.

Fatty Acid Metabolic Remodeling During Type 2 Diabetes Remission After Bariatric Surgery

Hypertrophic remodeling of white adipose tissues is associated with overexposure of lean organs to circulating triglycerides (TGs) and nonesterified fatty acids (NEFAs), ultimately leading to insulin resistance. Bariatric surgery promotes type 2 diabetes (T2D) remission through a succession of weight loss-dependent and -independent mechanisms. However, the longitudinal contribution of adipocyte size reduction and fatty acid metabolic handling remain unknown. Here we show that severely obese participants with T2D display hypertriglyceridemia and excessive systemic lipolysis during intravenous lipid overload. Three days after biliopancreatic diversion with duodenal switch (DS), whole-body glycerol turnover was normalized and associated with lower HOMA-insulin resistance index. A mean excess weight loss of 84% was achieved 12 months after DS. The smaller subcutaneous adipocyte size predicted better glycemic control in T2D. TG disposal and acylcarnitine production during lipid overload, along with muscle insulin sensitivity, improved with weight loss. Nevertheless, systemic NEFA fluxes and NEFA spillover remained similar, suggesting that increased NEFA storage capacity per volume of adipose tissue exactly compensated for the decrease in fat mass during weight loss. In conclusion, T2D remission after DS is mainly associated with greater circulating TG disposal, lower systemic lipolysis, and better fatty acid handling by lean tissues.

HNF1α defect influences post-prandial lipid regulation

Purpose

Hepatocyte nuclear factor 1 alpha (HNF1α) defects cause Mature Onset Diabetes of the Young type 3 (MODY3), characterized by defects in beta-cell insulin secretion. However, HNF1α is involved in many other metabolic pathways with relevance for monogenic or polygenic type 2 diabetes. We aimed to investigate gut hormones, lipids, and insulin regulation in response to a meal test in HNF1α defect carriers (MODY3) compared to non-diabetic subjects (controls) and type 2 diabetes (T2D).

Methods

We administered a standardized liquid meal to each participant. Over 6 hours, we measured post-meal responses of insulin regulation (blood glucose, c-peptide, insulin), gut hormones (ghrelin, glucose-dependent insulinotropic polypeptide, glucagon-like peptide-1) and lipids (non-esterified fatty acids [NEFA] and triglycerides).

Results

We found that MODY3 participants had lower insulin secretion indices than controls and T2D participants, showing the expected β-cell defect. MODY3 had similar glycated hemoglobin levels (HbA1c median [IQR]: 6.5 [5.6–7.6]%) compared to T2D (median: 6.6 [6.2–6.9]%; P<0.05). MODY3 had greater insulin sensitivity (Matsuda index: 71.9 [29.6; 125.5]) than T2D (3.2 [4.0; 6.0]; P<0.05). MODY3 experienced a larger decrease in the ratio of NEFA to insulin (NEFA 30–0 / insulin 30–0: -39 [-78; -30] x10⁴) in the early post-prandial period (0–30 minutes) compared to controls and to T2D (-2.0 [-0.6; -6.4] x10⁴; P<0.05). MODY3 had lower fasting (0.66 [0.46; 1.2] mM) and post-meal triglycerides levels compared to T2D (fasting: 2.3 [1.7; 2.7] mM; P<0.05). We did not detect significant post-meal differences in ghrelin and incretins between MODY3 and other groups.

Conclusion

In response to a standard meal test, MODY3 showed greater early post-prandial NEFA diminution in response to relatively low early insulin secretion, and they maintained very low post-prandial triglycerides levels.

Early Metabolic Improvement After Bariatric Surgery: The First Steps Toward Remission of Type 2 Diabetes

The introduction of bariatric surgery into clinical practice in the 1980s was followed by a relatively long watch-and-wait period before the very rapid accumulation of scientific literature, over the past decade, concerning its clinical effectiveness and safety and its mechanisms of action in the treatment of obesity. These surgical procedures now emerge as the most effective therapeutic modality to induce long-term remission of type 2 diabetes. Recent research has shed light on the potential mechanisms leading to the profound improvement of glucose homeostasis following most bariatric surgery procedures. These mechanisms can be classified as weight loss dependent and independent, both playing sequential and then synergistic antidiabetes roles. Many groups, including our own, have contributed to our understanding of the relative roles of these mechanisms at differing time periods following these procedures. Here we summarize what we currently know about the mechanisms underlying the very rapid, weight loss-independent improvement in glucose homeostasis after bariatric surgery. Beyond its impact in the field of bariatric surgery, this new knowledge about the very rapid in vivo "reverse engineering" of type 2 diabetes actually provides unique insights into the intricate and complex mechanisms linking nutrition and obesity with the development of this disease.

Impaired adipose tissue lipid storage, but not altered lipolysis, contributes to elevated levels of NEFA in type 2 diabetes. Degree of hyperglycemia and adiposity are important factors

BACKGROUND

Elevated levels of circulating non-esterified fatty acids (NEFA) mediate many adverse metabolic effects. In this work we aim to determine the impact of type 2 diabetes (T2D), glycemic control and obesity on lipolysis regulation.

DESIGN AND PARTICIPANTS

20 control and 20 metformin-treated T2D subjects were matched for sex (10M/10 F), age (58±11 vs 58±9 y) and BMI (30.8±4.6 vs 30.7±4.9kg/m²). In vivo lipolysis was assessed during a 3h-OGTT with plasma glycerol and NEFA levels. Subcutaneous adipose tissue (SAT) biopsies were obtained to measure mRNA and metabolite levels of factors related to lipolysis and lipid storage and to assess in vitro lipolysis in isolated subcutaneous adipocytes.

RESULTS

Plasma NEFA AUC during the OGTT where higher 30% (P=0.005) in T2D than in control subjects, but plasma glycerol AUC and subcutaneous adipocyte lipolysis in vitro were similar, suggesting that adipose tissue lipolysis is not altered. Expression in SAT of genes involved in lipid storage (FABP4, DGAT1, FASN) were reduced in T2D subjects compared with controls, but no differences were seen for genes involved in lipolysis. T2D subjects had elevated markers of beta-oxidation, α-hydroxybutyrate (1.4-fold, P<0.01) and β-hydroxybutyrate (1.7-fold, P<0.05) in plasma. In multivariate analysis, HbA1c, visceral adipose tissue volume and sex (male) were significantly associated with NEFA AUC in T2D subjects.

CONCLUSIONS

In T2D subjects, NEFA turnover is impaired, but not due to defects in lipolysis or lipid beta-oxidation. Impaired adipose NEFA re-esterification or de novo lipogenesis is likely to contribute to higher NEFA plasma levels in T2D. The data suggest that hyperglycemia and adiposity are important contributing factors for the regulation of plasma NEFA concentrations.

Adipose tissue dysfunction increases fatty liver association with pre diabetes and newly diagnosed type 2 diabetes mellitus

BACKGROUND

To evaluate the role of adipose tissue function on the association of fatty liver (FL) with impaired fasting glucose (IFG) or newly diagnosed type 2 diabetes mellitus (nT2D).

METHODS

In 1264 subjects, computed tomography was used to evaluate FL and elevated visceral adipose tissue (VAT). Fasting plasma glucose, <5.6, 5.6-6.9 and ≥7 mmol/l, were used to defined normoglycemic (NG), IFG or nT2D, respectively. Elevated free fatty acids, low serum adiponectin levels and adipose tissue insulin resistance (Adipo-IR), were used as markers of adipose tissue dysfunction.

RESULTS

Compared to NG subjects, those with IFG or nT2D had higher prevalence of FL and elevated VAT. FL was found to be independently associated with IFG and nT2D. Adipo-IR increased the association between FL and IFG [OR: 2.46 (95% I.C.: 1.73-3.49) to 5.42 (3.11-9.41)], whereas low adiponectin levels had a higher effect on the FL and nT2D association [OR: 4.26 (2.18-8.34) to 8.53 (2.96-24.55)].

CONCLUSION

Fatty liver was independently associated with IFG and nT2D. Our results indicate for the first time, that adipose tissue dysfunction increases these associations.

A Systematic Review and Meta-analysis of the Effect of Gastric Bypass Surgery on Plasma Lipid Levels

BACKGROUND

Obesity-related dyslipidaemia comprises hypercholesterolaemia, hypertriglyceridaemia, low HDL-cholesterol and normal to raised LDL-cholesterol levels. 40% of morbidly obese surgical patients have dyslipidaemia. Roux-en-Y gastric bypass (RYGB) surgery has many beneficial metabolic effects, but the full impact on plasma lipids has not been clearly defined.

METHODS

A systematic review of electronic databases (Ovid; Medline; PubMed; Embase) between 1960 and March 2012 was performed using search terms including the following: obesity surgery, bariatric surgery, gastric bypass, cholesterol, lipids, triglycerides and non-esterified fatty acids. A total of 2442 manuscripts were screened. Papers with paired plasma lipid levels around RYGB surgery were included. Exclusions included the following: editorials, dual publications, n < 10, resulting in 75 papers of relevance. A meta-analysis was performed of the effect of RYGB surgery upon plasma lipids at different time points up to 4 years following surgery, using a random effects model.

RESULTS

Paired data were available for 7815 subjects around RYGB surgery for morbid obesity with a baseline BMI 48 kg/m(2) (n = 2331). There was a reduction in plasma total cholesterol and LDL-C from 1 month up to 4 years post-RYGB (p < 0.00001, p < 0.00001). Following RYGB, HDL-C increased from 1 year onwards (p < 0.00001), and triglyceride levels were reduced postoperatively from 3 months up to 4 years (p < 0.00001). NEFA levels were increased at 1 month postoperatively (p = 0.003), but from 3 months onwards did not differ from preoperative levels (p = 0.07).

CONCLUSIONS

RYGB surgery reverses the dyslipidaemia of obesity. These findings support the use of RYGB in the management of high cardiovascular risk lipid profiles in morbid obesity.

Seven-Day Caloric and Saturated Fat Restriction Increases Myocardial Dietary Fatty Acid Partitioning in Impaired Glucose-Tolerant Subjects

Subjects with impaired glucose tolerance (IGT) have increased myocardial partitioning of dietary fatty acids (DFAs) with left ventricular dysfunction, both of which are improved by modest weight loss over 1 year induced by lifestyle changes. Here, we determined the effects of a 7-day hypocaloric diet (-500 kcal/day) low in saturated fat (<7% of energy) (LOWCAL study) versus isocaloric with the usual amount saturated fat (∼10% of energy) diet (ISOCAL) on DFA metabolism in subjects with IGT. Organ-specific DFA partitioning and cardiac and hepatic DFA fractional uptake rates were measured in 15 IGT subjects (7 males/8 females) using the oral 14(R,S)-[18F]-fluoro-6-thia-heptadecanoic acid positron emission tomography method after 7 days of an ISOCAL diet versus a LOWCAL diet using a randomized crossover design. The LOWCAL diet led to reductions in weight and postprandial insulin area under the curve. Myocardial DFA partitioning over 6 h was increased after the LOWCAL diet (2.3 ± 0.1 vs. 1.9 ± 0.2 mean standard uptake value, P < 0.04). However, the early (90-120 min) myocardial DFA fractional uptake was unchanged after the LOWCAL diet (0.055 ± 0.025 vs. 0.046 ± 0.009 min(-1), P = 0.7). Liver DFA partitioning was unchanged, but liver fractional uptake of DFA tended to be increased. Very short-term caloric and saturated fat dietary restrictions do not lead to the same changes in organ-specific DFA metabolism as those associated with weight loss in subjects with IGT.

Pathogenesis of selective insulin resistance in isolated hepatocytes

The development of insulin resistance (IR) in the liver is a key pathophysiologic event in the development of type 2 diabetes. Although insulin loses its ability to suppress glucose production, it largely retains its capacity to drive lipogenesis. This selective IR results in the characteristic hyperglycemia and dyslipidemia of type 2 diabetes. The delineation of two branched pathways of insulin receptor (InsR) signaling to glucose versus triglyceride production, one through FoxO and the other through SREBP-1c, provides a mechanism to account for this pathophysiological abnormality. We tested the complementary hypothesis that selective IR arises due to different intrinsic sensitivities of glucose production versus de novo lipogenesis to insulin as a result of cell-autonomous down-regulation of InsR number in response to chronic hyperinsulinemia. We demonstrate in mouse primary hepatocytes that chronic hyperinsulinemia abrogates insulin's inhibition of glucose production, but not its stimulation of de novo lipogenesis. Using a competitive inhibitor of InsR, we show that there is a 4-fold difference between levels of InsR inhibition required to cause resistance of glucose production versus lipogenesis to the actions of insulin. Our data support a parsimonious model in which differential InsR activation underlies the selective IR of glucose production relative to lipogenesis, but both processes require signaling through Akt1/2.

Peripheral adipose tissue insulin resistance alters lipid composition and function of hippocampal synapses

Compelling evidence indicates that type 2 diabetes mellitus, insulin resistance (IR), and metabolic syndrome are often accompanied by cognitive impairment. However, the mechanistic link between these metabolic abnormalities and CNS dysfunction requires further investigations. Here, we evaluated whether adipose tissue IR and related metabolic alterations resulted in CNS changes by studying synapse lipid composition and function in the adipocyte-specific ecto-nucleotide pyrophosphate phosphodiesterase over-expressing transgenic (AtENPP1-Tg) mouse, a model characterized by white adipocyte IR, systemic IR, and ectopic fat deposition. When fed a high-fat diet, AtENPP1-Tg mice recapitulate essential features of the human metabolic syndrome, making them an ideal model to characterize peripherally induced CNS deficits. Using a combination of gas chromatography and western blot analysis, we found evidence of altered lipid composition, including decreased phospholipids and increased triglycerides (TG) and free fatty acid in hippocampal synaptosomes isolated from high-fat diet-fed AtENPP1-Tg mice. These changes were associated with impaired basal synaptic transmission at the Schaffer collaterals to hippocampal cornu ammonis 1 (CA1) synapses, decreased phosphorylation of the GluN1 glutamate receptor subunit, down-regulation of insulin receptor expression, and up-regulation of the free fatty acid receptor 1.

Improved cardiac function and dietary fatty acid metabolism after modest weight loss in subjects with impaired glucose tolerance

Using a novel positron emission tomography (PET) method with oral administration of 14(R,S)-[¹⁸F]-fluoro-6-thia-heptadecanoic acid (¹⁸FTHA), we recently demonstrated that subjects with impaired glucose tolerance (IGT) display an impairment in cardiac function associated with increased myocardial uptake of dietary fatty acids. Here, we determined whether modest weight loss induced by lifestyle changes might improve these cardiac metabolic and functional abnormalities. Nine participants with IGT, enrolled in a one-year lifestyle intervention trial, were invited to undergo determination of organ-specific postprandial dietary fatty acids partition using the oral ¹⁸FTHA method, and cardiac function and oxidative metabolic index using PET [¹¹C]acetate kinetics with ECG-gated PET ventriculography before and after the intervention. The intervention resulted in significant weight loss and reduction of waist circumference, with reduced postprandial plasma glucose, insulin, and triglycerides excursion. We observed a significant increase in stroke volume, cardiac output, and left ventricular ejection fraction associated with reduced myocardial oxidative metabolic index and fractional dietary fatty acid uptake. Modest weight loss corrects the exaggerated myocardial channeling of dietary fatty acids and improves myocardial energy substrate metabolism and function in IGT subjects.

Improved plasma FFA/insulin homeostasis is independently associated with improved glucose tolerance after a 1-year lifestyle intervention in viscerally obese men

OBJECTIVE

Elevated plasma free fatty acids (FFAs) are one important link between excess visceral adiposity, insulin resistance, and the development of type 2 diabetes. Effects of lifestyle interventions on FFA metabolism are poorly known. This open-label study was conducted to test the effects of a 1-year healthy eating/physical activity intervention program on plasma FFA homeostasis in 117 viscerally obese men with dyslipidemia associated with insulin resistance (waist circumference≥90 cm, triglycerides≥1.69 mmol/L, and/or HDL-cholesterol<1.03 mmol/L).

RESEARCH DESIGN AND METHODS

Body weight, body composition, and fat distribution were assessed by dual-energy X-ray absorptiometry/computed tomography. Oral loads of lipid (60 g fat/m2 body surface area) and glucose (75 g) were measured before and after the intervention.

RESULTS

After 1 year of lifestyle intervention, visceral adiposity was reduced by -26% (95% CI -29 to -23), whereas cardiorespiratory fitness improved by +20% (95% CI +16 to +24). After 1 year, the suppression of FFAs after the glucose load improved, whereas insulin concentrations were drastically reduced. After the oral lipid load, the late increase in FFA was reduced together with reduced circulating insulin. We calculated an insulin sensitivity index to reflect the concentration of insulin needed to manage plasma FFAs after the oral lipid load, which increased after the intervention and was associated with improved glucose tolerance, independent of changes in visceral or total adiposity.

CONCLUSIONS

A 1-year healthy eating/physical activity intervention improved the suppression of FFAs after oral glucose and lipid load tests in viscerally obese men, possibly due to improved responsiveness to insulin. This insulin-mediated regulation of postprandial plasma FFA levels could be a link between visceral obesity and impaired glucose homeostasis.

The 2012 CDA-CIHR INMD young investigator award lecture: dysfunction of adipose tissues and the mechanisms of ectopic fat deposition in type 2 diabetes

Ectopic fat deposition in skeletal muscles, liver, heart, and other tissues has been closely linked with the development of lean tissues' insulin resistance and progression toward type 2 diabetes mellitus. Mechanisms of overexposure of these tissues to fatty acids include increased de novo lipogenesis, impaired fatty acid oxidation and increased fatty acid flux to these organs. White adipose tissues are the main organs responsible for the regulation of circulating fatty acids. It has been clearly demonstrated that pre-diabetes individuals and individuals with diabetes display impaired adipose tissue dietary fatty acid storage that may lead to increased circulating flux and exaggerated lean tissue fatty acid exposure. Additionally, brown adipose tissue depots are less metabolically active in individuals with type 2 diabetes. We have developed a series of novel in vivo investigative tools using positron emission tomography to comprehensively assess postprandial interorgan fatty acid partitioning and white and brown adipose tissue metabolism in subjects with pre-diabetes and type 2 diabetes. Our findings shed new lights into the sophisticated mechanisms that regulate fatty acid partitioning and energy homeostasis during the development of type 2 diabetes. New links between abnormal dietary fatty acid metabolism and early myocardial metabolic and functional defects are now being uncovered in humans with the hope to find novel ways to predict and avoid the devastating complications of diabetes.

Metabolic inflexibility of white and brown adipose tissues in abnormal fatty acid partitioning of type 2 diabetes

Type 2 diabetes (T2D) is characterized by a general dysregulation of postprandial energy substrate partitioning. Although classically described in regard to glucose metabolism, it is now evident that metabolic inflexibility of plasma lipid fluxes is also present in T2D. The organ that is most importantly involved in the latter metabolic defect is the white adipose tissue (WAT). Both catecholamine-induced nonesterified fatty acid mobilization and insulin-stimulated storage of meal fatty acids are impaired in many WAT depots of insulin-resistant individuals. Novel molecular imaging techniques now demonstrate that these defects are linked to increased dietary fatty acid fluxes toward lean organs and myocardial dysfunction in humans. Recent findings also demonstrate functional abnormalities of brown adipose tissues in T2D, thus suggesting that a generalized adipose tissue dysregulation of energy storage and dissipation may be at play in the development of lean tissue energy overload and lipotoxicity.

Increased myocardial uptake of dietary fatty acids linked to cardiac dysfunction in glucose-intolerant humans

Impaired cardiac systolic and diastolic function has been observed in preclinical models and in subjects with type 2 diabetes. Using a recently validated positron emission tomography (PET) imaging method with 14(R,S)-[(¹⁸F]-fluoro-6-thia-heptadecanoic acid to quantify organ-specific dietary fatty acid partitioning, we demonstrate in this study that overweight and obese subjects with impaired glucose tolerance (IGT⁺) display significant increase in fractional myocardial dietary fatty acid uptake over the first 6 h postprandial compared with control individuals (IGT⁻). Measured by [¹¹C]acetate with PET, IGT⁺ subjects have a significant increase in myocardial oxidative index. IGT⁺ subjects have significantly reduced left ventricular stroke volume and ejection fraction (LVEF) and tend to display impaired diastolic function, as assessed by PET ventriculography. We demonstrate an inverse relationship between increased myocardial dietary fatty acid partitioning and LVEF. Fractional dietary fatty acid uptake is reduced in subcutaneous abdominal and visceral adipose tissues in IGT⁺ directly associated with central obesity. Fractional dietary fatty acid uptake in skeletal muscles or liver is, however, similar in IGT⁺ versus IGT⁻. The current study demonstrates, for the first time, that excessive myocardial partitioning of dietary fatty acids occurs in prediabetic individuals and is associated with early impairment of left ventricular function and increased myocardial oxidative metabolism.

Adipose tissue insulin resistance in peripubertal girls with first-degree family history of polycystic ovary syndrome

OBJECTIVE

To assess metabolic and endocrine defects in girls genetically predisposed to polycystic ovary syndrome (PCOS).

DESIGN

Controlled cross-sectional study.

SETTING

University hospital.

PATIENT(S)

Nine girls, aged 8-14 years, having a first-degree relative diagnosed with PCOS (PCOSr) and 10 age-matched girls without a family history of PCOS.

INTERVENTION(S)

None.

MAIN OUTCOME MEASURE(S)

Insulin sensitivity (IS(FSIVGTT)) determined by frequently sampled IV glucose tolerance testing (GTT) and insulin-induced nonesterified fatty acid (NEFA) suppression, estimated by the log-linear slope of NEFA levels during the first 20 minutes of GTT.

RESULT(S)

In comparison to controls, PCOSr had higher body mass index (BMI) Z-score, waist circumference, and waist-to-height ratio. Levels of the androgen 17α-hydroxyprogesterone (17-OHP) were significantly increased in PCOSr, independent of adiposity, and inversely correlated with IS(FSIVGTT). The IS(FSIVGTT) was decreased and the NEFA suppression was less steep in PCOSr compared with controls, independent of BMI and 17-OHP. The NEFA suppression was more pronounced with increasing IS(FSIVGTT), independent of adiposity.

CONCLUSION(S)

Girls at high risk of developing PCOS display increased adiposity and 17-OHP levels, but are mainly characterized by global insulin resistance and resistance to insulin-induced suppression of lipolysis that were independent of adiposity and 17-OHP levels. Therefore, genetic predisposition to PCOS may be related to early insulin resistance and adipocyte dysfunction.

Update on adipose tissue blood flow regulation

According to Fick's principle, any metabolic or hormonal exchange through a given tissue depends on the product of the blood flow to that tissue and the arteriovenous difference. The proper function of adipose tissue relies on adequate adipose tissue blood flow (ATBF), which determines the influx and efflux of metabolites as well as regulatory endocrine signals. Adequate functioning of adipose tissue in intermediary metabolism requires finely tuned perfusion. Because metabolic and vascular processes are so tightly interconnected, any disruption in one will necessarily impact the other. Although altered ATBF is one consequence of expanding fat tissue, it may also aggravate the negative impacts of obesity on the body's metabolic milieu. This review attempts to summarize the current state of knowledge on adipose tissue vascular bed behavior under physiological conditions and the various factors that contribute to its regulation as well as the possible participation of altered ATBF in the pathophysiology of metabolic syndrome.

Effect of adipose tissue insulin resistance on metabolic parameters and liver histology in obese patients with nonalcoholic fatty liver disease

The role of adipose tissue insulin resistance in the pathogenesis of nonalcoholic fatty liver disease (NAFLD) remains unclear. To evaluate this, we measured in 207 patients with NAFLD (age = 51 ± 1, body mass index = 34.1 ± 0.3 kg/m(2) ) and 22 controls without NAFLD (no NAFLD) adipose tissue insulin resistance by means of a validated index (Adipo-IR(i) = plasma free fatty acids [FFA] x insulin [FPI] concentration) and as the suppression of plasma FFA during an oral glucose tolerance test and by a low-dose insulin infusion. We also explored the relationship between adipose tissue insulin resistance with metabolic and histological parameters by dividing them based on quartiles of adipose tissue insulin resistance (Adipo-IR(i) quartiles: Q1 = more sensitive; Q4 = more insulin resistant). Hepatic insulin resistance, measured as an index derived from endogenous glucose production x FPI (HIRi), and muscle insulin sensitivity, were assessed during a euglycemic insulin clamp with 3-[(3) H] glucose. Liver fat was measured by magnetic resonance imaging and spectroscopy, and a liver biopsy was performed to assess liver histology. Compared to patients without steatosis, patients with NAFLD were insulin resistant at the level of adipose tissue, liver, and skeletal muscle and had higher plasma aspartate aminotransferase and alanine aminotransferase, triglycerides, and lower high-density lipoprotein cholesterol and adiponectin levels (all P < 0.01). Metabolic parameters, hepatic insulin resistance, and liver fibrosis (but not necroinflammation) deteriorated as quartiles of adipose tissue insulin resistance worsened (all P < 0.01).

CONCLUSION

Adipose tissue insulin resistance plays a key role in the development of metabolic and histological abnormalities of obese patients with NAFLD. Treatment strategies targeting adipose tissue insulin resistance (e.g., weight loss and thiazolidinediones) may be of value in this population.

An oral lipid challenge and acute intake of caffeinated coffee additively decrease glucose tolerance in healthy men

Lipid-induced insulin resistance has been investigated primarily with i.v. infusions, and caffeine-induced insulin resistance, with alkaloid caffeine. The effects of orally consumed lipids and coffee have not been established and to our knowledge have never been simultaneously investigated. The goals of this study were to determine whether an oral lipid challenge and caffeinated coffee would disrupt glucose homeostasis and to characterize their respective incretin responses. It was hypothesized that oral ingestion of saturated lipids would impair glucose tolerance and that caffeinated coffee would further hinder glucose management. Ten young, healthy males participated in 5 trials in a randomized, cross-over design. At time 0 h, they underwent an oral fat tolerance test (OFTT: 1 g lipid/kg body weight) or consumed water, followed 5 h later by caffeinated (5 mg/kg) coffee, decaffeinated coffee, or water. At 6 h, volunteers underwent an oral glucose tolerance test (OGTT). Consumption of the OFTT increased glucose concentrations (P < 0.05) after a subsequent OGTT. At 7 h, caffeinated coffee produced the highest glucose concentrations (P < 0.05). Glucagon-like peptide-1 active (GLP-1a) and glucose-dependent insulinotropic polypeptide (GIP) were both increased for up to 6 h in all OFTT trials (P < 0.05). Compared to all other treatments, caffeinated and decaffeinated coffee produced higher GLP-1a response at 6.25 h (P < 0.05), whereas only caffeinated coffee increased GIP secretion (P < 0.05). These results show that oral consumption of lipids and caffeinated coffee can independently and additively decrease glucose tolerance. Incretin hormones could explain at least in part this impaired glucose homeostasis.

Organ-specific dietary fatty acid uptake in humans using positron emission tomography coupled to computed tomography

A noninvasive method to determine postprandial fatty acid tissue partition may elucidate the link between excess dietary fat and type 2 diabetes. We hypothesized that the positron-emitting fatty acid analog 14(R,S)-[(18)F]fluoro-6-thia-heptadecanoic acid ((18)FTHA) administered orally during a meal would be incorporated into chylomicron triglycerides, allowing determination of interorgan dietary fatty acid uptake. We administered (18)FTHA orally at the beginning of a standard liquid meal ingested in nine healthy men. There was no significant (18)FTHA uptake in the portal vein and the liver during the 1st hour. Whole body PET/CT acquisition revealed early appearance of (18)FTHA in the distal thoracic duct, reaching a peak at time 240 min. (18)FTHA mean standard uptake value increased progressively in the liver, heart, quadriceps, and subcutaneous and visceral adipose tissues between time 60 and 240 min. Most circulating (18)F activity between time 0 and 360 min was recovered into chylomicron triglycerides. Using Triton WR-1339 treatment in rats that received (18)FTHA by gavage, we confirmed that >90% of this tracer reached the circulation as triglycerides. This novel noninvasive method to determine tissue dietary fatty acid distribution in humans should prove useful in the study of the mechanisms leading to lipotoxicity.

Normal postprandial nonesterified fatty acid uptake in muscles despite increased circulating fatty acids in type 2 diabetes

OBJECTIVE

Postprandial plasma nonesterified fatty acid (NEFA) appearance is increased in type 2 diabetes. Our objective was to determine whether skeletal muscle uptake of plasma NEFA is abnormal during the postprandial state in type 2 diabetes.

RESEARCH DESIGN AND METHODS

Thigh muscle blood flow and oxidative metabolism indexes and NEFA uptake were determined using positron emission tomography coupled with computed tomography (PET/CT) with [(11)C]acetate and 14(R,S)-[(18)F]fluoro-6-thia-heptadecanoic acid ((18)FTHA) in seven healthy control subjects (CON) and seven subjects with type 2 diabetes during continuous oral intake of a liquid meal to achieve steady postprandial NEFA levels with insulin infusion to maintain similar plasma glucose levels in both groups.

RESULTS

In the postprandial state, plasma NEFA level was higher in type 2 diabetic subjects versus CON (P < 0.01), whereas plasma glucose was at the same level in both groups. Muscle NEFA fractional extraction and blood flow index levels were 56% (P < 0.05) and 24% (P = 0.27) lower in type 2 diabetes, respectively. However, muscle NEFA uptake was similar to that of CON (quadriceps femoris [QF] 1.47 ± 0.23 vs. 1.37 ± 0.24 nmol·g(-1)·min(-1), P = 0.77; biceps femoris [BF] 1.54 ± 0.26 vs. 1.46 ± 0.28 nmol·g(-1)·min(-1), P = 0.85). Muscle oxidative metabolism was similar in both groups. Muscle NEFA fractional extraction and blood flow index were strongly and positively correlated (r = 0.79, P < 0.005).

CONCLUSIONS

Postprandial muscle NEFA uptake is normal despite elevated systemic NEFA levels and acute normalization of plasma glucose in type 2 diabetes. Lower postprandial muscle blood flow with resulting reduction in muscle NEFA fractional extraction may explain this phenomenon.

Lipid-induced pancreatic β-cell dysfunction: focus on in vivo studies

The phenomenon of lipid-induced pancreatic β-cell dysfunction ("lipotoxicity") has been very well documented in numerous in vitro experimental systems and has become widely accepted. In vivo demonstration of β-cell lipotoxicity, on the other hand, has not been consistently demonstrated, and there remains a lack of consensus regarding the in vivo effects of chronically elevated free fatty acids (FFA) on β-cell function. Much of the disagreement relates to how insulin secretion is quantified in vivo and in particular whether insulin secretion is assessed in relation to whole body insulin sensitivity, which is clearly reduced by elevated FFA. By correcting for changes in in vivo insulin sensitivity, we and others have shown that prolonged elevation of FFA impairs β-cell secretory function. Prediabetic animal models and humans with a positive family history of type 2 diabetes are more susceptible to this impairment, whereas those with severe impairment of β-cell function (such as individuals with type 2 diabetes) demonstrate no additional impairment of β-cell function when FFA are experimentally raised. Glucolipotoxicity (i.e., the combined β-cell toxicity of elevated glucose and FFA) has been amply demonstrated in vitro and in some animal studies but not in humans, perhaps because there are limitations in experimentally raising plasma glucose to sufficiently high levels for prolonged periods of time. We and others have shown that therapies directed toward diminishing oxidative stress and ER stress have the potential to reduce lipid-induced β-cell dysfunction in animals and humans. In conclusion, lipid-induced pancreatic β-cell dysfunction is likely to be one contributor to the complex array of genetic and metabolic insults that result in the relentless decline in pancreatic β-cell function in those destined to develop type 2 diabetes, and mechanisms involved in this lipotoxicity are promising therapeutic targets.

The role of adipose tissue and lipotoxicity in the pathogenesis of type 2 diabetes

The widespread epidemics of obesity and type 2 diabetes mellitus (T2DM) suggest that both conditions are closely linked. An increasing body of evidence has shifted our view of adipose tissue from a passive energy depot to a dynamic "endocrine organ" that tightly regulates nutritional balance by means of a complex crosstalk of adipocytes with their microenvironment. Dysfunctional adipose tissue, particularly as observed in obesity, is characterized by adipocyte hypertrophy, macrophage infiltration, impaired insulin signaling, and insulin resistance. The result is the release of a host of inflammatory adipokines and excessive amounts of free fatty acids that promote ectopic fat deposition and lipotoxicity in muscle, liver, and pancreatic beta cells. This review focuses on recent work on how glucose homeostasis is profoundly altered by distressed adipose tissue. A better understanding of this relationship offers the best chance for early intervention strategies aimed at preventing the burden of T2DM.

Increased postprandial nonesterified fatty acid appearance and oxidation in type 2 diabetes is not fully established in offspring of diabetic subjects

Background

It has been proposed that abnormal postprandial plasma nonesterified fatty acid (NEFA) metabolism may participate in the development of tissue lipotoxicity and type 2 diabetes (T2D). We previously found that non-diabetic offspring of two parents with T2D display increased plasma NEFA appearance and oxidation rates during intravenous administration of a fat emulsion. However, it is currently unknown whether plasma NEFA appearance and oxidation are abnormal during the postprandial state in these subjects at high-risk of developing T2D.

Methodology

Palmitate appearance and oxidation rates and glycerol appearance rate were determined in eleven healthy offspring of two parents with T2D (positive family history, FH+), 13 healthy subjects without first-degree relatives with T2D (FH-) and 12 subjects with T2D at fasting, during normoglycemic hyperinsulinemic clamp and during continuous oral intake of a standard liquid meal to achieve steady postprandial NEFA and triacylglycerols (TG) without and with insulin infusion to maintain similar glycemia in all three groups.

Principal Findings

Plasma palmitate appearance and oxidation were higher at fasting and during the clamp conditions in the T2D group (all P<0.05). In the postprandial state, palmitate appearance, oxidative and non oxidative rates were all elevated in T2D (all P<0.05) but not in FH+. Both T2D and FH+ displayed elevated postprandial TG vs. FH- (P<0.001). Acute correction of hyperglycemia during the postprandial state did not affect these group differences. Increased waist circumference and BMI were positively associated with elevated postprandial plasma palmitate appearance and oxidation.

Conclusions/Significance

Postprandial plasma NEFA intolerance observed in subjects with T2D is not fully established in non-diabetic offspring of both parents with T2D, despite the presence of increased postprandial plasma TG in the later. Elevated postprandial plasma NEFA appearance and oxidation in T2D is observed despite acute correction of the exaggerated glycemic excursion in this group.

Abnormal in vivo myocardial energy substrate uptake in diet-induced type 2 diabetic cardiomyopathy in rats

The purpose of this study was to determine in vivo myocardial energy metabolism and function in a nutritional model of type 2 diabetes. Wistar rats rendered insulin-resistant and mildly hyperglycemic, hyperinsulinemic, and hypertriglyceridemic with a high-fructose/high-fat diet over a 6-wk period with injection of a small dose of streptozotocin (HFHFS) and control rats were studied using micro-PET (microPET) without or with a euglycemic hyperinsulinemic clamp. During glucose clamp, myocardial metabolic rate of glucose measured with [(18)F]fluorodeoxyglucose ([(18)F]FDG) was reduced by approximately 81% (P < 0.05), whereas myocardial plasma nonesterified fatty acid (NEFA) uptake as determined by [(18)F]fluorothia-6-heptadecanoic acid ([(18)F]FTHA) was not significantly changed in HFHFS vs. control rats. Myocardial oxidative metabolism as assessed by [(11)C]acetate and myocardial perfusion index as assessed by [(13)N]ammonia were similar in both groups, whereas left ventricular ejection fraction as assessed by microPET was reduced by 26% in HFHFS rats (P < 0.05). Without glucose clamp, NEFA uptake was approximately 40% lower in HFHFS rats (P < 0.05). However, myocardial uptake of [(18)F]FTHA administered by gastric gavage was significantly higher in HFHFS rats (P < 0.05). These abnormalities were associated with reduced Glut4 mRNA expression and increased Cd36 mRNA expression and mitochondrial carnitine palmitoyltransferase 1 activity (P < 0.05). HFHFS rats display type 2 diabetes complicated by left ventricular contractile dysfunction with profound reduction in myocardial glucose utilization, activation of fatty acid metabolic pathways, and preserved myocardial oxidative metabolism, suggesting reduced myocardial metabolic efficiency. In this model, increased myocardial fatty acid exposure likely occurs from circulating triglyceride, but not from circulating plasma NEFA.

Plasma nonesterified Fatty Acid intolerance and hyperglycemia are associated with intravenous lipid-induced impairment of insulin sensitivity and disposition index

CONTEXT

It is currently unclear why susceptibility to lipid-induced impairment of beta-cell function varies in different populations.

OBJECTIVE

The aim of the study was to determine whether mild hyperglycemia may be associated with nonesterified fatty acid (NEFA) intolerance and increased iv lipid-induced lipotoxic effect on the beta-cell.

DESIGN AND SETTING

The study consisted of an experimental design with control group conducted at an academic clinical research center.

PARTICIPANTS

Twenty-six overweight or obese individuals (12 with normal glucose tolerance, nine with impaired glucose tolerance or type 2 diabetes, and five subjects who previously had impaired glucose tolerance or type 2 diabetes but at the time of study had normal glucose tolerance after biliopancreatic diversion).

INTERVENTIONS

We assessed insulin sensitivity (S(I)) and beta-cell function [insulin disposition index (DI)] after an overnight iv infusion of heparin + Intralipid (HI) vs. normal saline for 16 h using a stepwise, incremental iv glucose infusion followed by a hyperglycemic clamp.

MAIN OUTCOME MEASURES

We measured S(I), DI, HI-induced change in plasma NEFA, and its association with HI-induced change in S(I) and DI.

RESULTS

HI resulted in significant reduction in S(I) and DI across the three groups of participants. HI-induced elevation of plasma NEFA was higher in hyperglycemic vs. normoglycemic groups. Both fasting glucose level and the magnitude of HI-induced NEFA elevation were associated with the reduction in S(I) (P = 0.007 and P = 0.01, respectively) and DI (P = 0.001 and P = 0.007, respectively).

CONCLUSION

Mild hyperglycemia and NEFA intolerance to iv lipid are associated with susceptibility to lipid-induced reduction in S(I) and DI.

Overactivation of NF-κB impairs insulin sensitivity and mediates palmitate-induced insulin resistance in C2C12 skeletal muscle cells

Lipid-induced insulin resistance is associated with inflammatory state in epidemiological studies. However, it is still unclear whether the activation of NF-κB, a pivotal transcription factor of inflammation, plays a crucial role in mediating skeletal muscle insulin resistance. This study addressed what was the role of NF-κB in lipid-induced insulin resistance and whether NF-κB activation was sufficient to cause insulin resistance in C2C12 myotubes. A 16 h exposure of myotubes to palmitate reduced net insulin-stimulated glucose uptake by 48%, GLUT4 translocation by 52%, Akt phosphorylation by 54%, induced a 1.8-fold increase in insulin-stimulated insulin receptor substrate (IRS) phosphorylation, and doubled NF-κB activation. Myotubes transfected with NF-κB p65 siRNA for 24 h and followed by a treatment with palmitate for 16 h efficiently blocked NF-κB activation, and prevented the detrimental effects of palmitate on the metabolic actions of insulin. Transfection of myotubes with I-κBα siRNA for 24 h also led to a twofold induction of NF-κB activation, and reduced net insulin-stimulated glucose uptake by 30%, GLUT4 translocation by 35%, Akt phosphorylation by 31%, induced a 0.7-fold increase in insulin-stimulated IRS phosphorylation. These findings suggest that NF-κB overexpression per se is sufficient to impair insulin sensitivity and palmitate-induced insulin resistance is mediated by NF-κB in skeletal muscle cells.

Lessons learned from studying families genetically predisposed to type 2 diabetes mellitus

Early interventions to prevent type 2 diabetes mellitus (T2DM) demand a better understanding of its underlying mechanisms. Nonobese healthy subjects with a strong family history of T2DM (FH(+) subjects) hold a key to this end by allowing the study of the disease before the development of confounding factors, such as obesity or hyperglycemia. In this article, we share our experience over the past decade in studying FH(+) subjects and how lipotoxicity alters glucose metabolism in such individuals, in particular pancreatic beta-cell function. FH(+) subjects have no obvious clinical abnormalities, but when carefully studied, reveal severe hepatic/muscle/adipose tissue insulin resistance and subtle defects in beta-cell function. In most subjects, metabolic adaption allows freedom from diabetes for decades. However, the obesity epidemic is drastically changing this. Given the unique susceptibility of pancreatic beta cells to free fatty acids in FH(+) subjects, interventions that protect against obesity-induced lipotoxicity may hold the greatest promise for preventing T2DM in genetically predisposed individuals.

Relationship between total and high molecular weight adiponectin levels and plasma nonesterified fatty acid tolerance during enhanced intravascular triacylglycerol lipolysis in men

CONTEXT

Increased plasma nonesterified fatty acid (NEFA) appearance during enhanced intravascular triacylglycerol (TG) lipolysis is a marker of metabolic adipose tissue dysfunction and may lead to the development of insulin resistance. The relationship between total and high molecular weight (HMW) adiponectin levels, NEFA appearance, and total TG lipolytic capacity has not been previously studied in humans.

OBJECTIVES

Our objective was to determine whether total and HMW adiponectin plasma levels are associated with plasma NEFA level and appearance, and with total TG lipolytic rate during enhanced intravascular TG lipolysis in men.

DESIGN

This was a cross-sectional metabolic study.

SETTING

The study was performed at an academic clinical research center.

PARTICIPANTS

There were 15 healthy men (mean +/- sd body mass index 25.5 +/- 4.7 kg/m(2)) aged 21-50 yr (mean +/- sd 31.1 +/- 10.2) without first-degree relatives with type 2 diabetes included in the study.

INTERVENTIONS

Pancreatic clamps and iv infusion of stable isotopic tracers ([1,1,2,3,3-(2)H(5)]glycerol and [U-(13)C]palmitate) were performed, whereas intravascular TG lipolysis was clamped by iv infusion of heparin plus Intralipid at low (fasting) and high insulin levels. Total and HMW adiponectin levels were measured using an ELISA.

MAIN OUTCOME MEASURES

Levels of total and HMW adiponectin, palmitate appearance (plasma palmitate appearance rate), and glycerol appearance (plasma glycerol appearance rate) were calculated.

RESULTS

During heparin plus Intralipid infusion, total and HMW adiponectin was inversely correlated with plasma palmitate appearance rate (r = -0.65; P = 0.01), but this association was lost when expressed per nonlean weight. Adiponectin levels were positively associated with plasma glycerol appearance rate per nonlean weight (r = 0.71 and r = 0.66, respectively; P < or = 0.01).

CONCLUSIONS

Increased adipose tissue mass likely explains the association between low adiponectin and reduced NEFA tolerance. Adiponectin level is a marker of total TG lipolytic rate per adipose tissue mass in men.