Validation of a new procedure to determine plasma fatty acid concentration and isotopic enrichment

Very-low-density lipoprotein triglyceride and free fatty acid plasma kinetics in women with high or low brown adipose tissue volume and overweight/obesity

Although a high amount of brown adipose tissue (BAT) is associated with low plasma triglyceride concentration, the mechanism responsible for this relationship in people is not clear. Here, we evaluate the interrelationships among BAT, very-low-density lipoprotein triglyceride (VLDL-TG), and free fatty acid (FFA) plasma kinetics during thermoneutrality in women with overweight/obesity who had a low (<20 mL) or high (≥20 mL) volume of cold-activated BAT (assessed by using positron emission tomography in conjunction with 2-deoxy-2-[18F]-fluoro-glucose). We find that plasma TG and FFA concentrations are lower and VLDL-TG and FFA plasma clearance rates are faster in women with high BAT than low BAT volume, whereas VLDL-TG and FFA appearance rates in plasma are not different between the two groups. These findings demonstrate that women with high BAT volume have lower plasma TG and FFA concentrations than women with low BAT volumes because of increased VLDL-TG and FFA clearance rates. This study was registered at ClinicalTrials.gov (NCT02786251).

Physiological interindividual variability in endogenous estradiol concentration does not influence adipose tissue and hepatic lipid kinetics in women

Objective

Increased triglyceride (TG) and apolipoprotein B-100 (apoB-100) concentrations in plasma are important risk factors for cardiovascular disease in women. Administration of some estrogen preparations raises plasma TG and apoB-100 concentrations by increasing hepatic very low-density lipoprotein (VLDL) TG and apoB-100 secretion rates. However, the influence of physiological variation in endogenous estradiol on VLDL-TG and VLDL-apoB-100 metabolism and on free fatty acid (FFA) release into plasma (the major source of fatty acids for VLDL-TG production) is not known.

Design and methods

We measured basal VLDL-TG, VLDL-apoB-100, and plasma FFA kinetics by using stable isotopically labeled tracers in 36 eumenorrheic, premenopausal women (age: 33 ± 2 years, BMI: 31 ± 1 kg/m2; mean ± s.e.m.) during the follicular phase of the menstrual cycle; participants were divided into two groups based on low (n = 18) or high (n = 18) plasma estradiol concentrations (defined as below or above the median value of 140 pmol/L in the whole group).

Results

Mean plasma estradiol concentration was >3-fold higher in the high-estradiol than in the low-estradiol group (299 ± 37 and 96 ± 7 pmol/L, P < 0.001); there was no difference in plasma progesterone concentrations between the two groups (P = 0.976). There were no significant differences in plasma FFA concentration, FFA rate of appearance in plasma, VLDL-TG and VLDL-apoB-100 concentrations, hepatic VLDL-TG and VLDL-apoB-100 secretion rates, VLDL-TG and VLDL-apoB-100 plasma clearance rates, and mean residence times (all P ≥ 0.45). No significant associations were found between plasma estradiol concentration and FFA, VLDL-TG, and VLDL-apoB-100 concentrations and kinetics (all P > 0.19).

Conclusions

Plasma estradiol concentration is not an important correlate of basal plasma FFA, VLDL-TG, and VLDL-apoB-100 kinetics in premenopausal women.

High-fructose feeding suppresses cold-stimulated brown adipose tissue glucose uptake independently of changes in thermogenesis and the gut microbiome

Diets rich in added sugars are associated with metabolic diseases, and studies have shown a link between these pathologies and changes in the microbiome. Given the reported associations in animal models between the microbiome and brown adipose tissue (BAT) function, and the alterations in the microbiome induced by high-glucose or high-fructose diets, we investigated the potential causal link between high-glucose or -fructose diets and BAT dysfunction in humans. Primary outcomes are changes in BAT cold-induced thermogenesis and the fecal microbiome (clinicaltrials.gov, NCT03188835). We show that BAT glucose uptake, but not thermogenesis, is impaired by a high-fructose but not high-glucose diet, in the absence of changes in the gastrointestinal microbiome. We conclude that decreased BAT glucose metabolism occurs earlier than other pathophysiological abnormalities during fructose overconsumption in humans. This is a potential confounding factor for studies relying on ¹⁸F-FDG to assess BAT thermogenesis.

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Fructose overfeeding decreases brown adipose tissue glucose metabolism

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These changes occur independently of oxidative metabolism

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No change is observed with glucose overfeeding

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The gut microbiome is not affected by fructose/glucose overfeeding

Selective and fast methylation of free fatty acids directly in plasma for their individual analysis by gas chromatography- mass spectrometry

An analytical procedure for the rapid and selective derivatization of free fatty acids into methyl esters directly in plasma without transmethylation of lipid-bound fatty acids was developed for their analysis by gas chromatography-mass spectrometry. The methyl esters of free fatty acids were obtained by reaction with methyl iodide in the solution of dipolar aprotic solvents and in the presence of solid bases. The mechanism of the methylation reaction with these reagents was investigated. Optimal conditions for the selective methylation of free fatty acids were established using different dipolar aprotic solvents and different solid bases. The possible transmethylation of covalently bonded fatty acids from plasma lipids has been investigated under different experimental conditions in order to be avoided. Total methylation of free fatty acids was achieved in 1 min at room temperature using methyl iodide and anhydrous potassium carbonate or sodium carbonate in dimethyl sulfoxide. Under these conditions, transmethylation of lipid-bound fatty acids was avoided. The methyl esters can be injected directly from the reaction solvents. A plasma volume of 50 μL was used without special purification. The detection limits were around 0.1 ng/μL. The proposed method avoids the drawbacks of the previous methods used for the one-step analysis of individual free fatty acids in human plasma.

Lysophospholipases cooperate to mediate lipid homeostasis and lysophospholipid signaling

Lysophospholipids (LysoPLs) are bioactive lipid species involved in cellular signaling processes and the regulation of cell membrane structure. LysoPLs are metabolized through the action of lysophospholipases, including lysophospholipase A1 (LYPLA1) and lysophospholipase A2 (LYPLA2). A new X-ray crystal structure of LYPLA2 compared with a previously published structure of LYPLA1 demonstrated near-identical folding of the two enzymes; however, LYPLA1 and LYPLA2 have displayed distinct substrate specificities in recombinant enzyme assays. To determine how these in vitro substrate preferences translate into a relevant cellular setting and better understand the enzymes' role in LysoPL metabolism, CRISPR-Cas9 technology was utilized to generate stable KOs of Lypla1 and/or Lypla2 in Neuro2a cells. Using these cellular models in combination with a targeted lipidomics approach, LysoPL levels were quantified and compared between cell lines to determine the effect of losing lysophospholipase activity on lipid metabolism. This work suggests that LYPLA1 and LYPLA2 are each able to account for the loss of the other to maintain lipid homeostasis in cells; however, when both are deleted, LysoPL levels are dramatically increased, causing phenotypic and morphological changes to the cells.

Plasma ferritin concentration is positively associated with in vivo fatty acid mobilization and insulin resistance in obese women

NEW FINDINGS

What is the central question of this study? Do obese women with relatively high whole-body iron stores exhibit elevated in vivo rates of fatty acid (FA) release from adipose tissue compared with a well-matched cohort of obese women with relatively low iron stores? What is the main finding and its importance? Obese women with high plasma [ferritin] (a marker of whole-body iron stores) had greater FA mobilization, lipolytic activation in adipose tissue and insulin resistance (IR) compared with obese women with lower plasma [ferritin]. Given that elevated FA mobilization is intimately linked with the development of IR, these findings suggest that elevated iron stores might contribute to IR in obesity by increasing systemic FA availability.

ABSTRACT

High rates of fatty acid (FA) mobilization from adipose tissue are associated with insulin resistance (IR) in obesity. In vitro evidence suggests that iron stimulates lipolysis in adipocytes, but whether iron is related to in vivo FA mobilization is unknown. We hypothesized that plasma ferritin concentration ([ferritin]), a marker of body iron stores, would be positively associated with FA mobilization. We measured [ferritin], the rate of appearance of FA in the systemic circulation (FA Ra; stable isotope dilution), key adipose tissue lipolytic proteins and IR (hyperinsulinaemic-euglycaemic clamp) in 20 obese, premenopausal women. [Ferritin] was correlated with FA Ra (r = 0.65; P = 0.002) and IR (r = 0.57; P = 0.008); these relationships remained significant after controlling for body mass index and plasma [C-reactive protein] (a marker of systemic inflammation) in multiple regression analyses. We then stratified subjects into tertiles based on [ferritin] to compare subjects with 'High-ferritin' versus 'Low-ferritin'. Plasma [hepcidin] was more than fivefold greater (P < 0.05) in the High-ferritin versus Low-ferritin group, but there was no difference in plasma [C-reactive protein] between groups, indicating that the large difference in plasma [ferritin] reflects a difference in iron stores, not systemic inflammation. We found that FA Ra, adipose protein abundance of hormone-sensitive lipase and adipose triglyceride lipase, and IR were significantly greater in subjects with High-ferritin versus Low-ferritin (all P < 0.05). These data provide the first evidence linking iron and in vivo FA mobilization and suggest that elevated iron stores might contribute to IR in obesity by increasing systemic FA availability.

Changes in markers for cardio-metabolic disease risk after only 1-2 weeks of a high saturated fat diet in overweight adults

Purpose

Diets high in saturated fat acids (SFA) have been linked with cardio-metabolic disease risk. The purpose of this study was to determine whether only 1–2 weeks of a high SFA diet could impact disease risk factors in overweight adults who normally eat a relatively low proportion of SFA (i.e., <40% of dietary fat).

Methods

Twelve overweight (BMI: 27±1 kg/m²) young adults were studied before and after a 2-week diet that increased the proportion of SFA (<40% to 60% of dietary fat), while maintaining their daily intake of total fat, carbohydrate, protein, and calories. Insulin resistance, blood pressure, plasma markers of liver damage, total plasma cholesterol concentrations, and fatty acid profile within plasma and skeletal muscle lipid pools were assessed before and after the intervention.

Results

Total plasma cholesterol concentration increased (148±5 vs. 164±8 mg/dl; P<0.05) after only one week, due exclusively to an increase in LDL-cholesterol (78±4 vs. 95±7 mg/dl; P<0.05). After two weeks, plasma aspartate amino transferase (AST) concentration increased (P<0.05) but we found no change in insulin resistance, or resting blood pressure. The diet increase the proportion of SFA in plasma (35±1% vs. 39±2%; P<0.05) and the intramyocellular triglyceride pool (32±1% vs. 37±1%; P<0.05) suggesting the fatty acids in these pools may readily exchange.

Conclusions

Although blood lipids remain within normal clinical range, increasing saturated fat in diet for only 2 weeks raises plasma markers of cardiovascular risk (LDL-cholesterol) and liver damage (AST). In overweight, but healthy-young adults SFA accumulate in plasma and muscle after only 1–2 weeks of dietary increase.

Diurnal Variation in PDK4 Expression Is Associated With Plasma Free Fatty Acid Availability in People

CONTEXT

Many biological pathways involved in regulating substrate metabolism display rhythmic oscillation patterns. In rodents, clock genes regulate circadian rhythms of metabolic genes and substrate metabolism. However, the interrelationships among substrate metabolism, metabolic genes, and clock genes have not been fully explored in people.

OBJECTIVE

We tested the hypothesis that the diurnal expression pattern of pyruvate dehydrogenase kinase 4 (PDK4), a key metabolic enzyme involved in fuel switching between glucose and free fatty acids (FFAs), is associated with plasma FFA concentration and clock genes.

DESIGN AND METHODS

We analyzed peripheral blood mononuclear cells (PBMCs), subcutaneous adipose tissue, and plasma samples obtained serially during 24 hours from metabolically healthy women (n = 10) and evaluated the interrelationships among PDK4, plasma FFA, and clock genes. We also determined the potential mechanisms responsible for PDK4 transcriptional regulation by using primary human PBMCs and adipocytes.

RESULTS

We found that PDK4 diurnal expression patterns were similar in PBMCs and adipose tissue (ρ = 0.84, P < 0.001). The diurnal variation in PBMC PDK4 expression correlated more strongly with plasma FFA and insulin (ρ = 0.86 and 0.63, respectively, both P < 0.001) concentrations than clock genes. Data obtained from primary culture experiments demonstrated that FFAs directly induced PDK4 gene expression, at least in part through activation of peroxisome proliferator-activated receptor α.

CONCLUSIONS

Our results suggest that plasma FFA availability is an important regulator of diurnal expression patterns of PDK4, and we identify a novel interaction between plasma FFA and cellular diurnal rhythms in regulating substrate metabolism.

Intramuscular triglyceride synthesis: importance in muscle lipid partitioning in humans

Intramuscular triglyceride (IMTG) concentration is elevated in insulin-resistant individuals and was once thought to promote insulin resistance. However, endurance-trained athletes have equivalent concentration of IMTG compared with individuals with type 2 diabetes, and have very low risk of diabetes, termed the "athlete's paradox." We now know that IMTG synthesis is positively related to insulin sensitivity, but the exact mechanisms for this are unclear. To understand the relationship between IMTG synthesis and insulin sensitivity, we measured IMTG synthesis in obese control subjects, endurance-trained athletes, and individuals with type 2 diabetes during rest, exercise, and recovery. IMTG synthesis rates were positively related to insulin sensitivity, cytosolic accumulation of DAG, and decreased accumulation of C18:0 ceramide and glucosylceramide. Greater rates of IMTG synthesis in athletes were not explained by alterations in FFA concentration, DGAT1 mRNA expression, or protein content. IMTG synthesis during exercise in Ob and T2D indicate utilization as a fuel despite unchanged content, whereas IMTG concentration decreased during exercise in athletes. mRNA expression for genes involved in lipid desaturation and IMTG synthesis were increased after exercise and recovery. Further, in a subset of individuals, exercise decreased cytosolic and membrane di-saturated DAG content, which may help explain insulin sensitization after acute exercise. These data suggest IMTG synthesis rates may influence insulin sensitivity by altering intracellular lipid localization, and decreasing specific ceramide species that promote insulin resistance.

HIV infection does not prevent the metabolic benefits of diet-induced weight loss in women with obesity

OBJECTIVE

To test the hypothesis that HIV infection impairs the beneficial effects of weight loss on insulin sensitivity, adipose tissue inflammation, and endoplasmic reticulum (ER) stress.

METHODS

A prospective clinical trial evaluated the effects of moderate diet-induced weight loss on body composition, metabolic function, and adipose tissue biology in women with obesity who were HIV-seronegative (HIV-) or HIV-positive (HIV+). Body composition, multiorgan insulin sensitivity (assessed by using a two-stage hyperinsulinemic-euglycemic clamp procedure with stable isotopically labeled tracer infusions), and adipose tissue expression of markers of inflammation, autophagy, and ER stress were evaluated in 8 HIV- and 20 HIV+ women with obesity before and after diet-induced weight loss of 6% to 8%.

RESULTS

Although weight loss was not different between groups (∼7.5%), the decrease in fat-free mass was greater in HIV+ than HIV- subjects (-4.4 ± 0.7% vs. -1.7 ± 1.0%, P < 0.05). Weight loss improved insulin sensitivity in adipose tissue (suppression of palmitate rate of appearance [Ra]), liver (suppression of glucose Ra), and muscle (glucose disposal) similarly in both groups. Weight loss did not affect adipose tissue expression of markers of inflammation or ER stress in either group.

CONCLUSIONS

Moderate diet-induced weight loss improves multiorgan insulin sensitivity in HIV+ women to the same extent as women who are HIV-. However, weight loss causes a greater decline in fat-free mass in HIV+ than HIV- women.

Detection of unusual very-long-chain fatty acid and ether lipid derivatives in the fibroblasts and plasma of patients with peroxisomal diseases using liquid chromatography-mass spectrometry

Metabolic changes occur in patients with peroxisomal diseases owing to impairments in the genes involved in peroxisome function. For diagnostic purposes, saturated very-long-chain fatty acids (VLCFAs) such as C24:0 and C26:0, phytanic acid, pristanic acid, and plasmalogens are often measured as metabolic hallmarks. As the direct pathology of peroxisomal disease is yet to be fully elucidated, we sought to explore the fatty acid species that accumulate in patients with peroxisomal diseases. We developed a method for detecting a range of fatty acids implicated in peroxisomal diseases such as Zellweger syndrome (ZS) and X-linked adrenoleukodystrophy (X-ALD). To this end, we employed an ultra-performance liquid chromatography-mass spectrometry (LC-MS) coupled with negatively charged electrospray ionization. Fatty acids from patients and control subjects were extracted from total lipids by acid-hydrolysis and compared. In accordance with previous results, the amounts of VLCFAs, phytanic acid, and pristanic acid differed between the two groups. We identified extremely long and highly polyunsaturated VLCFAs (ultra-VLC-PUFAs) such as C44:12 in ZS samples. Moreover, three unknown molecules were prominent in control samples but scarcely detectable in ZS samples. LC-MS/MS analysis identified these as 1-alkyl-sn-glycerol 3-phosphates derived from ether lipids containing fatty alcohols such as C16:0, C18:0, or C18:1. Our method provides an approach to observing a wide range of lipid-derived fatty acids and related molecules in order to understand the metabolic changes involved in peroxisomal diseases. This technique can therefore be used in identifying metabolic markers and potential clinical targets for future treatment.

Metabolomic Profiling Identifies Novel Circulating Biomarkers of Mitochondrial Dysfunction Differentially Elevated in Heart Failure With Preserved Versus Reduced Ejection Fraction: Evidence for Shared Metabolic Impairments in Clinical Heart Failure

BACKGROUND

Metabolic impairment is an important contributor to heart failure (HF) pathogenesis and progression. Dysregulated metabolic pathways remain poorly characterized in patients with HF and preserved ejection fraction (HFpEF). We sought to determine metabolic abnormalities in HFpEF and identify pathways differentially altered in HFpEF versus HF with reduced ejection fraction (HFrEF).

METHODS AND RESULTS

We identified HFpEF cases, HFrEF controls, and no-HF controls from the CATHGEN study of sequential patients undergoing cardiac catheterization. HFpEF cases (N=282) were defined by left ventricular ejection fraction (LVEF) ≥45%, diastolic dysfunction grade ≥1, and history of HF; HFrEF controls (N=279) were defined similarly, except for having LVEF <45%. No-HF controls (N=191) had LVEF ≥45%, normal diastolic function, and no HF diagnosis. Targeted mass spectrometry and enzymatic assays were used to quantify 63 metabolites in fasting plasma. Principal components analysis reduced the 63 metabolites to uncorrelated factors, which were compared across groups using ANCOVA. In basic and fully adjusted models, long-chain acylcarnitine factor levels differed significantly across groups (P<0.0001) and were greater in HFrEF than HFpEF (P=0.0004), both of which were greater than no-HF controls. We confirmed these findings in sensitivity analyses using stricter inclusion criteria, alternative LVEF thresholds, and adjustment for insulin resistance.

CONCLUSIONS

We identified novel circulating metabolites reflecting impaired or dysregulated fatty acid oxidation that are independently associated with HF and differentially elevated in HFpEF and HFrEF. These results elucidate a specific metabolic pathway in HF and suggest a shared metabolic mechanism in HF along the LVEF spectrum.

Physiological Mechanisms of Weight Gain-Induced Steatosis in People With Obesity

Weight gain is associated with an increase in intrahepatic triglycerides (IHTGs), and is the primary cause of nonalcoholic fatty liver disease in obese individuals. We combined imaging and stable isotope tracer techniques to evaluate the physiologic mechanisms of weight gain-induced steatosis in 27 obese people. Weight gain appeared to increase IHTG content by generating an imbalance between hepatic fatty acid availability and disposal, and resulted in increased hepatic de novo lipogenesis, decreased intrahepatic fatty acid oxidation, and inadequate increases in IHTG export via very low-density lipoprotein secretion. ClinicalTrials.gov ID NCT01184170.

Metabolic profiling in Prader-Willi syndrome and nonsyndromic obesity: sex differences and the role of growth hormone

OBJECTIVES

To identify metabolic factors controlling appetite and insulin sensitivity in PWS and assess effects of GH treatment.

METHODS

We compared amino acids, fatty acids and acylcarnitines in GH-treated and untreated PWS children and obese and lean controls to identify biomarkers associated with ghrelin, peptide YY and markers of insulin sensitivity (adiponectin and HOMA-IR).

RESULTS

Compared with obese controls (OC), children with PWS had fasting hyperghrelinaemia, hyperadiponectinaemia, hypoinsulinaemia and increased ghrelin/PYY. Hyperghrelinaemia, hyperadiponectinaemia and hypoinsulinaemia were more striking in PWS females than males, and decreases in BCAA were detected only in PWS females. GH-treated PWS subjects had lower leptin and higher IGF-1 and adiponectin than untreated subjects; fasting ghrelin, PYY and insulin levels were comparable. Ghrelin correlated inversely with BCAA in PWS but not OC. Adiponectin correlated negatively with BMIz and HOMA-IR in PWS; in contrast, adiponectin correlated more strongly with BCAA than BMIz or HOMA-IR in OC.

CONCLUSIONS

BCAA levels were lower in PWS females than OC females and correlated inversely with ghrelin. Low BCAA in PWS females may promote hyperghrelinaemia and hyperphagia, while hyperadiponectinaemia may maintain insulin sensitivity despite excess weight gain. GH treatment may reduce leptin and increase adiponectin, but does not affect fasting ghrelin or PYY.

Adrenergic control of lipolysis in women compared with men

Data suggest women are more sensitive to the lipolytic action of epinephrine compared with men while maintaining similar glucoregulatory effects (Horton et al. J Appl Physiol 107: 200-210, 2009). This study aimed to determine the specific adrenergic receptor(s) that may mediate these sex differences. Lean women (n = 14) and men (n = 16) were studied on 4 nonconsecutive days during the following treatment infusions: saline (S: control), epinephrine [E: mixed β-adrenergic (lipolytic) and α2-adrenergic (antilipolytic) stimulation], epinephrine + phentolamine (E + P: mixed β-adrenergic stimulation only), and terbutaline (T: selective β2-adrenergic stimulation). Tracer infusions of glycerol, palmitate, and glucose were administered to determine systemic lipolysis, free fatty acid (FFA) release, and glucose turnover, respectively. Following basal measurements, substrate and hormone concentrations were measured in all subjects over 90 min of treatment and tracer infusion. Women had greater increases in glycerol and FFA concentrations with all three hormone infusions compared with men (P < 0.01). Glycerol and palmitate rate of appearance (Ra) and rate of disappearance (Rd) per kilogram body weight were greater with E infusion in women compared with men (P < 0.05), whereas no sex differences were observed with other treatments. Glucose concentration and kinetics were not different between sexes with any infusion. In conclusion, these data support the hypothesis that the greater rate of lipolysis in women with infusion of E was likely due to lesser α2 antilipolytic activation. These findings may help explain why women have greater lipolysis and fat oxidation during exercise, a time when epinephrine concentration is elevated.

Diurnal variation in insulin sensitivity of glucose metabolism is associated with diurnal variations in whole-body and cellular fatty acid metabolism in metabolically normal women

CONTEXT

The mechanism(s) responsible for diurnal variations in insulin sensitivity of glucose metabolism in healthy people are unclear.

OBJECTIVE

The objective of the study was to evaluate whether diurnal variations in whole-body and cellular fatty acid metabolism could contribute to evening insulin resistance in metabolically normal people.

SUBJECTS AND DESIGN

We measured plasma the free fatty acid (FFA) concentration, palmitate kinetics, and skeletal muscle expression of genes involved in fatty acid metabolism at breakfast (7:00 am) and dinner (7:00 pm) in 13 overweight (body mass index 27.8 ± 1.2 kg/m(2)) but metabolically normal, women.

RESULTS

Plasma FFA concentration was approximately 30% greater just before consuming dinner than breakfast (P < .05) and remained greater after dinner than breakfast (FFA areas under the curve: 0.88 ± 0.33 and 0.51 ± 0.09 μmol/mL × 4 h, P = .001). However, adipose tissue lipolytic activity was not different in the evening and in the morning. Skeletal muscle expression of genes that regulate fatty acid oxidation were 38-82% lower, whereas genes involved in de novo lipogenesis were 51%-87 % higher before dinner than before breakfast (all P < .05), and these changes were associated with diurnal variation in the muscle expression of core clock genes that regulate fatty acid metabolism.

CONCLUSION

Metabolically normal women demonstrate diurnal variations in fatty acid metabolism, manifested by an increase in circulating FFAs, presumably derived from previous meal consumption rather than lipolysis of adipose tissue triglycerides, and a shift in muscle fatty acid metabolism from oxidation to lipogenesis. These metabolic alterations could be responsible for the known evening decline in insulin sensitivity.

A history of previous gestational diabetes mellitus is associated with adverse changes in insulin secretion and VLDL metabolism independently of increased intrahepatocellular lipid

AIMS/HYPOTHESIS

We have previously reported a high prevalence of non-alcoholic fatty liver disease (NAFLD) among women with previous gestational diabetes mellitus (pGDM). We wanted to confirm that intrahepatocellular lipid (IHCL) is associated with pGDM independently of adiposity and determine: (1) if VLDL metabolism is dysregulated; and (2) the extent to which NAFLD and IHCL account for the dysmetabolic phenotype in pGDM.

METHODS

We analysed data from a cohort of 234 women (114 with pGDM) and identified effects of pGDM on lipid and glucoregulation that were independent of ultrasound-diagnosed NAFLD. We then measured IHCL by MR spectroscopy in a representative subgroup (n = 36) and conducted detailed metabolic studies (IVGTT, VLDL apolipoprotein B [apoB] kinetics and palmitate turnover) and measurement of regional body fat by MRI to demonstrate effects of IHCL that were independent of a history of pGDM.

RESULTS

pGDM was associated with increased IHCL (p = 0.04) after adjustment for adiposity. Independently of IHCL, pGDM was associated with a lower IVGTT disposition index (p = 0.02) and acute insulin response to glucose (pGDM+/NAFLD-, 50% lower; pGDM+/NAFLD+, 36% lower; effect of pGDM, p = 0.03), increased VLDL apoB pool size (pGDM+/NAFLD-, 3.1-fold higher; pGDM+/NAFLD+, 1.2-fold higher; effect of pGDM, p = 0.02) and, at borderline significance (p = 0.05), increased rate of VLDL apoB synthesis.

CONCLUSIONS/INTERPRETATION

pGDM is associated with increased IHCL independently of adiposity. The increased liver fat contributes to the phenotype, but pGDM status is independently associated with diminished insulin secretion and (shown for the first time) augmented VLDL metabolism. IHCL with pGDM may compound a dysmetabolic phenotype.

A ~60-min brisk walk increases insulin-stimulated glucose disposal but has no effect on hepatic and adipose tissue insulin sensitivity in older women

The purpose of this study was to determine whether brisk walking improves multiorgan (liver, muscle, adipose tissue) insulin sensitivity in older women. Ten nonobese older women (age: 66.7 ± 1.5 yr, mean ± SE) completed two 2-stage hyperinsulinemic-euglycemic clamp procedures [insulin infusion rate stage 1: 10 mU/m(2) body surface area (BSA) per min; stage 2: 50 mU/m(2) BSA per min] in conjunction with stable isotope-labeled glucose and palmitate tracer infusions: one in the morning after a single, ∼1-h bout of brisk treadmill walking, the other after an equivalent period of rest in the late afternoon of the preceding day. We found that basal glucose rate of appearance (Ra) into plasma was not different after rest and after exercise (17.3 ± 0.8 and 17.1 ± 0.4 μmol/kg fat-free mass per min, respectively). The insulin-mediated decrease in glucose Ra during stage 1 of the clamp was also not different after rest and exercise (82.2% ± 3.4% and 77.7% ± 2.1%, respectively), but glucose rate of disappearance (Rd) during stage 2 of the clamp was significantly greater (P < 0.05) after exercise than rest (88.0 ± 5.9 and 78.4 ± 6.5 μmol/kg fat-free mass per min, respectively). There were no differences in palmitate Ra during basal conditions or insulin infusion after exercise and after rest. Therefore, we conclude that a single bout of brisk walking for ∼1 h improves muscle insulin sensitivity but has no effect on liver and adipose tissue insulin sensitivity in older women.

Role of fat body lipogenesis in protection against the effects of caloric overload in Drosophila

The Drosophila fat body is a liver- and adipose-like tissue that stores fat and serves as a detoxifying and immune responsive organ. We have previously shown that a high sugar diet leads to elevated hemolymph glucose and systemic insulin resistance in developing larvae and adults. Here, we used stable isotope tracer feeding to demonstrate that rearing larvae on high sugar diets impaired the synthesis of esterified fatty acids from dietary glucose. Fat body lipid profiling revealed changes in both carbon chain length and degree of unsaturation of fatty acid substituents, particularly in stored triglycerides. We tested the role of the fat body in larval tolerance of caloric excess. Our experiments demonstrated that lipogenesis was necessary for animals to tolerate high sugar feeding as tissue-specific loss of orthologs of carbohydrate response element-binding protein or stearoyl-CoA desaturase 1 resulted in lethality on high sugar diets. By contrast, increasing the fat content of the fat body by knockdown of king-tubby was associated with reduced hyperglycemia and improved growth and tolerance of high sugar diets. Our work supports a critical role for the fat body and the Drosophila carbohydrate response element-binding protein ortholog in metabolic homeostasis in Drosophila.

Novel and reversible mechanisms of smoking-induced insulin resistance in humans

Smoking is the most common cause of preventable morbidity and mortality in the United States, in part because it is an independent risk factor for the development of insulin resistance and type 2 diabetes. However, mechanisms responsible for smoking-induced insulin resistance are unclear. In this study, we found smokers were less insulin sensitive compared with controls, which increased after either 1 or 2 weeks of smoking cessation. Improvements in insulin sensitivity after smoking cessation occurred with normalization of IRS-1(ser636) phosphorylation. In muscle cell culture, nicotine exposure significantly increased IRS-1(ser636) phosphorylation and decreased insulin sensitivity, recapitulating the phenotype of smoking-induced insulin resistance in humans. The two pathways known to stimulate IRS-1(ser636) phosphorylation (p44/42 mitogen-activated protein kinase [MAPK] and mammalian target of rapamycin [mTOR]) were both stimulated by nicotine in culture. Inhibition of mTOR, but not p44/42 MAPK, during nicotine exposure prevented IRS-1(ser636) phosphorylation and normalized insulin sensitivity. These data indicate nicotine induces insulin resistance in skeletal muscle by activating mTOR. Therapeutic agents designed to oppose skeletal muscle mTOR activation may prevent insulin resistance in humans who are unable to stop smoking or are chronically exposed to secondhand smoke.

Influence of dietary supplementation with (L)-carnitine on metabolic rate, fatty acid oxidation, body condition, and weight loss in overweight cats

OBJECTIVE

To investigate the influence of dietary supplementation with l-carnitine on metabolic rate, fatty acid oxidation, weight loss, and lean body mass (LBM) in overweight cats undergoing rapid weight reduction.

ANIMALS

32 healthy adult neutered colony-housed cats.

PROCEDURES

Cats fattened through unrestricted ingestion of an energy-dense diet for 6 months were randomly assigned to 4 groups and fed a weight reduction diet supplemented with 0 (control), 50, 100, or 150 μg of carnitine/g of diet (unrestricted for 1 month, then restricted). Measurements included resting energy expenditure, respiratory quotient, daily energy expenditure, LBM, and fatty acid oxidation. Following weight loss, cats were allowed unrestricted feeding of the energy-dense diet to investigate weight gain after test diet cessation.

RESULTS

Median weekly weight loss in all groups was ≥ 1.3%, with no difference among groups in overall or cumulative percentage weight loss. During restricted feeding, the resting energy expenditure-to-LBM ratio was significantly higher in cats that received l-carnitine than in those that received the control diet. Respiratory quotient was significantly lower in each cat that received l-carnitine on day 42, compared with the value before the diet began, and in all cats that received l-carnitine, compared with the control group throughout restricted feeding. A significant increase in palmitate flux rate in cats fed the diet with 150 μg of carnitine/g relative to the flux rate in the control group on day 42 corresponded to significantly increased stoichiometric fat oxidation in the l-carnitine diet group (> 62% vs 14% for the control group). Weight gain (as high as 28%) was evident within 35 days after unrestricted feeding was reintroduced.

CONCLUSIONS AND CLINICAL RELEVANCE

Dietary l-carnitine supplementation appeared to have a metabolic effect in overweight cats undergoing rapid weight loss that facilitated fatty acid oxidation.

Fatty acid and very low density lipoprotein metabolism in obese African American and Caucasian women with type 2 diabetes

Type 2 diabetes mellitus (T2DM) is associated with increased plasma triglyceride (TG) concentrations, but African Americans (AA) have lower plasma TG than Caucasians (CC). We evaluated the hypothesis that obese AA women have lower plasma TG than obese CC women do because of differences in lipid kinetics. Eleven AA and 11 CC obese women with T2DM, matched on body mass index (BMI) (AA = 37 ± 1, CC = 37 ± 1 kg/m(2)), age, duration of diabetes, percentage body fat, and insulin sensitivity (S(I), determined by an intravenous glucose tolerance test), were studied. Plasma TG concentration (AA = 1.14 ± 0.11, CC = 1.88 ± 0.18 mmol/l), FFA rate of appearance (R(a)) into plasma (AA = 419 ± 27, CC = 503 ± 31 µmol·min(-1)), and total VLDL-TG secretion rate (AA = 18 ± 2, CC = 29 ± 4 µmol·min(-1)) were lower in AA than CC women (all P < 0.05). In contrast, plasma total apolipoprotein (apo)B-100 concentration (AA = 1,542 ± 179, CC = 1,620 ± 118 nmol/l) and VLDL-apoB-100 secretion rate (AA = 1.3 ± 0.1, CC = 1.3 ± 0.1 nmol·min(-1)) were similar in both groups, so the molar ratio of VLDL-TG secretion rate to VLDL-apoB-100 secretion rate was lower in AA women than in CC women. VLDL-TG concentration was lower in AA women due to lower total VLDL-TG secretion rate. However, the VLDL-apoB-100 secretion rate was the same in both groups, demonstrating that AA women secrete smaller VLDL particles containing less TG than do CC women.

Marginal vitamin B-6 deficiency decreases plasma (n-3) and (n-6) PUFA concentrations in healthy men and women

Previous animal studies showed that severe vitamin B-6 deficiency altered fatty acid profiles of tissue lipids, often with an increase of linoleic acid and a decrease of arachidonic acid. However, little is known about the extent to which vitamin B-6 deficiency affects human fatty acid profiles. The aim of this study was to determine the effects of marginal vitamin B-6 deficiency on fatty acid profiles in plasma, erythrocytes, and peripheral blood mononuclear cells (PBMC) of healthy adults fed a 28-d, low-vitamin B-6 diet. Healthy participants (n = 23) received a 2-d, controlled, vitamin B-6-adequate diet followed by a 28-d, vitamin B-6-restricted diet to induce a marginal deficiency. Plasma HDL and LDL cholesterol concentrations, FFA concentrations, and erythrocyte and PBMC membrane fatty acid compositions did not significantly change from baseline after the 28-d restriction. Plasma total arachidonic acid, EPA, and DHA concentrations decreased from (mean ± SD) 548 ± 96 to 490 ± 94 μmol/L, 37 ± 13 to 32 ± 13 μmol/L, and 121 ± 28 to 109 ± 28 μmol/L [positive false discovery rate (pFDR) adjusted P < 0.05], respectively. The total (n-6):(n-3) PUFA ratio in plasma exhibited a minor increase from 15.4 ± 2.8 to 16.6 ± 3.1 (pFDR adjusted P < 0.05). These data indicate that short-term vitamin B-6 restriction decreases plasma (n-3) and (n-6) PUFA concentrations and tends to increase the plasma (n-6):(n-3) PUFA ratio. Such changes in blood lipids may be associated with the elevated risk of cardiovascular disease in vitamin B-6 insufficiency.

Randomized trial of exercise effect on intrahepatic triglyceride content and lipid kinetics in nonalcoholic fatty liver disease

Nonalcoholic fatty liver disease (NAFLD) and alterations in hepatic lipoprotein kinetics are common metabolic complications associated with obesity. Lifestyle modification involving diet-induced weight loss and regular exercise decreases intrahepatic triglyceride (IHTG) content and very low density lipoprotein (VLDL) triglyceride (TG) secretion rate. The aim of this study was to evaluate the weight loss-independent effect of following the physical activity guidelines recommended by the Department of Health and Human Services on IHTG content and VLDL kinetics in obese persons with NAFLD. Eighteen obese people (body mass index [BMI]: 38.1 ± 4.6 kg/m(2)) with NAFLD were randomized to 16 weeks of exercise training (45%-55% VO(2peak) , 30-60 minutes × 5 days/week; n = 12) or observation (control; n = 6). Magnetic resonance spectroscopy and stable isotope tracer infusions in conjunction with compartmental modeling were used to evaluate IHTG content and hepatic VLDL-TG and apolipoprotein B-100 (apoB-100) secretion rates. Exercise training resulted in a 10.3% ± 4.6% decrease in IHTG content (P < 0.05), but did not change total body weight (103.1 ± 4.2 kg before and 102.9 ± 4.2 kg after training) or percent body fat (38.9% ± 2.1% before and 39.2% ± 2.1% after training). Exercise training did not change the hepatic VLDL-TG secretion rate (17.7 ± 3.9 μmol/min before and 16.8 ± 5.4 μmol/min after training) or VLDL-apoB-100 secretion rate (1.5 ± 0.5 nmol/min before and 1.6 ± 0.6 nmol/min after training).

CONCLUSION

Following the Department of Health and Human Services recommended physical activity guidelines has small but beneficial effects on IHTG content, but does not improve hepatic lipoprotein kinetics in obese persons with NAFLD.

Testosterone increases the muscle protein synthesis rate but does not affect very-low-density lipoprotein metabolism in obese premenopausal women

Men and women with hyperandrogenemia have a more proatherogenic plasma lipid profile [e.g., greater triglyceride (TG) and total and low-density lipoprotein-cholesterol and lower high-density lipoprotein-cholesterol concentrations] than healthy premenopausal women. Furthermore, castration of male rats markedly reduces testosterone availability below normal and decreases plasma TG concentration, and testosterone replacement reverses this effect. Testosterone is, therefore, thought to be an important regulator of plasma lipid homeostasis. However, little is known about the effect of testosterone on plasma TG concentration and kinetics. Furthermore, testosterone is a potent skeletal muscle protein anabolic agent in men, but its effect on muscle protein turnover in women is unknown. We measured plasma lipid concentrations, hepatic very low density lipoprotein (VLDL)-TG and VLDL-apolipoprotein B-100 secretion rates, and the muscle protein fractional synthesis rate in 10 obese women before and after trandermal testosterone (1.25 g of 1% AndroGel daily) treatment for 3 wk. Serum total and free testosterone concentrations increased (P < 0.05) by approximately sevenfold in response to testosterone treatment, reaching concentrations that are comparable to those in women with hyperandrogenemia, but lower than the normal range for eugonadal men. Except for a small (∼10%) decrease in plasma high-density lipoprotein particle and cholesterol concentrations (P < 0.04), testosterone therapy had no effect on plasma lipid concentrations, lipoprotein particle sizes, and hepatic VLDL-TG and VLDL-apolipoprotein B-100 secretion rates (all P > 0.05); the muscle protein fractional synthesis rate, however, increased by ∼45% (P < 0.001). We conclude that testosterone is a potent skeletal muscle protein anabolic agent, but not an important regulator of plasma lipid homeostasis in obese women.

The fatty acid translocase gene CD36 and lingual lipase influence oral sensitivity to fat in obese subjects

The precise orosensory inputs engaged for dietary lipids detection in humans are unknown. We evaluated whether a common single nucleotide polymorphism (rs1761667) in the CD36 gene that reduces CD36 expression and the addition of orlistat, a lipase inhibitor, to reduce FA release from triacylglycerols (TGs), the main component of dietary fats, would attenuate fat orosensory sensitivity in humans. Twenty-one obese subjects with different rs1761667 genotypes (6 AA, 7 AG, and 8 GG) were studied on two occasions in which oleic acid and triolein orosensory detection thresholds were measured using emulsions prepared with and without orlistat. Subjects homozygous for the G-allele had 8-fold lower oral detection thresholds for oleic acid and triolein than subjects homozygous for the A allele, which associates with lower CD36 expression (P = 0.03). Thresholds for heterozygous subjects were intermediate. The addition of orlistat increased detection thresholds for triolein (log threshold = -0.3 ± 0.2 vs. 0.3 ± 0.1; P < 0.001) but not oleic acid (log threshold = -1.0 ± 0.2 vs. -0.8 ± 0.2; P > 0.2). In conclusion, this is the first experimental evidence for a role of CD36 in fat gustatory perception in humans. The data also support involvement of lingual lipase and are consistent with the concept that FA and not TG is the sensed stimulus.

Validation of a novel index to assess insulin resistance of adipose tissue lipolytic activity in obese subjects

Insulin resistance in adipose tissue increases the release of free fatty acids into the circulation, which likely contributes to impaired insulin action in liver and skeletal muscle associated with obesity. However, reliable assessment of adipose tissue insulin resistance requires performing a hyperinsulinemic-euglycemic clamp procedure in conjunction with a fatty acid tracer infusion to determine insulin-mediated suppression of lipolytic rate. We developed a simpler method for evaluating adipose tissue insulin resistance in vivo, determined as the product of palmitate rate of appearance into the bloodstream and plasma insulin concentration during basal conditions. We validated our Adipose Tissue Insulin Resistance Index (ATIRI) by comparison with an assessment of adipose tissue insulin resistance determined by using the hyperinsulinemic-euglycemic clamp procedure in conjunction with a palmitate tracer infusion in 47 obese nondiabetic subjects (body mass index: 40.1 ± 9.3 kg/m(2)). We found the ATIRI correlated closely with adipose tissue insulin resistance assessed during the clamp procedure (r =-0.854, P < 0.001). These results demonstrate that the ATIRI provides a reliable index of adipose tissue insulin resistance in obese subjects.

Elevated serum α-linolenic acid levels are associated with decreased chance of pregnancy after in vitro fertilization

OBJECTIVE

To analyze relationships between serum free fatty acid (FFA) concentrations and pregnancy.

DESIGN

Prospective cohort.

SETTING

University hospital.

PATIENT(S)

Ninety-one women undergoing IVF.

INTERVENTION(S)

Serum was analyzed for total and specific serum FFAs, including myristic, palmitic, stearic, oleic, linoleic, and α-linolenic acids.

MAIN OUTCOME MEASURE(S)

Univariate analyses were used to identify specific FFAs and other factors associated with pregnancy after IVF. Logistic regression was performed modeling relationships between identified factors and chance of pregnancy.

RESULT(S)

In unadjusted analyses, women with elevated serum α-linolenic acid (ALA) levels (highest quartile) demonstrated a decreased chance of pregnancy compared with women with the lowest levels (odds ratio 0.24, 95% confidence interval 0.052-0.792). No associations between other FFAs and pregnancy were identified. In a multivariable regression model, associations between elevated serum ALA levels and decreased chance of pregnancy remained after adjusting for patient age, body mass index, and history of endometriosis or previous live birth (adjusted odds ratio 0.139, 95% confidence interval 0.028-0.686).

CONCLUSION(S)

Elevated serum ALA levels are associated with decreased chance of pregnancy in women undergoing IVF. Further work is needed to determine whether ALA is involved in early reproductive processes and whether the relationship between ALA and pregnancy is associated with excess ALA intake, impaired ALA metabolism, or both.

Fenofibrate administration does not affect muscle triglyceride concentration or insulin sensitivity in humans

Animal data suggest that males, in particular, rely on peroxisome proliferator activated receptor-α activity to maintain normal muscle triglyceride metabolism. We sought to examine whether this was also true in men vs women and its relationship to insulin sensitivity (Si). Normolipidemic obese men (n = 9) and women (n = 9) underwent an assessment of Si (intravenous glucose tolerance test) and intramuscular triglyceride (IMTG) metabolism (gas chromatography/mass spectrometry and gas chromatography-combustion isotope ratio mass spectrometry from plasma and muscle biopsies taken after infusion of [U-(13)C]palmitate) before and after 12 weeks of fenofibrate treatment. Women were more insulin sensitive (Si: 5.2 ± 0.7 vs 2.4 ± 0.4 ×10(-4)/ μU/mL, W vs M, P < .01) at baseline despite similar IMTG concentration (41.9 ± 15.5 vs 30.8 ± 5.1 μg/mg dry weight, W vs M, P = .43) and IMTG fractional synthesis rate (FSR) (0.27%/h ± 0.07%/h vs 0.35%/h ± 0.06%/h, W vs M, P = .41) as men. Fenofibrate enhanced FSR in men (0.35 ± 0.06 to 0.54 ± 0.06, P = .05), with no such change seen in women (0.27 ± 0.07 to 0.32 ± 0.13, P = .73) and no change in IMTG concentration in either group (23.0 ± 3.9 in M, P = .26 vs baseline; 36.3 ± 12.0 in W, P = .79 vs baseline). Insulin sensitivity was unaffected by fenofibrate (P ≥ .68). Lower percentage saturation of IMTG in women vs men before (29.1% ± 2.3% vs 35.2% ± 1.7%, P = .06) and after (27.3% ± 2.8% vs 35.1% ± 1.9%, P = .04) fenofibrate most closely related to their greater Si (R(2) = 0.34, P = .10) and was largely unchanged by the drug. Peroxisome proliferator activated receptor-α agonist therapy had little effect on IMTG metabolism in men or women. Intramuscular triglyceride saturation, rather than IMTG concentration or FSR, most closely (but not significantly) related to Si and was unchanged by fenofibrate administration.

Factors to consider in using [U-C]palmitate for analysis of sphingolipid biosynthesis by tandem mass spectrometry

This study describes the use of a stable-isotope labeled precursor ([U-¹³C]palmitate) to analyze de novo sphingolipid biosynthesis by tandem mass spectrometry. It also describes factors to consider in interpreting the data, including the isotope's location (¹³C appears in three isotopomers and isotopologues: [M + 16] for the sphingoid base or N-acyl fatty acid, and [M + 32] for both); the isotopic enrichment of palmitoyl-CoA; and its elongation, desaturation, and incorporation into N-acyl-sphingolipids. For HEK293 cells incubated with 0.1 mM [U-¹³C]palmitic acid, ∼60% of the total palmitoyl-CoA was ¹³C-labeled by 3 h (which was near isotopic equilibrium); with this correction, the rates of de novo biosynthesis of C16:0-ceramide, C16:0-monohexosylceramide, and C16:0-sphingomyelins were 62 ± 3, 13 ± 2, and 60 ± 11 pmol/h per mg protein, respectively, which are consistent with an estimated rate of appearance of C16:0-ceramide using exponential growth modeling (119 ± 11 pmol/h per mg protein). Including estimates for the very long-chain fatty acyl-CoAs, the overall rate of sphingolipid biosynthesis can be estimated to be at least ∼1.6-fold higher. Thus, consideration of these factors gives a more accurate picture of de novo sphingolipid biosynthesis than has been possible to-date, while acknowledging that there are inherent limitations to such approximations.

Associations between free fatty acids, cumulus oocyte complex morphology and ovarian function during in vitro fertilization

OBJECTIVE

To determine if follicular free fatty acid (FFA) levels are associated with cumulus oocyte complex (COC) morphology.

DESIGN

Prospective cohort study.

SETTING

University in vitro fertilization (IVF) practice.

PATIENT(S)

A total of 102 women undergoing IVF.

INTERVENTION(S)

Measurement of FFAs in serum and ovarian follicular fluid.

MAIN OUTCOME MEASURE(S)

Total and specific follicular and serum FFA levels, correlations between follicular and serum FFAs, and associations between follicular FFA levels and markers of oocyte quality, including COC morphology.

RESULT(S)

Predominant follicular fluid and serum FFAs were oleic, palmitic, linoleic, and stearic acids. Correlations between follicular and serum FFA concentrations were weak (r=0.252, 0.288, 0.236, 0.309, respectively for specific FFAs; r=0.212 for total FFAs). A receiver operating characteristic curve determined total follicular FFAs≥0.232 μmol/mL distinguished women with a lower versus higher percentage of COCs with favorable morphology. Women with elevated follicular FFAs (n=31) were more likely to have COCs with poor morphology than others (n=71; OR 3.3, 95% CI1.2-9.2). This relationship held after adjusting for potential confounders, including age, body mass index, endometriosis, and amount of gonadotropin administered (β=1.2; OR 3.4, 95% CI 1.1-10.4).

CONCLUSION(S)

Elevated follicular FFA levels are associated with poor COC morphology. Further work is needed to determine what factors influence follicular FFA levels and if these factors impact fertility.

Effects of acute hyperglucagonemia on hepatic and intestinal lipoprotein production and clearance in healthy humans

OBJECTIVE

The metabolism of hepatic- and intestinally derived lipoproteins is regulated in a complex fashion by nutrients, hormones, and neurologic and other factors. Recent studies in animal models suggest an important role for glucagon acting via the glucagon receptor in regulating hepatic triglyceride (TG) secretion. Here we examined the direct effects of glucagon on regulation of hepatic and intestinal lipoprotein metabolism in humans.

RESEARCH DESIGN AND METHODS

Eight healthy men underwent two studies each, in random order, 4-6 weeks apart in which de novo lipogenesis, kinetics of larger VLDL1 TG, and kinetics of VLDL1 and smaller VLDL2 apolipoprotein (apo)B100 and B48 were studied using established stable isotope enrichment methods. Subjects were studied in the constant fed state under conditions of a pancreatic clamp (with infusion of somatostatin, insulin, and growth hormone) at either basal glucagon (BG study, 64.5 ± 2.1 pg/mL) or hyperglucagonemia (high glucagon [HG] study, 183.2 ± 5.1 pg/mL).

RESULTS

There were no significant differences in plasma concentration of VLDL1 or VLDL2 TG, apoB100 or apoB48 between BG and HG studies. There was, however, lower (P < 0.05) VLDL1 apoB100 fractional catabolic rate (-39%) and production rate (-30%) in HG versus BG, but no difference in de novo lipogenesis or TG turnover, and glucagon had no effect on intestinal (B48-containing) lipoprotein metabolism.

CONCLUSIONS

Glucagon acutely regulates hepatic but not intestinal lipoprotein particle metabolism in humans both by decreasing hepatic lipoprotein particle production as well as by inhibiting particle clearance, with no net effect on particle concentration.

Metabolic pathways promoting intrahepatic fatty acid accumulation in methionine and choline deficiency: implications for the pathogenesis of steatohepatitis

The pathological mechanisms that distinguish simple steatosis from steatohepatitis (or NASH, with consequent risk of cirrhosis and hepatocellular cancer) remain incompletely defined. Whereas both a methionine- and choline-deficient diet (MCDD) and a choline-deficient diet (CDD) lead to hepatic triglyceride accumulation, MCDD alone is associated with hepatic insulin resistance and inflammation (steatohepatitis). We used metabolic tracer techniques, including stable isotope ([¹³C₄]palmitate) dilution and mass isotopomer distribution analysis (MIDA) of [¹³C₂]acetate, to define differences in intrahepatic fatty acid metabolism that could explain the contrasting effect of MCDD and CDD on NASH in C57Bl6 mice. Compared with control-supplemented (CS) diet, liver triglyceride pool sizes were similarly elevated in CDD and MCDD groups (24.37 ± 2.4, 45.94 ± 3.9, and 43.30 ± 3.5 μmol/liver for CS, CDD, and MCDD, respectively), but intrahepatic neutrophil infiltration and plasma alanine aminotransferase (31 ± 3, 48 ± 4, 231 ± 79 U/l, P < 0.05) were elevated only in MCDD mice. However, despite loss of peripheral fat in MCDD mice, neither the rate of appearance of palmitate (27.2 ± 3.5, 26.3 ± 2.3, and 28.3 ± 3.5 μmol·kg⁻¹·min⁻¹) nor the contribution of circulating fatty acids to the liver triglyceride pool differed between groups. Unlike CDD, MCDD had a defect in hepatic triglyceride export that was confirmed using intravenous tyloxapol (142 ± 21, 122 ± 15, and 80 ± 7 mg·kg⁻¹·h⁻¹, P < 0.05). Moreover, hepatic de novo lipogenesis was significantly elevated in the MCDD group only (1.4 ± 0.3, 2.3 ± 0.4, and 3.4 ± 0.4 μmol/day, P < 0.01). These findings suggest that important alterations in hepatic fatty acid metabolism may promote the development of steatohepatitis. Similar mechanisms may predispose to hepatocyte damage in human NASH.

Reproducibility of glucose, fatty acid and VLDL kinetics and multi-organ insulin sensitivity in obese subjects with non-alcoholic fatty liver disease

OBJECTIVE

Non-alcoholic fatty liver disease (NAFLD) is associated with abnormalities in basal glucose and free fatty acid (FFA) metabolism, multi-organ insulin resistance and alterations in lipoprotein kinetics. These metabolic outcomes can be evaluated in vivo by using stable isotopically labeled tracer methods. An understanding of the reproducibility of these measures is necessary to ensure adequate statistical power in studies designed to evaluate metabolic function in subjects with NAFLD.

METHODS

We determined the degree of intra-individual variability of skeletal muscle, adipose tissue, and hepatic insulin sensitivity and basal plasma glucose, FFA, and very-low-density lipoprotein triglyceride and apolipoprotein B-100 (apoB-100) kinetics in eight obese subjects with NAFLD (age: 44 ± 3 years; body mass index: 38.2 ± 1.7 kg m(-2); intrahepatic triglyceride content: 24.5 ± 3.9%), by using the hyperinsulinemic-euglycemic clamp technique and stable isotope-labeled tracer methods and mathematical modeling on two separate occasions ∼2 months apart.

RESULTS

The intra-individual variability (coefficient of variation) ranged from 6% for basal glucose production to 21% for insulin-stimulated glucose disposal (percentage increase from basal). We estimated that a 25% difference in any outcome measure can be detected with a sample size of ≤ 8 subjects for paired studies and ≤ 15 subjects per group for unpaired studies, assuming an α value of 0.05 and a β value of 0.20 (that is, 80% power).

CONCLUSION

These results demonstrate that only a small number of subjects are needed to detect clinically relevant effects in insulin sensitivity and hepatic lipoprotein metabolism in obese subjects with NAFLD, and will be useful to determine appropriate sample size for future metabolic studies.

Altered intramuscular lipid metabolism relates to diminished insulin action in men, but not women, in progression to diabetes

Whether sex differences in intramuscular triglyceride (IMTG) metabolism underlie sex differences in the progression to diabetes are unknown. Therefore, the current study examined IMTG concentration and fractional synthesis rate (FSR) in obese men and women with normal glucose tolerance (NGT) vs. those with prediabetes (PD). PD (n = 13 men and 7 women) and NGT (n = 7 men and 12 women) groups were matched for age and anthropometry. Insulin action was quantified using a hyperinsulinemic-euglycemic clamp with infusion of [6,6-(2)H(2)]-glucose. IMTG concentration was measured by gas chromatography/mass spectrometry (GC/MS) and FSR by GC/combustion isotope ratio MS (C-IRMS), from muscle biopsies taken after infusion of [U-(13)C]palmitate during 4 h of rest. In PD men, the metabolic clearance rate (MCR) of glucose was lower during the clamp (4.71 ± 0.77 vs. 8.62 ± 1.26 ml/kg fat-free mass (FFM)/min, P = 0.04; with a trend for lower glucose rate of disappearance (Rd), P = 0.07), in addition to higher IMTG concentration (41.2 ± 5.0 vs. 21.2 ± 3.4 µg/mg dry weight, P ≤ 0.01), lower FSR (0.21 ± 0.03 vs. 0.42 ± 0.06 %/h, P ≤ 0.01), and lower oxidative capacity (P = 0.03) compared to NGT men. In contrast, no difference in Rd, IMTG concentration, or FSR was seen in PD vs. NGT women. Surprisingly, glucose Rd during the clamp was not different between NGT men and women (P = 0.25) despite IMTG concentration being higher (42.6 ± 6.1 vs. 21.2 ± 3.4 µg/mg dry weight, P = 0.03) and FSR being lower (0.23 ± 0.04 vs. 0.42 ± 0.06 %/h, P = 0.02) in women. Alterations in IMTG metabolism relate to diminished insulin action in men, but not women, in the progression toward diabetes.

Direct ultrasound-assisted methylation of fatty acids in serum for free fatty acid determinations

A method for simultaneous determination of 16 free fatty acids (FFAs) in serum is described. The method involves conversion of FFAs to fatty acid methyl esters (FAMEs) using the heat of ultrasonic waves followed by gas chromatography and mass spectrometry (GC–MS) analysis. Optimum levels of the variables affecting the yield of FAMEs were investigated. The results indicate that the optimal levels are 55 °C, 60 W, 10% H2SO4/CH3OH, and 50 min. Recoveries ranged from 85.32% to 112.11%, with a detection limit ranging from 0.03 to 0.08 μg mL–1. The linearity, using the linear correlation coefficient, was higher than 0.9914.

Comprehensive analysis of the major lipid classes in sebum by rapid resolution high-performance liquid chromatography and electrospray mass spectrometry

Sebum is a complex lipid mixture that is synthesized in sebaceous glands and excreted on the skin surface. The purpose of this study was the comprehensive detection of the intact lipids that compose sebum. These lipids exist as a broad range of chemical structures and concentrations. Sebum was collected with SebuTape(TM) from the foreheads of healthy donors, and then separated by HPLC on a C8 stationary phase with sub 2 µm particle size. This HPLC method provided high resolution and excellent reproducibility of retention times (RT). Compound mining was performed with time of flight (TOF) and triple quadrupole (QqQ) mass spectrometers (MS), which allowed for the classification of lipids according to their elemental composition, degree of unsaturation, and MS/MS fragmentation. The combination of the two MS systems detected 95 and 29 families of triacylglycerols (TAG) and diacylglycerols (DAG), respectively. Assignment was carried out regardless of positional isomerism. Among the wax esters (WE), 28 species were found to contain the 16:1 fatty acyl moiety. This method was suitable for the simultaneous detection of squalene and its oxygenated derivative. A total of 9 cholesterol esters (CE) were identified and more than 48 free fatty acids (FFA) were detected in normal sebum. The relative abundance of each individual lipid within its own chemical class was determined for 12 healthy donors. In summary, this method provided the first characterization of the features and distribution of intact components of the sebum lipidome.

Inflexibility in intramuscular triglyceride fractional synthesis distinguishes prediabetes from obesity in humans

Whether intramuscular triglyceride (IMTG) concentration or flux is more important in the progression to type 2 diabetes is controversial. Therefore, this study examined IMTG concentration, as well as its fractional synthesis rate (FSR), in obese people with normal glucose tolerance (NGT; n = 20) vs. obese people with prediabetes (PD; n = 19), at rest and during exercise. Insulin action and secretion were assessed using an intravenous glucose tolerance test. [U-(13)C]palmitate was infused for 4 h before and throughout 1.5 h of treadmill walking at 50% VO(2(max)). IMTG concentration was measured by gas chromatograph/mass spectrometer, and FSR by gas chromatography-combustion isotope ratio mass spectrometer, from muscle biopsies taken immediately before and after exercise. Basal IMTG concentration was higher (43 +/- 5.7 vs. 27 +/- 3.9 mg/mg dry weight, P = 0.03) and FSR trended lower (0.23 +/- 0.04 vs. 0.32 +/- 0.05/h, P = 0.075), as did insulin action (S(i); 2.9 +/- 0.43 vs. 3.3 +/- 0.35 x 10(-4)/mU/ml, P = 0.07), in PD vs. NGT. IMTG concentration did not change significantly during exercise, but was no longer different in PD vs. NGT (45 +/- 7.7 vs. 37 +/- 5.8 mg/mg dry weight, P = 0.41). IMTG FSR suppressed during exercise in NGT (-81% to 0.06 +/- 0.13/h, P = 0.02), but not PD (+4% to 0.24 +/- 0.13%/h, P = 0.95). Palmitate oxidation was similar during rest (P = 0.92) and exercise (P = 0.94) between groups, but its source appeared different with more coming from muscle at rest and plasma during exercise in NGT, whereas the converse was true in PD. Altogether, higher basal IMTG concentration that is metabolically inflexible distinguishes obese people with PD from those with NGT.

Increased whole-body adiposity without a concomitant increase in liver fat is not associated with augmented metabolic dysfunction

Aim of this study was to determine whether an increase in adiposity, without a concomitant increase in intrahepatic triglyceride (IHTG) content, is associated with a deterioration in metabolic function. To this end, multiorgan insulin sensitivity, assessed by using a two-stage hyperinsulinemic-euglycemic clamp procedure in conjunction with stable isotopically labeled tracer infusion, and very low-density lipoprotein (VLDL) kinetics, assessed by using stable isotopically labeled tracer infusion and mathematical modeling, were determined in 10 subjects with class I obesity (BMI: 31.6 +/- 0.3 kg/m(2); 37 +/- 2% body fat; visceral adipose tissue (VAT): 1,225 +/- 144 cm(3)) and 10 subjects with class III obesity (BMI: 41.5 +/- 0.5 kg/m(2); 43 +/- 2% body fat; VAT: 2,121 +/- 378 cm(3)), matched on age, sex, and IHTG content (14 +/- 4 and 14 +/- 3%, respectively). No differences between class I and class III obese groups were detected in insulin-mediated suppression of palmitate (67 +/- 3 and 65 +/- 3%, respectively; P = 0.635) and glucose (67 +/- 3 and 73 +/- 5%, respectively; P = 0.348) rates of appearance in plasma, and the insulin-mediated increase in glucose disposal (218 +/- 18 and 193 +/- 30%, respectively; P = 0.489). In addition, no differences between class I and class III obese groups were detected in secretion rates of VLDL-triglyceride (6.5 +/- 1.0 and 6.0 +/- 1.4 micromol/l x min, respectively; P = 0.787) and VLDL-apolipoprotein B-100 (0.40 +/- 0.05 and 0.41 +/- 0.04 nmol/l x min, respectively; P = 0.866), and plasma clearance rates of VLDL-triglyceride (31 (16-59) and 29 (18-46) ml/min, respectively; P = 0.888) and VLDL-apolipoprotein B-100 (15 (11-19) and 17 (11-25) ml/min, respectively; P = 0.608). We conclude that increased adiposity without a concomitant increase in IHTG content does not cause additional abnormalities in adipose tissue, skeletal muscle, and hepatic insulin sensitivity, or VLDL metabolism.

Tauroursodeoxycholic Acid may improve liver and muscle but not adipose tissue insulin sensitivity in obese men and women

OBJECTIVE

Insulin resistance is commonly associated with obesity. Studies conducted in obese mouse models found that endoplasmic reticulum (ER) stress contributes to insulin resistance, and treatment with tauroursodeoxycholic acid (TUDCA), a bile acid derivative that acts as a chemical chaperone to enhance protein folding and ameliorate ER stress, increases insulin sensitivity. The purpose of this study was to determine the effect of TUDCA therapy on multiorgan insulin action and metabolic factors associated with insulin resistance in obese men and women.

RESEARCH DESIGN AND METHODS

Twenty obese subjects ([means +/- SD] aged 48 +/- 11 years, BMI 37 +/- 4 kg/m2) were randomized to 4 weeks of treatment with TUDCA (1,750 mg/day) or placebo. A two-stage hyperinsulinemic-euglycemic clamp procedure in conjunction with stable isotopically labeled tracer infusions and muscle and adipose tissue biopsies were used to evaluate in vivo insulin sensitivity, cellular factors involved in insulin signaling, and cellular markers of ER stress. RESULTS Hepatic and muscle insulin sensitivity increased by approximately 30% (P < 0.05) after treatment with TUDCA but did not change after placebo therapy. In addition, therapy with TUDCA, but not placebo, increased muscle insulin signaling (phosphorylated insulin receptor substrate(Tyr) and Akt(Ser473) levels) (P < 0.05). Markers of ER stress in muscle or adipose tissue did not change after treatment with either TUDCA or placebo.

CONCLUSIONS

These data demonstrate that TUDCA might be an effective pharmacological approach for treating insulin resistance. Additional studies are needed to evaluate the target cells and mechanisms responsible for this effect.

Estrogen deficiency after menopause does not result in male very-low-density lipoprotein metabolism phenotype

CONTEXT

Sex differences in lipid metabolism result in a less proatherogenic plasma lipid profile in premenopausal women than men. The mechanisms responsible for this are unclear but are thought to be related to differences in the sex hormone milieu in men and women.

OBJECTIVE

Our objective was to evaluate the effect of endogenous sex hormones on very-low-density lipoprotein (VLDL) triglyceride (TG) and apolipoprotein B-100 (apoB-100) metabolism. EXPERIMENTAL DESIGN AND MAIN OUTCOME MEASURES: We measured basal VLDL-TG and VLDL-apoB-100 concentrations and kinetics by using stable isotope-labeled tracers.

SETTING AND PARTICIPANTS

Eight premenopausal women [age, 43 + or - 8 yr; body mass index (BMI), 35 + or - 4 kg/m(2); mean + or - sd], eight postmenopausal women (age, 55 + or - 4 yr; BMI, 34 + or - 4 kg/m(2)), and eight men (age, 41 + or - 13 yr; BMI, 34 + or - 4 kg/m(2)) were studied at Washington University School of Medicine, St. Louis, MO.

RESULTS

VLDL-TG secretion rate was approximately double (P < 0.05) in postmenopausal women and men compared with premenopausal women but not different in postmenopausal women and men. The secretion rate of VLDL-apoB-100 was not different in pre- and postmenopausal women but was greater (P < 0.05) in men than in women.

CONCLUSIONS

Endogenous ovarian sex steroids are responsible for sexual dimorphism in VLDL-TG secretion, whereas VLDL-apoB-100 secretion is not regulated by female reproductive hormones.

Dissociation between intrahepatic triglyceride content and insulin resistance in familial hypobetalipoproteinemia

BACKGROUND & AIMS

Hepatic steatosis is associated with insulin resistance, but it is not clear whether increased intrahepatic triglyceride (IHTG) content causes the resistance or is a marker. Subjects with familial hypobetalipoproteinemia (FHBL) have high levels of IHTG because of a genetic defect in hepatic export of triglycerides, and provide a unique cohort to study the relationship between steatosis and insulin sensitivity.

METHODS

One group of lean subjects with normal IHTG content (2.2% +/- 0.6% of liver volume) (n = 6), and 3 groups of overweight and obese subjects matched for body mass index, were studied: (1) normal IHTG content (3.3% +/- 0.5%; n = 6), (2) high IHTG content (21.4% +/- 2.6%) due to nonalcoholic fatty liver disease (NAFLD; n = 6), and (3) high IHTG content (18.1% +/- 2.2%) due to FHBL (n = 3). A hyperinsulinemic-euglycemic clamp procedure, in conjunction with glucose tracer infusion, was used to determine multiorgan insulin sensitivity.

RESULTS

Hepatic insulin sensitivity (reciprocal of glucose rate of appearance [micromol x kg fat-free mass(-1) x min(-1)] x insulin [mU/L]) was greatest in the Lean group (2.0 +/- 0.4); it was the same among subjects with FHBL (0.8 +/- 0.1) and the group with normal IHTG content, matched for body mass index (0.7 +/- 0.1), but greater than the NAFLD group (0.3 +/- 0.1) (P < .01). Muscle insulin sensitivity (percent increase in glucose uptake during insulin infusion) was greatest in the Lean group (576% +/- 70%). Muscle insulin sensitivity was similar in subjects with FHBL and those with normal IHTG (319% +/- 77%, 326% +/- 27%, respectively), but greater than the NAFLD group (145% +/- 18%) (P < .01).

CONCLUSIONS

Steatosis is dissociated from insulin resistance in FHBL, which suggests that increased IHTG content is a marker, not a cause, of metabolic dysfunction.

Decrease in hepatic very-low-density lipoprotein-triglyceride secretion after weight loss is inversely associated with changes in circulating leptin

AIM

Although weight loss usually decreases very-low-density lipoprotein-triglyceride (VLDL-TG) secretion rate, the change in VLDL-TG kinetics is not directly related to the change in body weight. Circulating leptin also declines with weight loss and can affect hepatic lipid metabolism. The aim of this study was to determine whether circulating leptin is associated with weight loss-induced changes in VLDL-TG secretion.

METHODS

Ten extremely obese subjects were studied. VLDL-TG secretion rate and the contribution of systemic (derived from lipolysis of subcutaneous adipose tissue TG) and non-systemic fatty acids (derived primarily from lipolysis of intrahepatic and intraperitoneal TG, and de novo lipogenesis) to VLDL-TG production were determined by using stable isotopically labelled tracer methods before and 1 year after gastric bypass surgery.

RESULTS

Subjects lost 33 +/- 12% of body weight, and VLDL-TG secretion rate decreased by 46 +/- 23% (p = 0.001), primarily because of a decrease in the secretion of VLDL-TG from non-systemic fatty acids (p = 0.002). Changes in VLDL-TG secretion rates were not significantly related to reductions in body weight, body mass index, plasma palmitate flux, free fatty acid or insulin concentrations. The change in VLDL-TG secretion was inversely correlated with the change in plasma leptin concentration (r = -0.72, p = 0.013), because of a negative association between changes in leptin and VLDL-TG secretion from non-systemic fatty acids (r = -0.95, p < 0.001).

CONCLUSIONS

Weight loss-induced changes in plasma leptin concentration are inversely associated with changes in VLDL-TG secretion rate. Additional studies are needed to determine whether the correlation between circulating leptin and VLDL-TG secretion represents a cause-and-effect relationship.

Rapid measurement of plasma free fatty acid concentration and isotopic enrichment using LC/MS

Measurements of plasma free fatty acids (FFA) concentration and isotopic enrichment are commonly used to evaluate FFA metabolism. Until now, gas chromatography-combustion-isotope ratio mass spectrometry (GC/C/IRMS) was the best method to measure isotopic enrichment in the methyl derivatives of (13)C-labeled fatty acids. Although IRMS is excellent for analyzing enrichment, it requires time-consuming derivatization steps and is not optimal for measuring FFA concentrations. We developed a new, rapid, and reliable method for simultaneous quantification of (13)C-labeled fatty acids in plasma using high-performance liquid chromatography-mass spectrometry (HPLC/MS). This method involves a very quick Dole extraction procedure and direct injection of the samples on the HPLC system. After chromatographic separation, the samples are directed to the mass spectrometer for electrospray ionization (ESI) and analysis in the negative mode using single ion monitoring. By employing equipment with two columns connected parallel to a mass spectrometer, we can double the throughput to the mass spectrometer, reducing the analysis time per sample to 5 min. Palmitate flux measured using this approach agreed well with the GC/C/IRMS method. This HPLC/MS method provides accurate and precise measures of FFA concentration and enrichment.

Increased intramuscular lipid synthesis and low saturation relate to insulin sensitivity in endurance-trained athletes

Intramuscular triglyceride (IMTG) has received considerable attention as a potential mechanism promoting insulin resistance. Endurance-trained athletes have high amounts of IMTG but are insulin sensitive, suggesting IMTG content alone does not change insulin action. Recent data suggest increased muscle lipid synthesis protects against fat-induced insulin resistance. We hypothesized that rates of IMTG synthesis at rest would be increased in athletes compared with controls. Eleven sedentary men and 11 endurance-trained male cyclists participated in this study. An intravenous glucose tolerance test was performed to assess insulin action. After 3 days of dietary control and an overnight fast, [13C16]palmitate was infused at 0.0174 micromol.kg(-1).min(-1) for 4 h, followed by a muscle biopsy to measure isotope incorporation into IMTG and diacylglycerol. Compared with controls, athletes were twice as insulin sensitive (P=0.004) and had a significantly greater resting IMTG concentration (athletes: 20.4+/-1.6 microg IMTG/mg dry wt, controls: 14.5+/-1.8 microg IMTG/mg dry wt, P=0.04) and IMTG fractional synthesis rate (athletes: 1.56+/-0.37%/h, controls: 0.61+/-0.15%/h, P=0.03). Stearoyl-CoA desaturase 1 mRNA expression (P=0.02) and protein content (P=0.03) were also significantly greater in athletes. Diacylglycerol, but not IMTG, saturation was significantly less in athletes compared with controls (P=0.002). These data indicate endurance-trained athletes have increased synthesis rates of skeletal muscle IMTG and decreased saturation of skeletal muscle diacylglycerol. Increased synthesis rates are not due to recovery from exercise and are likely adaptations to chronic endurance exercise training.

Insulin acutely inhibits intestinal lipoprotein secretion in humans in part by suppressing plasma free fatty acids

OBJECTIVE

Intestinal lipoprotein production has recently been shown to be increased in insulin resistance, but it is not known whether it is regulated by insulin in humans. Here, we investigated the effect of acute hyperinsulinemia on intestinal (and hepatic) lipoprotein production in six healthy men in the presence and absence of concomitant suppression of plasma free fatty acids (FFAs).

RESEARCH DESIGN AND METHODS

Each subject underwent the following three lipoprotein turnover studies, in random order, 4-6 weeks apart: 1) insulin and glucose infusion (euglycemic-hyperinsulinemic clamp) to induce hyperinsulinemia, 2) insulin and glucose infusion plus Intralipid and heparin infusion to prevent the insulin-induced suppression of plasma FFAs, and 3) saline control.

RESULTS

VLDL1 and VLDL2-apoB48 and -apoB100 production rates were suppressed by 47-62% by insulin, with no change in clearance. When the decline in FFAs was prevented by concomitant infusion of Intralipid and heparin, the production rates of VLDL1 and VLDL2-apoB48 and -apoB100 were intermediate between insulin and glucose infusion and saline control.

CONCLUSIONS

This is the first demonstration in humans that intestinal apoB48-containing lipoprotein production is acutely suppressed by insulin, which may involve insulin's direct effects and insulin-mediated suppression of circulating FFAs.