Mechanisms of fatty acid-induced inhibition of glucose uptake

FFA cause hepatic insulin resistance by inhibiting insulin suppression of glycogenolysis

Free fatty acids (FFA) have been shown to inhibit insulin suppression of endogenous glucose production (EGP). To determine whether this is the result of stimulation by FFA of gluconeogenesis (GNG) or glycogenolysis (GL) or a combination of both, we have determined rates of GNG and GL (with (2)H(2)O) and EGP in 16 healthy nondiabetic volunteers (11 males, 5 females) during euglycemic-hyperinsulinemic (~450 pM) clamping performed either with or without simultaneous intravenous infusion of lipid plus heparin. During insulin infusion, FFA decreased from 571 to 30 micromol/l (P < 0.001), EGP from 15.7 to 2.0 micromol x kg(-1) x min(-1) (P < 0.01), GNG from 8.2 to 3.7 micromol x kg(-1). min(-1) (P < 0.05), and GL from 7.4 to -1.7 micromol x kg(-1). min(-1) (P < 0.02). During insulin plus lipid/heparin infusion, FFA increased from 499 to 1,247 micromol/l (P < 0.001). EGP decreased 64% less than during insulin alone (-5.1 +/- 0.7 vs. -13.7 +/- 3.4 micromol x kg(-1). min(-1)). The decrease in GNG was not significantly different from the decrease of GNG during insulin alone (-2.6 vs. -4.5 micromol x kg(-1). min(-1), not significant). In contrast, GL decreased 66% less than during insulin alone (-3.1 vs. -9.2 micromol x kg(-1). min(-1), P < 0.05). We conclude that insulin suppressed EGP by inhibiting GL more than GNG and that elevated plasma FFA levels attenuated the suppression of EGP by interfering with insulin suppression of GL.

Glycemic control determines hepatic and peripheral glucose effectiveness in type 2 diabetic subjects

Glucose effectiveness is impaired in type 2 diabetes. We hypothesize that chronic hyperglycemia and hyperlipidemia contribute importantly to this defect. To test this hypothesis, we compared the effect of acute hyperglycemia on glucose turnover in type 2 diabetic subjects in good control (GC) (n = 14, age 51.7 +/- 3.7 years, BMI 28.4 +/- 1.0 kg/ m(2), HbA(1c) 5.9 +/- 0.2%) and poor control (PC) (n = 10, age 50.0 +/- 2.5 years, BMI 27.9 +/- 1.5 kg/m(2), HbA(1c) 9.9 +/- 0.6%) with age- and weight-matched nondiabetic subjects (ND) (n = 11, age 47.0 +/- 4.4 years, BMI 28.5 +/- 1.0 kg/m(2), HbA(1c) 5.1 +/- 0.2%). Fixed hormonal conditions were attained by infusing somatostatin for 6 h with replacement of basal insulin, glucagon, and growth hormone. Glucose fluxes ([3-(3)H]glucose) were compared during euglycemic (5 mmol/l, t = 180-240 min) and hyperglycemic (Hy) (10 mmol/l, t = 300-360 min, variable glucose infusion) clamp intervals. Acute hyperglycemia suppressed hepatic glucose production (GP) by 43% and increased peripheral glucose uptake (GU) by 86% in the ND subjects. Conversely, GP failed to suppress (-7%) and GU was suboptimally increased (+34%) in response to Hy in the PC group. However, optimal glycemic control was associated with normal glucose effectiveness in GC subjects (GP -38%, GU +72%; P > 0.05 for GC vs. ND). To determine whether short-term correction of hyperglycemia and/or hyperlipidemia is sufficient to reverse the impairment in glucose effectiveness, five PC subjects were restudied after 72 h of normoglycemia ( approximately 100 mg/dl; variable insulin infusions). These subjects regained normal effectiveness of glucose to suppress GP and stimulate GU and in response to Hy (GP -47%, GU + 71%; P > 0.05 vs. baseline studies). Thus, chronic hyperglycemia and/or hyperlipidemia contribute to impaired effectiveness of glucose in regulating glucose fluxes in type 2 diabetes and hence to worsening of the overall metabolic condition. Short-term normalization of plasma glucose might break the vicious cycle of impaired glucose effectiveness in type 2 diabetes.

Incorporation of beta cell redifferentiation therapy into a lipoprivic strategy for reversing type 2 diabetes

Type 2 diabetes (non-insulin-dependent diabetes mellitus, NIDDM), at least in the majority of patients characterized by insulin resistance and increased visceral fat, appears to be precipitated by the exposure of tissues to excessive levels of free fatty acids; this can contribute to the muscle insulin resistance, excessive hepatic gluconeogenesis, and beta cell dysfunction that collaborate to impair glycemic control. The resultant hyperglycemia, in turn, exacerbates the insulin resistance and beta cells dysfunction. The failure of glucose-stimulated insulin secretion (GSIS) in beta cells helps to sustain the elevations of serum glucose and free fatty acids, which in turn reinforce the failure of GSIS, possibly by inhibiting expression of the transcription factor IDX-1; NIDDM thus represents a vicious cycle that is not easily broken. A new strategy for achieving rapid loss of body fat - hepatothermic therapy (HT), an integrated approach involving exercise training, low-fat, low-glycemic-index food choices, and a supplementation program that promotes hepatic fatty acid oxidation - shows promise for alleviating the excessive fat exposure at the root of the diabetic syndrome, as well as the underlying insulin resistance syndrome responsible for increased macrovascular risk. However, when HT proves incapable of breaking the vicious cycle sustaining beta cell dysfunction, a supplementary strategy, beta cell redifferentiation therapy (BRT), may be required. BRT consists of a protocol in which near-normoglycemia is maintained for several weeks through use of intensive insulin therapy (e.g. artificial pancreas) or other effective measures, during which time beta cell GSIS can be expected to substantially recover owing to relief from glucolipotoxicity. Clinical experience demonstrates that this improved beta cell function, in certain cases, can persist for months or years after temporary BRT. A portion of the improved glycemic control achieved with very low calorie diets in NIDDM is reflective of improved GSIS, presumably consequent to a sustained reduction in diurnal glycemia. Long-lived analogs of glucagon-like peptide-1 (GLP-1) may find a key role in BRT; this incretin hormone not only potentiates GSIS, but also appears to increase the expression and activity of IDX-1 in beta cells, thus promoting beta cell redifferentiation. If HT is instituted prior to and following BRT to alleviate the FFA overexposure that initially precipitated the diabetic syndrome, it seems likely that the benefits of BRT will be conserved in the long term, thus enabling a reversal of NIDDM - in other words, maintenance of normoglycemia without medication. Since NIDDM is inherently preventable, its reversal should be the fundamental goal of diabetes therapy.

Inhibition of glycogen synthesis by fatty acid in C(2)C(12) muscle cells is independent of PKC-alpha, -epsilon, and -theta

We have previously shown that glycogen synthesis is reduced in lipid-treated C(2)C(12) skeletal muscle myotubes and that this is independent of changes in glucose uptake. Here, we tested whether mitochondrial metabolism of these lipids is necessary for this inhibition and whether the activation of specific protein kinase C (PKC) isoforms is involved. C(2)C(12) myotubes were pretreated with fatty acids and subsequently stimulated with insulin for the determination of glycogen synthesis. The carnitine palmitoyltransferase-1 inhibitor etomoxir, an inhibitor of beta-oxidation of acyl-CoA, did not protect against the inhibition of glycogen synthesis caused by the unsaturated fatty acid oleate. In addition, although oleate caused translocation, indicating activation, of individual PKC isoforms, inhibition of PKC by pharmacological agents or adenovirus-mediated overexpression of dominant negative PKC-alpha, -epsilon, or -theta mutants was unable to prevent the inhibitory effects of oleate on glycogen synthesis. We conclude that neither mitochondrial lipid metabolism nor activation of PKC-alpha, -epsilon, or -theta plays a role in the direct inhibition of glycogen synthesis by unsaturated fatty acids.

Skeletal muscle triglyceride: marker or mediator of obesity-induced insulin resistance in type 2 diabetes mellitus?

The inability of insulin to stimulate glucose metabolism in skeletal muscle is a classic characteristic of type 2 diabetes, but this insulin resistance entails altered patterns of lipid metabolism as well. An association between intracellular triglyceride and insulin resistance has been well established in both human and animal studies of obesity-related insulin resistance and type 2 diabetes. Skeletal muscle's ability to select substrate for fuel metabolism, a metabolic flexibility, is also lost in insulin resistance, and defects in fatty acid metabolism during fasting or postabsorptive conditions likely play an important role in lipid oversupply to insulin-resistant muscle. These impairments appear to be at least indirectly centered on the ability of mitochondria to oxidize fatty acids, possibly through mediation of lipid metabolite levels such as ceramide or diacylglycerol, which are known to directly attenuate insulin signaling. Moreover, periodic use of muscle triglyceride by exercise may mediate the association between muscle triglyceride and insulin resistance. Thus, it appears that skeletal muscle triglyceride is perhaps a surrogate for other lipid species having a more direct effect on insulin action. Defining mechanisms by which dysregulation of fatty acid metabolism and persistent lipid oversupply alter insulin action may help to target more effective strategies to prevent or treat type 2 diabetes.

Female rats do not exhibit free fatty acid-induced insulin resistance

It is well described that excessive lipid metabolism can cause insulin resistance in both animals and humans, and this has been implicated as a causative factor in the development of insulin resistance and type 2 diabetes in humans. Recently, we have shown that intravenous lipid emulsion (liposyn) infusion during a 120-min euglycemic-hyperinsulinemic clamp led to significant reductions in insulin action and fatty acid translocase (FAT/CD36) skeletal muscle protein expression. After reviewing the literature, it became evident that essentially all past studies, including our own, were conducted in male animals. Therefore, to determine whether there were sex determinants of fat-induced insulin resistance, we assessed the impact of free fatty acid (FFA) elevation on insulin action in female rats. Here, we report that a fourfold elevation in plasma FFA concentration induced a 40% reduction in the insulin-stimulated glucose disposal rate, a 30% decline in insulin-stimulated skeletal muscle insulin substrate receptor-1 (IRS-1) phosphorylation, a 48% decrease in IRS-1-associated phosphatidylinositol (PI) 3-kinase activity, and a 50% reduction in muscle FAT/CD36 protein expression in male rats. In striking contrast, we found no effect of FFA elevation to cause insulin resistance, changes in IRS-1/PI 3-kinase, or FAT/CD36 protein levels in female animals. Our findings indicate that female animals are protected from lipid-induced reductions in insulin action.

Acute elevation of NEFA causes hyperinsulinemia without effect on insulin secretion rate in healthy human subjects

Increased circulating levels of nonesterified free fatty acids (NEFA) have been observed in such hyperinsulinemic states as obesity, impaired glucose tolerance, diabetes, and dyslipidemia where they have been causally linked to the development of insulin resistance and hyperinsulinemia. The concentration of NEFA in plasma is believed to have direct modifying effects on insulin secretion and clearance. It remains controversial whether acute increases in NEFA potentiate insulin secretion in human subjects. We studied the effect of an acute elevation of NEFA during lipid-heparin infusion compared to a glycerol-only control on glucose-stimulated insulin secretion and clearance during a 120-min hyperglycemic (10 mM) clamp in 7 healthy normoglucose-tolerant volunteers. The metabolic clearance rate of C-peptide (MCR(CP)) was measured in each subject during the study by simultaneous infusion of C-peptide. Insulin secretion rate (ISR) was calculated from deconvolution of C-peptide data after correction for the rate of C-peptide infusion. Clearance rate of insulin (MCR(INS)) was calculated based upon endogenous ISR. Plasma glucose (mg/dL): basal (90-115 min) 90.2 +/- 2.8 vs. 90.2 +/- 2.3; clamp (150-240 min) 180.5 +/- 2.8 vs. 180.9 +/- 1.3. Plasma insulin (pmol/L): prebasal (fasting) 29.6 +/- 10.0 vs. 29.8 +/- 10.6; basal (90-115 min) 30.1 +/- 9.2 vs. 34.5 +/- 12.1; second phase clamp (210-240 min) 127.6 +/- 18.2 vs. 182.5 +/- 17.3*. Plasma NEFA (mM): prebasal 0.47 +/- 0.08 vs. 0.52 +/- 0.09; basal 0.35 +/- 0.05 vs. 0.98 +/- 0.02*; clamp (122-240 min) 0.06 +/- 0.02 vs. 0.77 +/- 0.06*. ISR (pmol/min): prebasal 72.7 +/- 7.5 vs. 72.0 +/- 7.9; second phase clamp (210-240 min) 268.5 +/- 27.2 vs. 200.2 +/- 23.7. MCR(INS) (mL/min): prebasal 3393 +/- 488 vs. 3370 +/- 511; clamp 2284 +/- 505 vs. 1214 +/- 153* (*p < 0.05 glycerol vs. intralipid/heparin). This study demonstrates that acute NEFA elevation causes hyperinsulinemia due to a significant decrease in systemic insulin clearance without increasing rates of insulin secretion.

Free fatty acids in obesity and type 2 diabetes: defining their role in the development of insulin resistance and beta-cell dysfunction

Plasma free fatty acids (FFA) play important physiological roles in skeletal muscle, heart, liver and pancreas. However, chronically elevated plasma FFA appear to have pathophysiological consequences. Elevated FFA concentrations are linked with the onset of peripheral and hepatic insulin resistance and, while the precise action in the liver remains unclear, a model to explain the role of raised FFA in the development of skeletal muscle insulin resistance has recently been put forward. Over 30 years ago, Randle proposed that FFA compete with glucose as the major energy substrate in cardiac muscle, leading to decreased glucose oxidation when FFA are elevated. Recent data indicate that high plasma FFA also have a significant role in contributing to insulin resistance. Elevated FFA and intracellular lipid appear to inhibit insulin signalling, leading to a reduction in insulin-stimulated muscle glucose transport that may be mediated by a decrease in GLUT-4 translocation. The resulting suppression of muscle glucose transport leads to reduced muscle glycogen synthesis and glycolysis. In the liver, elevated FFA may contribute to hyperglycaemia by antagonizing the effects of insulin on endogenous glucose production. FFA also affect insulin secretion, although the nature of this relationship remains a subject for debate. Finally, evidence is discussed that FFA represent a crucial link between insulin resistance and beta-cell dysfunction and, as such, a reduction in elevated plasma FFA should be an important therapeutic target in obesity and type 2 diabetes.

Defective fatty acid uptake modulates insulin responsiveness and metabolic responses to diet in CD36-null mice

Deficiency of the membrane protein FAT/CD36 causes a marked defect in fatty acid uptake by various tissues and is genetically linked to insulin resistance in rats and humans. Here, we examined insulin responsiveness of CD36-/- mice. When fed a diet high in complex carbohydrates and low (5%) in fat, these animals cleared glucose faster than the wild-type. In vivo, uptake of 2-fluorodeoxyglucose by muscle was increased severalfold, and in vitro, insulin responsiveness of glycogenesis by the soleus was enhanced. Null mice had lower glycogen levels in muscle and liver, lower muscle triglyceride levels, and increased liver triglyceride content--all findings consistent with increased insulin-sensitivity. However, when the chow diet was switched to one high in fructose, CD36-/- mice but not wild-type mice developed marked glucose intolerance, hyperinsulinemia, and decreased muscle glucose uptake. High-fat diets impaired glucose tolerance equally in both groups, although CD36 deficiency helped moderate insulin-responsive muscle glucose oxidation. In conclusion, CD36 deficiency enhances insulin responsiveness on a high-starch, low-fat diet. It predisposes to insulin resistance induced by high fructose and partially protects from that induced by high-fat diets. In humans, CD36 deficiency may be an important factor in the metabolic adaptation to diet and in susceptibility to some forms of diet-induced pathology.

Tumor necrosis factor-alpha suppresses adipocyte-specific genes and activates expression of preadipocyte genes in 3T3-L1 adipocytes: nuclear factor-kappaB activation by TNF-alpha is obligatory

Tumor necrosis factor-alpha (TNF-alpha) is a contributing cause of the insulin resistance seen in obesity and obesity-linked type 2 diabetes, but the mechanism(s) by which TNF-alpha induces insulin resistance is not understood. By using 3T3-L1 adipocytes and oligonucleotide microarrays, we identified 142 known genes reproducibly upregulated by at least threefold after 4 h and/or 24 h of TNF-alpha treatment, and 78 known genes downregulated by at least twofold after 24 h of TNF-alpha incubation. TNF-alpha-induced genes include transcription factors implicated in preadipocyte gene expression or NF-kappaB activation, cytokines and cytokine-induced proteins, growth factors, enzymes, and signaling molecules. Importantly, a number of adipocyte-abundant genes, including GLUT4, hormone sensitive lipase, long-chain fatty acyl-CoA synthase, adipocyte complement-related protein of 30 kDa, and transcription factors CCAAT/enhancer binding protein-alpha, receptor retinoid X receptor-alpha, and peroxisome profilerator-activated receptor gamma were significantly downregulated by TNF-alpha treatment. Correspondingly, 24-h exposure of 3T3-L1 adipocytes to TNF-alpha resulted in reduced protein levels of GLUT4 and several insulin signaling proteins, including the insulin receptor, insulin receptor substrate 1 (IRS-1), and protein kinase B (AKT). Nuclear factor-kappaB (NF-kappaB) was activated within 15 min of TNF-alpha addition. 3T3-L1 adipocytes expressing IkappaBalpha-DN, a nondegradable NF-kappaB inhibitor, exhibited normal morphology, global gene expression, and insulin responses. However, absence of NF-kappaB activation abolished suppression of >98% of the genes normally suppressed by TNF-alpha and induction of 60-70% of the genes normally induced by TNF-alpha. Moreover, extensive cell death occurred in IkappaBalpha-DN-expressing adipocytes after 2 h of TNF-alpha treatment. Thus the changes in adipocyte gene expression induced by TNF-alpha could lead to insulin resistance. Further, NF-kappaB is an obligatory mediator of most of these TNF-alpha responses.

Acyl-coenzyme A dehydrogenases are localized on GLUT4-containing vesicles via association with insulin-regulated aminopeptidase in a manner dependent on its dileucine motif

Insulin-regulated aminopeptidase (IRAP, also termed vp165) is known to be localized on the GLUT4-containing vesicles and to be recruited to the plasma membrane after stimulation with insulin. The cytoplasmic region of IRAP contains two dileucine motifs and acidic regions, one of which (amino acid residues 55-82) is reportedly involved in retention of GLUT4-containing vesicles. The region of IRAP fused with glutathione-S-transferase [GST-IRAP(55-82)] was incubated with lysates from 3T3-L1 adipocytes, leading to identification of long-chain, medium-chain, and short-chain acyl-coenzyme A dehydrogenases (ACDs) as the proteins associated with IRAP. The association was nearly abolished by mutation of the dileucine motif of IRAP. Immunoblotting of fractions prepared from sucrose gradient ultracentrifugation and vesicles immunopurified with anti-GLUT4 antibody revealed these ACDs to be localized on GLUT4-containing vesicles. Furthermore, 3-mercaptopropionic acid and hexanoyl-CoA, inhibitors of long-chain and medium-chain ACDs, respectively, induced dissociation of long-chain acyl-coenzyme A dehydrogenase and/or medium-chain acyl-coenzyme A dehydrogenase from IRAP in vitro as well as recruitment of GLUT4 to the plasma membrane and stimulation of glucose transport activity in permeabilized 3T3-L1 adipocytes. These findings suggest that ACDs are localized on GLUT4-containing vesicles via association with IRAP in a manner dependent on its dileucine motif and play a role in retention of GLUT4-containing vesicles to an intracellular compartment.

Amino acid- and lipid-induced insulin resistance in rat heart: molecular mechanisms

Lipids compete with glucose for utilization by the myocardium. Amino acids are an important energetic substrate in the heart but it is unknown whether they reduce glucose disposal. The molecular mechanisms by which lipids and amino acids impair insulin-mediated glucose disposal in the myocardium are unknown. We evaluated the effect of lipids and amino acids on the insulin stimulated glucose uptake in the isolated rat heart and explored the involved target proteins. The hearts were perfused with 16 mM glucose alone or with 6% lipid or 10% amino acid solutions at the rate of 15 ml/min. After 1 h of perfusion (basal period), insulin (240 nmol/l) was added and maintained for an additional hour. Both lipids and amino acids blocked the insulin effect on glucose uptake (P<0.01) and reduced the activity of the IRSs/PI 3-kinase/Akt/GSK3 axis leading to the activation of glucose transport and glycogen synthesis. Amino acids, but not lipids, increased the activity of the p70 S6 kinase leading to the stimulation of protein synthesis. Amino acids induce myocardial insulin resistance recruiting the same molecular mechanisms as lipids. Amino acids retain an insulin-like stimulatory effect on p70 S6 kinase, which is independent from the PI 3-Kinase downstream effectors.

Disordered fat storage and mobilization in the pathogenesis of insulin resistance and type 2 diabetes

The primary genetic, environmental, and metabolic factors responsible for causing insulin resistance and pancreatic beta-cell failure and the precise sequence of events leading to the development of type 2 diabetes are not yet fully understood. Abnormalities of triglyceride storage and lipolysis in insulin-sensitive tissues are an early manifestation of conditions characterized by insulin resistance and are detectable before the development of postprandial or fasting hyperglycemia. Increased free fatty acid (FFA) flux from adipose tissue to nonadipose tissue, resulting from abnormalities of fat metabolism, participates in and amplifies many of the fundamental metabolic derangements that are characteristic of the insulin resistance syndrome and type 2 diabetes. It is also likely to play an important role in the progression from normal glucose tolerance to fasting hyperglycemia and conversion to frank type 2 diabetes in insulin resistant individuals. Adverse metabolic consequences of increased FFA flux, to be discussed in this review, are extremely wide ranging and include, but are not limited to: 1) dyslipidemia and hepatic steatosis, 2) impaired glucose metabolism and insulin sensitivity in muscle and liver, 3) diminished insulin clearance, aggravating peripheral tissue hyperinsulinemia, and 4) impaired pancreatic beta-cell function. The precise biochemical mechanisms whereby fatty acids and cytosolic triglycerides exert their effects remain poorly understood. Recent studies, however, suggest that the sequence of events may be the following: in states of positive net energy balance, triglyceride accumulation in "fat-buffering" adipose tissue is limited by the development of adipose tissue insulin resistance. This results in diversion of energy substrates to nonadipose tissue, which in turn leads to a complex array of metabolic abnormalities characteristic of insulin-resistant states and type 2 diabetes. Recent evidence suggests that some of the biochemical mechanisms whereby glucose and fat exert adverse effects in insulin-sensitive and insulin-producing tissues are shared, thus implicating a diabetogenic role for energy excess as a whole. Although there is now evidence that weight loss through reduction of caloric intake and increase in physical activity can prevent the development of diabetes, it remains an open question as to whether specific modulation of fat metabolism will result in improvement in some or all of the above metabolic derangements or will prevent progression from insulin resistance syndrome to type 2 diabetes.

Metabolic approaches to the treatment of ischemic heart disease: the clinicians' perspective

This review article discusses pharmacological approaches to optimizing myocardial metabolism during ischemia. Fatty acids are the main fuel for the healthy heart, with a lesser contribution coming from the oxidation of glucose and lactate. Myocardial ischaemia dramatically alters fuel metabolism, causing an accelerated rate of glucose conversion to lactate and a switch from lactate uptake by the heart to lactate production. This causes a dramatic disruption in cell homeostasis (e.g. lactate accumulation and a decrease in pH and ATP). Paradoxically, moderately ischemic tissue (approximately 50% of normal flow) continues to derive most of its energy (50-70%) from the oxidation of fatty acids despite a high rate of lactate production. This ischaemia-induced disruption in cardiac metabolism can be minimized by metabolic agents that reduce fatty acid oxidation and increase the combustion of glucose and lactate, resulting in clinical benefit to the ischemic patient. Agents that inhibit fatty acid beta-oxidation, such as ranolazine and trimetazidine, have proven to be effective in the treatment of stable angina. Treatment of acute myocardial infarction patients with an infusion of the glucose-insulin-potassium, which results in suppression of myocardial fatty acid oxidation and greater glucose combustion, has proven effective in reducing mortality. These metabolic therapies are free of direct hemodynamic or chronotropic effects, and thus are well positioned for use alongside traditional agents such as beta-adrenergic receptor antagonists or calcium channel antagonists.

Effect of a high-protein, high-monounsaturated fat weight loss diet on glycemic control and lipid levels in type 2 diabetes

OBJECTIVE

To determine the effect of a high-protein (HP) weight loss diet compared with a lower-protein (LP) diet on fat and lean tissue and fasting and postprandial glucose and insulin concentrations.

RESEARCH DESIGN AND METHODS

Replacing dietary protein for carbohydrate (CHO) during energy restriction and weight loss has been effective in sparing lean mass and improving insulin sensitivity in obese subjects but has not been tested in subjects with type 2 diabetes. We compared an HP diet (28% protein, 42% CHO, 28% fat [8% saturated fatty acids, 12% monounsaturated fatty acids, 5% polyunsaturated fatty acids]) with an LP diet (16% protein, 55% CHO, 26% fat [8% saturated fatty acids, 11% monounsaturated fatty acids, 5% polyunsaturated fatty acids]) in 54 obese men and women with type 2 diabetes during 8 weeks of energy restriction (1,600 kcal) and 4 weeks of energy balance. Body composition was determined by dual-energy X-ray absorptiometry at weeks 0 and 12.

RESULTS

Overall, weight loss of 5.2 +/- 1.8 kg was achieved independently of diet composition. However, women on the HP diet lost significantly more total (5.3 vs. 2.8 kg, P=0.009) and abdominal (1.3 vs. 0.7 kg, P=0.006) fat compared with the women on the LP diet, whereas, in men, there was no difference in fat loss between diets (3.9 vs. 5.1 kg). Total lean mass decreased in all subjects independently of diet composition. LDL cholesterol reduction was significantly greater on the HP diet (5.7%) than on the LP diet (2.7%) (P < 0.01).

CONCLUSIONS

Both dietary patterns resulted in improvements in the cardiovascular disease (CVD) risk profile as a consequence of weight loss. However, the greater reductions in total and abdominal fat mass in women and greater LDL cholesterol reduction observed in both sexes on the HP diet suggest that it is a valid diet choice for reducing CVD risk in type 2 diabetes.

Increased fatty acid re-esterification by PEPCK overexpression in adipose tissue leads to obesity without insulin resistance

Adipose tissue glyceroneogenesis generates glycerol 3-phosphate, which could be used for fatty acid esterification during starvation. To determine whether increased glyceroneogenesis leads to increased fat mass and to explore the role of obesity in the development of insulin resistance, we overexpressed PEPCK, a regulatory enzyme of glyceroneogenesis in adipose tissue. Transgenic mice showed a chronic increase in PEPCK activity, which led to increased glyceroneogenesis, re-esterification of free fatty acids (FFAs), increased adipocyte size and fat mass, and higher body weight. In spite of increased fat mass, transgenic mice showed decreased circulating FFAs and normal leptin levels. Moreover, glucose tolerance and whole-body insulin sensitivity were preserved. Skeletal muscle basal and insulin-stimulated glucose uptake and glycogen content were not affected, suggesting that skeletal muscle insulin sensitivity is normal in transgenic obese mice. Our results indicate the key role of PEPCK in the control of FFA re-esterification in adipose tissue and, thus, the contribution of glyceroneogenesis to fat accumulation. Moreover, they suggest that higher fat mass without increased circulating FFAs does not lead to insulin resistance or type 2 diabetes in these mice.

Systemic and local adrenergic regulation of muscle glucose utilization during hypoglycemia in healthy subjects

Adrenergic responses are crucial for hypoglycemic recovery. Epinephrine increases glucose production, lipolysis, and peripheral insulin resistance as well as blood flow and glucose delivery. Sympathetic activation causes vasoconstriction and reduces glucose delivery. To determine the effects of alpha- and beta-adrenergic activity on muscle glucose uptake during hypoglycemia, we studied forearm blood flow (FBF) (plethysmography), arteriovenous glucose difference (AV-diff), and forearm glucose uptake (FGU) during insulin infusion with 60 min of euglycemia followed by 60 min of hypoglycemia. Twelve healthy subjects (27 plus minus 5 years of age) were randomized to intravenous propranolol (IV PROP, 80 microg/min), intravenous phentolamine (IV PHEN, 500 microg/min), intra-arterial propranolol (IA PROP, 25 microg/min), intra-arterial phentolamine (IA PHEN, 12 microg/min per 100 ml forearm tissue), and saline (SAL). FBF increased during hypoglycemia with SAL (P < 0.001) but not with IA or IV PROP. FGU (P = 0.015) and AV-diff (P = 0.099) fell during hypoglycemia with IA PROP but not with IV PROP. FBF increased during hypoglycemia with IA and IV PHEN (P < 0.005). AV-diff fell during hypoglycemia with IA and IV PHEN (P < 0.01), but FGU was unchanged. Blood pressure fell (P < 0.001), and adrenergic and neuroglycopenic symptoms increased with IV PHEN (P < 0.01). Thus, systemic but not local propranolol prevents a decrease in forearm glucose extraction during hypoglycemia, suggesting that epinephrine increases peripheral muscular insulin resistance through systemic effects. alpha-Adrenergic activation inhibits vasodilation and helps maintain brain glucose delivery.

Plasma free fatty acid levels and the risk of ischemic heart disease in men: prospective results from the Québec Cardiovascular Study

Insulin resistance, through numerous related disturbances in glucose and lipoprotein-lipid metabolism, is associated with an increased risk of ischemic heart disease (IHD). The purpose of the present study was to examine the relationship between increased plasma free fatty acid (FFA) concentrations, as a feature of the insulin resistance syndrome, and the risk of IHD in men. Analyses were carried out in a nested, case-control sample of men selected from a population of 2103 individuals without IHD at baseline among whom 114 developed IHD during a 5-year follow-up period. Incident IHD cases were matched with controls for age, body mass index, smoking habits and alcohol intake. Analyses were performed while excluding (88 cases and 98 controls) and including (103 cases and 99 controls) patients with type 2 diabetes. Among non-diabetic individuals, elevated plasma FFA concentrations (3rd tertile of the distribution) yielded a twofold increase in the risk of IHD (odds ratio [OR] 2.1, P=0.05) compared with lower plasma FFA levels (lowest tertile) after adjusting for non-lipid risk factors. Further adjustment for insulin, triglycerides, apolipoprotein B, HDL cholesterol and small dense LDL attenuated significantly the relationship between plasma FFA concentrations and the risk of IHD. High plasma FFA levels showed no synergism with selected features of the insulin resistance syndrome in determining the risk of IHD. Inclusion of diabetic subjects in the study did not improve FFA independent prognostic value to the risk of IHD. These results suggest that elevated plasma FFA concentrations are associated with an increased risk of IHD. However, a single fasting measurement of plasma FFA levels does not appear to improve our ability to predict IHD onset in men when information on other risk factors is considered.

Effects of free fatty acids and glycerol on splanchnic glucose metabolism and insulin extraction in nondiabetic humans

The present study sought to determine whether elevated plasma free fatty acids (FFAs) alter the ability of insulin and glucose to regulate splanchnic as well as muscle glucose metabolism. To do so, FFAs were increased in 10 subjects to approximately 1 mmol/l by an 8-h Intralipid/heparin (IL/Hep) infusion, whereas they fell to levels near the detection limit of the assay (<0.05 mmol/l) in 13 other subjects who were infused with glycerol alone at rates sufficient to either match (n = 5, low glycerol) or double (n = 8, high glycerol) the plasma glycerol concentrations observed during the IL/Hep infusion. Glucose was clamped at approximately 8.3 mmol/l, and insulin was increased to approximately 300 pmol/l to stimulate both muscle and hepatic glucose uptake. Insulin secretion was inhibited with somatostatin. Leg and splanchnic glucose metabolism were assessed using a combined catheter and tracer dilution approach. Leg glucose uptake (21.7 +/- 3.5 vs. 48.3 +/- 9.3 and 57.8 +/- 11.7 micromol x kg(-1) leg x min(-1)) was lower (P < 0.001) during IL/Hep than the low- or high-glycerol infusions, confirming that elevated FFAs caused insulin resistance in muscle. IL/Hep did not alter splanchnic glucose uptake or the contribution of the extracellular direct pathway to UDP-glucose flux. On the other hand, total UDP-glucose flux (13.2 +/- 1.7 and 12.5 +/- 1.0 vs. 8.1 +/- 0.5 micromol x kg(-1) x min(-1)) and flux via the indirect intracellular pathway (8.4 +/- 1.2 and 8.1 +/- 0.6 vs. 4.8 +/- 0.05 micromol x kg(-1) x min(-1)) were greater (P < 0.05) during both the IL/Hep and high-glycerol infusions than the low-glycerol infusion. In contrast, only IL/Hep increased (P < 0.05) splanchnic glucose production, indicating that elevated FFAs impaired the ability of the liver to autoregulate. Splanchnic insulin extraction, directly measured using the arterial and hepatic vein catheters, did not differ (67 +/- 3 vs. 71 +/- 5 vs. 69 +/- 1%) during IL/Hep and high- and low-glycerol infusions. We conclude that elevated FFAs exert multiple effects on glucose metabolism. They inhibit insulin- and glucose-induced stimulation of muscle glucose uptake and suppression of splanchnic glucose production. They increase the contribution of the indirect pathway to glycogen synthesis and impair hepatic autoregulation. On the other hand, they do not alter either splanchnic glucose uptake or splanchnic insulin extraction in nondiabetic humans.

Accurate assessment of beta-cell function: the hyperbolic correction

Only in the last decade did modeling studies predict, and knockout experiments confirm, that type 2 diabetes is a "2-hit" disease in which insulin resistance is necessarily accompanied by beta-cell defect(s) preventing the compensatory upregulation of insulin secretion. This long- delayed insight was associated with the development of a constant, the "disposition index," describing the beta-cell sensitivity-secretion relationship as a rectangular hyperbola. Shifts in insulin sensitivity are accompanied by compensatory alterations in beta-cell sensitivity to glucose. Insulin-sensitive subjects do not require a massive insulin response to exogenous glucose to maintain a normal blood glucose. But if their insulin sensitivity decreases by 80%, as in late pregnancy, they need a fivefold greater insulin response to achieve an identical disposition index. Women with gestational diabetes have an insulin response similar to that of normal volunteers; at first glance, this suggests similar islet function, but the utility of the disposition index is to normalize this response to the amplitude of third trimester insulin resistance, revealing severe beta-cell deficiency. The index is a quantitative, convenient, and accurate tool in analyzing epidemiologic data and identifying incipient impaired glucose tolerance. Separate major issues remain, however: the causes of insulin resistance, the causes of the failure of adequate beta-cell compensation in type 2 diabetes, and the nature of the signal(s) from insulin-resistant tissues that fail to elicit the appropriate beta-cell increment in sensitivity to glucose and other stimuli. The disposition index is likely to remain the most accurate physiologic measure for testing hypotheses and solutions to these challenges in whole organisms.

High dietary lipids induce liver glucose-6-phosphatase expression in rainbow trout (Oncorhynchus mykiss)

To contribute to the understanding of the mechanisms involved in poor metabolic utilization of dietary carbohydrates by rainbow trout (Oncorhynchus mykiss), we explored in this study the effects of dietary lipids on the regulation of two hepatic key enzymes, i.e., glucokinase (GK, first enzyme of the glycolytic pathway) and glucose-6-phosphatase (G6Pase, last enzyme of the gluconeogenesis). Two groups of juvenile trout were pair-fed for 8 wk either a low (10%) or a high (25%) level of dietary lipids supplied as fish oil; the pair-feeding technique was adopted to vary fat intake while keeping the protein and carbohydrate intakes more or less constant. Fish fed the high level of dietary lipids had inefficient control of glycemia compared with fish fed the low level of lipids. Levels of dietary lipids did not affect GK activity even though there was a small increase of GK mRNA level at 3 h after feeding high levels of lipids. By contrast, the high level of dietary lipids significantly increased G6Pase mRNA expression at 3, 6 and 12 h and enzyme activity at 6 h after food consumption. Thus, these data suggest that poor dietary carbohydrate utilization in rainbow trout may be related at least in part to increased hepatic glucose production under conditions of high dietary fat intake.

Increasing fructose 2,6-bisphosphate overcomes hepatic insulin resistance of type 2 diabetes

Hepatic glucose production is increased as a metabolic consequence of insulin resistance in type 2 diabetes. Because fructose 2,6-bisphosphate is an important regulator of hepatic glucose production, we used adenovirus-mediated enzyme overexpression to increase hepatic fructose 2,6-bisphosphate to determine if the hyperglycemia in KK mice, polygenic models of type 2 diabetes, could be ameliorated by reduction of hepatic glucose production. Seven days after treatment with virus encoding a mutant 6-phosphofructo-2-kinase/fructose-2,6-bisphosphatase designed to increase fructose 2,6-bisphosphate levels, plasma glucose, lipids, and insulin were significantly reduced in KK/H1J and KK.Cg-A(y)/J mice. Moreover, high fructose 2,6-bisphosphate levels downregulated glucose-6-phosphatase and upregulated glucokinase gene expression, thereby reversing the insulin-resistant pattern of hepatic gene expression of these two key glucose-metabolic enzymes. The increased hepatic fructose 2,6-bisphosphate also reduced adiposity in both KK mice. These results clearly indicate that increasing hepatic fructose 2,6-bisphosphate overcomes the impairment of insulin in suppressing hepatic glucose production, and it provides a potential therapy for type 2 diabetes.

Endogenous glucose production is inhibited by the adipose-derived protein Acrp30

Intraperitoneal injection of purified recombinant Acrp30 lowers glucose levels in mice. To gain insight into the mechanism(s) of this hypoglycemic effect, purified recombinant Acrp30 was infused in conscious mice during a pancreatic euglycemic clamp. In the presence of physiological hyperinsulinemia, this treatment increased circulating Acrp30 levels by approximately twofold and stimulated glucose metabolism. The effect of Acrp30 on in vivo insulin action was completely accounted for by a 65% reduction in the rate of glucose production. Similarly, glucose flux through glucose-6-phosphatase (G6Pase) decreased with Acrp30, whereas the activity of the direct pathway of glucose-6-phosphate biosynthesis, an index of hepatic glucose phosphorylation, increased significantly. Acrp30 did not affect the rates of glucose uptake, glycolysis, or glycogen synthesis. These results indicate that an acute increase in circulating Acrp30 levels lowers hepatic glucose production without affecting peripheral glucose uptake. Hepatic expression of the gluconeogenic enzymes phosphoenolpyruvate carboxykinase and G6Pase mRNAs was reduced by more than 50% following Acrp30 infusion compared with vehicle infusion. Thus, a moderate rise in circulating levels of the adipose-derived protein Acrp30 inhibits both the expression of hepatic gluconeogenic enzymes and the rate of endogenous glucose production.

Pathogenesis of type 2 diabetes. Insulin resistance

This article addresses the role of insulin resistance in the pathogenesis of type 2 diabetes. The major causes of acquired insulin resistance including aging, pregnancy, lack of physical activity, and obesity are discussed briefly. The role of free fatty acids (FFAs) as a link between obesity and insulin resistance/type 2 diabetes is discussed in detail. Evidence is provided showing that increased plasma FFA levels produce insulin resistance dosage dependently, acutely, and chronically. Mechanisms by which FFA can cause insulin resistance are outlined. Lastly, normalizing plasma FFA levels is proposed as a new approach to reducing insulin resistance and the risk for type 2 diabetes mellitus, and atherosclerotic vascular disease.

Plasma fatty acids, adiposity, and variance of skeletal muscle insulin resistance in type 2 diabetes mellitus

Skeletal muscle insulin resistance (IR) is typically severe in type 2 diabetes mellitus (DM). However, the factors that account for interindividual differences in the severity of IR are not well understood. The current study was undertaken to examine the respective roles of plasma FFA, regional adiposity, and other metabolic factors as determinants of the severity of skeletal muscle IR in type 2 DM. Twenty-three subjects (12 women and 11 men) with type 2 DM underwent positron emission tomography imaging using [18F]2-fluoro-2-deoxyglucose during euglycemic insulin infusions (120 mU/min x m2) to measure skeletal muscle IR, using Patlak analysis of the tissue activity curves. Body composition analysis included body mass index, fat mass, and fat-free mass by dual energy x-ray tomography, and computed tomography determinations of visceral adiposity, thigh adipose tissue distribution, and muscle composition. Body mass index, fat mass, subfascial adiposity in the thigh, and visceral adipose tissue (VAT) were all significantly related to skeletal muscle IR (r = -0.48 to -0.63; P < 0.01). However, the strongest simple correlate of IR in skeletal muscle was insulin-suppressed plasma FFA (r = -0.81; P < 0.001). VAT was the sole component of adiposity that significantly correlated with insulin-suppressed plasma FFA concentration (r = 0.64; P < 0.001). These findings indicate that the severity of skeletal muscle IR in type 2 DM is closely related to the IR of suppressing lipolysis and that plasma fatty acids and VAT are key elements mediating the link between obesity and skeletal muscle IR in type 2 DM.

The polymorphism at codon 54 of the FABP2 gene increases fat absorption in human intestinal explants

Based on titration microcalorimetry and Caco-2 cell line transfection studies, it has been suggested that the A54T of the FABP2 gene plays a significant role in the assimilation of dietary fatty acids. However, reports were divergent with regard to the in vivo interaction between this polymorphism and postprandial lipemia. We therefore determined the influence of this intestinal fatty acid-binding protein polymorphism on intestinal fat transport using the human jejunal organ culture model, thus avoiding the interference of various circulating factors capable of metabolizing in vivo postprandial lipids. Analysis of DNA samples from 32 fetal intestines revealed 22 homozygotes for the wild-type Ala-54/Ala-54 genotype (0.83) and 10 heterozygotes for the polymorphic Thr-54/Ala-54 genotype (0.17). The Thr-encoding allele was associated with increased secretion of newly esterified triglycerides, augmented de novo apolipoprotein B synthesis, and elevated chylomicron output. On the other hand, no alterations were found in very low density lipoprotein and high density lipoprotein production, apolipoprotein A-I biogenesis, or microsomal triglyceride transfer protein mass and activity. Similarly, the alanine to threonine substitution at residue 54 did not result in changes in brush border hydrolytic activities (sucrase, glucoamylase, lactase, and alkaline phosphatase) or in glucose uptake or oxidation. Our data clearly document that the A54T polymorphism of FABP2 specifically influences small intestinal lipid absorption without modifying glucose uptake or metabolism. It is proposed that, in the absence of confounding factors such as environmental and genetic variables, the FABP2 polymorphism has an important effect on postprandial lipids in vivo, potentially influencing plasma levels of lipids and atherogenesis.

Incorporation of the fasting plasma FFA concentration into QUICKI improves its association with insulin sensitivity in nonobese individuals

Insulin resistance plays a major role in the pathophysiology of diabetes and is associated with obesity and cardiovascular disease. Excellent methods exist for the assessment of insulin sensitivity in the laboratory setting, such as the glucose clamp. However, these methods are not suitable for large population studies, and, thus, surrogate estimates of insulin sensitivity based on measurements in a single blood sample have been developed. Recently an index based on the logarithm and the reciprocal of the insulin-glucose product (QUICKI) has been proposed. QUICKI correlated with insulin sensitivity across the entire spectrum of glucose tolerance, but its performance was less satisfactory in normal subjects. Aim of this study was to ascertain whether the inclusion of fasting plasma free fatty acids concentration into QUICKI improves its association with insulin sensitivity in nonobese subjects. To test this hypothesis, we performed a euglycemic hyperinsulinemic clamp [40 mU/(m(2).min)] in 57 young, healthy, nonobese individuals with (n = 17) or without (n = 40) first-degree relatives affected by type 2 diabetes (the former group being an in vivo model of mild insulin resistance). We then compared the clamp-based index of insulin sensitivity with both QUICKI and a revised QUICKI, the latter index including the contribution of fasting free fatty acid concentration as well. The revised QUICKI considerably improved the relationship with the clamp-based index of insulin sensitivity (r = 0.51, P < 0.0001) with respect to QUICKI (r = 0.27, P < 0.05). In addition, the revised QUICKI revealed a reduction of insulin sensitivity in the offspring of type 2 diabetes (10%; P < 0.006) that QUICKI was unable to detect (3%; P = 0.28). In conclusion, this study suggests that the incorporation of fasting free fatty acid level into QUICKI is useful to improve its correlation with the clamp-based index of insulin sensitivity and its discriminatory power in case of mild insulin resistance. Further investigation is needed to ascertain its applicability to patients with obesity and type 2 diabetes.

Management of the insulin resistance syndrome

The insulin resistance syndrome (IRS) is a common disorder, which has important clinical implications. It is a cluster of cardiovascular risk factors that include obesity, hypertension, dyslipidemia, glucose intolerance, and type 2 diabetes mellitus. Lifestyle modifications and insulin sensitizers are among the several therapeutic strategies available for the treatment of the IRS. Optimal treatment will not only improve glycemic control, but may also significantly lower cardiovascular disease.

Thiazolidinedione treatment prevents free fatty acid-induced insulin resistance in male wistar rats

We sought to ascertain whether pretreatment with troglitazone (20 days) could prevent acute free fatty acid (FFA)-induced insulin resistance in male Wistar rats. Animals were divided into three groups: 1) control, 2) FFA infusion alone (FFA1), and 3) thiazolidinedione (TZD)-treated + FFA infusion (FFA1). Days before a hyperinsulinemic-euglycemic clamp, all animals were cannulated in the jugular vein (infusion) and carotid artery (sampling). Animals were allowed 5 days to recover from surgery and fasted 12 h before the experiment. Glucose (variable), insulin (40 mU. kg(-1). min(-1)), and Liposyn (heparinized 10% lipid emulsion) infusions were initiated simultaneously and continued from 0-120 min. Steady-state glucose, 8.3 +/- 0.14 mmol/l, and insulin concentrations, 7.3 +/- 2.45 nmol/l, were the same between groups. Interestingly, steady-state FFA levels were significantly lower in animals pretreated with TZD compared with FFA alone (1.83 +/- 0.26 vs. 2.96 +/- 0.25 mmol/l; P = 0.009), despite matched intralipid infusion rates. A second group of TZD-treated animals (TZD + FFA2) were infused with intralipid at a higher infusion rate (44%) to match the arterial concentrations of FFA1. The glucose infusion and insulin-stimulated glucose disposal rates (GDRs) were significantly decreased (40%) for untreated Liposyn infused (FFA1) compared with control rats. In addition, insulin receptor substrate-1 (IRS-1) phosphorylation and IRS-1-associated phosphatidylinositol (PI) 3-kinase activity was significantly reduced, 30-50%, in FFA1 rats. TZD pretreatment prevented the FFA-induced decrement in insulin signaling. Fatty acid translocase (FAT/CD36) also was significantly reduced (56%) in untreated FFA1 rats after the clamp but remained identical to control values for TZD-treated rats. In conclusion, acutely elevated FFA levels 1) induced a significant reduction in tracer-determined GDR paralleled by impaired tyrosine phosphorylation of IRS-1 and reduced IRS-1-associated PI 3-kinase activity and 2) induced a significant reduction in FAT/CD36 total protein. TZD pretreatment prevented FFA-induced decrements in insulin action and prevented the reduction in FAT/CD36 protein.

Glucose-induced insulin hypersecretion in lipid-infused healthy subjects is associated with a decrease in plasma norepinephrine concentration and urinary excretion

We investigated the effect of a 48 h triglyceride infusion on the subsequent insulin secretion in response to glucose in healthy men. We measured the variations in plasma concentration and urinary excretion of catecholamines as an indirect estimation of sympathetic tone. For 48 h, 20 volunteers received a triglyceride/heparin or a saline solution, separated by a 1-month interval. At time 48 h, insulin secretion in response to glucose was investigated by a single iv glucose injection (0.5 g/kg(-1)) followed by an hyperglycemic clamp (10 mg.kg(-1).min(-1), during 50 min). The triglyceride infusion resulted in a 3-fold elevation in plasma free fatty acids and an increase in insulin and C-peptide plasma concentrations (1.5- and 2.5-fold, respectively, P < 0.05), compared with saline. At time 48 h of lipid infusion, plasma norepinephrine (NE) concentration and urinary excretion levels were lowered compared with saline (plasma NE: 0.65 +/- 0.08 vs. 0.42 +/- 0.06 ng/ml, P < 0.05; urinary excretion: 800 +/- 70 vs. 620 +/- 25 nmol/24 h, P < 0.05). In response to glucose loading, insulin and C-peptide plasma concentrations were higher in lipid compared with saline infusion (plasma insulin: 600 +/- 98 vs. 310 +/- 45 pM, P < 0.05; plasma C-peptide 3.5 +/- 0.2 vs. 1.7 +/- 0.2 nM, P < 0.05). In conclusion, in healthy subjects, a 48-h lipid infusion induces basal hyperinsulinemia and exaggerated insulin secretion in response to glucose which may be partly related to a decrease in sympathetic tone.

Activation of mouse phosphodiesterase 3B gene promoter by adipocyte differentiation in 3T3-L1 cells

Activation of phosphodiesterase (PDE) 3B reduces free fatty acid output from adipocytes. Induction of PDE3B gene expression by adipocyte differentiation could improve insulin resistance. To examine whether the PDE3B promoter is activated by this differentiation, the 5' flanking sequence of the mouse PDE3B gene was isolated. The transcription initiation site was determined to be located 195 bp upstream of the translation start site. No putative binding site for peroxisome proliferator-activated receptor gamma was found within 2 kb upstream of the transcription initiation site. This region had promoter activity, which was further activated on adipocyte differentiation in 3T3-L1 cells.

Relationship between abdominal visceral fat and metabolic risk factors in obese adolescents

The distribution of body fat may play an important role in determining the risk of obesity-related morbidity in obese adults. In view of the alarming increase in adolescent obesity, this article addresses whether defects in insulin action and secretion and increased intra-abdominal fat that typify central obesity in adults are expressed early in the course of developing obesity. The euglycemic hyperinsulinemic clamp technique with stable isotopes was used to determine insulin effects on glucose and glycerol turnover in obese and lean adolescents. The hyperglycemic clamp was used to quantitate insulin secretion, while magnetic resonance imaging was used to directly assess abdominal subcutaneous and visceral fat. The results indicate that obese adolescent girls have the following characteristics: 1) insulin resistance with major defects in oxidative and nonoxidative glucose metabolism; 2) hyperinsulinemia in the fasting state and in response to intravenous glucose; and 3) impaired suppression of total body lipid oxidation and plasma FFA concentrations in response to insulin. Am. J. Hum. Biol. 11:259-266, 1999. Copyright 1999 Wiley-Liss, Inc.

Dehydroepiandrosterone alters Zucker rat soleus and cardiac muscle lipid profiles

High levels of serum free fatty acids (FFA) and lower proportions of polyunsaturated (PU) FAs, specifically arachidonic acid (AA), are common in obesity, insulin resistance (IR), and type 2 diabetes mellitus. Dehydrepiandrosterone (DHEA) decreases body fat content, dietary fat consumption, and insulin levels in obese Zucker rats (ZR), a genetic model of human youth onset obesity and type 2 diabetes. This study was conducted to investigate DHEA's effects on lean and obese ZR serum FFA levels and total lipid (TL) FA profiles in heart and soleus muscle. We postulated that DHEA alters serum FFA levels and tissue TL FA profiles of obese ZR so that they resemble the levels and profiles of lean ZR. If so, DHEA may directly or indirectly alter tissue lipids, FFA flux, and perhaps lower IR in obese ZR. Lean and obese male ZR were divided into six groups with 10 animals in each: obese ad libitum control, obese pair-fed, obese DHEA, lean ad libitum control, lean pair-fed, and lean DHEA. All animals had ad libitum access to a diet whose calories were 50% fat, 30% carbohydrate, and 20% protein. Only the diets of the DHEA treatment groups were supplemented with 0.6% DHEA. Pair-fed groups were given the average number of calories per day consumed by their corresponding DHEA group, and ad libitum groups had 24-h access to the DHEA-free diet. Serum FFA levels and heart and soleus TL FA profiles were measured. Serum FFA levels were higher in obese (approximately 1 mmol/L) compared to lean (approximately 0.6 mmol/L) ZR, regardless of group. In hearts, monounsaturated (MU) FA were greater and PU FA were proportionally lower in obese compared to the lean rats. In soleus, saturated and MU FA were greater and PU FA were proportionally lower in the obese compared to the lean rats. DHEA groups displayed significantly increased proportions of TL AA and decreased oleic acid in both muscle types. Mechanisms by which DHEA alters TL FA profiles are a reflection of changes occurring within specific lipid fractions such as FFA, phospholipid, and triglyceride. This study provides initial insights into DHEA's lipid altering effects.

Adipose tissue and the insulin resistance syndrome

Obesity is associated with insulin resistance. Insulin resistance underlies a constellation of adverse metabolic and physiological changes (the insulin resistance syndrome) which is a strong risk factor for development of type 2 diabetes and CHD. The present article discusses how accumulation of triacylglycerol in adipocytes can lead to deterioration of the responsiveness of glucose metabolism in other tissues. Lipodystrophy, lack of adipose tissue, is also associated with insulin resistance. Any plausible explanation for the link between excess adipose tissue and insulin resistance needs to be able to account for this observation. Adipose tissue in obesity becomes refractory to suppression of fat mobilization by insulin, and also to the normal acute stimulatory effect of insulin on activation of lipoprotein lipase (involved in fat storage). The net effect is as though adipocytes are 'full up' and resisting further fat storage. Thus, in the postprandial period especially, there is an excess flux of circulating lipid metabolites that would normally have been 'absorbed' by adipose tissue. This situation leads to fat deposition in other tissues. Accumulation of triacylglycerol in skeletal muscles and in liver is associated with insulin resistance. In lipodystrophy there is insufficient adipose tissue to absorb the postprandial influx of fatty acids, so these fatty acids will again be directed to other tissues. This view of the link between adipose tissue and insulin resistance emphasises the important role of adipose tissue in 'buffering' the daily influx of dietary fat entering the circulation and preventing excessive exposure of other tissues to this influx.

Mechanism of fat-induced hepatic gluconeogenesis: effect of metformin

High-fat feeding has been shown to cause hepatic insulin resistance. The aims of this study were to investigate the biochemical steps responsible for enhanced gluconeogenesis as a result of increased dietary fat intake and the site or sites at which the antihyperglycemic agent metformin acts to inhibit this process. Male Hooded Wistar rats were fed either a standard chow diet (5% fat by weight) or a high-fat diet (60% fat by weight) for 14 days with or without metformin. Total endogenous glucose production and gluconeogenesis were determined using [6-(3)H]glucose and [U-(14)C]alanine, respectively. Gluconeogenic enzyme activity and, where appropriate, protein and mRNA levels were measured in liver tissues. The high-fat diet increased endogenous glucose production (21.9 +/- 4.4 vs. 32.2 +/- 4.8 micromol x kg(-1) x min(-1), P < 0.05) and alanine gluconeogenesis (4.5 +/- 0.9 vs. 9.6 +/- 1.9 micromol x kg(-1) x min(-1), P < 0.05). Metformin reduced both endogenous glucose production (32.2 +/- 4.8 vs. 16.1 +/- 2.1 micromol x kg(-1) x min(-1), P < 0.05) and alanine gluconeogenesis (9.6 +/- 1.9 vs. 4.7 +/- 0.8 micromol x kg(-1) x min(-1), P < 0.05) after high-fat feeding. These changes were reflected in liver fructose-1,6-bisphosphatase protein levels (4.5 +/- 0.9 vs. 9.6 +/- 1.9 arbitrary units, P < 0.05 chow vs. high-fat feeding; 9.5 +/- 1.9 vs. 4.7 +/- 0.8 arbitrary units, P < 0.05 high fat fed in the absence vs. presence of metformin) but not in changes to the activity of other gluconeogenic enzymes. There was a significant positive correlation between alanine gluconeogenesis and fructose-1,6-bisphosphatase protein levels (r = 0.56, P < 0.05). Therefore, excess supply of dietary fat stimulates alanine gluconeogenesis via an increase in fructose-1,6-bisphosphatase protein levels. Metformin predominantly inhibits alanine gluconeogenesis by preventing the fat-induced changes in fructose-1,6-bisphosphatase levels.

Weight-related differences in glucose metabolism and free fatty acid production in two South African population groups

OBJECTIVE

The effects of free fatty acids (FFA), leptin, tumour necrosis factor (TNF) alpha and body fat distribution on in vivo oxidation of a glucose load were studied in two South African ethnic groups.

DESIGN AND MEASUREMENTS

Anthropometric and various metabolic indices were measured at fasting and during a 7 h oral glucose tolerance test (OGTT). Body composition was measured using bioelectrical impedance analysis and subcutaneous and visceral fat mass was assessed using a five- and two-level CT-scan respectively. Glucose oxidation was evaluated by measuring the ratio of (13)CO(2) to (12)CO(2) in breath following ingestion of 1-(13)C-labelled glucose.

SUBJECTS

Ten lean black women (LBW), ten obese black women (OBW), nine lean white women (LWW) and nine obese white women (OWW) were investigated after an overnight fast.

RESULTS

Visceral fat levels were significantly higher (P<0.01) in obese white than black women, despite similar body mass indexes (BMIs). There were no ethnic differences in glucose oxidation however; in the lean subjects of both ethnic groups the area under the curve (AUC) was higher than in obese subjects (P<0.05 for both) and was found to correlate negatively with weight (r=-0.69, P<0.01) after correcting for age. Basal TNF alpha concentrations were similar in all groups. Percentage suppression of FFAs at 30 min of the OGTT was 24+/-12% in OWW and -38+/-23% (P<0.05) in OBW, ie the 30 min FFA level was higher than the fasting level in the latter group. AUC for FFAs during the late postprandial period (120--420 min) was significantly higher in OWW than OBW (P<0.01) and LWW (P<0.01) and correlated positively with visceral fat mass independent of age (r=0.78, P<0.05) in the OWW only. Leptin levels were higher (P<0.01) both at fasting and during the course of the OGTT in obese women from both ethnic groups compared to the lean women.

CONCLUSIONS

Glucose oxidation is reduced in obese subjects of both ethnic groups; inter- and intra-ethnic differences were observed in visceral fat mass and FFA production and it is possible that such differences may play a role in the differing prevalences of obesity-related disorders that have been reported in these two populations.

Influence of dietary fat on postprandial glucose metabolism (exogenous and endogenous) using intrinsically (13)C-enriched durum wheat

The present study evaluates the influence of different amounts of fat added to starch on postprandial glucose metabolism (exogenous and endogenous). Nine women (24 (se 2) years old, BMI 20.4 (se 0.7) kg/m(2)) ingested 1 week apart 75 g glucose equivalent of (13)C-labelled starch in the form of pasta without (low fat; LF) or with 15 (medium fat; MF) or 40 (high fat; HF) g sunflower oil. During the 7 h following meal consumption, plasma glucose, non-esterified fatty acids, triacylglycerols (TG) and insulin concentrations, and endogenous (using [6,6-(2)H(2)]glucose) and exogenous glucose turnover were determined. With MF and HF meals, a lower postprandial glucose peak was observed, but with a secondary recovery. A decrease in exogenous glucose appearance explained lower glycaemia in HF. At 4 h after the HF meal the insulin, insulin:glucose and postprandial blood TG were higher than those measured after the LF and MF meals. Despite higher insulinaemia, total glucose disappearance was similar and endogenous glucose production was suppressed less than after the LF and MF meals, suggesting insulin resistance. Thus, the addition of a large amount of fat appears to be unfavourable to glucose metabolism because it leads to a feature of insulin resistance. On the contrary, the MF meal did not have these adverse effects, but it was able to decrease the initial glycaemic peak.

Effects of acute changes of plasma free fatty acids on intramyocellular fat content and insulin resistance in healthy subjects

The reason for the 3- to 4-h delay between a rise in plasma free fatty acid (FFA) levels and the development of insulin resistance remains unknown. In the current study, we have tested the hypothesis that the delay may be caused by the need for plasma FFAs to first enter muscle cells and to be re-esterified there before causing insulin resistance. To this end, we have determined intramyocellular triglyceride (IMCL-TG) content with proton nuclear magnetic resonance ((1)H-NMR) spectroscopy in healthy volunteers before and 4 h after lowering of plasma FFAs (with euglycemic-hyperinsulinemic clamping) or after increasing plasma FFAs (with lipid plus heparin infusions). Increasing plasma FFAs (from 516 to 1,207 micromol/l or from 464 to 1,857 micromol/l, respectively) was associated with acute increases in IMCL-TG from 100 to 109 +/- 5% (P < 0.05) or to 133 +/- 11% (P < 0.01), respectively, and with a significant increase in insulin resistance (P < 0.05 after 3.5 h). Lowering of plasma FFAs from 560 to 41 micromol/l was associated with a tendency for IMCL-TG to decrease (from 100 to 95 +/- 3%). Changes in plasma FFAs correlated linearly with IMCL-TG (r = 0.74, P < 0.003). The demonstration that acute changes in plasma FFAs were accompanied by corresponding changes in IMCL-TG and with the development of insulin resistance, taken together with previous reports of a close correlation between IMCL-TG and insulin resistance, supported the notion that accumulation of IMCL-TG is a step in the development of FFA-induced insulin resistance.

Non-invasive studies of glycogen metabolism in human skeletal muscle using nuclear magnetic resonance spectroscopy

Nuclear magnetic resonance spectroscopy provides non-invasive and real-time assessment of the metabolic fluxes in skeletal muscle during exercise, recovery from exercise and stimulation by insulin. Carbon-13 nuclear magnetic resonance spectroscopy has proved that reduced glycogen synthesis is a consistent feature of insulin-resistant type 2 diabetic patients, their offspring, and obesity. Low intracellular glucose and glucose-6-phosphate concentrations indicate that decreased glucose transport is mainly responsible for common insulin resistance. An elevation of plasma free fatty acids causes similar alterations of muscle glucose metabolism, and could play a central role in the development of impaired muscle glucose transport associated with insulin resistance.

Mechanisms linking diet and colorectal cancer: the possible role of insulin resistance

Diet is clearly implicated in the origin of colorectal cancer, with risk factors for the disease including reduced consumption of vegetables, fiber, and starch and increased consumption of red meat and animal fat. Several hypotheses have been developed to explain these associations. Most recently, McKeown-Eyssen and Giovannucci noted the similarity of the risk factors for colorectal cancer and those for insulin resistance and suggested that insulin resistance leads to colorectal cancer through the growth-promoting effect of elevated levels of insulin, glucose, or triglycerides. We briefly review the evidence from observational, epidemiological, and experimental animal studies linking diet with insulin resistance and colorectal cancer. The evidence suggests that diets high in energy and saturated fat and with high glycemic index carbohydrate and low levels of fiber and n-3 fatty acids lead to insulin resistance with hyperinsulinemia, hyperglycemia, and hypertriglyceridemia. We then consider how insulin, the related insulin-like growth factors, triglycerides, and nonesterified fatty acids could lead to increased growth of colon cancer precursor lesions and the development of colorectal cancer. Finally, we consider the implications of this scheme on possible future research directions, including studies of satiety and clinical tests of the importance of insulin resistance in the colon carcinogenesis process.

Decreased susceptibility to fatty acid-induced peripheral tissue insulin resistance in women

Elevation of plasma nonesterified fatty acid (NEFA) levels has been shown in various studies to induce peripheral tissue insulin resistance and impair the suppression of endogenous glucose production (EGP). These studies have been conducted predominantly in men. We compared the effects of elevated plasma NEFA levels on basal and insulin-stimulated glucose metabolism in 8 normal women (age 42 +/- 8 years [mean +/- SD], BMI 25 +/- 3 kg/m(2)) and 10 normal men (35 +/- 6 years, 24 +/- 3 kg/m(2)). Each subject underwent two 5-h 80 mU. m(-2). min(-1) hyperinsulinemic-euglycemic clamps with measurement of glucose kinetics (intravenous [3-(3)H]glucose) and substrate oxidation. Plasma NEFA levels were elevated in one study for 3 h before and during the clamp ( approximately 1 mmol/l in both groups) by infusion of 20% Intralipid (60 ml/h) and heparin (900 U/h). In the control studies, the men and women had similar insulin-stimulated glucose disposal rates (R(d)) and substrate oxidation rates. In the men, elevated NEFA levels decreased insulin-stimulated glucose R(d) during the final 40 min of the clamp by 23% (P < 0.001). By contrast, no significant change in glucose R(d) was found in the women (control 10.4 +/- 1.1, lipid study 9.9 +/- 1.3 mg. kg(-1). min(-1)). Glucose R(d) was also unchanged in six women studied at a lower insulin dose (40 mU. m(-2). min(-1)). During the last 40 min of the high-insulin dose clamps with elevated NEFA, glucose oxidation was decreased by 33% in the men (P < 0.001) and by 23% in the women (P < 0.02). Nonoxidative glucose R(d) at this time was decreased by 15% in the men (P = 0.02) but was not significantly affected in women. Basal EGP was unaffected by elevation of plasma NEFA levels in both groups. Suppression of EGP during the glucose clamps, however, was impaired. At the insulin infusion rate used, the magnitude of this defect was comparable in men and women. In summary, our findings suggest that although the effects on EGP appear comparable, the inhibitory effects of NEFA on peripheral tissue insulin sensitivity are observed in men but cannot be demonstrated in women.

Systemic and regional free fatty acid metabolism in type 2 diabetes

To determine whether type 2 diabetes mellitus alters systemic and regional free fatty acid ([3H]palmitate) metabolism, 14 nondiabetic (ND) and 14 type 2 diabetic (D) subjects underwent hyperinsulinemic-hyperglycemic (approximately 9.3 mM) clamps. The subjects were matched for age, body mass index, percent body fat, and fat-free mass. D subjects had more (P < 0.05) visceral fat than ND. During somatostatin, replacement growth hormone, and glucagon infusions, insulin was infused to achieve moderate (approximately 75 pmol/l) and high (approximately 150 pmol/l) physiological insulin levels. D subjects had greater (P < 0.02) systemic and regional (splanchnic and leg) palmitate release than ND subjects during both insulin infusion intervals. The relative contributions of splanchnic, leg, and nonsplanchnic upper body regions to systemic palmitate release did not differ between groups, although the last contributed the most (approximately 75%) to systemic palmitate release. Visceral fat area correlated with systemic palmitate flux (r = 0.45, P < 0.03) during both insulin infusions. We conclude that type 2 diabetes is associated with a generalized impairment in insulin suppression of lipolysis compared with equally obese ND individuals.