Mechanisms of fatty acid-induced inhibition of glucose uptake

Mechanism of free fatty acid-induced insulin resistance in humans

To examine the mechanism by which lipids cause insulin resistance in humans, skeletal muscle glycogen and glucose-6-phosphate concentrations were measured every 15 min by simultaneous 13C and 31P nuclear magnetic resonance spectroscopy in nine healthy subjects in the presence of low (0.18 +/- 0.02 mM [mean +/- SEM]; control) or high (1.93 +/- 0.04 mM; lipid infusion) plasma free fatty acid levels under euglycemic (approximately 5.2 mM) hyperinsulinemic (approximately 400 pM) clamp conditions for 6 h. During the initial 3.5 h of the clamp the rate of whole-body glucose uptake was not affected by lipid infusion, but it then decreased continuously to be approximately 46% of control values after 6 h (P < 0.00001). Augmented lipid oxidation was accompanied by a approximately 40% reduction of oxidative glucose metabolism starting during the third hour of lipid infusion (P < 0.05). Rates of muscle glycogen synthesis were similar during the first 3 h of lipid and control infusion, but thereafter decreased to approximately 50% of control values (4.0 +/- 1.0 vs. 9.3 +/- 1.6 mumol/[kg.min], P < 0.05). Reduction of muscle glycogen synthesis by elevated plasma free fatty acids was preceded by a fall of muscle glucose-6-phosphate concentrations starting at approximately 1.5 h (195 +/- 25 vs. control: 237 +/- 26 mM; P < 0.01). Therefore in contrast to the originally postulated mechanism in which free fatty acids were thought to inhibit insulin-stimulated glucose uptake in muscle through initial inhibition of pyruvate dehydrogenase these results demonstrate that free fatty acids induce insulin resistance in humans by initial inhibition of glucose transport/phosphorylation which is then followed by an approximately 50% reduction in both the rate of muscle glycogen synthesis and glucose oxidation.

Enzymes of carbohydrate metabolism in young and adult rats fed diets differing in fat and carbohydrate

Glycogen content as well as glycolytic, gluconeogenic and fatty acid synthesis enzyme activities were monitored in young and adult male rats fed diets differing in fat content: 11% (low), 22% (medium) and 42% (high) of total energy from fat. The results showed significant differences in the responses of young and adult rats to changes in dietary fat and carbohydrate. In young animals, increasing dietary fat decreased total liver glycogen phosphorylase (GP), pyruvate kinase (PK), glycerol 3-phosphate dehydrogenase, glucose 6-phosphate dehydrogenase, malic enzyme (ME), ATP-citrate lyase (ATP-CL) and fatty acid synthase (FAS). Increasing dietary fat also affected enzyme levels in other tissues: hexokinase (HK) and pyruvate dehydrogenase (PDH) activities decreased whereas skeletal muscle PK activity increased. The pattern of enzyme changes was similar in livers of fed adults with the exception that liver GP was not affected by dietary manipulations. Overnight food deprivation decreased liver glucokinase (GK), ME, ATP-CL, and FAS activities and increased liver phosphoenolpyruvate carboxykinase (PEPCK) and phosphofructokinase in both young and adult animals. In young animals, food deprivation also: (i) reduced liver GK and PK, (ii) increased kidney PEPCK, (iii) decreased muscle PEPCK and (iv) decreased kidney PDH. Food-deprived adults had increased skeletal muscle PEPCK and kidney glycogen synthetase as well as decreased kidney PEPCK muscle GP activity. These differences suggest that young animals are somewhat more responsive to changes in dietary manipulations. They also show that overnight food restriction causes a more profound metabolic re-organization in younger than in older animals.

Age-related changes in body composition are associated with hepatic insulin resistance in conscious rats

Age-dependent changes in body composition and hepatic ([3H]glucose) glucose metabolism were examined in 2-, 4-, and 14-mo-old (n = 26) conscious Sprague-Dawley rats. Hepatic glucose production (HGP) and hepatic glucose-6-phosphatase maximum velocity were decreased 18 and 30%, respectively, between 2 and 4 mo but were unchanged with further aging. However, between 4 and 14 mo, twofold higher plasma insulin levels were required to maintain similar HGP, suggesting that hepatic insulin resistance develops with age. Utilizing hepatic-pancreatic clamp technique, we showed that a much higher rate of insulin infusion (1.6 +/- 0.1 vs. 0.8 +/- 0.1 mU.kg-1.min-1) was needed to achieve similar plasma glucose levels and HGP. Furthermore, when 4-mo-old rats were infused with insulin at similar rates as the 14-mo-old rats, HGP was decreased by approximately 30%. Because hepatic insulin sensitivity was inversely related to the increase in body weight (r2 = 0.876) and free fatty acid levels (r2 = 0.843), we suggest that age-related changes in body composition may lead to the impairment of hepatic glucose metabolism.

Increased hepatic pyruvate dehydrogenase kinase activity in fed hyperthyroid rats: studies in vivo and with cultured hepatocytes

Experimental hyperthyroidism induced by the administration of tri-iodothyronine (T3; 100 micrograms/100 g body wt; 3 days) increased plasma non-esterified fatty acids in the fed state in the rat. At the same time, hepatic PDH kinase responded with a persistent (1.6-fold) increase in activity. The exposure of hepatocytes from fed euthyroid rats to T3 (100 nM) in culture for 21 h increased PDH kinase activity to an extent comparable to that observed in vivo in response to hyperthyroidism. The in vitro increase in PDH kinase activity was suppressed by insulin (100 microU/ml) and by inhibition of mitochondrial fatty acid oxidation. The results demonstrate a direct hepatic action of T3 to increase PDH kinase activity, which is mediated by intramitochondrial fatty acyl-CoA or a product of beta-oxidation, and facilitated by hepatic insulin resistance.

Hyperinsulinemia, hyperproinsulinemia and insulin resistance in the metabolic syndrome

For better comprehension of the metabolic syndrome, it is necessary to differentiate the effect of insulin on glucose metabolism on the one hand, and on other metabolic activities on the other hand. Whereas glucose utilization is affected by insulin resistance, the effect of insulin on lipid metabolism, ion and aminoacid transport does not seem to be diminished. Lipid metabolism, however, seems to play a crucial role in the induction of the vicious cycle. Increased energy and fat ingestion may be due to an increased number of galanin secreting cells in the hypothalamus. The excessive fat intake results in an increased rate of release of insulin and increased influx of triglycerides into the blood. From these triglycerides an excess of free fatty acids is released by the action of lipoprotein lipase. The increased plasma free fatty acid level then results in insulin resistance affecting glucose metabolism. Also, these free fatty acids may impair the secretion of insulin. Induction of insulin resistance results in higher glucose levels, which may cause hyperinsulinemia. Hyperinsulinemia maintains the elevation of triglycerides. When diabetes becomes overt and elevated glucose levels prevail, the hyperinsulinism acts on the metabolic pathways which are still sensitive to insulin, namely lipid metabolism, aminoacid transport and ion transport.

A polymorphism in the human intestinal fatty acid binding protein alters fatty acid transport across Caco-2 cells

The human intestinal fatty acid binding protein (IFABP) binds long-chain fatty acids in vitro, but its intracellular function has remained speculative. A polymorphism in the gene that encodes IFABP results in an alanine (Ala54) to threonine (Thr54) substitution at codon 54 that alters the in vitro binding affinity of the protein for long-chain fatty acids. To identify potential functional variability between Ala54 and Thr54 IFABP, we established permanently transfected Caco-2 cell lines that express either Ala54 or Thr54 IFABP. We found that Caco-2 cells expressing Thr54 IFABP transport long-chain fatty acids and secrete triglycerides to a greater degree than Caco-2 cells expressing Ala54 IFABP. These results provide the first demonstration that IFABP participates in the intracellular transport of long-chain fatty acids. In addition, the observed increase in transport of fatty acids across cells expressing Thr54 IFABP suggests a plausible physiologic mechanism for our prior observation that Pima Indians with a Thr54 IFABP genotype have increased post-absorptive lipid oxidation rates and are more insulin-resistant than Pimas with a Ala54 IFABP genotype.

Glucose and insulin-induced inhibition of fatty acid oxidation: the glucose-fatty acid cycle reversed

In this study we have investigated a hypothesis that proposes the reverse of the so-called "glucose-fatty acid cycle, " i.e., that accelerated carbohydrate metabolism directly inhibits fatty acid oxidation. We studied normal volunteers in the basal state and during a hyperinsulinemic, hyperglycemic clamp (plasma insulin = 1,789 +/- 119 pmol/l, plasma glucose = 7.7 +/- 0.2 mmol/l). We quantified fat oxidation using indirect calorimetry and stable isotopes ([1-13C]oleate). Plasma oleate enrichment and free fatty acid (FFA) concentration were kept constant by means of infusion of lipids and heparin. Glucose oxidation increased from basal 6.2 +/- 0.8 to 22.3 +/- 1.4 mumol.kg-1.min-1 during the clamp (P < 0.01). Total (indirect calorimetry) and plasma fatty acid oxidation (isotopic determination) decreased from 2.6 +/- 0.2 to 0.4 +/- 0.3 (P < 0.01) and 2.2 +/- 0.2 to 1.4 +/- 0.1 mumol.kg-1.min-1 (P <0.05), respectively. We conclude that under the conditions of the present experiment, glucose and/or insulin directly inhibits fatty acid oxidation. Our findings suggest that, contrary to the prediction of the glucose-fatty acid cycle, the intracellular availability of glucose (rather than FFA) determines the nature of substrate oxidation in human subjects.

Fuel metabolism in pregnancy and in gestational diabetes mellitus

Fuel metabolism during pregnancy and in gestational diabetes mellitus (GDM) is reviewed with emphasis on carbohydrate and fat metabolism. In early pregnancy, insulin secretion in response to glucose is increased, peripheral insulin sensitivity is normal or increased, glucose tolerance is normal or slightly enhanced. In addition, there is maternal fat accumulation. During late pregnancy, there is increased fetal growth and increased fetal demand for nutrients. Maternal responses to these demands consist of an accelerated switch from carbohydrate to fat utilization that is facilitated by peripheral insulin resistance and by high blood levels of lipolytic hormones. In patients with GDM, insulin resistance is either comparable or greater than in nondiabetic pregnancy whereas insulin secretion appears to be compromised. Important short term consequences of GDM are perinatal complications, whereas long term complications include an increased rate of development of maternal non-insulin-dependent diabetes mellitus.

Role of lipids in development of noninsulin-dependent diabetes mellitus: lessons learned from Pima Indians

We studied the role of lipids in the pathogenesis of noninsulin-dependent diabetes mellitus (NIDDM) in Pima Indians. High plasma levels of nonesterified fatty acid (NEFA) predicted development of NIDDM, but this effect cannot entirely be explained by the glucose-fatty acid cycle. Dyslipidemia, although often associated with diabetes, did not seem to predict NIDDM and might rather be associated with, or the consequence of insulin resistance. In some individuals, a single amino acid substitution in the intestinal fatty acid binding protein could result in increased rates of intestinal absorption of dietary NEFA and thereby contribute to increased lipid-oxidation rates and insulin resistance.

Chronic exogenous hyperinsulinaemia does not modify the acute inhibitory effect of insulin on the secretion of very-low-density lipoprotein triacylglycerol and apolipoprotein B in primary cultures of rat hepatocytes

Male Wistar rats were fitted with subcutaneous osmotic mini-pumps that delivered insulin at a constant rate of 0.20 i.u./h for 7 days. This treatment raised the plasma insulin concentration from 31 +/- 4 to 201 +/- 64 micro-i.u./ml. Hepatocytes prepared from the hyperinsulinaemic animals secreted very-low-density lipoprotein (VLDL) triacylglycerol (TAG) at a higher rate (172 +/- 21 microgram per 24 h per mg cell protein) than did those from sham-operated controls (109 +/- 12 microgram per 24 h per mg) (P<0.05). However, chronic exogenous hyperinsulinaemia had no stimulatory effect on the secretion of VLDL apolipoprotein B (apoB) in derived hepatocytes compared with those from the sham-operated controls (2.32 +/- 0.38 compared with 3.09 +/- 0.40 microgram per 24 h per mg). Hepatocytes from the hyperinsulinaemic rats thus secreted larger VLDL particles as evidenced by the increased TAG:apoB ratio (78.4 +/- 13.1 compared with 38.4 +/- 7.6; P<0.05). In hepatocytes from the hyperinsulinaemic rats a larger proportion of the newly synthesized TAG was secreted as VLDL. Hepatocytes from the hyperinsulinaemic and the sham-operated control animals were equally sensitive to the inhibitory effect of insulin added in vitro on the secretion of VLDL TAG. Insulin added in vitro to the culture medium of hepatocytes from hyperinsulinaemic animals significantly decreased the TAG:apoB ratio of the secreted VLDL. This change did not occur in hepatocytes from sham-operated rats. These results suggest that, in vivo, chronic hyperinsulinaemia is not in itself sufficient to desensitize the liver to the acute inhibitory effect of insulin on the secretion of VLDL.

A model to measure insulin effects on glucose transport and phosphorylation in muscle: a three-tracer study

We studied five healthy subjects with perfused forearm and euglycemic clamp techniques in combination with a three-tracer (D-[12C]mannitol, not transportable; 3-O-[14C]methyl-D-glucose, transportable but not metabolizable; D-[3-3H]glucose, transportable and metabolizable) intra-arterial pulse injection to assess transmembrane transport and intracellular phosphorylation of glucose in vivo in human muscle. The washout curves of the three tracers were analyzed with a multicompartmental model. A priori identifiability analysis of the tracer model shows that the rate constants of glucose transport into and out of the cells and of glucose phosphorylation are uniquely identifiable. Tracer model parameters were estimated by a nonlinear least-squares parameter estimation technique. We then solved for the tracee model and estimated bidirectional transmembrane transport glucose fluxes, glucose intracellular phosphorylation, extracellular and intracellular volumes of glucose distribution, and extracellular and intracellular glucose concentrations. Physiological hyperinsulinemia (473 +/- 22 pM) caused 2.7-fold (63.1 +/- 7.2 vs. 23.4 +/- 6.1 mumol.min-1.kg-1, P < 0.01) and 5.1-fold (42.5 +/- 5.8 vs. 8.4 +/- 2.2 mumol.min-1.kg-1, P < 0.01) increases in transmembrane influx and intracellular phosphorylation of glucose, respectively. Extracellular distribution volume and concentration of glucose were unchanged, whereas intracellular distribution volume of glucose was increased (approximately 2-fold) and intracellular glucose concentration was almost halved by hyperinsulinemia. In summary, 1) a multicompartment model of three-tracer kinetic data can quantify transmembrane glucose fluxes and intracellular glucose phosphorylation in human muscle; and 2) physiological hyperinsulinemia stimulates both transport and phosphorylation of glucose and, in doing so, amplifies the role of glucose transport as a rate-determining step of muscle glucose uptake.

Decreased non-esterified fatty acid suppression and features of the insulin resistance syndrome occur in a sub-group of individuals with normal glucose tolerance

To investigate causes of increased triglyceride concentrations in subjects with normal glucose tolerance (determined by oral glucose tolerance testing using World Health Organization criteria) 883 healthy subjects (389 men and 494 women) between 40 and 65 years of age were studied. Subjects were divided by gender into four groups according to 120-min glucose concentrations. Individuals in the highest quartile of glucose concentration had the highest mean triglyceride concentrations (p < 0.0001) and highest mean non-esterified fatty acid (NEFA) concentrations (p < 0.0001). There was also a clustering of cardiovacular risk factors normally associated with the insulin resistance syndrome in subjects in this group. Regression analysis showed that the most important determinants of triglyceride levels were smoking (men p = 0.001, women p = 0.005), waist:hip ratio (men p = 0.01, women p < 0.001) and NEFA suppression (men p = 0.02, women p = 0.005). NEFAs suppressed 16.7% in women compared to 2.4% in men during the first 30 min of the oral glucose tolerance test (p < 0.001). These results show that a clustering of cardiovascular risk factors associated with decreased NEFA suppression occurs in a sub-group of subjects with normal glucose tolerance and that the pattern of NEFA suppression differs between men and women.

Route of nutrient delivery affects insulin sensitivity and liver glucose transporter expression in rat

To optimize artificial nutrition (AN) techniques for patients suffering from malnutrition or reduced intestinal absorption, utilization of energy fuels, especially glucose, requires better understanding. Because the liver plays a key role in glucose homeostasis, the aim of this study was to assess the effects of continuous intragastric and intravenous nutrition on insulin secretion and several markers of liver glucose metabolism, especially glucose transporter GLUT-2. Wistar male rats underwent catheterization of either stomach (intragastric) or vena cava (intravenous) and received 24 h/day the same all-in-one formula over 7 to 14 days. The metabolic parameters from intragastrically fed rats did not differ significantly from those from orally fed control rats. Intravenous nutrition induced insulin resistance (marked hyperinsulinemia and/or mild hyperglycemia) and reduced liver GLUT-2 protein and mRNA levels. The decrease in liver GLUT-2 gene expression might be mediated either by an inhibitory role of hyperinsulinemia or by the decrease in gut or portal factors. These results suggest that the route of nutrient delivery influences their utilization by the liver.

Bromocriptine inhibits in vivo free fatty acid oxidation and hepatic glucose output in seasonally obese hamsters (Mesocricetus auratus)

Seasonally obese hyperinsulinemic hamsters were treated for 5 weeks with bromocriptine (500 to 600 micrograms per animal) and tested for drug effects on energy balance, body fat stores, nocturnal whole-body free fatty acid (FFA) metabolism and hepatic glucose output, and diurnal glucose tolerance. After 5 weeks, bromocriptine treatment reduced retroperitoneal fat pad weight by 45% without altering either daily food consumption or end-treatment total daily energy expenditure. Also, 5 weeks of treatment improved the diurnal glucose tolerance, resulting in a 47% and 33% decrease in the area under glucose and insulin curves, respectively. After 4 weeks, bromocriptine treatment reduced nocturnal lipolysis by 28%, palmitate rate of appearance into plasma by 30%, palmitate oxidation by 33%, and hepatic glucose output by 28%. Moreover, these reductions were accompanied by a 75% reduction in plasma insulin concentration. The data suggest that bromocriptine may improve diurnal glucose tolerance in part by inhibiting the preceding nocturnal lipolysis and FFA oxidation. Reductions in nocturnal FFA oxidation and hepatic glucose production may result from bromocriptine's influences on circadian organization of hypothalamic centers known to regulate these activities. Available evidence suggests that bromocriptine may impact this neuroendocrine organization of metabolism by increasing the dopamine to noradrenaline activity ratio in central (hypothalamic) and peripheral (eg, liver and adipose) target tissues.

A high concentration of fasting plasma non-esterified fatty acids is a risk factor for the development of NIDDM

To assess the role of fasting plasma non-esterified fatty acids (NEFA) in the development of non-insulin-dependent diabetes mellitus (NIDDM), data were analysed from annual examinations of 190 non-diabetic Pima Indians. Glucose tolerance was measured by a 75-g oral glucose tolerance test, insulin action by a euglycaemic hyperinsulinaemic (40 mU x m-2 x min-1) clamp and in vitro lipolysis using isolated abdominal fat cells. After a mean follow-up period of 4.0 +/- 2.4 years (mean +/- SD), 47 subjects developed NIDDM. Risk factors for NIDDM were estimated by proportional-hazards analysis and risk ratios (RR) with 95% confidence intervals (95% CI) calculated at the 90th and 10th percentile of the predictor variables. A large average fat-cell volume was predictive of NIDDM (RR=2.4; 95% CI=1.2-4.8) independent of age, sex, percent body and body fat distribution. A high fasting plasma NEFA concentration was also a risk factor for NIDDM (RR=2.3; 95% CI=1.1-4.7) independent of sex, percent body fat, waist/thigh ratio, insulin-mediated glucose uptake and fasting triglyceride concentration. We conclude that large fat cells and the resulting increased plasma NEFA concentrations are risk factors for the development of NIDDM.

Effects of fasting on hepatic and peripheral glucose metabolism in conscious rats with near-total fat depletion

Experimental diabetes and fasting are both associated with hypoinsulinaemia and share several other metabolic features. We investigated hepatic and peripheral glucose metabolism in young rats after near-total depletion of their fat mass. Conscious rats were fasted for 72 h (n = 13), while 6 h-fasted animals (n = 14) served as controls. Rats were studied either during saline infusion or insulin (18 m-units/kg per min)-clamp studies. In fasting, despite a 2-fold increase in hepatic glucose-6-phosphatase (Glc-6-Pase) Vmax. (from 16 +/- 2 mumol/g of liver per min in control; P < 0.001), the basal hepatic glucose production (HGP) decreased by 47% [from 88 +/- 3 mumol/kg lean body mass (LBM) per min in control; P < 0.01]. The decreased HGP in fasting was associated with a 70% decrease in the hepatic levels of glucose 6-phosphate (Glc-6-P) (from 366 +/- 53 nmol/g wet wt. in control; P < 0.01). Thus Glc-6-Pase activity assayed in the presence of the Glc-6-P levels found in vivo was decreased by 44%. During hyperinsulinaemia, peripheral glucose uptake was decreased by 15% with 3 days of fasting (from 272 +/- 17 mumol/kg LBM per min in control; P < 0.01). This was completely accounted for by a 42% decrease in whole-body glycolysis (P < 0.01), while the rate of glycogen synthesis was unchanged. Thus fasting (after near-total fat depletion) differs from experimental diabetes because: (1) despite markedly increased Glc-6-Pase, HGP is decreased in fasting, due to a marked decrease in the substrate level (Glc-6-P) in vivo; and (2) the impairment in peripheral insulin sensitivity in fasting is due to a decrease in the glycolytic, and not the glycogen-synthetic, pathway.

Effects of fat on glucose uptake and utilization in patients with non-insulin-dependent diabetes

It was the aim of this study to determine whether FFA inhibit insulin-stimulated whole body glucose uptake and utilization in patients with non-insulin-dependent diabetes. We performed five types of isoglycemic (approximately 11mM) clamps: (a) with insulin; (b) with insulin plus fat/heparin; (c) with insulin plus glycerol; (d) with saline; (e) with saline plus fat/heparin and two types of euglycemic (approximately 5mM) clamps: (a) with insulin; (b) with insulin plus fat/heparin. During these studies, we determined rates of glucose uptake, glycolysis (both with 3[3H] glucose), glycogen synthesis (determined as glucose uptake minus glycolysis), carbohydrate oxidation (by indirect calorimetry) and nonoxidative glycolysis (determined as glycolysis minus carbohydrate oxidation). Fat/heparin infusion did not affect basal glucose uptake, but inhibited total stimulated (insulin stimulated plus basal) glucose uptake by 40-50% in isoglycemic and in euglycemic patients at plasma FFA concentration of approximately 950 and approximately 550 microM, respectively. In isoglycemic patients, the 40-50% inhibition of total stimulated glucose uptake was due to near complete inhibition of the insulin-stimulated part of glucose uptake. Proportional inhibition of glucose uptake, glycogen synthesis, and glycolysis suggested a major FFA-mediated defect involving glucose transport and/or phosphorylation. In summary, fat produced proportional inhibitions of insulin-stimulated glucose uptake and of intracellular glucose utilization. We conclude, that physiologically elevated levels of FFa could potentially be responsible for a large part of the peripheral insulin resistance in patients with non-insulin-dependent diabetes mellitus.

Marked hyperinsulinaemia in postmenopausal, healthy Indian (Asian) women

The effect of the menopause on insulin metabolism has not received specific attention in populations prone to non-insulin-dependent (Type 2) diabetes mellitus (NIDDM). Insignificant or slight alterations in insulin levels have been reported in postmenopausal women of mainly European ancestry. We thus report on the results of a cross-sectional study on the correlates of fasting insulin levels in 177 healthy, Indian nurses aged between 25 and 55 years. Fasting insulin concentration was markedly higher in the 75 postmenopausal subjects (23.9 mU I-1) than in the 102 premenopausal women (11.7 mU I-1 (p < 0.0001). Forty-three (57%) of the postmenopausal subjects had insulin values more than 20 mU I-1 (the upper normal limit). Stepwise regression analysis on the entire group revealed menopause (p < 0.0001), waist:hip ratio (p = 0.0001), apolipoprotein E genotype (p = 0.002), and the testosterone: sex hormone binding globulin ratio (p = 0.0002) as statistically significant, independent predictors of log insulin levels. Age did not account for the difference between premenopausal and postmenopausal subjects. The apolipoprotein E genotype emerged as a significant correlate of insulin levels, only in postmenopausal women: epsilon 3/3, 26.3 mU I-1; epsilon 3/4, 51.8 mU I-1 (p = 0.0007). Hyperinsulinaemic postmenopausal subjects had higher fasting glucose levels than normoinsulinemic nurses (p = 0.03), but glycosylated haemoglobin and fructosamine values were all within the normal range. Thus fasting hyperinsulinaemia was marked and common among a group of healthy, postmenopausal Indian nurses below the age of 55 years, suggesting that the menopausal transition may permit or provoke insulin resistance in this susceptible population.

Insulin regulation of glucose turnover and lipid levels in obese children with fasting normoinsulinaemia

To evaluate the early metabolic alterations induced by obesity, we studied glucose turnover and lipid levels in obese children with fasting normoinsulinaemia. Two experimental protocols were carried out. Protocol I consisted of a euglycaemic glucose clamp at two rates of insulin infusion. Protocol II was similar to protocol I except for a variable lipid infusion used to maintain basal non-esterified fatty acid (NEFA) levels. During protocol I, the glucose disappearance rates were lower in obese children, while no differences were found in hepatic glucose release. NEFA response to insulin was not substantially altered in obese children either at low or high insulin infusion. During protocol II, the NEFA clamp induced a 25% reduction in peripheral insulin sensitivity in control children whereas no changes were observed in obese children. Interestingly, lipid infusion in control children was not sufficient to reproduce the same degree of insulin resistance observed in obese children, suggesting that NEFA are only one of the determinants of insulin resistance at this stage of obesity. In conclusion, the present study provides a portrait of glucose metabolism and lipid levels in normoinsulinaemic obese children. Our results document that peripheral insulin resistance is the first alteration at this stage of obesity, whereas an increase in insulin secretion and a defect in the inhibition of hepatic glucose release by insulin may develop at a later stage. In addition, primarily receptor and post-receptor defects and some alterations of NEFA metabolism are likely to coexist in the induction of insulin resistance at this stage of obesity.

Impaired free fatty acid utilization by skeletal muscle in non-insulin-dependent diabetes mellitus

This study was undertaken to assess utilization of FFA by skeletal muscle in patients with non-insulin-dependent diabetes mellitus (NIDDM). 11 NIDDM and 9 nondiabetic subjects were studied using leg balance methods to measure the fractional extraction of [3H]oleate. Limb indirect calorimetry was used to estimate RQ. Percutaneous muscle biopsy samples of vastus lateralis were analyzed for muscle fiber type distribution, capillary density, and metabolic potential as reflected by measurements of the activity of seven muscle enzyme markers of glycolytic and aerobic-oxidative pathways. During postabsorptive conditions, fractional extraction of oleate across the leg was lower in NIDDM subjects (0.31 +/- 0.08 vs. 0.43 +/- 0.10, P < 0.01), and there was reduced oleate uptake across the leg (66 +/- 8 vs. 82 +/- 13 nmol/min, P < 0.01). Postabsorptive leg RQ was increased in NIDDM (0.85 +/- 0.03 vs. 0.77 +/- 0.02, P < 0.01), and rates of lipid oxidation by skeletal muscle were lower while glucose oxidation was increased (P < 0.05). In subjects with NIDDM, proportions of type I, IIa, and IIb fibers were 37 +/- 2, 37 +/- 6, and 26 +/- 5%, respectively, which did not differ from nondiabetics; and capillary density, glycolytic, and aerobic-oxidative potentials were similar. During 6 h after ingestion of a mixed meal, arterial FFA remained greater in NIDDM subjects. Therefore, despite persistent reduced fractional extraction of oleate across the leg in NIDDM (0.34 +/- 0.04 vs. 0.38 +/- 0.03, P < 0.05), rates of oleate uptake across the leg were greater in NIDDM (54 +/- 7 vs. 45 +/- 8 nmol/min, P < 0.01). In summary, during postabsorptive conditions there is reduced utilization of FFA by muscle, while during postprandial conditions there is impaired suppression of FFA uptake across the leg in NIDDM. During both fasting and postprandial conditions, NIDDM subjects have reduced rates of lipid oxidation by muscle.