Assessment of postprandial glucose metabolism: conventional dual- vs. triple-tracer method

The dual-tracer method has been used conventionally for assessment of postprandial fluxes, i.e., appearance in plasma of ingested glucose (R(a meal)), endogenous glucose production (EGP), and disposal (R(d)). To quantify the magnitude of errors affecting the calculations and their dependence on model assumptions, this method was assessed and compared with the triple-tracer method, which provides model-independent estimates. For this purpose, the dual-tracer protocol was performed twice in eight normal subjects, with [1-(13)C]glucose to trace ingested glucose and [6,6-(2)H(2)]glucose constantly infused. A third tracer, [6-(3)H]glucose, was infused at variable rates to render the calculation of R(a meal) and EGP virtually model independent. The dual-tracer method analyzed with a one-compartment model performed poorly, since R(a meal) peak was significantly lower and delayed compared with triple-tracer reference, resulting in a significantly lower estimation of the amount of absorbed glucose (9,036 +/- 558 vs. 11,316 +/- 823 micromol/kg, P = 0.0117). EGP showed a paradoxical pattern, with an initial overshoot followed by a rapid decay to negative values, resulting in a significant underestimation of EGP suppression (57 +/- 3 vs. 65 +/- 4%, P = 0.0117). A two-compartment model performed better but did not overcome the limitations of the dual-tracer approach, since the amount of absorbed glucose was still significantly underestimated (10,231 +/- 661 vs. 12,169 +/- 838 micromol/kg, P = 0.0117) and EGP still showed a paradoxical behavior. R(d), estimated from R(a meal) and EGP, was significantly underestimated with the dual-tracer method, irrespective of adopted model. We conclude that three suitably infused tracers are required for accurate assessment of postprandial R(a meal), EGP, and R(d).     

Validity of triple- and dual-tracer techniques to estimate glucose appearance

The triple-tracer (TT) dilution technique has been proposed to be the gold standard method to measure postprandial glucose appearance. However, validation against an independent standard has been missing. We addressed this issue and also validated the simpler dual-tracer (DT) technique. Sixteen young subjects with type 1 diabetes (age 19.5 ± 3.8 yr, BMI 23.4 ± 1.5 kg/m(2), HbA(1c) 8.7 ± 1.7%, diabetes duration 9.0 ± 6.9 yr, total daily insulin 0.9 ± 0.2 U·kg(-1)·day(-1), mean ± SD) received a variable intravenous 20% dextrose infusion enriched with [U-(13)C]glucose over 8 h to achieve postprandial-resembling glucose excursions while intravenous insulin was administered to achieve postprandial-resembling levels of plasma insulin. Primed [6,6-(2)H(2)]glucose was infused in a manner that mimicked the expected endogenous glucose production and [U-(13)C; 1,2,3,4,5,6,6-(2)H(7)]glucose was infused in a manner that mimicked the expected glucose appearance from a standard meal. Plasma glucose enrichment was measured by gas chromatography-mass spectrometry. The intravenous dextrose infusion served as an independent standard and was reconstructed using the TT and DT techniques with the two-compartment Radziuk/Mari model and an advanced stochastic computational method. The difference between the infused and reconstructed dextrose profile was similar for the two methods (root mean square error 6.6 ± 1.9 vs. 8.0 ± 3.5 μmol·kg(-1)·min(-1), TT vs. DT, P = NS, paired t-test). The TT technique was more accurate in recovering the overall dextrose infusion (100 ± 9 and 92 ± 12%; P = 0.02). The root mean square error associated with the mean dextrose infusion profile was 2.5 and 3.3 μmol·kg(-1)·min(-1) for the TT and DT techniques, respectively. We conclude that the TT and DT techniques combined with the advanced computational method can measure accurately exogenous glucose appearance. The TT technique tends to outperform slightly the DT technique, but the latter benefits from reduced experimental and computational complexity.     

Acute inhibition of lipolysis does not affect postprandial suppression of endogenous glucose production

To test the hypothesis that intrahepatic availability of fatty acid could modify the rate of suppression of endogenous glucose production (EGP), acipimox or placebo was administered before and during a test meal. We used a modified isotopic methodology to measure EGP in 11 healthy subjects, and (1)H magnetic resonance spectroscopic measurement of hepatic triglyceride stores was also undertaken. Acipimox suppressed plasma free fatty acids markedly before the meal (0.05 +/- 0.01 mmol/l at -10 min, P = 0) and throughout the postprandial period (0.03 +/- 0.01 mmol/l at 150 min). Mean peak plasma glucose was significantly lower after the meal on acipimox days (8.9 +/- 0.4 vs. 10.1 +/- 0.5 mmol/l, P < 0.01), as was mean peak serum insulin (653.1 +/- 99.9 vs. 909 +/- 118 pmol/l, P < 0.01). Fasting EGP was similar (11.15 +/- 0.58 micromol.kg(-1).min(-1) placebo vs. 11.17 +/- 0.89 mg.kg(-1).min(-1) acipimox). The rate of suppression of EGP after the meal was almost identical on the 2 test days (4.36 +/- 1.52 vs. 3.69 +/- 1.21 micromol.kg(-1).min(-1) at 40 min). There was a significant negative correlation between the acipimox-induced decrease in peak plasma glucose and liver triglyceride content (r = -0.827, P = 0.002), suggesting that, when levels of liver fat were low, inhibition of lipolysis was able to affect glucose homeostasis. Acute pharmacological sequestration of fatty acids in triglyceride stores improves postprandial glucose homeostasis without effect on the immediate postprandial suppression of EGP.     

Partitioning glucose distribution/transport, disposal, and endogenous production during IVGTT

We have separated the effect of insulin on glucose distribution/transport, glucose disposal, and endogenous production (EGP) during an intravenous glucose tolerance test (IVGTT) by use of a dual-tracer dilution methodology. Six healthy lean male subjects (age 33 +/- 3 yr, body mass index 22.7 +/- 0.6 kg/m(2)) underwent a 4-h IVGTT (0.3 g/kg glucose enriched with 3-6% D-[U-(13)C]glucose and 5-10% 3-O-methyl-D-glucose) preceded by a 2-h investigation under basal conditions (5 mg/kg of D-[U-(13)C]glucose and 8 mg/kg of 3-O-methyl-D-glucose). A new model described the kinetics of the two glucose tracers and native glucose with the use of a two-compartment structure for glucose and a one-compartment structure for insulin effects. Insulin sensitivities of distribution/transport, disposal, and EGP were similar (11.5 +/- 3.8 vs. 10.4 +/- 3.9 vs. 11.1 +/- 2.7 x 10(-2) ml small middle dot kg(-1) small middle dot min(-1) per mU/l; P = nonsignificant, ANOVA). When expressed in terms of ability to lower glucose concentration, stimulation of disposal and stimulation of distribution/transport accounted each independently for 25 and 30%, respectively, of the overall effect. Suppression of EGP was more effective (P < 0.01, ANOVA) and accounted for 50% of the overall effect. EGP was suppressed by 70% (52-82%) (95% confidence interval relative to basal) within 60 min of the IVGTT; glucose distribution/transport was least responsive to insulin and was maximally activated by 62% (34-96%) above basal at 80 min compared with maximum 279% (116-565%) activation of glucose disposal at 20 min. The deactivation of glucose distribution/transport was slower than that of glucose disposal and EGP (P < 0.02) with half-times of 207 (84-510), 12 (7-22), and 29 (16-54) min, respectively. The minimal-model insulin sensitivity was tightly correlated with and linearly related to sensitivity of EGP (r = 0.96, P < 0.005) and correlated positively but nonsignificantly with distribution/transport sensitivity (r = 0.73, P = 0.10) and disposal sensitivity (r = 0.55, P = 0.26). We conclude that, in healthy subjects during an IVGTT, the two peripheral insulin effects account jointly for approximately one-half of the overall insulin-stimulated glucose lowering, each effect contributing equally. Suppression of EGP matches the effect in the periphery.     

Pathways for glucose disposal after meal ingestion in humans

To characterize postprandial glucose disposal more completely, we used the tritiated water technique, a triple-isotope approach (intravenous [3-H(3)]glucose and [(14)C]bicarbonate and oral [6,6-(2)H(2)]glucose) and indirect calorimetry to assess splanchnic and peripheral glucose disposal, direct and indirect glucose storage, oxidative and nonoxidative glycolysis, and the glucose entering plasma via gluconeogenesis after ingestion of a meal in 11 normal volunteers. During a 6-h postprandial period, a total of approximately 98 g of glucose were disposed of. This was more than the glucose contained in the meal ( approximately 78 g) due to persistent endogenous glucose release ( approximately 21 g): splanchnic tissues initially took up approximately 23 g, and an additional approximately 75 g were removed from the systemic circulation. Direct glucose storage accounted for approximately 32 g and glycolysis for approximately 66 g (oxidative approximately 43 g and nonoxidative approximately 23 g). About 11 g of glucose appeared in plasma as a result of gluconeogenesis. If these carbons were wholly from glucose undergoing glycolysis, only approximately 12 g would be available for indirect pathway glycogen formation. Our results thus indicate that glycolysis is the main initial postprandial fate of glucose, accounting for approximately 66% of overall disposal; oxidation and storage each account for approximately 45%. The majority of glycogen is formed via the direct pathway ( approximately 73%).     

Hydroxycitric acid delays intestinal glucose absorption in rats

In this study, we investigated in rats if hydroxycitric acid (HCA) reduces the postprandial glucose response by affecting gastric emptying or intestinal glucose absorption. We compared the effect of regulator HCA (310 mg/kg) and vehicle (control) on the glucose response after an intragastric or intraduodenal glucose load to investigate the role of altered gastric emptying. Steele's one-compartment model was used to investigate the effect of HCA on systemic glucose appearance after an intraduodenal glucose load, using [U-(13)C]-labeled glucose and d-[6,6-(2)H(2)]-labeled glucose. Because an effect on postabsorptive glucose clearance could not be excluded, the effect of HCA on the appearance of enterally administered glucose in small intestinal tissue, liver, and portal and systemic circulation was determined by [U-(14)C]glucose infusion. Data show that HCA treatment delays the intestinal absorption of enterally administered glucose at the level of the small intestinal mucosa in rats. HCA strongly attenuated postprandial blood glucose levels after both intragastric (P < 0.01) and intraduodenal (P < 0.001) glucose administration, excluding a major effect of HCA on gastric emptying. HCA delayed the systemic appearance of exogenous glucose but did not affect the total fraction of glucose absorbed over the study period of 150 min. HCA treatment decreased concentrations of [U-(14)C]glucose in small intestinal tissue at 15 min after [U-(14)C]glucose administration (P < 0.05), in accordance with the concept that HCA delays the enteral absorption of glucose. These data support a possible role for HCA as food supplement in lowering postprandial glucose profiles.     

Calculating glucose fluxes during meal tolerance test: a new computational approach

A new calculation method is proposed to quantify the endogenous glucose production (EGP), the glucose appearance rate due to meal ingestion (R(a meal)), and the glucose disposal (R(d)) during a three-tracer study design. The method utilizes the maximum likelihood theory combined with a regularization method to achieve a theoretically coherent computational framework. The method uses the two-compartment formulation of the glucose kinetics. Instead of assuming smoothness of unlabeled and labeled glucose concentrations, the method assumes that the EGP, the R(a meal), and the fractional glucose clearance are smooth, increasing plausibility of their individual estimates. The method avoids transformation of the measurement errors, which may skew the estimates of the EGP, R(a meal), and R(d) with the traditional approach. Finally, the sequential nature of the calculations is replaced by calculating the EGP, R(a meal), and R(d) in "one go" to avoid the propagation of the errors from the EGP and R(a meal) into R(d). An example study is shown demonstrating the utility of the approach. A better performance of the new method is demonstrated in a simulation study.     

Stereoselective synthesis of chirally deuterated (S)-D-(6-(2)H(1))glucose

Chirally deuterated (S)-D-(6-(2)H(1))glucose has been prepared in good overall yield from d-(6,6'-(2)H(2))glucose by a short, five-step synthesis from D-(6,6-(2)H(2))glucose utilizing (R)-(+)-Alpine-Borane [(R)-9-[(6,6-dimethylbicyclo[3.1.1]hept-2-yl)methyl]-9-borabicyclo[3.3.1]nonane]. Suitably protected methyl 2,3,4-tri-O-benzyl-D-(6,6-(2)H(2))glucopyranoside was prepared and the deuterated O-6 primary alcohol was oxidized to an aldehyde by Swern oxidation. Stereoselective reduction with nondeuterated (R)-(+)-Alpine-Borane gave methyl 2,3,4-tri-O-benzyl-(6S)-D-(6-(2)H(1))glucopyranoside, which was deprotected under standard conditions to afford the title compound. The key stereoselective reduction step was achieved in 90% yield. The preparation uses economical, commercially available starting materials and will be useful for elucidating biosynthetic mechanisms.     

Mechanism of action of exenatide to reduce postprandial hyperglycemia in type 2 diabetes

We examined the contributions of insulin secretion, glucagon suppression, splanchnic and peripheral glucose metabolism, and delayed gastric emptying to the attenuation of postprandial hyperglycemia during intravenous exenatide administration. Twelve subjects with type 2 diabetes (3 F/9 M, 44 +/- 2 yr, BMI 34 +/- 4 kg/m2, Hb A(1c) 7.5 +/- 1.5%) participated in three meal-tolerance tests performed with double tracer technique (iv [3-3H]glucose and oral [1-14C]glucose): 1) iv saline (CON), 2) iv exenatide (EXE), and 3) iv exenatide plus glucagon (E+G). Acetaminophen was given with the mixed meal (75 g glucose, 25 g fat, 20 g protein) to monitor gastric emptying. Plasma glucose, insulin, glucagon, acetaminophen concentrations and glucose specific activities were measured for 6 h post meal. Post-meal hyperglycemia was markedly reduced (P < 0.01) in EXE (138 +/- 16 mg/dl) and in E+G (165 +/- 12) compared with CON (206 +/- 15). Baseline plasma glucagon ( approximately 90 pg/ml) decreased by approximately 20% to 73 +/- 4 pg/ml in EXE (P < 0.01) and was not different from CON in E+G (81 +/- 2). EGP was suppressed by exenatide [231 +/- 9 to 108 +/- 8 mg/min (54%) vs. 254 +/- 29 to189 +/- 27 mg/min (26%, P < 0.001, EXE vs. CON] and partially reversed by glucagon replacement [247 +/- 15 to 173 +/- 18 mg/min (31%)]. Oral glucose appearance was 39 +/- 4 g in CON vs. 23 +/- 6 g in EXE (P < 0.001) and 15 +/- 5 g in E+G, (P < 0.01 vs. CON). The glucose retained within the splanchnic bed increased from approximately 36g in CON to approximately 52g in EXE and to approximately 60g in E+G (P < 0.001 vs. CON). Acetaminophen((AUC)) was reduced by approximately 80% in EXE vs. CON (P < 0.01). We conclude that exenatide infusion attenuates postprandial hyperglycemia by decreasing EGP (by approximately 50%) and by slowing gastric emptying.     

Mechanisms for abnormal postprandial glucose metabolism in type 2 diabetes

To assess mechanisms for postprandial hyperglycemia, we used a triple-isotope technique ([\3-(3)H]glucose and [(14)C]bicarbonate and oral [6,6-dideutero]glucose iv) and indirect calorimetry to compare components of glucose release and pathways for glucose disposal in 26 subjects with type 2 diabetes and 15 age-, weight-, and sex-matched normal volunteers after a standard meal. The results were as follows: 1) diabetic subjects had greater postprandial glucose release (P<0.001) because of both increased endogenous and meal-glucose release; 2) the greater endogenous glucose release (P<0.001) was due to increased gluconeogenesis (P<0.001) and glycogenolysis (P=0.01); 3) overall tissue glucose uptake, glycolysis, and storage were comparable in both groups (P>0.3); 4) glucose clearance (P<0.001) and oxidation (P=0.004) were reduced, whereas nonoxidative glycolysis was increased (P=0.04); and 5) net splanchnic glucose storage was reduced by approximately 45% (P=0.008) because of increased glycogen cycling (P=0.03). Thus in type 2 diabetes, postprandial hyperglycemia is primarily due to increased glucose release; hyperglycemia overcomes the effects of impaired insulin secretion and sensitivity on glucose transport, but intracellular defects persist so that pathways of glucose metabolism are abnormal and glucose is shunted away from normal sites of storage (e.g., liver and muscle) into other tissues.     

Effect of IGF-I on FFA and glucose metabolism in control and type 2 diabetic subjects

The effects of insulin-like growth factor I (IGF-I) and insulin on free fatty acid (FFA) and glucose metabolism were compared in eight control and eight type 2 diabetic subjects, who received a two-step euglycemic hyperinsulinemic (0.25 and 0.5 mU x kg(-1) x min(-1)) clamp and a two-step euglycemic IGF-I (26 and 52 pmol x kg(-1) x min(-1)) clamp with [3-(3)H]glucose, [1-(14)C]palmitate, and indirect calorimetry. The insulin and IGF-I infusion rates were chosen to augment glucose disposal (R(d)) to a similar extent in control subjects. In type 2 diabetic subjects, stimulation of R(d) (second clamp step) in response to both insulin and IGF-I was reduced by approximately 40-50% compared with control subjects. In control subjects, insulin was more effective than IGF-I in suppressing endogenous glucose production (EGP) during both clamp steps. In type 2 diabetic subjects, insulin-mediated suppression of EGP was impaired, whereas EGP suppression by IGF-I was similar to that of controls. In both control and diabetic subjects, IGF-I-mediated suppression of plasma FFA concentration and inhibition of FFA turnover were markedly impaired compared with insulin (P < 0.01-0.001). During the second IGF-I clamp step, suppression of plasma FFA concentration and FFA turnover was impaired in diabetic vs. control subjects (P < 0.05-0.01). CONCLUSIONS: 1) IGF-I is less effective than insulin in suppressing EGP and FFA turnover; 2) insulin-resistant type 2 diabetic subjects also exhibit IGF-I resistance in skeletal muscle. However, suppression of EGP by IGF-I is not impaired in diabetic individuals, indicating normal hepatic sensitivity to IGF-I.     

Enteral infusion of glucose at rates approximating EGP enhances glucose disposal but does not cause hypoglycemia

Portal infusion of glucose at rates approximating endogenous glucose production (EGP) causes paradoxical hypoglycemia in wild-type but not GLUT2 null mice, implying activation of a specific portal glucose sensor. To determine whether this occurs in humans, glucose containing [3-3H]glucose was infused intraduodenally at rates of 3.1 mg. kg-1. min-1 (n = 5), 1.55 mg. kg-1. min-1 (n = 9), or 0/0.1 mg. kg-1. min-1 (n = 9) for 7 h in healthy nondiabetic subjects. [6,6-2H2]glucose was infused intravenously to enable simultaneous measurement of EGP, glucose disappearance, and the rate of appearance of the intraduodenally infused glucose. Plasma glucose concentrations fell (P < 0.01) from 90 +/- 1 to 84 +/- 2 mg/dl during the 0/0.1 mg. kg-1. min-1 id infusions but increased (P < 0.001) to 104 +/- 5 and 107 +/- 3 mg/dl, respectively, during the 1.55 and 3.1 mg. kg-1. min-1 id infusions. In contrast, insulin increased (P < 0.05) during the 1.55 and 3.0 mg. kg-1. min-1 infusions, reaching a peak of 10 +/- 2 and 18 +/- 5 micro U/ml, respectively, by 2 h. Insulin concentrations then fell back to concentrations that no longer differed by study end (7 +/- 1 vs. 8 +/- 1 micro U/ml). This resulted in comparable suppression of EGP by study end (0.84 +/- 0.2 and 0.63 +/- 0.1 mg. kg-1. min-1). Glucose disappearance was higher (P < 0.01) during the final hour of the 3.1 than 1.55 mg. kg-1. min-1 id infusion (4.47 +/- 0.2 vs. 2.6 +/- 0.1 mg. kg-1. min-1), likely because of the slightly, but not significantly, higher glucose and insulin concentrations. We conclude that, in contrast to mice, selective portal glucose delivery at rates approximating EGP does not cause hypoglycemia in humans.     

Autoregulation of glucose production in men with a glycerol load during rest and exercise

Related to hepatic autoregulation we evaluated hypotheses that 1) glucose production would be altered as a result of a glycerol load, 2) decreased glucose recycling rate (Rr) would result from increased glycerol uptake, and 3) the absolute rate of gluconeogenesis (GNG) from glycerol would be positively correlated to glycerol rate of disappearance (R(d)) during a glycerol load. For these purposes, glucose and glycerol kinetics were determined in eight men during rest and during 90 min of leg cycle ergometry at 45 and 65% of peak O2 consumption (.VO2 (peak)). Trials were conducted after an overnight fast, with exercise commencing 12 h after the last meal. Subjects received a continuous infusion of [6,6-(2)H(2)]glucose, [1-(13)C]glucose, and [1,1,2,3,3-(2)H(5)]glycerol without (CON) or with an additional 1,000 mg (rest: 20 mg/min; exercise: 40 mg/min) of [2-(13)C]- or unlabeled glycerol added to the infusate (GLY). Infusion of glycerol dampened glucose Rr, calculated as the difference between [6,6-(2)H(2)]- and [1-(13)C]glucose rates of appearance (R(a)), at rest [0.35 +/- 0.12 (CON) vs. 0.12 +/- 0.10 mg. kg(-1). min(-1) (GLY), P < 0.05] and during exercise at both intensities [45%: 0.63 +/- 0.14 (CON) vs. 0.04 +/- 0.12 (GLY); 65%: 0.73 +/- 0.14 (CON) vs. 0.04 +/- 0.17 mg. kg(-1). min(-1) (GLY), P < 0.05]. Glucose R(a) and oxidation were not affected by glycerol infusion at rest or during exercise. Throughout rest and both exercise intensities, glycerol R(d) was greater in GLY vs. CON conditions (rest: 0.30 +/- 0.04 vs. 0.58 +/- 0.04; 45%: 0.57 +/- 0.07 vs. 1.19 +/- 0.04; 65%: 0.73 +/- 0.06 vs. 1.27 +/- 0.05 mg. kg(-1). min(-1), CON vs. GLY, respectively). Differences in glycerol R(d) (DeltaR(d)) between protocols equaled the unlabeled glycerol infusion rate and correlated with plasma glycerol concentration (r = 0.97). We conclude that infusion of a glycerol load during rest and exercise at 45 and 65% of .VO2(peak) 1) does not affect glucose R(a) or R(d), 2) blocks glucose Rr, 3) increases whole body glycerol R(d) in a dose-dependent manner, and 4) results in gluconeogenic rates from glycerol equivalent to CON glucose recycling rates.     

Effect of macronutrients, age, and obesity on 6- and 24-h postprandial glucose metabolism in cats

Obesity and age are risk factors for feline diabetes. This study aimed to test the hypothesis that age, long-term obesity, and dietary composition would lead to peripheral and hepatorenal insulin resistance, indicated by higher endogenous glucose production (EGP) in the fasted and postprandial state, higher blood glucose and insulin, and higher leptin, free thyroxine, and lower adiponectin concentrations. Using triple tracer-(2)H(2)O, [U-(13)C(3)] propionate, and [3,4-(13)C(2)] glucose infusion, and indirect calorimetry-we investigated carbohydrate and fat metabolic pathways in overnight-fasted neutered cats (13 young lean, 12 old lean, and 12 old obese), each fed three different diets (high protein with and without polyunsaturated fatty acids, and high carbohydrate) in a crossover design. EGP was lowest in fasted and postprandial obese cats despite peripheral insulin resistance, indicated by hyperinsulinemia. Gluconeogenesis was the most important pathway for EGP in all groups, but glycogen contributed significantly. Insulin and leptin concentrations were higher in old than in young lean cats; adiponectin was lowest in obese cats but surprisingly highest in lean old cats. Diet had little effect on metabolic parameters. We conclude that hepatorenal insulin resistance does not develop in the fasted or postprandial state, even in long-term obese cats, allowing the maintenance of euglycemia through lowering EGP. Glycogen plays a major role in EGP, especially in lean fasted cats, and in the postprandial state. Aging may predispose to insulin resistance, which is a risk factor for diabetes in cats. Mechanisms underlying the high adiponectin of healthy old lean cats need to be further explored.     

Glucose oxidation and nonoxidative glucose disposal during prolonged fasts of the northern elephant seal pup (Mirounga angustirostris)

Elephant seal weanlings demonstrate rates of endogenous glucose production (EGP) during protracted fasts that are higher than predicted on the basis of mass and time fasting. To determine the nonoxidative and oxidative fate of endogenously synthesized glucose, substrate oxidation, metabolic rate, glycolysis, and EGP were measured in fasting weanlings. Eight weanlings were sampled at 14 days of fasting, and a separate group of nine weanlings was sampled at 49 days of fasting. Metabolic rate was determined via flow-through respirometry, and substrate-specific oxidation was determined from the respiratory quotient and urinary nitrogen measurements. The rate of glucose disposal (Glu((R)(d))) was determined through a primed, constant infusion of [3-(3)H]glucose, and glycolysis was determined from the rate of appearance of (3)H in the body water pool. Glu((R)(d)) was 1.41 ± 0.27 and 0.95 ± 0.21 mmol/min in the early and late fasting groups, respectively. Nearly all EGP went through glycolysis, but the percentage of Glu((R)(d)) oxidized to meet the daily metabolic demand was only 24.1 ± 4.4% and 16.7 ± 5.9% between the early and late fasting groups. Glucose oxidation was consistently less than 10% of the metabolic rate in both groups. This suggests that high rates of EGP do not support substrate provisions for glucose-demanding tissues. It is hypothesized that rates of EGP may be ancillary to the upregulation of the tricarboxylic acid cycle to meet high rates of lipid oxidation while mitigating ketosis.     

Regulation of endogenous glucose production after a mixed meal in type 2 diabetes

The extent and time course of suppression of endogenous glucose production (EGP) in type 2 diabetes after a mixed meal have been determined using a new tracer methodology. Groups of age-, sex-, and weight-matched normal controls (n = 8) and diet-controlled type 2 diabetic subjects (n = 8) were studied after ingesting a standard mixed meal (550 kcal; 67% carbohydrate, 19% fat, 14% protein). There was an early insulin increment in both groups such that, by 20 min, plasma insulin levels were 266 +/- 54 and 190 +/- 53 pmol/l, respectively. EGP was similar basally [2.55 +/- 0.12 mg x kg(-1) x min(-1) in control subjects vs. 2.92 +/- 0.16 mg x kg(-1) x min(-1) in the patients (P = 0.09)]. After glucose ingestion, EGP declined rapidly in both groups to approximately 50% of basal within 30 min of the meal. Despite the initial rapid decrease, the EGP was significantly greater in the diabetic group at 60 min (1.75 +/- 0.12 vs. 1.05 +/- 0.14 mg x kg(-1) x min(-1); P < 0.01) and did not reach nadir until 210 min (0.96 +/- 0.17 mg x kg(-1) x min(-1)). Between 60 and 240 min, EGP was 47% higher in the diabetic group (0.89 +/- 0.09 vs. 1.31 +/- 0.13 mg x kg(-1) x min(-1), P < 0.02). These data quantitate the initial rapid suppression of EGP after a mixed meal in type 2 diabetes and the contribution of continuing excess glucose production to subsequent hyperglycemia.     

Measurement of selective effect of insulin on glucose disposal from labeled glucose oral test minimal model

The oral glucose minimal model (OMM) measures insulin sensitivity (S(I)) and the glucose rate of appearance (R(a)) of ingested glucose in the presence of physiological changes of insulin and glucose concentrations. However, S(I) of OMM measures the overall effect of insulin on glucose utilization and glucose production. In this study we show that, by adding a tracer to the oral dose, e.g., of a meal, and by using the labeled version of OMM, OMM* to interpret the data, one can measure the selective effect of insulin on glucose disposal, S(I)*. Eighty-eight individuals underwent both a triple-tracer meal with the tracer-to-tracee clamp technique, providing a model-independent reference of the R(a) of ingested glucose (R(a meal)(ref)) and an insulin-modified labeled intravenous glucose tolerance test (IVGTT*). We show that OMM* provides not only a reliable means of tracing the R(a) of ingested glucose (R(a meal)) but also accurately measures S(I)*. We do so by comparing OMM* R(a meal) with the model-independent R(a meal)(ref) provided by the tracer-to-tracee clamp technique, while OMM* S(I)* is compared with both S(I)(* ref), obtained by using as known input R(a meal)(ref), and with S(I)* measured during IVGTT*.     

Differential effect of saturated and polyunsaturated fatty acids on hepatic glucose metabolism in humans

Prolonged infusions of lipid and heparin that achieve high physiological free fatty acid (FFA) concentrations inhibit hepatic (and peripheral) insulin sensitivity in humans. These infusions are composed largely of polyunsaturated fatty acids (PUFA; linoleic and linolenic). It is not known whether fatty acid composition per se affects hepatic glucose metabolism in humans. To address this issue, we examined the impact of enteral infusions of either palm oil (48% palmitic, 35% oleic, and 8% linoleic acids) or safflower oil (6% palmitic, 12% oleic, 74% linoleic acids) in 14 obese nondiabetic subjects. (2)H(2)O was administered to determine the contribution of gluconeogenesis to endogenous glucose production (EGP), and a primed continuous infusion of [6,6-(2)H]glucose was administered to assess glucose appearance. As a result of the lipid infusions, plasma FFA concentrations increased significantly in both the palm oil (507.5 +/- 47.4 to 939.3 +/- 61.3 micromol/l, P < 0.01) and safflower oil (588.2.0 +/- 43.0 to 857.8 +/- 68.7 micromol/l, P < 0.01) groups after 4 h. EGP was similar at baseline (12.4 +/- 1.8 vs. 11.2 +/- 1.0 micromol x kg FFM(-1) x min(-1)). During a somatostatin-insulin clamp, the glucose infusion rate was significantly lower (AUC glucose infusion rate 195.8 +/- 50.7 vs. 377.8 +/- 38.0 micromol/kg FFM, P < 0.01), and rates of EGP were significantly higher (10.7 +/- 1.4 vs. 6.5 +/- 1.5 micromol x kg FFM(-1) x min(-1), P < 0.01) after palm oil compared with safflower oil, respectively. Baseline rates of gluconeogenesis and glycogenolysis were also similar. However, after lipid infusion, rates of glycogenolysis were suppressed by safflower oil but not by palm oil. Thus these studies demonstrate, for the first time in humans, a differential effect of saturated fatty acids and PUFA on hepatic glucose metabolism.