Effects of type 2 diabetes on insulin secretion, insulin action, glucose effectiveness, and postprandial glucose metabolism

OBJECTIVE

In this study, we sought to determine whether postprandial insulin secretion, insulin action, glucose effectiveness, and glucose turnover were abnormal in type 2 diabetes.

RESEARCH DESIGN AND METHODS

Fourteen subjects with type 2 diabetes and 11 nondiabetic subjects matched for age, weight, and BMI underwent a mixed-meal test using the triple-tracer technique. Indexes of insulin secretion, insulin action, and glucose effectiveness were assessed using the oral "minimal" and C-peptide models.

RESULTS

Fasting and postprandial glucose concentrations were higher in the diabetic than nondiabetic subjects. Although peak insulin secretion was delayed (P < 0.001) and lower (P < 0.05) in type 2 diabetes, the integrated total postprandial insulin response did not differ between groups. Insulin action, insulin secretion, disposition indexes, and glucose effectiveness all were lower (P < 0.05) in diabetic than in nondiabetic subjects. Whereas the rate of meal glucose appearance did not differ between groups, the percent suppression of endogenous glucose production (EGP) was slightly delayed and the increment in glucose disappearance was substantially lower (P < 0.01) in diabetic subjects during the first 3 h after meal ingestion. Together, these defects resulted in an excessive rise in postprandial glucose concentrations in the diabetic subjects.

CONCLUSIONS

When measured using methods that avoid non-steady-state error, the rate of appearance of ingested glucose was normal and suppression of EGP was only minimally impaired. However, when considered in light of the prevailing glucose concentration, both were abnormal. In contrast, rates of postprandial glucose disappearance were substantially decreased due to defects in insulin secretion, insulin action, and glucose effectiveness.

A gene network simulator to assess reverse engineering algorithms

In the context of reverse engineering of biological networks, simulators are helpful to test and compare the accuracy of different reverse-engineering approaches in a variety of experimental conditions. A novel gene-network simulator is presented that resembles some of the main features of transcriptional regulatory networks related to topology, interaction among regulators of transcription, and expression dynamics. The simulator generates network topology according to the current knowledge of biological network organization, including scale-free distribution of the connectivity and clustering coefficient independent of the number of nodes in the network. It uses fuzzy logic to represent interactions among the regulators of each gene, integrated with differential equations to generate continuous data, comparable to real data for variety and dynamic complexity. Finally, the simulator accounts for saturation in the response to regulation and transcription activation thresholds and shows robustness to perturbations. It therefore provides a reliable and versatile test bed for reverse engineering algorithms applied to microarray data. Since the simulator describes regulatory interactions and expression dynamics as two distinct, although interconnected aspects of regulation, it can also be used to test reverse engineering approaches that use both microarray and protein-protein interaction data in the process of learning. A first software release is available at http://www.dei.unipd.it/~dicamill/software/netsim as an R programming language package.

beta-Cell function and insulin sensitivity in adolescents from an OGTT

Given the increase in the incidence of insulin resistance, obesity, and type 2 diabetes in children and adolescents, it would be of paramount importance to assess quantitative indices of insulin secretion and action during a physiological perturbation, such as a meal or an oral glucose-tolerance test (OGTT). A minimal model method is proposed to measure quantitative indices of insulin secretion and action in adolescents from an oral test. A 7 h, 21-sample OGTT was performed in 11 adolescents. The C-peptide minimal model was identified on C-peptide and glucose data to quantify indices of beta-cell function: static phi(s) and dynamic phi(d) responsivity to glucose from which total responsivity phi was also measured. The glucose minimal model was identified on glucose and insulin data to estimate insulin sensitivity, S(I), which was compared to a reference measure, S(I)(ref), provided by a tracer method. Disposition indices, which adjust insulin secretion for insulin action, were then calculated. Indices of beta-cell function were phi(s) = 51.35 +/- 8.89 x 10(-9)min(-1), phi(d) = 1,392 +/- 258 x 10(-9), and phi = 82.09 +/- 17.70 x 10(-9)min(-1). Insulin sensitivity was S(I) = 14.19 +/- 2.73 x 10(-4), not significantly different from S(I)(ref) = 14.96 +/- 3.04 x 10(-4) dl/kg.min per microU/ml, and well correlated: r = 0.98, P < 0.0001, thus indicating that S(I) can be accurately measured from an oral test. Disposition indices were DI(s) = 1,040 +/- 201 x 10(-14) dl/kg/min(2) per pmol/l, DI(d) = 33,178 +/- 10,720 x 10(-14) dl/kg/min per pmol/l, DI = 1,844 +/- 522 x 10(-14) dl/kg/min(2) per pmol/l. Virtually the same minimal model assessment was obtained with a reduced 3 h, 9-sample protocol. OGTT interpreted with C-peptide and glucose minimal model has the potential to provide novel insight regarding the regulation of glucose metabolism in adolescents, and to evaluate the effect of obesity and interventions such as diet and exercise.

Dipeptidyl peptidase-4 inhibition by vildagliptin and the effect on insulin secretion and action in response to meal ingestion in type 2 diabetes

OBJECTIVE

The purpose of this study was to determine the mechanism by which dipeptidyl peptidase-4 inhibitors lower postprandial glucose concentrations.

RESEARCH DESIGN AND METHODS

We measured insulin secretion and action as well as glucose effectiveness in 14 subjects with type 2 diabetes who received vildagliptin (50 mg b.i.d.) or placebo for 10 days in random order separated by a 3-week washout. On day 9 of each period, subjects ate a mixed meal. Insulin sensitivity (S(I)), glucose effectiveness, and beta-cell responsivity indexes were estimated using the oral glucose and C-peptide minimal models. At 300 min 0.02 unit/kg insulin was administered intravenously.

RESULTS

Vildagliptin reduced postprandial glucose concentrations (905 +/- 94 vs. 1,008 +/- 104 mmol/6 h, P = 0.02). Vildagliptin did not alter net S(I) (7.71 +/- 1.28 vs. 6.41 +/- 0.84 10(-4) dl x kg(-1) x min(-1) x muU(-1) x ml(-1), P = 0.13) or glucose effectiveness (0.019 +/- 0.002 vs. 0.018 +/- 0.002 dl x kg(-1) x min(-1), P = 0.65). However, the net beta-cell responsivity index was increased (35.7 +/- 5.2 vs. 28.9 +/- 5.2 10(-9) min(-1), P = 0.03) as was total disposition index (381 +/- 48 vs. 261 +/- 35 10(-14) dl x kg(-1) x min(-2) x pmol(-1) x l(-1), P = 0.006). Vildagliptin lowered postprandial glucagon concentrations (27.0 +/- 1.1 vs. 29.7 +/- 1.5 microg x l(-1) x 6 h(-1), P = 0.03), especially after administration of exogenous insulin (81.5 +/- 6.4 vs. 99.3 +/- 5.6 ng/l, P = 0.02).

CONCLUSIONS

Vildagliptin lowers postprandial glucose concentrations by stimulating insulin secretion and suppressing glucagon secretion but not by altered insulin action or glucose effectiveness. A novel observation is that vildagliptin alters alpha-cell responsiveness to insulin administration, but the significance of this action is as yet unclear.

Use of labeled oral minimal model to measure hepatic insulin sensitivity

The ability to accurately quantify indexes of the individual role of glucose (GE(L)) and insulin (S(I)(L)) in the suppression of endogenous glucose production (EGP) would improve the understanding of liver metabolism. Measuring these indexes during an IVGTT by minimal modeling of tracer labeled and unlabeled glucose data is often unreliable, possibly due to an inadequate description of EGP included in the Minimal Model. Moreover, a validation of the assumptions of the Minimal Model on EGP data has never been done. Recently, Krudys et al. (Krudys KM, Dodds MG, Nissen SM, Vicini P. Am J Physiol Endocrinol Metab 288: E1038-E1046, 2005) have proposed a PK/PD (pharmacokinetic/pharmacodynamic) model of the EGP profile that occurs during an intravenous glucose tolerance test (IVGTT); however, this model has also not been validated. The aim of this study was thus to test the Minimal Model, the PK/PD model, and six alternative EGP descriptions on recent model-independent EGP data of 20 subjects obtained with a triple-tracer meal protocol. Model performance was compared in terms of data fit, precision of the estimated parameters, and physiological plausibility. Neither the PK/PD nor the traditional Minimal Model were able to accurately describe EGP data or provide reliable estimates of the indexes. In contrast, one of the new models performed best by showing a good fit and providing accurate and precise estimates of hepatic sensitivity indexes: GE(L) = 0.013 +/- 0.001 dl x kg(-1) x min(-1); S(I)(L) = 5.34 +/- 0.47 10(-4) dl x kg(-1) x min(-1) per microU/ml (42 and 34%, respectively, of total sensitivity indexes GE(TOT) and S(I)(TOT)). Although this model requires further validation, it has the potential to improve our understanding of the role of the liver in pathophysiological states.

A model of glucose production during a meal

The efficiency of glucose and insulin control on glucose production (EGP) plays an important role in glucose homeostasis and its derangement in diabetes. Therefore the ability to accurately quantify indices of the individual role of glucose (GE(L)) and insulin (S(I)(L)) in the suppression of EGP would allow to improve the understanding of liver metabolism. Measuring these indices by minimal modelling of tracer labelled and unlabelled glucose data is often unreliable, possibly due to an inadequate description of EGP included in the minimal model (EGP(MM)). Moreover a validation of EGP(MM) on EGP data has never been done. Here EGP(MM) and alternative EGP descriptions were tested on recent model-independent EGP data of 20 subjects obtained with a triple-tracer meal protocol. Model performances were compared in terms of data fit and physiological plausibility. EGP(MM) was not able to describe EGP data, while one of the new model showed a good fit and provided accurate and precise estimates of hepatic sensitivity indices: GE(L) = 0.013 +/- 0.001 dl/kg/min; S(I)(L) =5.71 +/- 0.48 10(-4) dl/kg/min per microU/ml (36% and 41%, respectively, of total sensitivity indices GE(TOT) and S(I)(TOT)). This novel approach will allow to enhance our understanding of the role of the liver in pathophysiological states.

Glucose production by deconvolution in intravenous and oral glucose tolerance tests: role of output variable

Endogenous glucose production (EGP) after a glucose stimulus can be estimated by deconvolution of the endogenous component of glucose concentration, which is computed from noisy measurements. This study analyzes how measurement errors propagate to endogenous glucose and affect EGP reconstruction during intravenous (IVGTT) and oral (MEAL) glucose tolerance tests. Monte Carlo simulations show that the effect of errors on endogenous glucose and thus on EGP is more critical during IVGTT than during MEAL. A two regularization-parameter deconvolution technique for IVGTT is proposed, which successfully handles this added difficulty.

Short-term high dietary fructose intake had no effects on insulin sensitivity and secretion or glucose and lipid metabolism in healthy, obese adolescents

There is virtually no information on the metabolic impact of dietary fructose intake in adolescents despite their high fructose consumption, particularly via sweetened beverages.

AIM

To determine the short-term metabolic effects of dietary fructose intake in obese adolescents.

METHODS

Six volunteers (3 M/3 F; 15.2 +/- 0.5 yr; 35 +/- 2 kg/m2; 39 +/- 2% body fat) were studied twice following 7 d of isocaloric, isonitrogenous high carbohydrate (60% CHO; 25% fat) diets with fructose accounting for 6% and 24% of total energy intake, respectively (random order). Insulin sensitivity and secretion were analyzed by the stable labeled intravenous glucose tolerance test and glucose and lipid kinetics using GCMS.

RESULTS

A fourfold increase in dietary fructose intake did not affect insulin sensitivity or secretion, glucose kinetics, lipolysis or glucose, insulin, C-peptide, triglycerides, HDL- and LDL-cholesterol concentrations.

CONCLUSIONS

In the short term, when energy intake is constant, dietary fructose per se is not a contributor to insulin resistance and hypersecretion in obese adolescents.

Glucose fluxes during OGTT in adolescents assessed by a stable isotope triple tracer method

Virtually no information is available on glucose fluxes during a meal or glucose ingestion in adolescents.

AIM

To use a triple tracer approach to measure rates of appearance of ingested glucose (Ra(ogtt)), endogenous glucose production (EGP) and glucose disappearance (Rd) following an oral glucose bolus in adolescents.

METHODS

Eleven adolescents (4 M/7 F, 15 +/- 1 yr; 67.3 +/- 4.7 kg; 24 +/-2 kg/m2) underwent a frequent sampled oral glucose tolerance test (OGTT) (labelled with [6,6-2H2]glucose) combined with intravenous infusion of [1-(13)C]glucose and [U-(13)C6]glucose following an overnight fast. Formulas were developed to estimate glucose fluxes using one- or two-compartment models.

RESULTS

During the 7 h following the OGTT bolus, 9.8 +/- 2.3% of the ingested glucose was extracted by the liver, EGP was suppressed by 45 +/- 4% and Rd increased by 21 +/- 5%.

CONCLUSIONS

The triple tracer method provided accurate assessment of Ra(ogtt), EGP and Rd fluxes during an OGTT in adolescents. Thus, this method might provide novel insight on postprandial glucose fluxes in children/adolescents under various conditions.

Effect of 2 years of testosterone replacement on insulin secretion, insulin action, glucose effectiveness, hepatic insulin clearance, and postprandial glucose turnover in elderly men

OBJECTIVE

We sought to determine whether, and if so the mechanism by which, testosterone replacement improves carbohydrate tolerance.

RESEARCH DESIGN AND METHODS

Fifty-five elderly men with relative testosterone deficiency ingested a labeled mixed meal and underwent a frequently sampled labeled intravenous glucose tolerance test before and after either placebo or treatment with testosterone patch (5 mg/day) for 2 years.

RESULTS

Despite restoring bioavailable testosterone to values observed in young men, the change (24 months minus baseline values) in fasting and postprandial glucose, insulin, and C-peptide concentrations and meal appearance, glucose disposal, and endogenous glucose production were virtually identical to those observed after 2 years of placebo. The change over time in insulin and C-peptide concentrations post-intravenous glucose injection also did not differ. Furthermore, the change over time in insulin action and glucose effectiveness (measured with the unlabeled and labeled "oral" and "intravenous" minimal models), as well as insulin secretion and hepatic insulin clearance (measured with the C-peptide model), did not differ in the testosterone and placebo groups.

CONCLUSIONS

We conclude that 2 years of treatment with testosterone in elderly men does not improve carbohydrate tolerance or alter insulin secretion, insulin action, glucose effectiveness, hepatic insulin clearance, or the pattern of postprandial glucose metabolism. Thus, testosterone deficiency is unlikely the cause of the age-associated deterioration in glucose tolerance commonly observed in elderly men.

Contribution of hepatic and extrahepatic insulin resistance to the pathogenesis of impaired fasting glucose: role of increased rates of gluconeogenesis

OBJECTIVE

To determine the contribution of hepatic insulin resistance to the pathogenesis of impaired fasting glucose (IFG).

RESEARCH DESIGN AND METHODS

Endogenous glucose production (EGP) and glucose disposal were measured in 31 subjects with IFG and 28 subjects with normal fasting glucose (NFG) after an overnight fast and during a clamp when endogenous secretion was inhibited with somatostatin and insulin infused at rates that approximated portal insulin concentrations present in IFG subjects after an overnight fast (approximately 80 pmol/l, "preprandial") or within 30 min of eating (approximately 300 pmol/l, "prandial").

RESULTS

Despite higher (P < 0.001) insulin and C-peptide concentrations and visceral fat (P < 0.05), fasting EGP and glucose disposal did not differ between IFG and NFG subjects, implying hepatic and extrahepatic insulin resistance. This was confirmed during preprandial insulin infusion when glucose disposal was lower (P < 0.05) and EGP higher (P < 0.05) in IFG than in NFG subjects. Higher EGP was due to increased (P < 0.05) rates of gluconeogenesis in IFG. EGP was comparably suppressed in IFG and NFG groups during prandial insulin infusion, indicating that hepatic insulin resistance was mild. Glucose disposal remained lower (P < 0.01) in IFG than in NFG subjects.

CONCLUSIONS

Hepatic and extrahepatic insulin resistance contribute to fasting hyperglycemia in IFG with the former being due at least in part to impaired insulin-induced suppression of gluconeogenesis. However, since hepatic insulin resistance is mild and near-maximal suppression of EGP occurs at portal insulin concentrations typically present in IFG subjects within 30 min of eating, extrahepatic (but not hepatic) insulin resistance coupled with accompanying defects in insulin secretion is the primary cause of postprandial hyperglycemia.

Effects of nonglucose nutrients on insulin secretion and action in people with pre-diabetes

To determine whether nonglucose nutrient-induced insulin secretion is impaired in pre-diabetes, subjects with impaired or normal fasting glucose were studied after ingesting either a mixed meal containing 75 g glucose or 75 g glucose alone. Despite comparable glucose areas above basal, glucose-induced insulin secretion was higher (P < 0.05) and insulin action lower (P < 0.05) during the meal than the oral glucose tolerance test (OGTT) in all subgroups regardless of whether they had abnormal or normal glucose tolerance (NGT). However, the nutrient-induced delta (meal minus OGTT) in insulin secretion and glucagon concentrations did not differ among groups. Furthermore, the decrease in insulin action after meal ingestion was compensated in all groups by an appropriate increase in insulin secretion resulting in disposition indexes during meals that were equal to or greater than those present during the OGTT. In contrast, disposition indexes were reduced (P < 0.01) during the OGTT in the impaired glucose tolerance groups, indicating that reduced glucose induced insulin secretion. We conclude that, whereas glucose-induced insulin secretion is impaired in people with abnormal glucose tolerance, nonglucose nutrient-induced secretion is intact, suggesting that a glucose-specific defect in the insulin secretory pathway is an early event in the evolution of type 2 diabetes.

Significance analysis of microarray transcript levels in time series experiments

Background

Microarray time series studies are essential to understand the dynamics of molecular events. In order to limit the analysis to those genes that change expression over time, a first necessary step is to select differentially expressed transcripts. A variety of methods have been proposed to this purpose; however, these methods are seldom applicable in practice since they require a large number of replicates, often available only for a limited number of samples. In this data-poor context, we evaluate the performance of three selection methods, using synthetic data, over a range of experimental conditions. Application to real data is also discussed.

Results

Three methods are considered, to assess differentially expressed genes in data-poor conditions. Method 1 uses a threshold on individual samples based on a model of the experimental error. Method 2 calculates the area of the region bounded by the time series expression profiles, and considers the gene differentially expressed if the area exceeds a threshold based on a model of the experimental error. These two methods are compared to Method 3, recently proposed in the literature, which exploits splines fit to compare time series profiles. Application of the three methods to synthetic data indicates that Method 2 outperforms the other two both in Precision and Recall when short time series are analyzed, while Method 3 outperforms the other two for long time series.

Conclusion

These results help to address the choice of the algorithm to be used in data-poor time series expression study, depending on the length of the time series.

Two years of treatment with dehydroepiandrosterone does not improve insulin secretion, insulin action, or postprandial glucose turnover in elderly men or women

To determine if dehydroepiandrosterone (DHEA) replacement improves insulin secretion, insulin action, and/or postprandial glucose metabolism, 112 elderly subjects with relative DHEA deficiency ingested a labeled mixed meal and underwent a frequently sampled intravenous glucose tolerance test before and after 2 years of either DHEA or placebo. Despite restoring DHEA sulphate concentrations to values observed in young men and women, the changes over time in fasting and postprandial glucose concentrations, meal appearance, glucose disposal, and endogenous glucose production were identical to those observed after 2 years of placebo. The change over time in postmeal and intravenous glucose tolerance test insulin and C-peptide concentrations did not differ in men treated with DHEA or placebo. In contrast, postmeal and intravenous glucose tolerance test change over time in insulin and C-peptide concentrations were greater (P < 0.05) in women after DHEA than after placebo. However, since DHEA tended to decrease insulin action, the change over time in disposition indexes did not differ between DHEA- and placebo-treated women, indicating that the slight increase in insulin secretion was a compensatory response to a slight decrease in insulin action. We conclude that 2 years of replacement of DHEA in elderly men and women does not improve insulin secretion, insulin action, or the pattern of postprandial glucose metabolism.

Incretin effect potentiates beta-cell responsivity to glucose as well as to its rate of change: OGTT and matched intravenous study

The aim of this study is to gain greater insight into the mechanism whereby "incretins" (greater insulinemia after oral than intravenous glucose) enhance insulin secretion. To do so, we use a model of C-peptide secretion to reanalyze data from a previously published study in which glycemic profiles observed following glucose ingestion were matched in the same 10 subjects by means of an intravenous glucose infusion. We report that incretins increase insulin secretion by enhancing both the dynamic (to the rate of increase of glucose) and static (to given glucose concentration) response with an increase of 58% for the static (Phi(s) = 16.4 +/- 1.8 vs. 24.6 +/- 2.0 10(-9) min(-1), P = 0.01) and 63% for the dynamic (Phi(d) = 278 +/- 32 vs. 463 +/- 86 10(-9), P = 0.02) indexes. Since increases in the dynamic response to glucose are believed to be due to an increase in the rate of docking, and exocytosis of insulin containing granules and increases in the static response to glucose are believed to be caused by a shift in the sensitivity of the beta-cell to glucose, these results suggest that incretins may modulate more than one step in the beta-cell insulin secretory cascade.

Pathogenesis of pre-diabetes: mechanisms of fasting and postprandial hyperglycemia in people with impaired fasting glucose and/or impaired glucose tolerance

Thirty-two subjects with impaired fasting glucose (IFG) and 28 subjects with normal fasting glucose (NFG) ingested a labeled meal and 75 g glucose (oral glucose tolerance test) on separate occasions. Fasting glucose, insulin, and C-peptide were higher (P < 0.05) in subjects with IFG than in those with NFG, whereas endogenous glucose production (EGP) did not differ, indicating hepatic insulin resistance. EGP was promptly suppressed, and meal glucose appearance comparably increased following meal ingestion in both groups. In contrast, glucose disappearance (R(d)) immediately after meal ingestion was lower (P < 0.001) in subjects with IFG/impaired glucose tolerance (IGT) and IFG/diabetes but did not differ in subjects with IFG/normal glucose tolerance (NGT) or NFG/NGT. Net insulin action (S(i)) and insulin-stimulated glucose disposal (S(i)*) were reduced (P < 0.001, ANOVA) in subjects with NFG/IGT, IFG/IGT, and IFG/diabetes but did not differ in subjects with NFG/NGT or IFG/NGT. Defective insulin secretion also contributed to lower postprandial R(d) since disposition indexes were lower (P < 0.001, ANOVA) in subjects with NFG/IGT, IFG/IGT, and IFG/diabetes but did not differ in subjects with NFG/NGT and IFG/NGT. We conclude that postprandial hyperglycemia in individuals with early diabetes is due to lower rates of glucose disappearance rather than increased meal appearance or impaired suppression of EGP, regardless of their fasting glucose. In contrast, insulin secretion, action, and the pattern of postprandial turnover are essentially normal in individuals with isolated IFG.

Prediction of postprandial glycemic exposure: utility of fasting and 2-h glucose measurements alone and in combination with assessment of body composition, fitness, and strength

OBJECTIVE

To determine the best predictors of total postprandial glycemic exposure and peak glucose concentrations in nondiabetic humans.

RESEARCH DESIGN AND METHODS

Data from 203 nondiabetic volunteers who ingested a carbohydrate-containing mixed meal were analyzed.

RESULTS

Fasting glucose and insulin concentrations were poor predictors of postprandial glucose area above basal (R2 = approximately 0.07, P < 0.001). The correlation was stronger for 2-h glucose concentration (R2 = 0.55, P < 0.001) and improved slightly but significantly (P < 0.001) with the addition of fasting glucose, insulin, age, sex, and body weight to the model (r2 = 0.58). The 2-h glucose concentration also predicted the peak glucose concentration (R2 = 0.37, P < 0.001) with strength of the prediction increasing (P < 0.001) modestly with the addition of fasting glucose, insulin, age, sex, and body weight to the model (R2 = 0.48, P < 0.001). On the other hand, addition of measures of body function and composition did not improve prediction of total glycemic exposure or peak glucose concentration.

CONCLUSIONS

Isolated measures of fasting or 2-h glucose concentrations alone or in combination with more complex measures of body composition and function are poor predictors of postprandial glycemic exposure or peak glucose concentration. This may explain, at least in part, the weak and at times inconsistent relationship between these parameters and cardiovascular risk.

DHEA in elderly women and DHEA or testosterone in elderly men

BACKGROUND

Dehydroepiandrosterone (DHEA) and testosterone are widely promoted as antiaging supplements, but the long-term benefits, as compared with potential harm, are unknown.

METHODS

We performed a 2-year, placebo-controlled, randomized, double-blind study involving 87 elderly men with low levels of the sulfated form of DHEA and bioavailable testosterone and 57 elderly women with low levels of sulfated DHEA. Among the men, 29 received DHEA, 27 received testosterone, and 31 received placebo. Among the women, 27 received DHEA and 30 received placebo. Outcome measures included physical performance, body composition, bone mineral density (BMD), glucose tolerance, and quality of life.

RESULTS

As compared with the change from baseline to 24 months in the placebo group, subjects who received DHEA for 2 years had an increase in plasma levels of sulfated DHEA by a median of 3.4 microg per milliliter (9.2 micromol per liter) in men and by 3.8 microg per milliliter (10.3 micromol per liter) in women. Among men who received testosterone, the level of bioavailable testosterone increased by a median of 30.4 ng per deciliter (1.1 nmol per liter), as compared with the change in the placebo group. A separate analysis of men and women showed no significant effect of DHEA on body-composition measurements. Neither hormone altered the peak volume of oxygen consumed per minute, muscle strength, or insulin sensitivity. Men who received testosterone had a slight increase in fat-free mass, and men in both treatment groups had an increase in BMD at the femoral neck. Women who received DHEA had an increase in BMD at the ultradistal radius. Neither treatment improved the quality of life or had major adverse effects.

CONCLUSIONS

Neither DHEA nor low-dose testosterone replacement in elderly people has physiologically relevant beneficial effects on body composition, physical performance, insulin sensitivity, or quality of life. (ClinicalTrials.gov number, NCT00254371 [ClinicalTrials.gov].).

Mechanism of insulin's anabolic effect on muscle: measurements of muscle protein synthesis and breakdown using aminoacyl-tRNA and other surrogate measures

Despite being an anabolic hormone in skeletal muscle, insulin's anticatabolic mechanism in humans remains controversial, with contradictory reports showing either stimulation of protein synthesis (PS) or inhibition of protein breakdown (PB) by insulin. Earlier measurements of muscle PS and PB in humans have relied on different surrogate measures of aminoacyl-tRNA and intracellular pools. We report that insulin's effect on muscle protein turnover using aminoacyl-tRNA as the precursor of PS and PB is calculated by mass balance of tracee amino acid (AA). We compared the results calculated from various surrogate measures. To determine the physiological role of insulin on muscle protein metabolism, we infused tracers of leucine and phenylalanine into 18 healthy subjects, and after 3 h, 10 subjects received a 4-h femoral arterial infusion of insulin (0.125 mUxkg(-1)xmin(-1)), while eight subjects continued with saline. Tracer-to-tracee ratios of leucine, phenylalanine, and ketoisocaproate were measured in the arterial and venous plasma, muscle tissue fluid, and AA-tRNA to calculate muscle PB and PS. Insulin infusion, unlike saline, significantly reduced the efflux of leucine and phenylalanine from muscle bed, based on various surrogate measures which agreed with those based on leucyl-tRNA (-28%), indicating a reduction in muscle PB (P < 0.02) without any significant effect on muscle PS. In conclusion, using AA-tRNA as the precursor pool, it is demonstrated that, in healthy humans in the postabsorptive state, insulin does not stimulate muscle protein synthesis and confirmed that insulin achieves muscle protein anabolism by inhibition of muscle protein breakdown.

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).

Effects of age and sex on postprandial glucose metabolism: differences in glucose turnover, insulin secretion, insulin action, and hepatic insulin extraction

To determine the effects of age and sex on the regulation of postprandial glucose metabolism, glucose turnover, insulin secretion, insulin action, and hepatic insulin extraction were concurrently measured in 145 healthy elderly (aged 70 +/- 1 years) and in 58 young (aged 28 +/- 1 years) men and women before and after ingestion of a mixed meal containing [1-(13)C]glucose. At the time of meal ingestion, [6-(3)H]glucose and [6,6-(2)H(2)]glucose were infused intravenously to enable concurrent measurement of the rates of postprandial endogenous glucose production (EGP), meal appearance, and glucose disappearance. Fasting and postprandial glucose concentrations were higher (P < 0.001) in both elderly women and elderly men compared with young individuals of the same sex. The higher postprandial glucose concentrations in the elderly than young women were caused by higher rates of meal appearance (P < 0.01) and slightly lower (P < 0.05) rates of glucose disappearance immediately after eating. In contrast, higher glucose concentrations in the elderly than young men were solely due to decreased (P < 0.001) glucose disappearance. Although postprandial glucose concentrations did not differ in elderly women and elderly men, rates of meal appearance and glucose disappearance rates both were higher (P < 0.001) in the women. Fasting EGP was higher (P < 0.05) in elderly than young subjects of both sexes and in women than men regardless of age. On the other hand, postprandial suppression of EGP was rapid all groups. Insulin action and secretion were lower (P < 0.001) in the elderly than young men but did not differ in the elderly and young women. This resulted in lower (P < 0.001) meal disposition indexes in elderly than young men but no difference in elderly and young women. Total meal disposition indexes were lower (P < 0.05) in elderly men than elderly women, indicating impaired insulin secretion, whereas disposition indexes were higher (P < 0.05) in young men than young women. Hepatic insulin clearance was greater (P < 0.001) in the elderly than young subjects of both sexes but did not differ between men and women regardless of age. In contrast, the ability of glucose to facilitate its own uptake (glucose effectiveness) was higher (P < 0.001) in women than men but did not differ in elderly and young subjects. Thus, age and sex impact on insulin secretion, insulin action, hepatic insulin extraction, and glucose effectiveness, resulting in substantial differences in the regulation of postprandial glucose metabolism in men and women and in elderly and young subjects.

Brain oxygen utilization is unchanged by hypoglycemia in normal humans: lactate, alanine, and leucine uptake are not sufficient to offset energy deficit

During hypoglycemia, substrates other than glucose have been suggested to serve as alternate neural fuels. We evaluated brain uptake of endogenously produced lactate, alanine, and leucine at euglycemia and during insulin-induced hypoglycemia in 17 normal subjects. Cross-brain arteriovenous differences for plasma glucose, lactate, alanine, leucine, and oxygen content were quantitated. Cerebral blood flow (CBF) was measured by Fick methodology using N(2)O as the dilution indicator gas. Substrate uptake was measured as the product of CBF and the arteriovenous concentration difference. As arterial glucose concentration fell, cerebral oxygen utilization and CBF remained unchanged. Brain glucose uptake (BGU) decreased from 36.3+/-2.6 to 26.6+/-2.1 micromol.100 g of brain(-1).min(-1) (P<0.001), equivalent to a drop in ATP of 291 micromol.100 g(-1).min(-1). Arterial lactate rose (P<0.001), whereas arterial alanine and leucine fell (P<0.009 and P<0.001, respectively). Brain lactate uptake (BLU) increased from a net release of -1.8+/- 0.6 to a net uptake of 2.5+/-1.2 micromol.100 g(-1).min(-1) (P<0.001), equivalent to an increase in ATP of 74 micromol.100 g(-1).min(-1). Brain leucine uptake decreased from 7.1+/-1.2 to 2.5 +/- 0.5 micromol.100 g(-1).min(-1) (P<0.001), and brain alanine uptake trended downward (P<0.08). We conclude that the ATP generated from the physiological increase in BLU during hypoglycemia accounts for no more than 25% of the brain glucose energy deficit.

A minimal model of insulin secretion and kinetics to assess hepatic insulin extraction

The liver is the principal site of insulin degradation, and assessing its ability to extract insulin is important to understand several pathological states. Noninvasive quantification of hepatic extraction (HE) in an individual requires comparing the profiles of insulin secretion (ISR) and posthepatic insulin delivery rate (IDR). To do this, we propose here the combined use of the classical C-peptide minimal model with a new minimal model of insulin delivery and kinetics. The models were identified on insulin-modified intravenous glucose tolerance test (IM-IVGTT) data of 20 healthy subjects. C-peptide kinetics were fixed to standard population values, whereas insulin kinetics were assessed in each individual, along with IDR parameters, thanks to the presence of insulin decay data observed after exogenous insulin administration. From the two models, profiles of ISR and IDR were predicted, and ISR and IDR indexes of beta-cell responsivity to glucose in the basal state, as well as during first- and second-phase secretion, were estimated. HE profile, obtained by comparing ISR and IDR profiles, showed a rapid suppression immediately after the glucose administration. HE indexes, obtained by comparing ISR and IDR indexes, indicated that the liver is able to extract 70 +/- 9% of insulin passing through it in the basal state and 54 +/- 14% during IM-IVGTT. In conclusion, insulin secretion, kinetics, and hepatic extraction can be reliably assessed during an IM-IVGTT by using insulin and C-peptide minimal models.

Insulin sensitivity by oral glucose minimal models: validation against clamp

Measuring insulin sensitivity in the presence of physiological changes in glucose and insulin concentrations, e.g., during a meal or OGTT, is important to better understand insulin resistance in a variety of metabolic conditions. Recently, two oral minimal models have been proposed to measure overall insulin sensitivity (S(I)) and its selective effect on glucose disposal (S(I)*) from oral tests. S(I) and S(I)* have been successfully validated against multiple tracer meal estimates, but validation against euglycemic hyperinsulinemic clamp estimates is lacking. Here, we do so in 21 subjects who underwent both a multiple-tracer OGTT and a labeled euglycemic hyperinsulinemic clamp. Correlation between minimal-model S(I), S(I) and corresponding clamp estimates S(I)(*clamp), S(I)(*clamp) was satisfactory, respectively r = 0.81, P < 0.001, and r = 0.71, P < 0.001. S(I) was significantly lower than S(I)(clamp) (8.08 +/- 0.89 vs. 13.66 +/- 1.69 10(-4) dl.kg(-1).min(-1) per microU/ml, P = 0.0002), whereas S(I) and S(I)(*clamp) were very similar (8.17 +/- 1.59 vs. 8.84 +/- 1.39 10(-4) dl.kg(-1).min(-1) per microU/ml, P = 0.52). These results add credibility to the oral minimal-model method as a simple and reliable physiological tool to estimate S(I) and S(I)*, also in large-scale clinical trials.