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

IVGTT glucose minimal model covariate selection by nonlinear mixed-effects approach

Population approaches, traditionally employed in pharmacokinetic-pharmacodynamic studies, have shown value also in the context of glucose-insulin metabolism models by providing more accurate individual parameters estimates and a compelling statistical framework for the analysis of between-subject variability (BSV). In this work, the advantages of population techniques are further explored by proposing integration of covariates in the intravenous glucose tolerance test (IVGTT) glucose minimal model analysis. A previously published dataset of 204 healthy subjects, who underwent insulin-modified IVGTTs, was analyzed in NONMEM, and relevant demographic information about each subject was employed to explain part of the BSV observed in parameter values. Demographic data included height, weight, sex, and age, but also basal glycemia and insulinemia, and information about amount and distribution of body fat. On the basis of nonlinear mixed-effects modeling, age, visceral abdominal fat, and basal insulinemia were significant predictors for SI (insulin sensitivity), whereas only age and basal insulinemia were significant for P2 (insulin action). The volume of distribution correlated with sex, age, percentage of total body fat, and basal glycemia, whereas no significant covariate was detected to explain variability in SG (glucose effectiveness). The introduction of covariates resulted in a significant shrinking of the unexplained BSV, especially for SI and P2 and considerably improved the model fit. These results offer a starting point for speculation about the physiological meaning of the relationships detected and pave the way for the design of less invasive and less expensive protocols for epidemiological studies of glucose-insulin metabolism.

Does abnormal insulin action or insulin secretion explain the increase in prevalence of impaired glucose metabolism with age in populations of different ethnicities?

BACKGROUND

Age is associated with both impaired glucose and insulin metabolism. To what extent the age-related changes in insulin resistance (IR) and beta-cell function contribute to the increase in prevalence of impaired fasting glucose (IFG) and impaired glucose tolerance (IGT) is less known, and this is investigated in this study.

METHODS

This study included 6610 men and 7664 women of different ethnic groups aged 30-69 years. IR and beta-cell function were examined by the homeostasis model assessment of insulin resistance (HOMA-IR) and homeostasis model assessment of beta-cell function (HOMA-B). Odds ratios (ORs) and 95% confidence intervals (95% CIs) were estimated using logistic regression analysis adjusting for body mass index and study.

RESULTS

In Chinese men, the ORs (95% CIs) for IFG were 2.69 (1.70, 4.26), 2.51 (1.49, 4.21) and 2.89 (1.68, 4.97), respectively, in age groups of 40-49, 50-59 and 60-69 years compared with 30-39 years (p < 0.001 for trend); the corresponding figures for IGT were 1.73 (1.25, 2.38), 2.54 (1.78, 3.63) and 3.57 (2.46, 5.19) (p < 0.001 for trend). Similar trends for IGT were observed also in Chinese women and other ethnic groups, but not for IFG in Mauritius Indian and Creole men. Adjustment for HOMA-IR and HOMA-B reduced the ORs in all age groups of all ethnicities for both IFG and IGT, but the risk gradient between age groups remained particularly for the IGT.

CONCLUSIONS

The age-related increase in glucose intolerance may not be fully explained by the defect in HOMA-IR and HOMA-B. As HOMA-IR and HOMA-B are only surrogate measures of insulin sensitivity and insulin secretion, the results need to be further investigated.

Metabolic actions of insulin in men and women

Insulin is an important regulator of glucose, lipid, and protein metabolism. It suppresses hepatic glucose and triglyceride production, inhibits adipose tissue lipolysis and whole-body and muscle proteolysis, and stimulates glucose uptake in muscle. In this review we discuss what is currently known about the control of substrate metabolism by insulin in men and women. The data available so far indicate that women are more sensitive to insulin with regards to glucose metabolism (both in the liver and in muscle), whereas there are no differences between men and women in insulin action on lipolysis. Potential differences exist in the regulation of plasma triglyceride concentration and protein metabolism by insulin and in changes in insulin action in response to stimuli (e.g., weight loss and exercise) that are known to alter insulin sensitivity. However, these areas have not been studied comprehensively enough to draw firm conclusions.

Impact of rs361072 in the phosphoinositide 3-kinase p110beta gene on whole-body glucose metabolism and subunit protein expression in skeletal muscle

OBJECTIVE

Phosphoinositide 3-kinase (PI3K) is a major effector in insulin signaling. rs361072, located in the promoter of the gene (PIK3CB) for the p110beta subunit, has previously been found to be associated with homeostasis model assessment for insulin resistance (HOMA-IR) in obese subjects. The aim was to investigate the influence of rs361072 on in vivo glucose metabolism, skeletal muscle PI3K subunit protein levels, and type 2 diabetes.

RESEARCH DESIGN AND METHODS

The functional role of rs361072 was studied in 196 Danish healthy adult twins. Peripheral and hepatic insulin sensitivity was assessed by a euglycemic-hyperinsulinemic clamp. Basal and insulin-stimulated biopsies were taken from the vastus lateralis muscle, and tissue p110beta and p85alpha proteins were measured by Western blotting. The genetic association with type 2 diabetes and quantitative metabolic traits was investigated in 9,316 Danes with glucose tolerance ranging from normal to overt type 2 diabetes.

RESULTS

While hepatic insulin resistance was similar in the fasting state, carriers of the minor G allele had lower hepatic glucose output (per-allele effect: -16%, P(add) = 0.004) during high physiological insulin infusion. rs361072 did not associate with insulin-stimulated peripheral glucose disposal despite a decreased muscle p85alpha:p110beta protein ratio (P(add) = 0.03) in G allele carriers. No association with HOMA-IR or type 2 diabetes (odds ratio 1.07, P = 0.5) was identified, and obesity did not interact with rs361072 on these traits.

CONCLUSIONS

Our study suggests that the minor G allele of PIK3CB rs361072 associates with decreased muscle p85alpha:p110beta ratio and lower hepatic glucose production at high plasma insulin levels. However, no impact on type 2 diabetes prevalence was found.

Assessment of hepatic insulin degradation, in normoglycemic hypertensive patients, by minimal modelling of standard intravenous glucose tolerance test data

Role of hepatic insulin degradation in modulating insulin delivery to peripheral circulation, in insulin-resistant hypertensive patients, is not yet fully understood. This issue was investigated here by a novel application to hypertension of a previously proposed minimal modelling of insulin and C-peptide data, using population values for insulin and C-peptide kinetics parameters. Data, from frequently sampled intravenous glucose tolerance test (FSIGTT), were analysed in ten normoglycemic, hypertensive patients (H-group), compared with eight normoglycemic, normotensive subjects (N-group), matched for age, gender and body mass index. Minimal modelling of C-peptide and insulin data provided beta-cell responsiveness to glucose perturbation (first, Phi(1), second, Phi(2), and basal, Phi(b), phase), insulin secretion rate, ISR(t) and total pre-hepatic insulin secretion, TIS, as well as insulin delivery rate, IDR(t), and total insulin delivery, TID, into plasma, over 5-h test. Instantaneous normalized hepatic insulin degradation rate was computed as HIDR(t)=1-[IDR(t)/ISR(t)]. In our H-group, insulin sensitivity, S(I), assessed by minimal model of glucose kinetics, showed a 56% reduction, which confirmed deterioration of insulin action in hypertension. This was associated with significant increase in Phi(1) (105%), TIS (55%) and TID (62%). No significant alterations were observed in other characteristic parameters of secretion and hepatic degradation of insulin, such that no significant difference was observed in HIDR(t) between our H and N groups. In conclusion, an increase of first phase and total insulin secretion occurring, in our H-group, in the presence of no alteration of hepatic insulin degradation, resulted in up-regulation of total insulin delivered to plasma (TID) for insulin-resistance compensation.

Contribution of skeletal muscle mass on sex differences in 2-hour plasma glucose levels after oral glucose load in Thai subjects with normal glucose tolerance

Women have higher 2-hour plasma glucose levels after oral glucose challenge than men. The smaller skeletal muscle mass in women may contribute to the higher postload glucose levels. The objective of this study was to test the hypothesis that the different amount of skeletal muscle mass between men and women contributed to sex difference in postload plasma glucose levels in subjects with normal glucose tolerance. Forty-seven Thai subjects with normal glucose tolerance, 23 women and 24 age- and body mass index-matched men, were studied. Body fat, abdominal fat, and appendages lean mass were measured by dual-energy x-ray absorptiometry. Skeletal muscle insulin sensitivity was determined by euglycemic-hyperinsulinemic clamp. First-phase insulin secretion and hepatic insulin sensitivity were determined from oral glucose tolerance data. beta-Cell function was estimated from the homeostasis model assessment of %B by the homeostasis model assessment 2 model. Correlation and linear regression analysis were performed to identify factors contributing to variances of postload 2-hour plasma glucose levels. This study showed that women had significantly higher 2-hour plasma glucose levels and smaller skeletal muscle mass than men. Measures of insulin secretion and insulin sensitivity were not different between men and women. Male sex (r = -0.360, P = .013) and appendages lean mass (r = -0.411, P = .004) were negatively correlated with 2-hour plasma glucose, whereas log 2-hour insulin (r = 0.571, P < .0001), total body fat (r = 0.348, P = .016), and log abdominal fat (r = 0.298, P = .042) were positively correlated with 2-hour plasma glucose. The correlation of 2-hour plasma glucose and sex disappeared after adjustment for appendages lean mass. By multivariate linear regression analysis, log 2-hour insulin (beta = 18.9, P < .0001), log 30-minute insulin (beta = -36.3, P = .001), appendages lean mass (beta = -1.0 x 10(-3), P = .018), and hepatic insulin sensitivity index (beta = -17.3, P = .041) explained 54.2% of the variance of 2-hour plasma glucose. In conclusion, the higher postload 2-hour plasma glucose levels in women was not sex specific but was in part a result of the smaller skeletal muscle mass. The early insulin secretion, hepatic insulin sensitivity, and skeletal muscle mass were the significant factors negatively predicting 2-hour postload plasma glucose levels in Thai subjects with normal glucose tolerance.

Specific relation between abdominal obesity and early-phase hyperglycemia is modulated by hepatic insulin resistance in healthy older women

OBJECTIVE

To describe the impact of abdominal obesity and hepatic insulin resistance on phase-specific glycemic responses in older women.

RESEARCH DESIGN AND METHODS

We studied 23 healthy older women (60-88 years old). Abdominal obesity was defined by an abdominal circumference > or =95 cm. Plasma glucose and insulin were measured in response to a 3-h oral glucose tolerance test. Insulin suppression of hepatic glucose production was determined using in vivo clamp techniques.

RESULTS

Despite identical prevailing insulin concentrations, glucose excursions 30 min postchallenge (but not later) were greater in women with abdominal obesity than in those without (162 +/- 19 vs. 132 +/- 16 mg/dl; P < 0.01). There was a strong correlation between hepatic glucose production suppression under low-dose insulin infusion and early-phase glucose excursions from the oral glucose tolerance test (r = -0.83; P < 0.001) in women with abdominal obesity, but not in women without (r = 0.44; P < 0.11).

CONCLUSIONS

Abdominal obesity relates specifically to early-phase hyperglycemia via hepatic insulin resistance, even in healthy older women.

An integrated model for the glucose-insulin system

The integrated glucose-insulin model was originally developed on a variety of intravenous glucose provocation experiments in healthy volunteers and type 2 diabetic patients. The model, which simultaneously describes time-courses of glucose and insulin based on mechanism-based components for production, elimination and homeostatic feedback, has been further extended to oral glucose provocations, meal tests and insulin administration. The model has been used to describe experiments ranging from 4 to 24 hr. Applications of the integrated glucose-insulin model include the clinical assessment of the mechanism of action of anti-diabetic drugs and the magnitude of their effects. Finally, the model was used for optimizing the design of provocation experiments.

Minimal model assessment of hepatic insulin extraction during an oral test from standard insulin kinetic parameters

In this article, a first aim was to develop a minimal modeling approach to noninvasively assess hepatic insulin extraction in 204 healthy subjects studied with a standard meal by coupling the already available meal C-peptide minimal model with a new insulin model. The ingredients of this model are posthepatic IDR, which in turn is described in terms of pancreatic ISR and hepatic insulin extraction HE, and a linear monocompartmental model of insulin kinetics. Even if ISR is provided by the C-peptide minimal model, the simultaneous assessment of HE and insulin kinetics is critical, since compensations may arise between parameters describing these two processes. Therefore, as a second aim of this study, a method was developed to predict standard values of insulin kinetic parameters in an individual on the basis of the individual's anthropometric characteristics. The statistical analysis, based on linear regression of insulin kinetic parameters estimated from IM-IVGTT data performed on the same subjects, demonstrated that insulin kinetic parameters can be accurately predicted from age and body surface area. Once kinetic parameters of the new insulin model were fixed to these values, HE profile and indexes during a meal were reliably estimated in each individual, indicating a significant suppression during the meal since the overall index of HE, equal to 60 +/- 1% in the basal state, is reduced to 40 +/- 1% during a meal. However, standard parameters provide an approximation of the individual one; thus, the third aim was to define the impact on estimated indexes of using standard instead of individually estimated values. Our results showed that the 25% uncertainty affecting as an average insulin kinetic parameters of an individual, when they are predicted from age and body surface area, translates into a similar relative uncertainty in the individual's hepatic insulin extraction indexes.

High prevalence of diabetes and impaired fasting glucose in urban Latin America: the CARMELA Study

AIMS

Cardiovascular risk is increased with glucose metabolism abnormalities. Prevalence data can support public health initiatives required to address this risk. The Cardiovascular Risk Factor Multiple Evaluation in Latin America (CARMELA) study was designed to estimate the prevalence of Type 2 diabetes, impaired fasting glucose and related risk factors in seven urban Latin American populations.

METHODS

CARMELA was a cross-sectional, population-based study of 11 550 adults 25-64 years of age. With a multi-stage sample design of a probabilistic nature, approximately 1600 subjects were randomly selected in each city.

RESULTS

Overall, the prevalence of diabetes was 7.0% (95% confidence intervals 6.5-7.6%). The prevalence of individuals with diabetes or impaired fasting glucose increased with increasing age. In the oldest age category, 55-64 years of age, prevalence of diabetes ranged from 9 to 22% and prevalence of impaired fasting glucose ranged from 3 to 6%. Only 16.3% of people with prior diagnosis of diabetes and who were receiving pharmacologic treatment, were in good glycaemic control (fasting glucose < 6.1 mmol/l). The prevalence of diabetes in individuals with abdominal obesity was approximately twofold higher. Participants with hypertension, elevated serum triglycerides and increased common carotid artery intima-media thickness were also more likely to have diabetes.

CONCLUSIONS

The prevalence of diabetes and impaired fasting glucose is high in seven major Latin American cities; intervention is needed to avoid substantial medical and socio-economic consequences. CARMELA supports the associations of abdominal obesity, hypertension, elevated serum triglycerides and carotid intima-media thickness with diabetes.

Fatty acid metabolism in the elderly: effects of dehydroepiandrosterone and testosterone replacement in hormonally deficient men and women

CONTEXT

Aging, low dehydroepiandrosterone (DHEA), and testosterone are associated with increased adiposity and metabolic risk. Treatment with these hormones may improve these abnormalities.

OBJECTIVE

The objective of the study was to determine effects of aging, DHEA, or testosterone replacement on adiposity, meal fat partitioning, and postabsorptive lipolysis.

DESIGN

This was a cross-sectional, 2-yr, double-blind, randomized, placebo-controlled trial.

SETTING

The study was conducted in the general community.

PATIENTS

Elderly women and men (>or=60 yr) with low DHEA sulfate (women and men) and bioavailable testosterone (men) concentrations and young adults.

INTERVENTIONS

Thirty elderly women each received 50 mg DHEA or placebo daily for 2 yr. Thirty elderly men received 75 mg DHEA, 29 received 5 mg testosterone (patch), and 32 received placebo daily for 2 yr. Thirty young women and 32 young men served as controls.

MAIN OUTCOME MEASURES

In vivo measures of meal fat storage into sc fat, postabsorptive lipolysis, and regional adiposity at baseline and after treatment.

RESULTS

At baseline, the elderly had more body fat, greater systemic lipolysis (women, P = 0.0003; men, P < 0.0001) adjusted for resting energy expenditure, greater meal fat oxidation (women, P = 0.026; men, P = 0.0025), and less meal fat storage in sc fat (women, P = 0.0139; men, P= 0.0006). Although testosterone treatment increased meal fat storage into upper- vs. lower-body fat in elderly men, neither hormone affected regional adiposity, meal fat oxidation, or systemic lipolysis.

CONCLUSIONS

Aging, in the context of low DHEA sulfate (women and men) and bioavailable testosterone (men) concentrations, is associated with changes in meal fat partitioning and postabsorptive lipolysis that are not corrected by DHEA and only partly corrected by testosterone replacement.

Overnight closed-loop insulin delivery with model predictive control: assessment of hypoglycemia and hyperglycemia risk using simulation studies

BACKGROUND

Hypoglycemia and hyperglycemia during closed-loop insulin delivery based on subcutaneous (SC) glucose sensing may arise due to (1) overdosing and underdosing of insulin by control algorithm and (2) difference between plasma glucose (PG) and sensor glucose, which may be transient (kinetics origin and sensor artifacts) or persistent (calibration error [CE]). Using in silico testing, we assessed hypoglycemia and hyperglycemia incidence during over-night closed loop. Additionally, a comparison was made against incidence observed experimentally during open-loop single-night in-clinic studies in young people with type 1 diabetes mellitus (T1DM) treated by continuous SC insulin infusion.

METHODS

Simulation environment comprising 18 virtual subjects with T1DM was used to simulate overnight closed-loop study with a model predictive control (MPC) algorithm. A 15 h experiment started at 17:00 and ended at 08:00 the next day. Closed loop commenced at 21:00 and continued for 11 h. At 18:00, protocol included meal (50 g carbohydrates) accompanied by prandial insulin. The MPC algorithm advised on insulin infusion every 15 min. Sensor glucose was obtained by combining model-calculated noise-free interstitial glucose with experimentally derived transient and persistent sensor artifacts associated with FreeStyle Navigator (FSN). Transient artifacts were obtained from FSN sensor pairs worn by 58 subjects with T1DM over 194 nighttime periods. Persistent difference due to FSN CE was quantified from 585 FSN sensor insertions, yielding 1421 calibration sessions from 248 subjects with diabetes.

RESULTS

Episodes of severe (PG < or = 36 mg/dl) and significant (PG < or = 45 mg/dl) hypoglycemia and significant hyperglycemia (PG > or = 300 mg/dl) were extracted from 18,000 simulated closed-loop nights. Severe hypoglycemia was not observed when FSN CE was less than 45%. Hypoglycemia and hyperglycemia incidence during open loop was assessed from 21 overnight studies in 17 young subjects with T1DM (8 males; 13.5 +/- 3.6 years of age; body mass index 21.0 +/- 4.0 kg/m2; duration diabetes 6.4 +/- 4.1 years; hemoglobin A1c 8.5% +/- 1.8%; mean +/- standard deviation) participating in the Artificial Pancreas Project at Cambridge. Severe and significant hypoglycemia during simulated closed loop occurred 0.75 and 17.11 times per 100 person years compared to 1739 and 3479 times per 100 person years during experimental open loop, respectively. Significant hyperglycemia during closed loop and open loop occurred 75 and 15,654 times per 100 person years, respectively.

CONCLUSIONS

The incidence of severe and significant hypoglycemia reduced 2300- and 200-fold, respectively, during stimulated overnight closed loop with MPC compared to that observed during open-loop overnight clinical studies in young subjects with T1DM. Hyperglycemia was 200 times less likely. Overnight closed loop with the FSN and the MPC algorithm is expected to reduce substantially the risk of hypoglycemia and hyperglycemia.

The effect of age on insulin sensitivity and insulin secretion in first-degree relatives of type 1 diabetic patients: a population analysis

CONTEXT

Understanding the role of insulin resistance in type 1 diabetes may lead to new prevention strategies. Estimates of insulin resistance in first-degree relatives of those with type 1 diabetes may be obtained using the minimal model of glucose kinetics incorporating a population approach.

OBJECTIVE

The objective of the study was to explore parameters contributing to glucose homeostasis in a cross-sectional study of first-degree relatives across a wide age range.

DESIGN

Insulin sensitivity (SI) was assessed using the minimal model of glucose kinetics after an oral glucose tolerance test combined with nonlinear mixed-effects modeling. Beta-cell function was measured from the insulinogenic index at 30 min (IGI(30)). Disposition index (DI) was estimated as the product of SI and IGI(30).

SETTING

The study was conducted at an academic center.

SUBJECTS

Subjects included 1241 first-degree relatives (aged 2-75 yr).

RESULTS

SI was found to be negatively correlated with age, whereas IGI(30) increased until young adulthood. The increase IGI(30) was apparently insufficient to compensate for the insulin resistance because DI decreased linearly at the rate of 0.035 (10(-2) min(-1) mmol(-1) liter per year) after young adulthood. Both IGI(30) and DI were significantly lower in those with vs. without autoantibodies, whereas there was no difference between these groups with respect to SI.

CONCLUSIONS

Beta-cell function, adjusted for age-related insulin resistance, decreases throughout life in first-degree relatives. This deterioration may be exacerbated in the presence of autoantibodies. Oral glucose tolerance test data combined with a nonlinear mixed-effect modeling population approach may be a useful technique to evaluate SI and secretion in a population.

Are we optimizing gestational diabetes treatment with glyburide? The pharmacologic basis for better clinical practice

Glyburide's pharmacokinetics (PK) and pharmacodynamics have not been studied in women with gestational diabetes mellitus (GDM). The objective of this study was to assess steady-state PK of glyburide, as well as insulin sensitivity, beta-cell responsivity, and overall disposition indices after a mixed-meal tolerance test (MMTT) in women with GDM (n = 40), nonpregnant women with type 2 diabetes mellitus (T2DM) (n = 26), and healthy pregnant women (n = 40, MMTT only). At equivalent doses, glyburide plasma concentrations were approximately 50% lower in pregnant women than in nonpregnant subjects. The average umbilical cord/maternal plasma glyburide concentration ratio at the time of delivery was 0.7 +/- 0.4. Insulin sensitivity was approximately fivefold lower in women with GDM as compared with healthy pregnant women. Despite comparable beta-cell responsivity indices, the average beta-cell function corrected for insulin resistance was more than 3.5-fold lower in women with glyburide-treated GDM than in healthy pregnant women. Women with GDM in whom glyburide treatment has failed may benefit from alternative medication or dosage escalation; however, fetal safety should be kept in mind.

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.

Nonlinear mixed effects to improve glucose minimal model parameter estimation: a simulation study in intensive and sparse sampling

Intravenous glucose tolerance test (IVGTT) minimal model parameters are commonly estimated by weighted least squares (WLSs) on each subject data. Sometimes, with sparse data, individual parameters cannot be satisfactorily obtained. In such cases, a population approach could be preferable. These methods allow borrowing information across all subjects simultaneously, quantifying population features directly, and subsequently, deriving individual parameter estimates. In this paper, we assessed different estimation methods on simulated datasets. Besides the standard WLS approach, we applied iterative procedures (iterative two-stage (ITS) and global two-stage (GTS) methods) as well as nonlinear mixed-effects models (NLMEMs), where the likelihood is based on model linearization: first-order (FO), FO conditional estimation (FOCE), and Laplace (LAP) approximations. The synthetic dataset, initially very rich, was progressively reduced (by 50% and 75%) in order to assess the robustness of the results in sparsely sampled situations. Our results show that, even with intensive sampling, population approaches provide more reliable parameter estimates. Moreover, these estimates are remarkably more robust when the data become scarce. ITS and GTS encounter critical problems when single subjects have very poor sampling schedules, whereas the NLMEM (excluding FO) methods are more versatile and able to cope with such situations. FOCE appears as the most satisfactory approach.

Diabetes: Models, Signals, and Control

The control of diabetes is an interdisciplinary endeavor, which includes a significant biomedical engineering component, with traditions of success beginning in the early 1960s. It began with modeling of the insulin-glucose system, and progressed to large-scale in silico experiments, and automated closed-loop control (artificial pancreas). Here, we follow these engineering efforts through the last, almost 50 years. We begin with the now classic minimal modeling approach and discuss a number of subsequent models, which have recently resulted in the first in silico simulation model accepted as substitute to animal trials in the quest for optimal diabetes control. We then review metabolic monitoring, with a particular emphasis on the new continuous glucose sensors, on the analyses of their time-series signals, and on the opportunities that they present for automation of diabetes control. Finally, we review control strategies that have been successfully employed in vivo or in silico, presenting a promise for the development of a future artificial pancreas and, in particular, discuss a modular architecture for building closed-loop control systems, including insulin delivery and patient safety supervision layers. We conclude with a brief discussion of the unique interactions between human physiology, behavioral events, engineering modeling and control relevant to diabetes.

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.

Secretion of insulinotropic proteins by commensal bacteria: rewiring the gut to treat diabetes

Here, we show that commensal bacteria can stimulate intestinal epithelial cells to secrete insulin in response to glucose. Commensal strains were engineered to secrete the insulinotropic proteins GLP-1 and PDX-1. Epithelia stimulated by engineered strains and glucose secreted up to 1 ng ml(-1) of insulin with no significant background secretion.

Higher muscle protein synthesis in women than men across the lifespan, and failure of androgen administration to amend age-related decrements

We investigated age and sex effects and determined whether androgen replacement in elderly individuals (> or = 60 yr) could augment protein synthesis. Thirty young men and 32 young women (18-31 yr) were studied once, whereas 87 elderly men were studied before and after 1 yr of treatment with 5 mg/day testosterone (T), 75 mg/day dehydroepiandrosterone (DHEA), or placebo (P); and 57 elderly women were studied before and after 1 yr of treatment with 50 mg/day DHEA or P. [(15)N]Phenylalanine and [(2)H(4)]tyrosine tracers were infused, with measurements in plasma and vastus lateralis muscle. Whole-body protein synthesis per fat-free mass and muscle protein fractional synthesis rate (FSR) were lower in elderly than in young individuals (P<0.001), not significantly affected by hormone treatments, and higher in women than in men (P<0.0001), with no sex x age interaction. In regression analyses, peak O2 consumption (VO2peak), resting energy expenditure (REE), and sex were independently associated with muscle FSR, as were VO2peak, REE, and interactions of sex with insulin-like growth factor-II and insulin for whole-body protein synthesis. Women maintain higher protein synthesis than men across the lifespan as rates decline in both sexes, and neither full replacement of DHEA (in elderly men and women) nor partial replacement of bioavailable T (in elderly men) is able to amend the age-related declines.

Sex differences in the control of glucose homeostasis

PURPOSE OF REVIEW

A markedly higher prevalence of impaired glucose tolerance has been reported in women than in men, whereas the opposite was seen for impaired fasting glucose. The present review focuses on the underlying mechanisms.

RECENT FINDINGS

An increased meal glucose appearance and disturbances in postprandial glucose disposal may contribute to higher glucose concentrations in women. An increased, similar or reduced insulin sensitivity has been reported in women than in men, which makes it unclear to what extent a disturbed insulin-mediated glucose disposal may contribute to increased postprandial glucose concentrations in women. This discrepancy may be explained by differences in the phase of menstrual cycle during the study, the use of oral contraceptives and different degrees of physical fitness. Nevertheless, there are consistent data indicating that women are protected against fatty acid-induced insulin resistance. Furthermore, both disturbances in endogenous glucose output and metabolic clearance of glucose may contribute to the reduced fasting glucose concentrations in women.

SUMMARY

There is an urgent need for studies that test whether sex-related disturbances in glucose metabolism may be involved in the pathogenesis of type 2 diabetes and the metabolic syndrome, taking age, menstrual cycle, the use of oral contraceptives and physical activity into account.

Evaluating the efficacy of closed-loop glucose regulation via control-variability grid analysis

BACKGROUND

Advancements in subcutaneous continuous glucose monitoring and subcutaneous insulin delivery are stimulating the development of a minimally invasive artificial pancreas that facilitates optimal glycemic regulation in diabetes. The key component of such a system is the blood glucose controller for which different design strategies have been investigated in the literature. In order to evaluate and compare the efficacy of the various algorithms, several performance indices have been proposed.

METHODS

A new tool-control-variability grid analysis (CVGA)-for measuring the quality of closed-loop glucose control on a group of subjects is introduced. It is a method for visualization of the extreme glucose excursions caused by a control algorithm in a group of subjects, with each subject presented by one data point for any given observation period. A numeric assessment of the overall level of glucose regulation in the population is given by the summary outcome of the CVGA.

RESULTS

It has been shown that CVGA has multiple uses: comparison of different patients over a given time period, of the same patient over different time periods, of different control laws, and of different tuning of the same controller on the same population.

CONCLUSIONS

Control-variability grid analysis provides a summary of the quality of glycemic regulation for a population of subjects and is complementary to measures such as area under the curve or low/high blood glucose indices, which characterize a single glucose trajectory for a single subject.

Effect of aging on glucose homeostasis: accelerated deterioration of beta-cell function in individuals with impaired glucose tolerance

OBJECTIVE

To examine the effect of aging on insulin secretion (first- and second-phase insulin release) and insulin sensitivity in people with normal glucose tolerance (NGT) or impaired glucose tolerance (IGT).

RESEARCH DESIGN AND METHODS

First- and second-phase insulin secretion and insulin sensitivity were assessed in hyperglycemic clamp experiments in 266 individuals with NGT and 130 individuals with IGT, ranging in age from approximately 20 to approximately 70 years. Changes in beta-cell function were compared using the disposition index to adjust for differences in insulin sensitivity.

RESULTS

As expected, both phases of insulin release and insulin sensitivity were reduced in individuals with IGT (all P < 0.01). Insulin sensitivity was not independently correlated with age in either group. In people with NGT, the disposition index for first- and second-phase insulin release decreased similarly at a rate of approximately 0.7% per year. In people with IGT, the disposition indexes for first- and second-phase insulin release decreased at greater rates ( approximately 2.2 and 1.4% per year, P = 0.002 and 0.009, respectively, vs. NGT), with the decrease in first phase being greater than that of second phase (P = 0.025).

CONCLUSIONS

Insulin secretion (both first and second phase) normally decreases at a rate of approximately 0.7% per year with aging; this decrease in beta-cell function is accelerated about two-fold in people with impaired glucose tolerance-first phase to a greater extent than second phase. Finally, aging per se has no effect on insulin sensitivity independent of changes in body composition.

Glucose Minimal Model population analysis: likelihood function profiling via Monte Carlo sampling

Population kinetic modeling approaches, implemented as nonlinear mixed effects models, are attracting growing interest in many fields of biomedicine thanks to their value in estimating population features from sparsely sampled data. However, their application often entails approximations of the original model function, whose effect is difficult to gauge in general. We apply negative log-likelihood profiling to assess the effect of model approximation on the glucose-insulin Minimal Model, and compare nonlinear mixed-effects approximate methods to two-stage methods. Our preliminary findings suggest that nonlinear mixed effects models provide accurate parameter estimates, but also point out that the reliability of such estimates may be affected by large population variability and small sample size.

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.

Meal simulation model of the glucose-insulin system

A simulation model of the glucose-insulin system in the postprandial state can be useful in several circumstances, including testing of glucose sensors, insulin infusion algorithms and decision support systems for diabetes. Here, we present a new simulation model in normal humans that describes the physiological events that occur after a meal, by employing the quantitative knowledge that has become available in recent years. Model parameters were set to fit the mean data of a large normal subject database that underwent a triple tracer meal protocol which provided quasi-model-independent estimates of major glucose and insulin fluxes, e.g., meal rate of appearance, endogenous glucose production, utilization of glucose, insulin secretion. By decomposing the system into subsystems, we have developed parametric models of each subsystem by using a forcing function strategy. Model results are shown in describing both a single meal and normal daily life (breakfast, lunch, dinner) in normal. The same strategy is also applied on a smaller database for extending the model to type 2 diabetes.

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.

Assessment of beta-cell function in humans, simultaneously with insulin sensitivity and hepatic extraction, from intravenous and oral glucose tests

Assessment of insulin secretion in humans under physiological conditions has been a challenge because of its complex interplay with insulin action and hepatic insulin extraction. The possibility of simultaneously assessing beta-cell function, insulin sensitivity, and hepatic insulin extraction under physiological conditions using a simple protocol is appealing, since it has the potential to provide novel insights regarding the regulation of fasting and postprandial glucose metabolism in diabetic and nondiabetic humans. In this Perspective, we review data indicating that an oral glucose tolerance test (OGTT) or a meal test is able to accomplish this goal when interpreted with the oral beta-cell minimal model. We begin by using the well-established intravenous minimal model to highlight how the oral minimal model was developed and how the oral assessment parallels that of an intravenous glucose tolerance test (IVGTT). We also point out the unique aspects of both approaches in relation to their ability to assess different aspects of the beta-cell secretory cascade. We review the ability of the oral model to concurrently measure insulin sensitivity and hepatic insulin extraction, thereby enabling it to quantitatively portray the complex relationship among beta-cell function, hepatic insulin extraction, and insulin action. In addition, data from 204 individuals (54 young and 159 elderly) who underwent both IVGTT and meal tolerance tests are used to illustrate how these different approaches provide complementary but differing insights regarding the regulation of beta-cell function in humans.

GIM, simulation software of meal glucose-insulin model

BACKGROUND

A simulation model of the glucose-insulin system in normal life conditions can be very useful in diabetes research, e.g., testing insulin infusion algorithms and decision support systems and assessing glucose sensor performance and patient and student training. A new meal simulation model has been proposed that incorporates state-of-the-art quantitative knowledge on glucose metabolism and its control by insulin at both organ/tissue and whole-body levels. This article presents the interactive simulation software GIM (glucose insulin model), which implements this model.

METHODS

The model is implemented in MATLAB, version 7.0.1, and is designed with a windows interface that allows the user to easily simulate a 24-hour daily life of a normal, type 2, or type 1 diabetic subject. A Simulink version is also available. Three meals a day are considered. Both open- and closed-loop controls are available for simulating a type 1 diabetic subject.

RESULTS

Software options are described in detail. Case studies are presented to illustrate the potential of the software, e.g., compare a normal subject vs an insulin-resistant subject or open-loop vs closed-loop insulin infusion in type 1 diabetes treatment.

CONCLUSIONS

User-friendly software that implements a state-of-the-art physiological model of the glucose-insulin system during a meal has been presented. The GIM graphical interface makes its use extremely easy for investigators without specific expertise in modeling.

Influence of the glycaemic index of an evening meal on substrate oxidation following breakfast and during exercise the next day in healthy women

OBJECTIVE

To investigate whether the 'overnight second-meal effect' results in altered substrate oxidation during the postprandial period following breakfast and subsequent sub-maximal exercise in women.

SUBJECTS/METHODS

Seven recreationally active women were recruited for the study. In each trial, participants were provided with their evening meal on day 1, which was composed of either high glycaemic index (HGI) or low glycaemic index (LGI) carbohydrates (CHO). On day 2, participants were provided with a standard HGI breakfast and then performed a 60 min run at 65% \[V.]O(2 max) 3 h later.

RESULTS

The incremental area under the curve (IAUC) for plasma glucose concentrations during the postprandial period following breakfast was greater in the HGI trial compared to the LGI trial (P<0.01). Similarly, the IAUC for serum insulin concentrations was greater in the HGI trial than the LGI trial (P<0.05). No differences in plasma free-fatty acids (FFA) or plasma glycerol concentrations were found between trials during the postprandial period. During subsequent exercise, there were no significant differences in substrate metabolism.

CONCLUSION

The glycaemic index of an evening meal does not alter substrate oxidation at rest following breakfast or during subsequent submaximal exercise in women. This study provides further evidence for the overnight second-meal effect on glycaemic responses following a LGI mixed evening meal.

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.

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.