Temporal optimization of exercise to lower fasting glucose levels.

Exercise stimulates glucose uptake and increases insulin sensitivity acutely. Temporally optimizing exercise timing may minimize the nocturnal rise in glucose levels. This study examined the effect of exercise timing on evening and overnight glucose concentrations in individuals who were non-obese with normal fasting glucose levels (Non-Ob; n = 18) and individuals with obesity (OB) with impaired fasting glucose levels (OB+IFG) and without (n = 16 and n = 18, respectively). Subjects were studied on three occasions (no exercise (NOEX)), morning exercise (AMEX; 0700 h) and evening exercise (PMEX; 2000 h). The evening meal was provided (1800 h) and blood samples were taken from 1740 to 0700 h and morning endogenous glucose production (EGP) was measured. Glucose and insulin concentrations increased with the dinner meal with peak concentrations being higher in OB+IFG than in OB and Non-Ob (P = 0.04). In OB+IFG, evening glucose concentrations rose above baseline levels at about 2300 h, with the glucose concentrations staying somewhat lower with AMEX and PMEX until ∼0500 h than with NOEX. In OB+IFG, insulin concentrations decreased following the dinner meal and waned throughout the night, despite the rising glucose concentrations. In the OB and Non-Ob individuals following the dinner meal, no increase in glucose concentrations occurred in the evening period and insulin levels mirrored this. No difference was observed in the morning fasting glucose levels between study days or between groups. Regardless of time of day, exercise delays the evening rise in glucose concentrations in adults with OB+IFG but does not lower morning fasting glucose levels or improve the synchrony between glucose and insulin concentrations. KEY POINTS: Insulin resistance and type 2 diabetes have been linked to disturbances of the core clock, and glucose tolerance demonstrates a diurnal rhythm in healthy humans with better glucose tolerance in the morning than in the afternoon and evening. Skeletal muscle is a primary site for insulin resistance in people with impaired glucose tolerance. In individuals with obesity and impaired fasting glucose levels (OB+IFG), following a dinner meal, glucose concentrations started to rise and continues throughout the night, resulting in elevated glucose levels, while concomitantly, insulin levels are waning. Exercise, regardless of the time of day, suppressed the rise in glucose levels in OB+IFG for many hours during the night but did not lower morning fasting glucose levels. Morning exercise was not quite as effective as evening exercise.

A new oral model of free fatty acid kinetics to assess lipolysis in subjects with and without type 2 diabetes.

Assessing free fatty acids (FFAs) kinetics and the role of insulin and glucose on FFA lipolysis and disposal may improve our understanding of the pathogenesis of type 2 diabetes (T2D). Some models have been proposed to describe FFA kinetics during an intravenous glucose tolerance test and only one during an oral glucose tolerance test. Here, we propose a model of FFA kinetics during a meal tolerance test and use it to assess possible differences in postprandial lipolysis in individuals with type 2 diabetes (T2D) and individuals with obesity without type 2 diabetes (ND). We studied 18 obese ND and 16 T2D undergoing three meal tolerance tests (MTT) on three occasions (breakfast, lunch, and dinner). We used plasma glucose, insulin, and FFA concentrations collected at breakfast to test a battery of models and selected the best one based on physiological plausibility, ability to fit the data, precision of parameter estimates, and the Akaike parsimony criterion. The best model assumes that the postprandial suppression of FFA lipolysis is proportional to the above basal insulin, while FFA disposal is proportional to FFA concentration. It was used to compare FFA kinetics in ND and T2D along the day. The maximum lipolysis suppression occurred significantly earlier in ND than T2D (39 ± 6 min vs. 102 ± 13 min, 36 ± 4 min vs. 78 ± 11 min, and 38 ± 6 min vs. 84 ± 13 min, P < 0.01, at breakfast, lunch, and dinner, respectively), making lipolysis significantly lower in ND than T2D. This is mainly attributable to the lower insulin concentration in the second group. This novel FFA model allows to assess lipolysis and insulin antilipolytic effect in postprandial conditions.NEW & NOTEWORTHY In this study, we propose a new mathematical model able to quantify postprandial FFA kinetics and adipose tissue insulin sensitivity in both subjects with obesity without type 2 diabetes (ND) and subjects with type 2 diabetes (T2D). Results show that the slower postprandial suppression of lipolysis in T2D contributes to the higher free fatty acid (FFA) concentration that, in turn, may contribute to hyperglycemia.

Inducing remission of Type 2 diabetes in the Caribbean: findings from a mixed methods feasibility study of a low-calorie liquid diet-based intervention in Barbados.

AIM: In a high proportion of people with recently diagnosed Type 2 diabetes, a short (2-3-month) low-calorie diet is able to restore normal glucose and insulin metabolism. The aim of this study was to determine the feasibility of this approach in Barbados. METHODS: Twenty-five individuals with Type 2 diabetes diagnosed within past 6 years, not on insulin, BMI ≥ 27 kg/m2 were recruited. Hypoglycaemic medication was stopped on commencement of the 8-week liquid (760 calorie) diet. Insulin response was assessed in meal tests at baseline, 8 weeks and 8 months. Semi-structured interviews, analysed thematically, explored participants' experiences. 'Responders' were those with fasting plasma glucose (FPG) < 7 mmol/l at 8 weeks. RESULTS: Ten men and 15 women (mean age 48, range 26-68 years) participated. Mean (sd) BMI was 34.2 kg/m2 (6.0); FPG 9.2 mmol/l (2.2). Mean weight loss at 8 weeks and 8 months was 10.1 kg [95% confidence interval (CI) 8.1, 12.0] and 8.2 kg (95% CI 5.8, 10.6); FPG was lower by 2.2 mmol/l (95% CI 1.2, 3.2) and 1.7 mmol/l (95% CI 0.8, 2.7) respectively. Nine of 11 (82%) of those who lost ≥ 10 kg were 'responders' compared with 6 of 14 (43%) who lost < 10 kg (P = 0.048). The 30-min insulin increment was higher in responders at baseline and follow-up (P ≤ 0.01). A food culture based on starchy foods and pressures to eat large amounts at social events were among the challenges identified by participants. CONCLUSIONS: The feasibility of this approach to weight loss and diabetes remission in a predominantly black population in Barbados was demonstrated.

Psychological outcomes of evening and night closed-loop insulin delivery under free living conditions in people with Type 1 diabetes: a 2-month randomized crossover trial.

AIM: To assess the impact on fear of hypoglycaemia and treatment satisfaction with an artificial pancreas system used for 2 consecutive months, as well as participant acceptance of the artificial pancreas system. METHODS: In a randomized crossover trial patient-related outcomes associated with an evening-and-night artificial pancreas and sensor-augmented pump therapy were compared. Both intervention periods lasted 8 weeks. The artificial pancreas acceptance questionnaire (range 0-90, higher scores better), Hypoglycaemia Fear Survey II (range 0-72, higher scores worse) and Diabetes Treatment Satisfaction Questionnaire (range 0-36, higher scores better) were completed by 32 participants. Semi-structured interviews were conducted after study completion in a subset of six participants. Outcomes were compared using a repeated-measures anova model or paired t-test when appropriate. RESULTS: The total artificial pancreas acceptance questionnaire score at the end of the artificial pancreas period was 69.1 (sd 14.7; 95% CI 63.5, 74.7), indicating a positive attitude towards the artificial pancreas. No significant differences were found among the scores at baseline, end of sensor-augmented pump therapy period or end of the artificial pancreas period with regard to fear of hypoglycaemia [28.2 (sd 17.5), 23.5 (sd 16.6) and 23.5 (sd 16.7), respectively; P = 0.099] or diabetes treatment satisfaction [29.0 (sd 3.9), 28.2 (sd 5.2) and 28.0 (sd 7.1), respectively; P = 0.43]. Themes frequently mentioned in the interviews were 'positive effects at work', 'improved blood glucose', 'fewer worries about blood glucose', but also 'frequent alarms', 'technological issues' and 'demand for an all-in-one device'. CONCLUSIONS: The psychological outcomes of artificial pancreas and sensor-augmented pump therapy were similar. Current artificial pancreas technology is promising but user concerns should be taken into account to ensure utility of these systems.

Multicenter outpatient dinner/overnight reduction of hypoglycemia and increased time of glucose in target with a wearable artificial pancreas using modular model predictive control in adults with type 1 diabetes.

AIMS: To test in an outpatient setting the safety and efficacy of continuous subcutaneous insulin infusion (CSII) driven by a modular model predictive control (MMPC) algorithm informed by continuous glucose monitoring (CGM) measurement. METHODS: 13 patients affected by type 1 diabetes participated to a non-randomized outpatient 42-h experiment that included two evening meals and overnight periods (in short, dinner & night periods). CSII was patient-driven during dinner & night period 1 and MMPC-driven during dinner&night period 2. The study was conducted in hotels, where patients could move around freely. A CGM system (G4 Platinum; Dexcom Inc., San Diego, CA, USA) and insulin pump (AccuChek Combo; Roche Diagnostics, Mannheim, Germany) were connected wirelessly to a smartphone-based platform (DiAs, Diabetes Assistant; University of Virginia, Charlottesville, VA, USA) during both periods. RESULTS: A significantly lower percentage of time spent with glucose levels <3.9 mmol/l was achieved in period 2 compared with period 1: 1.96 ± 4.56% vs 12.76 ± 15.84% (mean ± standard deviation, p < 0.01), together with a greater percentage of time spent in the 3.9-10 mmol/l target range: 83.56 ± 14.02% vs 62.43 ± 29.03% (p = 0.04). In addition, restricting the analysis to the overnight phases, a lower percentage of time spent with glucose levels <3.9 mmol/l (1.92 ± 4.89% vs 12.7 ± 19.75%; p = 0.03) was combined with a greater percentage of time spent in 3.9-10 mmol/l target range in period 2 compared with period 1 (92.16 ± 8.03% vs 63.97 ± 2.73%; p = 0.01). Average glucose levels were similar during both periods. CONCLUSIONS: The results suggest that MMPC managed by a wearable system is safe and effective during evening meal and overnight. Its sustained use during this period is currently being tested in an ongoing randomized 2-month study.

Assessment of insulin action on carbohydrate metabolism: physiological and non-physiological methods.

Carbohydrate metabolism in humans is regulated by insulin secretion from pancreatic ß-cells and glucose disposal by insulin-sensitive tissues. Insulin facilitates glucose utilization in peripheral tissues and suppresses hepatic glucose production. Any defects in insulin action predispose an individual to glucose intolerance and Type 2 diabetes mellitus. Early detection of defects in insulin action could provide opportunities to prevent or delay progression of the disease state. There are different approaches to assess insulin action. Initial methods, such as peripheral insulin concentration and simple indices, have several limitations. Subsequently, researchers developed methodologies using intravenous glucose infusion to determine glucose fluxes. However, these methodologies are limited by being non-physiological. Newer, innovative techniques that have been developed are more sophisticated and physiological. By modelling glucose kinetics using isotope dilution techniques, several robust parameters can be obtained that are physiologically relevant and sound. This brief review summarizes most of the non-physiological and physiological methodologies used to measure the variables of insulin action.

Mechanism of insulin resistance in normal pregnancy.

Normal pregnancy is associated with insulin resistance although the mechanism is not understood. Increased intramyocellular lipid is closely associated with the insulin resistance of type 2 diabetes and obesity, and the aim of this study was to determine whether this was so for the physiological insulin resistance of pregnancy. Eleven primiparous healthy pregnant women (age: 27-39 years, body mass index 24.0±3.1 kg/m2) and no personal or family history of diabetes underwent magnetic resonance studies to quantify intramyocellular lipid, plasma lipid fractions, and insulin sensitivity. The meal-related insulin sensitivity index was considerably lower in pregnancy (45.6±9.9 vs. 193.0±26.1; 10(-4) dl/kg/min per pmol/l, p=0.0002). Fasting plasma triglyceride levels were elevated 3-fold during pregnancy (2.3±0.2 vs. 0.8±0.1 mmol/l, p<0.01) and the low-density density lipoprotein fraction, responsible for fatty acid delivery to muscle and other tissues, was 6-fold elevated (0.75±0.43 vs. 0.12±0.09 mmol/l; p=0.001). However, mean intramyocellular lipid concentrations of the soleus muscle were not different during pregnancy (20.0±2.3 vs. 19.1±3.2 mmol/l, p=0.64). The pregnancy effect on muscle insulin resistance is distinct from that underlying type 2 diabetes.

Tracking the time course of reproduction number and lockdown's effect on human behaviour during SARS-CoV-2 epidemic: nonparametric estimation.

Understanding the SARS-CoV-2 dynamics has been subject of intense research in the last months. In particular, accurate modeling of lockdown effects on human behaviour and epidemic evolution is a key issue in order e.g. to inform health-care decisions on emergency management. In this regard, the compartmental and spatial models so far proposed use parametric descriptions of the contact rate, often assuming a time-invariant effect of the lockdown. In this paper we show that these assumptions may lead to erroneous evaluations on the ongoing pandemic. Thus, we develop a new class of nonparametric compartmental models able to describe how the impact of the lockdown varies in time. Our estimation strategy does not require significant Bayes prior information and exploits regularization theory. Hospitalized data are mapped into an infinite-dimensional space, hence obtaining a function which takes into account also how social distancing measures and people's growing awareness of infection's risk evolves as time progresses. This also permits to reconstruct a continuous-time profile of SARS-CoV-2 reproduction number with a resolution never reached before in the literature. When applied to data collected in Lombardy, the most affected Italian region, our model illustrates how people behaviour changed during the restrictions and its importance to contain the epidemic. Results also indicate that, at the end of the lockdown, around [Formula: see text] of people in Lombardy and [Formula: see text] in Italy was affected by SARS-CoV-2, with the fatality rate being 1.14%. Then, we discuss how the situation evolved after the end of the lockdown showing that the reproduction number dangerously increased in the summer, due to holiday relax, reaching values larger than one on August 1, 2020. Finally, we also document how Italy faced the second wave of infection in the last part of 2020. Since several countries still observe a growing epidemic and others could be subject to other waves, the proposed reproduction number tracking methodology can be of great help to health care authorities to prevent SARS-CoV-2 diffusion or to assess the impact of lockdown restrictions on human behaviour to contain the spread.

Adaptive changes in pancreatic beta cell fractional area and beta cell turnover in human pregnancy.

AIMS/HYPOTHESIS: We sought to establish the extent and basis for adaptive changes in beta cell numbers in human pregnancy. METHODS: Pancreas was obtained at autopsy from women who had died while pregnant (n = 18), post-partum (n = 6) or were not pregnant at or shortly before death (controls; n = 20). Pancreases were evaluated for fractional pancreatic beta cell area, islet size and islet fraction of beta cells, beta cell replication (Ki67) and apoptosis (TUNEL), and indirect markers of beta cell neogenesis (insulin-positive cells in ducts and scattered beta cells in pancreas). RESULTS: The pancreatic fractional beta cell area was increased by approximately 1.4-fold in human pregnancy, with no change in mean beta cell size. In pregnancy there were more small islets rather than an increase in islet size or beta cells per islet. No increase in beta cell replication or change in beta cell apoptosis was detected, but duct cells positive for insulin and scattered beta cells were increased with pregnancy. CONCLUSIONS/INTERPRETATION: The adaptive increase in beta cell numbers in human pregnancy is not as great as in most reports in rodents. This increase in humans is achieved by increased numbers of beta cells in apparently new small islets, rather than duplication of beta cells in existing islets, which is characteristic of pregnancy in rodents.

Relationship between fractional pancreatic beta cell area and fasting plasma glucose concentration in monkeys.

AIMS/HYPOTHESIS: We sought to establish the relationship between fasting plasma glucose concentrations and pancreatic fractional beta cell area in adult cynomolgus monkeys (Macaca fascicularis). METHODS: Fasting plasma glucose and pancreatic fractional beta cell area were measured in 18 control and 17 streptozotocin-treated adult primates (17.0 +/- 1.2 vs 15.4 +/- 1.2 years old). RESULTS: Fasting plasma glucose was increased (12.0 +/- 2.0 vs 3.4 +/- 0.1 mmol/l, p < 0.01) and fractional beta cell area was decreased (0.62 +/- 0.13% vs 2.49 +/- 0.35%, p < 0.01) in streptozotocin-treated monkeys. The relationship between fasting plasma glucose and pancreatic fractional beta cell area was described by a wide range of beta cell areas in controls. In streptozotocin-treated monkeys there was an inflection of fasting blood glucose at approximately 50% of the mean beta cell area in controls with a steep increase in blood glucose for each further decrement in beta cell area. CONCLUSIONS/INTERPRETATION: In adult non-human primates a decrement in fractional beta cell area of approximately 50% or more leads to loss of glycaemic control.

Combination peroxisome proliferator-activated receptor gamma and alpha agonist treatment in Type 2 diabetes prevents the beneficial pioglitazone effect on liver fat content.

AIMS: Peroxisome proliferator-activated receptor (PPAR)-gamma and PPAR-alpha agonists individually reduce intra-organ triglyceride content and improve insulin sensitivity. However, the precise effects of combined PPAR-gamma and PPAR-alpha therapy on intra-organ triglyceride content and insulin sensitivity in subjects with Type 2 diabetes have not yet been determined. METHODS: Diet-controlled Type 2 subjects (n = 9) were studied before and after 16 weeks of combined PPAR-gamma [pioglitazone (PIO), 45 mg daily] and PPAR-alpha [bezafibrate (BEZA), modified release 400 mg daily] agonist therapy. Glucose metabolism and endogenous glucose production were measured following a standard liquid test meal. Liver and muscle triglyceride levels were measured by (1)H magnetic resonance spectroscopy. RESULTS: Combined PIO and BEZA therapy reduced mean fasting (7.5 +/- 0.5 vs. 6.5 +/- 0.2 mmol/l, P = 0.04) and peak postprandial plasma glucose (15.3 +/- 1.1 vs. 11.7 +/- 0.6 mmol/l, P = 0.007). No significant change in hepatic or muscle triglyceride content was observed. Postprandial suppression of endogenous glucose production remained similar on both study days. Both subcutaneous and visceral fat content increased following therapy. CONCLUSIONS: Combined PIO and BEZA therapy in Type 2 diabetes does not decrease intrahepatic triglyceride content or postprandial endogenous glucose production. This study demonstrates an unexpected adverse interaction of PPAR-alpha with PPAR-gamma agonist therapy.

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.

Effect of sitagliptin, a dipeptidyl peptidase-4 inhibitor, on beta-cell function in patients with type 2 diabetes: a model-based approach.

PURPOSE: The purpose of this exploratory analysis was to assess the effect of sitagliptin, a dipeptidyl peptidase-4 inhibitor, on pancreatic beta-cell function using a model-based analysis. METHODS: Data for this analysis were from three large, placebo-controlled clinical studies that examined sitagliptin 100 mg q.d. as add-on to metformin therapy or as monotherapy over 18 or 24 weeks. In these studies, subsets of patients consented to undergo extensive blood sampling as part of a nine-point meal tolerance test performed at baseline and study end-point. Blood samples were collected at -10, 0, 10, 20, 30, 60, 90, 120 and 180 min relative to the start of a meal and subsequently were assayed for plasma glucose and serum C-peptide concentrations. Parameters for beta-cell function were calculated using the C-peptide minimal model, which estimates insulin secretion rate (ISR) and partitions the ISR into basal (Phi(b); ISR at basal glucose concentrations), static (Phi(s); ISR at above basal glucose concentrations following a meal) and dynamic (Phi(d); ISR in response to the rate of increase in above basal glucose concentrations following a meal) components. The total responsivity index (Phi(total); average ISR over the average glucose concentration) is calculated as a function of Phi(s), Phi(d )and Phi(b. )Insulin sensitivity was assessed with a validated composite index (ISI). Disposition indices (DI), which assess insulin secretion in the context of changes in insulin sensitivity, were calculated as the product of Phiand ISI. RESULTS: When administered in combination with ongoing metformin therapy or as monotherapy, sitagliptin was associated with substantial reductions in postprandial glycaemic excursion following a meal challenge relative to placebo. Sitagliptin produced significant (p < 0.05 vs. placebo) improvements in Phi(s )and Phi(total), regardless of treatment regimen (add-on to metformin or as monotherapy). For Phi(d), there was a numerical, but not statistically significant, improvement with sitagliptin relative to placebo. Treatment with sitagliptin increased Phi(b), but the difference relative to placebo was only significant with monotherapy. ISI was not significantly different between sitagliptin and placebo. The DIs for the static, dynamic and total measures were significantly (p < 0.05) increased with sitagliptin treatment relative to placebo. CONCLUSIONS: In this model-based analysis, sitagliptin improved beta-cell function relative to placebo in both fasting and postprandial states in patients with type 2 diabetes.

Parental evaluation of a telemonitoring service for children with Type 1 Diabetes.

Introduction In the past years, we developed a telemonitoring service for young patients affected by Type 1 Diabetes. The service provides data to the clinical staff and offers an important tool to the parents, that are able to oversee in real time their children. The aim of this work was to analyze the parents' perceived usefulness of the service. Methods The service was tested by the parents of 31 children enrolled in a seven-day clinical trial during a summer camp. To study the parents' perception we proposed and analyzed two questionnaires. A baseline questionnaire focused on the daily management and implications of their children's diabetes, while a post-study one measured the perceived benefits of telemonitoring. Questionnaires also included free text comment spaces. Results Analysis of the baseline questionnaires underlined the parents' suffering and fatigue: 51% of total responses showed a negative tendency and the mean value of the perceived quality of life was 64.13 in a 0-100 scale. In the post-study questionnaires about half of the parents believed in a possible improvement adopting telemonitoring. Moreover, the foreseen improvement in quality of life was significant, increasing from 64.13 to 78.39 ( p-value = 0.0001). The analysis of free text comments highlighted an improvement in mood, and parents' commitment was also proved by their willingness to pay for the service (median = 200 euro/year). Discussion A high number of parents appreciated the telemonitoring service and were confident that it could improve communication with physicians as well as the family's own peace of mind.

Physiologic evaluation of factors controlling glucose tolerance in man: measurement of insulin sensitivity and beta-cell glucose sensitivity from the response to intravenous glucose.

The quantitative contributions of pancreatic responsiveness and insulin sensitivity to glucose tolerance were measured using the "minimal modeling technique" in 18 lean and obese subjects (88-206% ideal body wt). The individual contributions of insulin secretion and action were measured by interpreting the dynamics of plasma glucose and insulin during the intravenous glucose tolerance test in terms of two mathematical models. One, the insulin kinetics model, yields parameters of first-phase (phi 1) and second-phase (phi 2) responsivity of the beta-cells to glucose. The other glucose kinetics model yields the insulin sensitivity parameters, SI. Lean and obese subjects were subdivided into good (KG greater than 1.5) and lower (KG less than 1.5) glucose tolerance groups. The etiology of lower glucose tolerance was entirely different in lean and obese subjects. Lean, lower tolerance was related to pancreatic insufficiency (phi 2 77% lower than in good tolerance controls [P less than 0.03]), but insulin sensitivity was normal (P greater than 0.5). In contrast, obese lower tolerance was entirely due to insulin resistance (SI diminished 60% [P less than 0.01]); pancreatic responsiveness was not different from lean, good tolerance controls (phi 1: P greater than 0.06; phi 2: P greater than 0.40). Subjects (regardless of weight) could be segregated into good and lower tolerance by the product of second-phase beta-cell responsivity and insulin sensitivity (phi 2 . SI). Thus, these two factors were primarily responsible for overall determination of glucose tolerance. The effect of phi 1 was to modulate the KG value within those groups whose overall tolerance was determined by phi 2 . SI. This phi 1 modulating influence was more pronounced among insulin sensitive (phi 1 vs. KG, r = 0.79) than insulin resistant (obese, low tolerance; phi 1 vs. KG, r = 0.91) subjects. This study demonstrates the feasibility of the minimal model technique to determine the etiology of impaired glucose tolerance.

Effect of insulin on system A amino acid transport in human skeletal muscle.

Transmembrane transport of neutral amino acids in skeletal muscle is mediated by at least four different systems (system A, ASC, L, and Nm), and may be an important target for insulin's effects on amino acid and protein metabolism. We have measured net amino acid exchanges and fractional rates of inward (k(in), min-1) and outward (kout, min-1) transmembrane transport of 2-methylaminoisobutyric acid (MeAIB, a nonmetabolizable amino acid analogue, specific for system A amino acid transport) in forearm deep tissues (skeletal muscle), by combining the forearm perfusion technique and a novel dual tracer ([1-H3]-D-mannitol and 2-[1-14C]-methylaminoisobutyric acid) approach for measuring in vivo the activity of system A amino acid transport. Seven healthy lean subjects were studied. After a baseline period, insulin was infused into the brachial artery to achieve local physiologic hyperinsulinemia (76 +/- 8 microU/ml vs 6.4 +/- 1.6 microU/ml in the basal period, P < 0.01) without affecting systemic hormone and substrate concentrations. Insulin switched forearm amino acid exchange from a net output (-2,630 +/- 1,100 nmol/min per kig of forearm tissue) to a net uptake (1,610 +/- 600 nmol/min per kg, P < 0.01 vs baseline). Phenylalanine and tyrosine balances simultaneously shifted from a net output (-146 +/- 47 and -173 +/- 34 nmol/min per kg, respectively) to a zero balance (16.3 +/- 51 for phenylalanine and 15.5 +/- 14.3 nmol/min per kg for tyrosine, P < 0.01 vs baseline for both), showing that protein synthesis and breakdown were in equilibrium during hyperinsulinemia. Net negative balances of alanine, methionine, glycine, threonine and asparagine (typical substrates for system A amino acid transport) also were decreased by insulin, whereas serine (another substrate for system A transport) shifted from a zero balance to net uptake. Insulin increased k(in) of MeAIB from a basal value of 11.8.10(-2) +/- 1.7.10(-2).min-1 to 13.7.10(-2) +/- 2.2.10(-2).min-1 (P < 0.02 vs the postabsorptive value), whereas kout was unchanged. We conclude that physiologic hyperinsulinemia stimulates the activity of system A amino acid transport in human skeletal muscle, and that this effect may play a role in determining the overall concomitant response of muscle amino acid/protein metabolism to insulin.

Effects of a change in the pattern of insulin delivery on carbohydrate tolerance in diabetic and nondiabetic humans in the presence of differing degrees of insulin resistance.

While it is well established that people with non-insulin dependent diabetes mellitus have defects in both insulin secretion and action, the relative contribution of each to glucose intolerance is not known. Therefore, nondiabetic (lean and obese) and non-insulin dependent diabetes mellitus subjects were studied on two occasions. On each occasion, insulin secretion was inhibited with somatostatin and glucose was infused in a pattern and amount that mimicked the systemic delivery rate normally observed after ingestion of 50 g of glucose. Insulin also was infused so as to mimic postprandial insulin profiles observed in separate groups of diabetic and nondiabetic subjects after food ingestion. Glucose turnover was measured using the isotope dilution method. A delayed pattern of insulin delivery (i.e., a "diabetic" insulin profile) led to higher (P < 0.05) glucose concentrations in all groups; however, the effects were transient, resulting in only a modest increase in the integrated glycemic responses. An isolated defect in insulin action had little effect on peak glucose concentration; however, it prolonged the duration of hyperglycemia, leading to a 2.5-4.2-fold increase (P < 0.05) in the integrated glycemic response. A combined defect in the pattern of insulin secretion and action was additive rather than synergistic. Both defects caused hyperglycemia by altering suppression of endogenous glucose release and stimulation of glucose disposal. Whereas obese diabetic and nondiabetic subjects had comparable defects in glucose clearance, non-insulin dependent diabetes mellitus subjects also had defects in hepatic insulin action. Thus, abnormalities in the pattern of insulin secretion and action alone or in combination impair glucose tolerance. An isolated defect in insulin action has a more pronounced and prolonged effect than does an isolated change in the pattern of insulin secretion. Hepatic and extrahepatic insulin resistance results in marked and sustained hyperglycemia.

Direct assessment of liver glycogen storage by 13C nuclear magnetic resonance spectroscopy and regulation of glucose homeostasis after a mixed meal in normal subjects.

Despite extensive recent studies, understanding of the normal postprandial processes underlying immediate storage of substrate and maintenance of glucose homeostasis in humans after a mixed meal has been incomplete. The present study applied 13C nuclear magnetic resonance spectroscopy to measure sequential changes in hepatic glycogen concentration, a novel tracer approach to measure postprandial suppression of hepatic glucose output, and acetaminophen to trace the pathways of hepatic glycogen synthesis to elucidate the homeostatic adaptation to the fed state in healthy human subjects. After the liquid mixed meal, liver glycogen concentration rose from 207 +/- 22 to 316 +/- 19 mmol/liter at an average rate of 0.34 mmol/liter per min and peaked at 318 +/- 31 min, falling rapidly thereafter (0.26 mmol/liter per min). The mean increment at peak represented net glycogen synthesis of 28.3 +/- 3.7 g (approximately 19% of meal carbohydrate content). The contribution of the direct pathway to overall glycogen synthesis was 46 +/- 5 and 68 +/- 8% between 2 and 4 and 4 and 6 h, respectively. Hepatic glucose output was completely suppressed within 30 min of the meal. It increased steadily from 60 to 255 min from 0.31 +/- 32 to 0.49 +/- 18 mg/kg per min then rapidly returned towards basal levels (1.90 +/- 0.04 mg/kg per min). This pattern of change mirrored precisely the plasma glucagon/insulin ratio. These data provide for the first time a comprehensive picture of normal carbohydrate metabolism in humans after ingestion of a mixed meal.

Assessment of insulin action and glucose effectiveness in diabetic and nondiabetic humans.

Insulin concentrations in humans continuously change and typically increase only when glucose also increases such as with eating. In this setting, it is not known whether the severity of hepatic and extrahepatic insulin resistance is comparable and whether the ability of glucose to regulate its own uptake and release is defective in non-insulin-dependent diabetes mellitus (NIDDM). To address this question, NIDDM and nondiabetic subjects were studied when glucose concentrations were clamped at either 5 mM (euglycemia) or varied so as to mimic the glucose concentrations observed in nondiabetic humans after food ingestion (hyperglycemia). Insulin was infused so as to simulate a "nondiabetic" postprandial profile. During euglycemia, insulin increased glucose disposal in nondiabetic but not diabetic subjects indicating marked extrahepatic resistance. In contrast, insulin-induced suppression of glucose release was only minimally less (P < 0.05) in diabetic than nondiabetic subjects (-1.06 +/- 0.09 vs. -1.47 +/- 0.21 nmol.kg-1 per 4 h). Hyperglycemia substantially enhanced disposal in both groups. Glucose effectiveness measured as the magnitude of enhancement of disposal (0.59 +/- 0.18 vs. 0.62 +/- 0.17 nmollkg-1 per 4 h) and suppression of release (-0.36 +/- 0.12 vs. -0.14 +/- 0.12 nmol.kg-1 per 4 h) did not differ in the diabetic and nondiabetic subjects. In conclusion, when assessed in the presence of a physiological insulin profile, people with NIDDM demonstrate: (a) profound extrahepatic insulin resistance, (b) modest hepatic insulin resistance, and (c) normal ability of glucose to stimulate its own uptake and suppress its own release.

Saturable transport of insulin from plasma into the central nervous system of dogs in vivo. A mechanism for regulated insulin delivery to the brain.

By acting in the central nervous system, circulating insulin may regulate food intake and body weight. We have previously shown that the kinetics of insulin uptake from plasma into cerebrospinal fluid (CSF) can best be explained by passage through an intermediate compartment. To determine if transport kinetics into this compartment were consistent with an insulin receptor-mediated transport process, we subjected overnight fasted, anesthetized dogs to euglycemic intravenous insulin infusions for 90 min over a wide range of plasma insulin levels (69-5,064 microU/ml) (n = 10). Plasma and CSF samples were collected over 8 h for determination of immunoreactive insulin levels, and the kinetics of insulin uptake from plasma into CSF were analyzed using a compartmental model with three components (plasma-->intermediate compartment-->CSF). By sampling frequently during rapid changes of plasma and CSF insulin levels, we were able to precisely estimate three parameters (average standard deviation 14%) characterizing the uptake of insulin from plasma, through the intermediate compartment and into CSF (k1k2); insulin entry into CSF and insulin clearance from the intermediate compartment (k2 + k3); and insulin clearance from CSF (k4). At physiologic plasma insulin levels (80 +/- 7.4 microU/ml), k1k2 was determined to be 10.7 x 10(-6) +/- 1.3 x 10(-6) min-2. With increasing plasma levels, however, k1k2 decreased progressively, being reduced sevenfold at supraphysiologic levels (5,064 microU/ml). The apparent KM of this saturation curve was 742 microU/ml (approximately 5 nM). In contrast, the rate constants for insulin removal from the intermediate compartment and from CSF did not vary with plasma insulin (k2 + k3 = 0.011 +/- 0.0019 min-1 and k4 = 0.046 +/- 0.021 min-1). We conclude that delivery of plasma insulin into the central nervous system is saturable, and is likely facilitated by an insulin-receptor mediated transport process.