Minimal model estimation of glucose absorption and insulin sensitivity from oral test: validation with a tracer method

Measuring insulin sensitivity during the physiological milieu of oral glucose perturbation, e.g., a meal or an oral glucose tolerance test, would be extremely valuable but difficult since the rate of appearance of absorbed glucose is unknown. The reference method is a tracer two-step one: first, the rate of appearance of glucose (R(a meal)(ref)) is reconstructed by employing the tracer-to-tracee ratio clamp technique with two tracers and a model of non-steady-state glucose kinetics; next, this R(a meal)(ref) is used as the known input of a model describing insulin action on glucose kinetics to estimate insulin sensitivity (SI(ref)). Recently, a nontracer method based on the oral minimal model (OMM) has been proposed to estimate simultaneously the above quantities, denoted R(a meal) and SI, respectively, from plasma glucose and insulin concentrations measured after an oral glucose perturbation. This last method has obvious advantages over the tracer method, but its domain of validity has never been assessed against a reference method. It is thus important to establish whether or not the "nontracer" R(a meal) and SI compare well with the "tracer" R(a meal)(ref) and SI(ref). We do this comparison on a database of 88 subjects, and it is very satisfactory: R(a meal) profiles agree well with the R(a meal)(ref) and correlation of SI(ref) with SI is r = 0.86 (P < 0.0001). We conclude that OMM candidates as a reliable tool to measure both the rate of glucose absorption and insulin sensitivity from oral glucose tests without employing tracers.     

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.     

Use of a novel triple-tracer approach to assess postprandial glucose metabolism

Numerous studies have used the dual-tracer method to assess postprandial glucose metabolism. The present experiments were undertaken to determine whether the marked tracer nonsteady state that occurs with the dual-tracer approach after food ingestion introduces error when it is used to simultaneously measure both meal glucose appearance (R(a meal)) and endogenous glucose production (EGP). To do so, a novel triple-tracer approach was designed: 12 subjects ingested a mixed meal containing [1-(13)C]glucose while [6-(3)H]glucose and [6,6-(2)H(2)]glucose were infused intravenously in patterns that minimized the change in the plasma ratios of [6-(3)H]glucose to [1-(13)C]glucose and of [6,6-(2)H(2)]glucose to endogenous glucose, respectively. R(a meal) and EGP measured with this approach were essentially model independent, since non-steady-state error was minimized by the protocol. Initial splanchnic glucose extraction (ISE) was 12.9% +/- 3.4%, and suppression of EGP (EGPS) was 40.3% +/- 4.1%. In contrast, when calculated with the dual-tracer one-compartment model, ISE was higher (P < 0.05) and EGPS was lower (P < 0.005) than observed with the triple-tracer approach. These errors could only be prevented by using time-varying volumes different for R(a meal) and EGP. Analysis of the dual-tracer data with a two-compartment model reduced but did not totally avoid the problems associated with marked postprandial changes in the tracer-to-tracee ratios. We conclude that results from previous studies that have used the dual-tracer one-compartment model to measure postprandial carbohydrate metabolism need to be reevaluated and that the triple-tracer technique may provide a useful approach for doing so.     

Free fatty acid-induced peripheral insulin resistance augments splanchnic glucose uptake in healthy humans

To investigate the effect of elevated plasma free fatty acid (FFA) concentrations on splanchnic glucose uptake (SGU), we measured SGU in nine healthy subjects (age, 44 +/- 4 yr; body mass index, 27.4 +/- 1.2 kg/m(2); fasting plasma glucose, 5.2 +/- 0.1 mmol/l) during an Intralipid-heparin (LIP) infusion and during a saline (Sal) infusion. SGU was estimated by the oral glucose load (OGL)-insulin clamp method: subjects received a 7-h euglycemic insulin (100 mU x m(-2) x min(-1)) clamp, and a 75-g OGL was ingested 3 h after the insulin clamp was started. After glucose ingestion, the steady-state glucose infusion rate (GIR) during the insulin clamp was decreased to maintain euglycemia. SGU was calculated by subtracting the integrated decrease in GIR during the period after glucose ingestion from the ingested glucose load. [3-(3)H]glucose was infused during the initial 3 h of the insulin clamp to determine rates of endogenous glucose production (EGP) and glucose disappearance (R(d)). During the 3-h euglycemic insulin clamp before glucose ingestion, R(d) was decreased (8.8 +/- 0.5 vs. 7.6 +/- 0.5 mg x kg(-1) x min(-1), P < 0.01), and suppression of EGP was impaired (0.2 +/- 0.04 vs. 0.07 +/- 0.03 mg x kg(-1) x min(-1), P < 0.01). During the 4-h period after glucose ingestion, SGU was significantly increased during the LIP vs. Sal infusion study (30 +/- 2 vs. 20 +/- 2%, P < 0.005). In conclusion, an elevation in plasma FFA concentration impairs whole body glucose R(d) and insulin-mediated suppression of EGP in healthy subjects but augments SGU.     

Postnatal ontogeny of glucose homeostasis and insulin action in sheep

Glucose tolerance declines with maturation and aging in several species, but the time of onset and extent of changes in insulin sensitivity and insulin secretion and their contribution to changes in glucose tolerance are unclear. We therefore determined the effect of maturation on glucose tolerance, insulin secretion, and insulin sensitivity in a longitudinal study of male and female sheep from preweaning to adulthood, and whether these measures were related across age. Glucose tolerance was assessed by intravenous glucose tolerance test (IVGTT, 0.25 g glucose/kg), insulin secretion as the integrated insulin concentration during IVGTT, and insulin sensitivity by hyperinsulinemic-euglycemic clamp (2 mU insulin.kg(-1).min(-1)). Glucose tolerance, relative insulin secretion, and insulin sensitivity each decreased with age (P < 0.001). The disposition index, the product of insulin sensitivity, and various measures of insulin secretion during fasting or IVGTT also decreased with age (P < 0.001). Glucose tolerance in young adult sheep was independently predicted by insulin sensitivity (P = 0.012) and by insulin secretion relative to integrated glucose during IVGTT (P = 0.005). Relative insulin secretion before weaning was correlated positively with that in the adult (P = 0.023), whereas glucose tolerance, insulin sensitivity, and disposition indexes in the adult did not correlate with those at earlier ages. We conclude that glucose tolerance declines between the first month of life and early adulthood in the sheep, reflecting decreasing insulin sensitivity and absence of compensatory insulin secretion. Nevertheless, the capacity for insulin secretion in the adult reflects that early in life, suggesting that it is determined genetically or by persistent influences of the perinatal environment.     

Estimates of in vivo insulin action in humans: comparison of the insulin clamp and the minimal model techniques

The insulin clamp technique, which is often assumed to measure the ability of insulin to stimulate glucose uptake, actually measures both insulin-independent and insulin-dependent glucose uptake. In contrast, the minimal model technique, recently introduced by Bergman, Philips and Cobelli (1981), attempts to directly estimate insulin sensitivity (insulin-dependent glucose uptake = S1) by measurement of plasma glucose and insulin values during a 3 hour intravenous glucose tolerance test (IVGTT). In the present study estimates of insulin action derived from the insulin clamp and the minimal model technique were compared in 20 humans with varying degrees of glucose tolerance. The insulin response during the IVGTT was too low to permit calculation of S1 in 5 subjects - 4 with Type II diabetes and 1 with normal glucose tolerance. Although the correlation coefficient between the two tests in the other 15 patients was statistically significant (r = 0.53, P less than 0.05), this statement is somewhat misleading. Thus, S1 in the 4/7 patients with Type II diabetes in whom it could be measured was zero, and the correlation between estimates of insulin action with the two techniques in the 11 non-diabetic patients was not statistically significant (r = 0.41, P = NS) when these 4 patients were removed from the analysis. In conclusion, these data indicate that there was only a weak correlation between estimates of insulin action assessed with the insulin clamp and the minimal model techniques. One explanation for this observation is that the insulin-independent component of total glucose disposal both varies widely among patients and contributes significantly to glucose uptake as assessed by the insulin clamp technique.(ABSTRACT TRUNCATED AT 250 WORDS)     

Population approaches to estimate minimal model indexes of insulin sensitivity and glucose effectiveness using full and reduced sampling schedules

The intravenous glucose tolerance test (IVGTT) interpreted with the minimal model provides individual indexes of insulin sensitivity (S(I)) and glucose effectiveness (S(G)). In population studies, the traditional approach, the standard two-stage (STS) method, fails to account for uncertainty in individual estimates, resulting in an overestimation of between-subject variability. Furthermore, in the presence of reduced sampling and/or insulin resistance, individual estimates may be unobtainable, biasing population information. Therefore, we investigated the use of two population approaches, the iterative two-stage (ITS) method and nonlinear mixed-effects modeling (NM), in a population (n = 235) of insulin-sensitive and insulin-resistant subjects under full (FSS, 33 samples) and reduced [RSS(240-min), 13 samples and RSS(180-min), 12 samples] IVGTT sampling schedules. All three population methods gave similar results with the FSS. Using RSS(240), the three methods gave similar results for S(I), but S(G) population means were overestimated. With RSS(180), S(I) and S(G) population means were higher for all three methods compared with their FSS counterparts. NM estimated similar between-subject variability (19% S(G), 53% S(I)) with RSS(180), whereas ITS showed regression to the mean for S(G) (0.01% S(G), 56% S(I)) and STS provided larger population variability in S(I) (29% S(G), 91% S(I)). NM provided individual estimates for all subjects, whereas the two-stage methods failed in 16-18% of the subjects using RSS(180) and 6-14% using RSS(240). We conclude that population approaches, specifically NM, are useful in studies with a sparsely sampled IVGTT ( approximately 12 samples) of short duration ( approximately 3 h) and when individual parameter estimates in all subjects are desired.     

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).     

Evaluation of nonlinear regression approaches to estimation of insulin sensitivity by the minimal model with reference to Bayesian hierarchical analysis

Minimal model analysis of intravenous glucose tolerance test (IVGTT) glucose and insulin concentrations offers a validated approach to measuring insulin sensitivity, but model identification is not always successful. Improvements may be achieved by using alternative settings in the modeling process, although results may differ according to setting, and care must be exercised in combining results. IVGTT data (12 samples, regular test) from 533 men without diabetes was modeled by the traditional nonlinear regression (NLR) approach, using five different permutations of settings. Results were evaluated with reference to the more robust Bayesian hierarchical (BH) approach to model identification and to the proportion of variance they explained in known correlates of insulin sensitivity (age, BMI, blood pressure, fasting glucose and insulin, serum triglyceride, HDL cholesterol, and uric acid concentration). BH analysis was successful in all cases. With NLR analysis, between 17 and 35 IVGTTs were associated with parameter coefficients of variation (PCVs) for minimal model parameters S(I) (insulin sensitivity) and S(G) (glucose effectiveness) of >100%. Systematic use of each different approach in combination reduced this number to five. Mean (interquartile range) S(I)(NLR) was then 3.14 (2.29-4.63) min(-1).mU(-1).l x 10(-4) and 2.56 (1.74-3.83) min(-1).mU(-1).l x 10(-4) for S(I)(BH) (correlation 0.86, P < 0.0001). S(I)(NLR) explained, on average, 10.6% of the variance in known correlates of insulin sensitivity, whereas S(I)(BH) explained 8.5%. In a large body of data, which BH analysis demonstrated could be fully identified, use of alternative modeling settings in NLR analysis could substantially reduce the number of analyses with PCVs >100%. S(I)(NLR) compared favorably with S(I)(BH) in the proportion of variance explained in known correlates of insulin sensitivity.     

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

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

EPR properties of a g=2 broad signal trapped in an S1 state in Ca2+-depleted Photosystem II

We investigated a new EPR signal that gives a broad line shape around g=2 in Ca(2+)-depleted Photosystem (PS) II. The signal was trapped by illumination at 243 K in parallel with the formation of Y(Z)*. The ratio of the intensities between the g=2 broad signal and the Y(Z)* signal was 1:3, assuming a Gaussian line shape for the former. The g=2 broad signal and the Y(Z)* signal decayed together in parallel with the appearance of the S(2) state multiline at 243 K. The g=2 broad signal was assigned to be an intermediate S(1)X* state in the transition from the S(1) to the S(2) state, where X* represents an amino acid radical nearby manganese cluster, such as D1-His337. The signal is in thermal equilibrium with Y(Z)*. Possible reactions in the S state transitions in Ca(2+)-depleted PS II were discussed.     

Contribution to glucose tolerance of insulin-independent vs. insulin-dependent mechanisms in mice

To study the contributions of insulin-dependent vs. insulin-independent mechanisms to intravenous glucose tolerance (K(G)), 475 experiments in mice were performed. An intravenous glucose bolus was given either alone or with exogenous insulin or with substances modulating insulin secretion and sensitivity. Seven samples were taken over 50 min. Insulin [suprabasal area under the curve (DeltaAUC(ins))] ranged from 0 to 100 mU. ml(-1). 50 min. After validation against the euglycemic hyperinsulinemic clamp, the minimal model of net glucose disappearance was exploited to analyze glucose and insulin concentrations to measure the action of glucose per se independent of dynamic insulin (S(G)) and the combined effect of insulin sensitivity (S(I)) and secretion. Sensitivity analysis showed that insulin [through disposition index (DI)] contributed to glucose tolerance by 29 +/- 4% in normal conditions. In conditions of elevated hyperinsulinemia, contribution by insulin increased on average to 69%. K(G) correlated with DI but was saturated for DeltaAUC(ins) above 15 mU. ml(-1). 50 min. Insulin sensitivity related to DeltaAUC(ins) in a hyperbolic manner, whereas S(G) did not correlate with the insulin peak in the physiological range. Thus glucose tolerance in vivo is largely mediated by mechanisms unrelated to dynamic insulin and saturates with high insulin.     

Reduced sampling schedule for the glucose minimal model: importance of Bayesian estimation

The minimal model (MM) of glucose kinetics during an intravenous glucose tolerance test (IVGTT) is widely used in clinical studies to measure metabolic indexes such as glucose effectiveness (S(G)) and insulin sensitivity (S(I)). The standard (frequent) IVGTT sampling schedule (FSS) for MM identification consists of 30 points over 4 h. To facilitate clinical application of the MM, reduced sampling schedules (RSS) of 13-14 samples have also been derived for normal subjects. These RSS are especially appealing in large-scale studies. However, with RSS, the precision of S(G) and S(I) estimates deteriorates and, in certain cases, becomes unacceptably poor. To overcome this difficulty, population approaches such as the iterative two-stage (ITS) approach have been recently proposed, but, besides leaving some theoretical issues open, they appear to be oversized for the problem at hand. Here, we show that a Bayesian methodology operating at the single individual level allows an accurate determination of MM parameter estimates together with a credible measure of their precision. Results of 16 subjects show that, in passing from FSS to RSS, there are no significant changes of point estimates in nearly all of the subjects and that only a limited deterioration of parameter precision occurs. In addition, in contrast with the previously proposed ITS method, credible confidence intervals (e.g., excluding negative values) are obtained. They can be crucial for a subsequent use of the estimated MM parameters, such as in classification, clustering, regression, or risk analysis.     

Identifiability and online estimation of diagnostic parameters with in the glucose insulin homeostasis

Today, diagnostic decisions about pre-diabetes or diabetes are made using static threshold rules for the measured plasma glucose. In order to develop an alternative diagnostic approach, dynamic models as the Minimal Model may be deployed. We present a novel method to analyze the identifiability of model parameters based on the interpretation of the empirical observability Gramian. This allows a unifying view of both, the observability of the system's states (with dynamics) and the identifiability of the system's parameters (without dynamics). We give an iterative algorithm, in order to find an optimized set of states and parameters to be estimated. For this set, estimation results using an Unscented Kalman Filter (UKF) are presented. Two parameters are of special interest for diagnostic purposes: the glucose effectiveness S(G) characterizes the ability of plasma glucose clearance, and the insulin sensitivity S(I) quantifies the impact from the plasma insulin to the interstitial insulin subsystem. Applying the identifiability analysis to the trajectories of the insulin glucose system during an intravenous glucose tolerance test (IVGTT) shows the following result: (1) if only plasma glucose G(t) is measured, plasma insulin I(t) and S(G) can be estimated, but not S(I). (2) If plasma insulin I(t) is captured additionally, identifiability is improved significantly such that up to four model parameters can be estimated including S(I). (3) The situation of the first case can be improved, if a controlled external dosage of insulin is applied. Then, parameters of the insulin subsystem can be identified approximately from measurement of plasma glucose G(t) only.     

Effect of maternal feed restriction during pregnancy on glucose tolerance in the adult guinea pig

Maternal nutrient restriction and impaired fetal growth are associated with postnatal insulin resistance, hyperinsulinemia, and glucose intolerance in humans but not consistently in other species, such as the rat or sheep. We therefore determined the effect of mild (85% ad libitum intake/kg body wt) or moderate (70% ad libitum intake/kg body wt) maternal feed restriction throughout pregnancy on glucose and insulin responses to an intravenous glucose tolerance test (IVGTT) in the young adult guinea pig. Maternal feed restriction reduced birth weight (mild and moderate: both P < 0.02) in male offspring. Moderate restriction increased plasma glucose area under the curve (P < 0.04) and decreased the glucose tolerance index (K(G)) (P < 0.02) during the IVGTT in male offspring compared with those of mildly restricted but not of ad libitum-fed mothers. Moderate restriction increased fasting plasma insulin (P < 0.04, adjusted for litter size) and the insulin response to IVGTT (P < 0.001), and both moderate and mild restriction increased the insulin-to-glucose ratio during the IVGTT (P < 0.003 and P < 0.02) in male offspring. When offspring were classed into tertiles according to birth weight, glucose tolerance was not altered, but fasting insulin concentrations were increased in low compared with medium birth weight males (P < 0.03). The insulin-to-glucose ratio throughout the IVGTT was increased in low compared with medium (P < 0.01) or high (P < 0.05) birth weight males. Thus maternal feed restriction in the guinea pig restricts fetal growth and causes hyperinsulinemia in young adult male offspring, suggestive of insulin resistance. These findings suggest that mild to moderate prenatal perturbation programs postnatal glucose homeostasis adversely in the guinea pig, as in the human.     

Intravenous glucose tolerance, insulin response to glucose, peripheral sensitivity to insulin, and serum lipoproteins in post-myocardial infarct patients with normal fasting blood glucose

Intravenous glucose tolerance (IVGTT), basal insulin and insulin response to glucose infusion (GIT), insulin sensitivity, and lipoprotein patterns were determined in non-obese post-coronary subjects, 3-6 months after myocardial infarction. Twelve had decreased and 31 normal IVGTT. The control group comprised 31 subjects with normal IVGTT, who did not display any signs of coronary disease. The post-coronary patients were not taking any drugs except for furosamide, which was shown not to influence insulin response to GIT or glucose tolerance. Decreased IVGTT in the post-coronary patients could be ascribed to decreased insulin response and insulin resistance. These two derangements are considered as hereditary markers in glucose intolerance and type 2 diabetes. Accordingly, our findings suggest that glucose intolerance in subjects with myocardial infarcts has the same background. The post-coronary patients demonstrated elevated triglycerides (TG) and cholesterol in total serum and in very low density lipoproteins (VLDL), the lipoprotein patterns being almost identical in post-coronary patients with or without decreased IVGTT. No relationship was found in the control and post-coronary groups between IVGTT, basal insulin, stimulated insulin (KI, IP), and insulin sensitivity (KG), on the one hand, and total or VLDL TG or any other lipoprotein particle, on the other. Thus, the derangements in glucose, insulin, and serum triglyceride metabolism were independent abnormalities (risk factors) in these non-obese post-coronary patients.     

Effect of orally administered phenylalanine with and without glucose on insulin, glucagon and glucose concentrations.

OBJECTIVE: As part of our studies of the metabolic effects of ingested proteins, we are currently investigating the effects of ingestion of individual amino acids. The objective of the present study was to determine whether ingested phenylalanine stimulates insulin and/or glucagon secretion, and if phenylalanine ingested with glucose modifies the insulin, glucagon or glucose response to the ingested glucose. DESIGN: Six healthy subjects were tested on 4 separate occasions. Plasma phenylalanine, glucose, insulin, glucagon, and total alpha amino nitrogen (AAN) (i.e., total amino acids) concentrations were measured at various times during a 2.5 h period after ingestion of 1 mmol phenylalanine/kg lean body mass, 25 g glucose, 1 mmol phenylalanine/kg lean body mass+25 g glucose, or water only, given in random order. RESULTS: Following phenylalanine ingestion, the circulating phenylalanine concentration increased approximately 14 fold and remained elevated for the duration of the experiment. Glucagon and AAN increased, insulin increased modestly, and glucose was unchanged when compared to water ingestion. When glucose was ingested with phenylalanine, the circulating phenylalanine, glucagon, AAN, and insulin area responses were approximately the sum of the responses to phenylalanine alone and glucose alone. However, the plasma glucose area response was decreased 66% when phenylalanine was co-ingested with glucose. CONCLUSION: In summary, phenylalanine in an amount moderately greater than that in a large protein meal stimulates an increase in insulin and glucagon concentration. It markedly attenuates the glucose-induced rise in plasma glucose when ingested with glucose.     

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.