DOI: 10.2337/dc06-1240 © 2007 by the American Diabetes Association
The BIGTT TestA novel test for simultaneous measurement of pancreatic ß-cell function, insulin sensitivity, and glucose tolerance
1 Steno Diabetes Center, Copenhagen, Denmark Address correspondence and reprint requests to Torben Hansen, MD, PhD, Steno Diabetes Center, Niels Steensens Vej 2, DK-2820 Gentofte, Denmark. E-mail: toha{at}steno.dk
OBJECTIVEInsulin resistance and impaired ß-cell function are key elements in the pathogenesis of type 2 diabetes. We aimed to develop valid algorithms for estimation of the insulin sensitivity index (SI) and acute insulin response (AIR) derived from simple and cheap physiological measurements that could be used in large-scale metabolic, genetic, and epidemiological studies. RESEARCH DESIGN AND METHODSFor our purpose, data from an oral glucose tolerance test (OGTT) (18 samples during 240 min) and a tolbutamide-modified intravenous glucose tolerance test (IVGTT) (33 samples during 180 min) from 258 individuals with fasting plasma glucose <7 mmol/l and 2-h plasma glucose <7.8 mmol/l were used for model development and internal validation. Data from an additional 28 individuals were used for external validation. Bergmans minimal model was used to calculate SI, and the trapezoidal method was used to calculate AIR08 min. Multiple linear regression was applied to derive predictive equations of log(SI) and log(AIR08 min) using data on sex, BMI, plasma glucose, and serum insulin levels obtained during the OGTT. RESULTSWe demonstrate that it is possible to obtain estimates of SI (BIGTT-SI) and AIR (BIGTT-AIR) that are highly correlated to IVGTT-derived values of SI (R2 = 0.77) and AIR (R2 = 0.54). In the two validation datasets we obtained similar results. CONCLUSIONSData from OGTTs can provide accurate measures of insulin sensitivity and ß-cell function, which can be used in large scale metabolic, genetic, and epidemiological studies.
Abbreviations: AIR, acute insulin response HOMA, homeostasis model assessment IGT, impaired glucose tolerance IVGTT, intravenous glucose tolerance test OGTT, oral glucose tolerance test RCPH, Research Centre for Prevention and Health
The maintenance of normal glucose homeostasis involves the simultaneous and coordinated roles of insulin release from the pancreatic ß-cells and insulin action on peripheral tissues, primarily muscle (1,2). For estimation of whole-body insulin sensitivity, the euglycemic-hyperinsulinemic clamp is considered the "gold standard" (3). The assessment of the insulin sensitivity index (SI) by the minimal model for the intravenous glucose tolerance test (IVGTT) has also been used in numerous studies (4), because the method has the advantage of providing simultaneous information on the SI and acute insulin response (AIR). Both methods are considered to provide exact and, among individuals with normal glucose tolerance, comparable measurements of insulin sensitivity (5). However, because these methods are time consuming and expensive and cannot be performed on the same day as an oral glucose tolerance test (OGTT), neither is suitable for large-scale studies. In the present study, we examine nondiabetic individuals without impaired glucose tolerance (IGT) to derive equations that are more accurate than those currently available for the SI and the AIR from an OGTT (1,612). We have used detailed information from both plasma glucose and serum insulin levels during an extended and frequently sampled OGTT combined with information on anthropometric measures and sex to generate equations and data from a frequently sampled IVGTT performed in the same individuals as reference. The approach is empirical and data driven, with multiple regression statistics being applied to the physiological data obtained.
Individuals for model development and internal validation OGTT data from 258 individuals with fasting plasma glucose <7.0 mmol/l and 2-h plasma glucose <7.8 mmol/l (nondiabetic without IGT) were used to develop (75% of the data) and validate (25% of the data) models. All participants were from 1 of 60 families as described previously (13). The performance of the model was further examined in 28 individuals with IGT identified in the same 60 families. Before participation in the study, all individuals provided informed consent. The study was approved by the Ethical Committee of Copenhagen and was in accordance with the principles of the Declaration of Helsinki.
OGTT
IVGTT
Individuals for external validation
Anthropometric measurements
Statistical methods
Of the total data, A combination of backwards elimination and forward selection was chosen in the modeling phase to handle the large number of variables in each model. A model was initially based on plasma glucose and serum insulin levels at 0, 30, 60, 90, 120, and 180 min as regression variables to describe the established indexes of insulin sensitivity and ß-cell function. Depending on which of these variables was statistically significant at a 10% significance level, values at adjacent time points were added sequentially to the model. We have thus used plasma glucose and serum insulin responses from several time points during the OGTT to generate the optimal equations for OGTT-derived indexes of insulin sensitivity (BIGTT-SI|full) and ß-cell function (BIGTT-AIRfull). Also, we generated and tested simple indexes of SI and AIR using time points that are routinely measured in most metabolic, genetic, and epidemiological studies, i.e., at 0, 30, and 120 min (BIGTT-SI|0-30-120 and BIGTT-AIR0-30-120), and 0, 60, and 120 min (BIGTT-SI|0-60-120 and BIGTT-AIR0-60-120), respectively. Evaluations and comparisons between models were based on the residual SD and the correlation coefficient (R2) obtained from the regressions. R2 describes the degree of variation explained by the model relative to the total variation in the population subjected to analysis. Because R2 is population dependent, it can only be used for comparisons of models within a dataset, not between datasets. However, the SD is comparable between models applied to various datasets, as the SD on the log scale corresponds approximately to the coefficient of variation of the predicted SI and AIR on the original scale.
Calculations of insulin sensitivity and acute insulin response using previously published models
Characteristics of participants are given in Table 1. Table 2 shows the predictive equations that were generated for calculations of BIGTT-SI and BIGTT-AIR. Estimates of SI (BIGTT-SI|full) and AIR (BIGTT-AIRfull) were highly correlated to IVGTT-derived values of SI (R2 = 0.77) and AIR (R2 = 0.54) (Table 3).
Each of the OGTT-derived estimates of SI was compared to estimates of SI obtained in the internal and external validation dataset as well as to estimates of insulin sensitivity using the fasting insulin level, the homeostasis model assessment (HOMA), and four other previously published models (Table 3). On the basis of the SD of the estimates, both the full model (BIGTT-SI|full) and the two more simple models (BIGTT-SI|0-30-120 and BIGTT-SI|0-60-120) for estimation of SI had an 30% higher accuracy than two widely used simple indexes of insulin sensitivity, i.e., fasting insulin and HOMA (Table 3). The OGTT-derived estimates of SI were almost similar in the two validation datasets (Table 3). The relationship between SI obtained from the IVGTT and BIGTT-SI|0-60-120 is presented in Fig. 1A, which shows a high degree of correlation at all levels of insulin sensitivity. A similar correlation at all levels of insulin sensitivity is seen by applying the optimal model (BIGTT-SI|full) as well as the BIGTT-SI|0-30-120 (data not shown).
The SD and R2 for various indexes of ß-cell function are also shown in Table 3. The novel BIGTT-AIR methods are about 2030% more accurate than three currently used models for estimation of ß-cell function, and the estimates were very similar in the two validation datasets (Table 3). The relationship between ß-cell function estimated by the IVGTT and by the BIGTT-AIR0-60-120 is shown in Fig. 1B. For subjects with IGT, the BIGTT models for the estimation of indexes of insulin sensitivity and ß-cell function were less accurate than when they were applied to data from nondiabetic individuals without IGT (see Table 1 of the online appendix available at http://dx.doi.org/10.2337/dc06-1240). This was a general observation for all applied models. However, the Matsuda, Cederholm, and Belfiori indexes did seem to capture the insulin sensitivity for subjects with IGT in a more satisfactory manner than the BIGTT model. Regarding the indexes of ß-cell function, the BIGTT model was more accurate than the alternative models (see Table 1 of the online appendix).
We present novel and validated models for calculation of SI and AIR with data available from an OGTT. The models are developed for accurate measurement of SI and AIR in nondiabetic individuals without IGT. The plasma glucose level obtained at 120 min after the ingestion of the glucose load identifies individuals for whom the method is applicable and at the same time identifies individuals having IGT or overt diabetes. Patients with type 2 diabetes and IGT are characterized by various degrees of abnormalities in both insulin action and insulin secretion (19), and it is therefore unlikely that models for measurements of insulin sensitivity and ß-cell function derived from an OGTT could be accurate throughout the whole spectrum of glucose tolerance (20). As expected, our model did not work as well in individuals with IGT (see Table 1 of the online appendix). If detailed information on ß-cell function and/or insulin sensitivity is essential in these individuals, other methods have to be used in addition to the OGTT. All three BIGTT-SI models were highly positively correlated (R2 = 0.690.77) to the SI derived from an IVGTT. This was the case for both the estimation dataset and the validation datasets. As correlation coefficients should not be used for comparisons between studies, we also report the residual SD of log(SI) for the various models derived from the present dataset. We find that all of our three models provide more accurate estimates of insulin sensitivity than various previously published methods (712). The higher accuracy of the present models might be due to 1) the fact that the models were developed using detailed data from a relatively large study population (n = 194), 2) only data from nondiabetic individuals without IGT were used for model development to avoid the confounding effect of overt hyperglycemia on estimates of insulin secretion and action, and 3) the fact that information on sex and BMI were included. It is noteworthy that for log(SI) the estimated parameters for sex and BMI show a high degree of consistency among the different models, indicating that adjustment for these factors is indeed relevant. However, adding information about sex and BMI to previously used indexes did not improve their accuracy. In the present study we have chosen to use insulin sensitivity estimates from a frequently sampled IVGTT as reference for the BIGTT-SI models. Most other published OGTT-derived models for estimation of insulin sensitivity are developed on the basis of data from euglycemic-hyperinsulinemic clamp studies. We believe, however, that our models provide an estimate of insulin sensitivity that could be compared with clamp data, as previous reports have shown a strong positive correlation between SI obtained from the frequently sampled IVGTT and SI obtained from the euglycemic-hyperinsulinemic clamp in normoglycemic individuals (5,21). The models derived from OGTT data for measurement of ß-cell function were developed with the AIR08 min response from the IVGTT, which is the gold standard, as reference. The models are highly accurate compared with other available methods using OGTT data for calculation of insulin secretion (Table 3) (9,12,2226). Both our models and other models for measurement of ß-cell function were less accurate when applied to individuals with IGT, but the BIGTT model was still more accurate than the alternative models (see Table 1 of the online appendix). Adding information on sex, height, and weight to previously used indexes did not improve their accuracy. In contrast to anthropometric variables, which are reproducible among laboratories, assay characteristics for measurement of serum insulin such as linearity, recovery, accuracy, precision, and cross-reactivity to proinsulin and its primary conversion intermediates vary among laboratories (27). In the present model we have used an insulin assay with no cross-reactivity to proinsulin and its primary conversion intermediates (14). Whether our model is valid when serum insulin levels are analyzed by other assays remains to be studied. In summary, we have developed and validated novel models for assessment of OGTT-derived estimates of SI and AIR. Information on sex and BMI combined with analysis of plasma glucose and serum insulin levels in the fasting state and up to eight time points during 4 h (full models) provides indexes for SI and AIR that are highly correlated to indexes obtained from a frequently sampled IVGTT. Furthermore, we have validated more simple OGTT-based models that can be used to predict estimates of SI and AIR, incorporating information on sex, BMI, and measurement of plasma glucose and serum insulin levels at 0, 30, 60, and 120 min. The models are designed for nondiabetic individuals without IGT; glucose tolerance is also being estimated from the OGTT. Because of the simplicity of these models, they can be implemented in large scale metabolic, genetic, and epidemiological studies.
This study was supported by grants from the Danish Medical Research Council, the European Union (BMH4-CT98-3084 and QLK-CT-200-01038), the Velux Foundation, the Danish Diabetes Association, and The Danish Heart Foundation. We thank the participants who took part in the studies and the staff at the Steno Diabetes Center and Research Centre for Prevention and Health.
T.H. and T.D. contributed equally to this work. Additional information for this article can be found in an online appendix at http://dx.doi.org/10.2337/dc06-1240. A table elsewhere in this issue shows conventional and Système International (SI) units and conversion factors for many substances. The costs of publication of this article were defrayed in part by the payment of page charges. This article must therefore be hereby marked "advertisement" in accordance with 18 U.S.C Section 1734 solely to indicate this fact. Received for publication June 15, 2006. Accepted for publication October 19, 2006.
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