Both insulin resistance and deficient

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1 P a t h o p h y s i o l o g y / C o m p l i c a t i o n s O R I G I N A L A R T I C L E Reduced -Cell Compensation to the Insulin Resistance Associated Wi t h O b e s ity in Members of Caucasian Familial Type 2 Diabetic Kindre d s STEVEN C. ELBEIN MD KIMBERLEY WEGNER BA STEVEN E. KAHN MB CHB O B J E C T I V E Both obesity and a family history of diabetes reduce insulin sensitivity but the impact of obesity on insulin secretion among individuals predisposed to diabetes is uncertain. We used a pedigree-based approach to test the hypothesis that -cell compensation to the insulin resistance associated with obesity is defective among individuals predisposed to diabetes by virtue of a strong family history of type 2 diabetes before the development of diabetes or glucose intolerance. RESEARCH DESIGN AND METHODS A total of 126 members of 26 families ascertained for at least a sib pair with type 2 diabetes with onset before age 65 years underwent a tolbutamide-modified frequently sampled intravenous glucose tolerance test (FSIGT). Family members included 26 individuals with impaired glucose tolerance and 100 individuals with no rmal glucose tolerance (NGT). The acute insulin response to glucose (AIR g l u c o s e ) was determ i n e d and insulin sensitivity ( ) estimated by minimal model analysis of FSIGT data. The -cell compensation for insulin sensitivity was estimated from the disposition index (DI) calculated as the product of. Obesity was measured by BMI. R E S U LT S Among all individuals BMI was a significant predictor of both as expected. However BMI also significantly predicted DI (P = 0.002) after correcting for age sex family membership and glucose tolerance status. The relationship of BMI and DI was confirm e d in 85 individuals with NGT who were aged 45 (P = 0.002) but not in 91 unrelated control individuals without a family history of diabetes. When normoglycemic individuals aged 45 were separated into three classes by BMI ( ) d e c reased pro g ressively and significantly with o b e s i t y where as A I R g u c o s e rose significantly from lean to most obese classes. In cont r a s t t o t h e e x p e c t a t i o n o f c o m p l e te -cell compensation with o b e s i t y D I fell significantly (P = 0.004) among obese family members. This relationship was not observed in control subjects. C O N C L U O N S Individuals with a genetic predisposition to diabetes show a re d u c e d -cell compensatory response to the reduced insulin sensitivity associated with obesity. We propose that this impaired compensation may be one manifestation of the underlying genetic defect in susceptible individuals. This finding helps explain the multiplicative effects of family history and obesity on risk of type 2 diabetes. Diabetes Care 2 3 : F rom the Endocrinology Section (S.C.E. K.W.) Central Arkansas Veterans Healthcare System and the D e p a rtment of Medicine (S.C.E. K.W.) University of Arkansas for Medical Sciences Little Rock Arkansas; and the Division of Metabolism Endocrinology and Nutrition (S.E.K.) University of Washington and VA Puget Sound Health Care System Seattle Wa s h i n g t o n. A d d ress correspondence and reprint requests to Steven C. Elbein MD Endocrinology 111J/lR John L. McClellan Memorial Veterans Hospital 4700 W. 7th St. Little Rock AR elbeinstevenc@ e x c h a n g e. u a m s. e d u. Received for publication 10 June 1999 and accepted in revised form 13 October A b b re v i a t i o n s : acute insulin response to glucose; DI disposition index; FSIGT frequently sampled intravenous glucose tolerance test; IGT impaired glucose tolerance; NGT normal glucose tolerance; insulin sensitivity index. A table elsewhere in this issue shows conventional and Système International (SI) units and conversion factors for many substances. Both insulin resistance and deficient insulin secretion are present in individuals with established type 2 diabetes (1) but the relative roles of peripheral insulin sensitivity and diminished pancreatic - c e l l function in the early pathogenesis of the disease remain controversial. Studies of individuals at high risk for type 2 diabetes off e r one means to understand the early pathop h y s i o l o g y. Considerable data demonstrate that reduced insulin sensitivity precedes and p redicts the development of diabetes (23) and is inherited (4 6). Recently we (7) and others (5) demonstrated the familiality of insulin secretion a finding that is consistent with other studies showing impairment in m e a s u res of -cell function in relatives of diabetic patients (8 10). Genetic heterogeneity may help e x p l a i n the conflicting roles of insulin secre t i o n and insulin sensitivity among study populations. However few studies have examined insulin secretion in the context of insulin sensitivity. Bergman et al. (11) hypothesized and Kahn et al. (12) first demonstrated in unrelated nondiabetic people that insulin secretion is related to insulin sensitivity as a hyperbolic function. Individuals with normal glucose tolerance (NGT) have a high insulin response to glucose when insulin sensitivity is low and vice versa. An apparently increased insulin response to glucose may nonetheless be a b n o rmal in the presence of markedly reduced insulin sensitivity while a very low insulin response may be appropriate in the setting of high insulin sensitivity (12). Thus an appropriate measure of insulin s e c retion must account for insulin sensitivi t y. The hyperbolic relationship between insulin sensitivity and secretion pro v i d e s one means to compare individuals with d i ff e rent degrees of insulin re s i s t a n c e. When -cell function is measured as the acute (2 10 min) insulin response to intravenous glucose (AIR g l u c o s e ) and insulin sensitivity is estimated as the insulin sensitivity index ( ) from minimal model analysis of f requently sampled intravenous glucose tolerance test (FSIGT) data an individual s DIABETES CARE VOLUME 23 NUMBE FEBRUARY

2 Reduced insulin secretion with obesity in familial diabetes Table 1 Characteristics of all members of type 2 diabetic kindreds by obesity status -cell response to insulin sensitivity can be defined as the disposition index (DI) which is determined as the product of ( ). Obesity consistently results in markedly reduced insulin sensitivity (1314). Although some investigators have suggested a linear relationship BMI and insulin sensitivity (15) Kahn et al. (12) showed a complex nonlinear relationship between insulin sensitivity measured as and obesity measure d as BMI. At a BMI 27 insulin sensitivity is reduced (14) and the variance in is also d e c reased (12). Thus obese individuals re p resent an ideal group in which to examine the pancreatic -cell compensation for reduced insulin sensitivity. Based on the hyperbolic relationships of insulin secre t i o n and insulin sensitivity AIR g l u c o s e s h o u l d i n c rease sufficiently with obesity to c o m- pensate for the reduced insulin sensitivity with the DI remaining normal. Howe v e r F e rrannini et al. (15) re p o rted hypers e c re t i o n in nondiabetic obese subjects even without insulin resistance. In contrast Ishikawa et al. (16) re p o rted the loss of this re l a t i o n s h i p between insulin secretion and obesity in response to oral glucose in individuals with a family history of diabetes. The interpre t a- tion of these studies is complicated by the f a i l u re to consider insulin secretion in the context of insulin sensitivity. Because the pathogenesis of type 2 diabetes appears to involve a loss of the normal adaptive response of the -cell to insulin resistance we hypothesized that N o n o b e s e O b e s e (BMI 27 kg/m 2 ) (BMI 27 kg/m 2 ) P n I G T 9 (13.8) 17 (30.9) Age (years) 37.8 ± ± 9.8 N S BMI (kg/m 2 ) ( ) ( ) Hip-to-waist ratio ± ± Fasting glucose (mmol/l) 4.89 ± ± Fasting insulin (pmol/l) ( ) 57.7 ( ) ( 10 5 m i n 1 /[ p m o l / l ]) ( ) 3.98 ( ) ( p m o l / l ) ( ) ( ) DI (min 1 ) ( ) ( ) P e rcentile score of DI Lowest decile of DI 23 (34.3) 34 (57.6) Data are n n (%) means ± SD or means ± SD (95% CIs). All family members who underwent FSIGT are comp a red by obesity (BMI 27.0). All comparisons of skewed variables are by Mann-Whitney U test or by Stud e n t s t test of natural log transformation of skewed variables. Percentile score is based on comparison with n o rmal unrelated control subjects a g e d 45 years with varying degrees of obesity (12). Lowest decile of DI c o m p a res numbers of individuals falling in the lowest decile of this normal distribution of DI. nondiabetic members of families with a s t rong history of type 2 diabetes would show a reduced DI in response to obesity w h e reas DI would be pre s e rved in norm a l obese subjects without a family history of diabetes. The present study supports this hypothesis and suggests that -cell compensation to the increased secre t o ry demand of obesity is disturbed in subjects with a family history of diabetes. RESEARCH DESIGN AND M E T H O D S Study population A total of 126 individuals were studied f rom 26 families ascertained for a minimum of a sib pair with type 2 diabetes with onset before age 65 years and grandpare n t s of Nort h e rn European extraction. Only siblings offspring or cousins of the diabetic sib pair were included; additional details a re described elsewhere (7). All individuals p rovided written informed consent before the study under a protocol approved by the University of Utah Institutional Review B o a rd and were studied at the General Clinical Research Center of the same institution. A control population comprised 91 nondiabetic Caucasian individuals age 45 who were studied in Seattle (12) and p rovided informed consent under a pro t o- col approved by the University of Wa s h- ington Human Subjects Review Committee. This population has been described pre v i- ously (12). Study protocol All family members underwent a standard 75-g oral glucose tolerance test at the first visit and were classified as having NGT i m p a i red glucose tolerance (IGT) or type 2 diabetes by World Health Organization criteria (17). Nondiabetic individuals were invited to participate in the second study if they were the offspring of a type 2 diabetic p a rent and also had a sibling available for study or if they had previously diagnosed IGT and had not developed diabetes. Mens t ruating women were studied during the follicular phase of their menstrual cycles. No subjects engaged in regular aerobic athletics although activity was otherwise unre s t r i c t e d. All subjects maintained their usual diet b e f o re both visits and all subjects were weight stable. At both first and second visits height was determined by the mean of thre e m e a s u res by a stadiometer and weight meas u red by calibrated digital scale. On the second study day subjects u n d e rwent a tolbutamide-modified FSIGT after an overnight fast as described in detail e l s e w h e re (18). Briefly 20 min after initiating intravenous lines in both arms thre e baseline samples were drawn for glucose and insulin measurements. Glucose was then administered intravenously at a dose of 11.4 g/m 2 of body surface area over 60 s. Samples were drawn at and 19 min. At 20 min after initiation of the glucose injection tolbutamide (125 mg/m 2 ) was administered intravenously over 30 s and sampling continued at and 180 min. Subjects with a pre v i- ous diagnosis of IGT those whose glucose continued to change between 150 and 180 min or those whose glucose deviated by m o re than 0.56 mmol/l from the baseline fasting glucose underwent additional sampling at 210 and 240 min. We have re p o rted that this 25-sample pro t o c o l retains the precision of the original 33- sample protocol (19). Glucose and insulin w e re measured by Penn Medical Laboratories (Washington DC) as described elsew h e re (18). The control population was similarly examined using the full-sample FSIGT protocol and insulin and glucose w e re measured in laboratories at the University of Wa s h i n g t o n Seattle WA (12). Calculations Obesity was quantified as BMI defined as weight (kg)/[height (m)] 2. For both the c o n t rol and family populations we used the computer program MINMOD (20) to cal- 222 DIABETES CARE VOLUME 23 NUMBE FEBRUARY 2000

3 Elbein Wegner and Kahn culate the. The AIR g l u c o s e was calculated as the mean of the insulin response above baseline from 2 to 10 min. We used the defined hyperbolic relationship of AIR g l u c o s e and in humans (12) to calculate the DI a s *. This parameter offers a meas u re o f -cell function in the context of insulin sensitivity (1112). Because insulin is a unit of both the eff e c t of any diff e rences in insulin assays should be negated assuming a constant re l a t i o n- ship between insulin assays. Thus unlike DI is theoretically independent of any specific laboratory. Based on this assumption we assigned a percentile rank for DI for each Utah study subject based on the normative database of Kahn et al. (12). Nonetheless to avoid errors based on this assumption key analyses examined the re l a- tionships of DI and BMI in each population without re f e rence to the normative database or comparison between data sets. A B Statistical analysis Analyses were perf o rmed using the SPSS statistical packages (SPSS Chicago). BMI AIR g l u c o s e and DI were natural logarithmically (ln)transformed to achieve norm a l- ity before testing between subject effects in general linear models and for determ i n a- tion of correlation coefficients using part i a l c o rrelation and re g ression analyses. General linear (mixed effects) models were examined for main effects of age and ln(bmi) as covariates sex and diagnosis as fixed factors and family membership as a random factor on ln(di) as the dependent variable. Most analyses excluded IGT individuals and thus dropped diagnosis as a fixed factor. Correlation estimates were adjusted for age sex diagnosis and family membership where family membership was defined by the kindred number to which the individual belonged. To furt h e r test the hypotheses without transform a- tion we conducted nonparametric analyses on the subset of 85 nondiabetic subjects age 45 years using the Kru s k a l - Wa l l i s test for AIR g l u c o s e and DI. Numbers of individuals falling in the lowest decile of p e rcentile score were compared using the 2 test. Individuals were divided into the following three groups based on BMI: nonobese ( 27 kg/m 2 ) moderately obese (27 30 kg/m 2 ) and very obese ( 30 k g / m 2 ). This population of subjects 45 years is comparable to the control population published by Kahn et al. (12) except for the family history of diabetes and slightly increased test age. C Figure 1 Scatterplot of 85 euglycemic family members a g e 45 showing the relationship of (A) AIR glucose (B) and DI (C) with BMI. Ln-transformed values are shown for all plots where units before transformation were kg/m 2 for BMI; 10 5 min -1 /[pmol/l] for ; pmol/l for AIR glucose ; 10 1 min 1 for DI. Regression lines are shown for each relationship based on ln transformations with r 2 values shown in the margin. Significance for simple regression without correction for age sex and family membership are as follows: versus BMI P 0.001; AIR glucose versus BMI P 0.001; DI versus BMI P = DIABETES CARE VOLUME 23 NUMBE FEBRUARY

4 Reduced insulin secretion with obesity in familial diabetes Table 2 Characteristics of 91 normal control individuals by obesity status R E S U LT S Characteristics of all study individuals are shown in Table 1 by obesity status. Although was decreased and A IR g l u c o s e was increased in absolute terms a significantly lower DI among obese individuals suggested impaired -cell compensation for the insulin resistance associated with obesity. In a mixed effect model we found expected relationships of (P )a n d (P = 0.002) with BMI. H o w e v e r B M I (P = 0.009) age (P ) and diagnosis (IGT vs. NGT P ) w e re all significant determinants of DI. Because age and diagnosis (IGT) were potential confounding covariates and factors in the analysis of DI in the full population we restricted subsequent analyses to 85 nondiabetic individuals who were age 45 years. This population diff e red fro m the control group only in having a family h i s t o ry of diabetes and a diff e rent study site. Figure 1 shows the distribution of lnt r a n s f o rmed values of AIR g l u c o s e and DI versus BMI for these 85 norm o g l y c e m i c individuals 45 years as a simple re g re s- sion where correlation coefficients were and for BMI and and DI re s p e c t i v e l y. Under a mixed effects model that accounted for age sex and family membership as main e ffects BMI was a significant determ i n a n t of AIR g l u c o s e (P = 0.001) (P ) and -cell compensation (DI P = 0.002). Age was not a significant factor in any model as expected in these young ( 45 years) individuals. The characteristics of the control population by obesity status are shown in Table 2. Like relatives of a diabetic sibling pair the c o n t rol population showed a significant correlation between ln-transformed BMI and both ln-transformed AIR g l u c o s e (P ) and (P 0.001) in a partial corre l a t i o n c o n t rolling for age. In contrast we found N o n o b e s e O b e s e (BMI 27 kg/m 2 ) (BMI 27 kg/m 2 ) P n Age (years) 27.8 ± ± BMI (kg/m 2 ) 22.7 ( ) 32.3 ( ) ( 10 5 m i n 1 /[ p m o l / l ]) 7.82 ( ) 3.55 ( ) ( p m o l / l ) 305 ( ) 574 ( ) DI (min 1 ) ( ) ( ) N S Data are n means ± SD or means ± SD (95% CIs). All statistical comparisons were examined by both Student s t test and Kru s k a l - Wallis test of analysis of variance. no significant relationship between BMI and DI (P = 0.61 Fig. 2). Correlation coefficients in a simple re g ression of ln-transf o rmed measure s (Fig. 2) B M I a nd AIR g l u c o s e and DI were a n d ( N S ) re s p e c t i v e l y. To further explore the relationship of BMI and parameters of insulin sensitivity insulin secretion and -cell compensation we divided the normoglycemic family members who were a g e d 45 into the following three groups by BMI: lean (BMI 27) moderately obese (BMI 27 30) and very obese (BMI 30). The re s u l t s c o n f i rmed that fell ro s e p ro g ressively with increasing obesity class ( Table 3). We re -cell compensation a p p ropriate DI would be pre s e rved in the moderately obese and very obese gro u p s. Instead we observed a significant (P = 0.004) fall in DI among obese family members. In contrast although the same relationships of w e re o b s e rved in the control population no significant decrement in DI was noted with obesity (Table 2). C O N C L U O N S Many investigators have attempted to understand the early stages in the pathogenesis of type 2 diabetes by comparing individuals with a family hist o ry of diabetes with a normal control population (282122). Most (22324) but not all (8) have suggested that family members a re insulin resistant when compared with a nondiabetic control population without a family history of diabetes and that insulin sensitivity is familial (3623). The role of insulin secretion is more controversial. We have demonstrated in this population that both AIR g l u c o s e and DI are familial (7) thus c o n f i rming similar work of Sakul et al. (5) in Pima Indians. Others have demonstrated i m p a i red first- and second-phase insulin release when family members are compare d with control subjects without considering insulin sensitivity (821). However to our knowledge this is the first study that examines the role of obesity as determined by BMI on an index of -cell compensation that takes insulin sensitivity into account among euglycemic family members who a re at high risk of future diabetes. The results of our study may offer a solution to the controversy re g a rding the role of insulin secretion in early diabetes. Even in the context of a strong family history of type 2 diabetes obesity resulted in a striking decrease in insulin sensitivity that was comparable to that seen in the control population. In both populations absolute firstphase insulin release increased as BMI i n c reased and declined. However when the hyperbolic relationship of (1112) was taken into account the constant that defines this hyperbolic curve for each individual (DI) was clearly decre a s e d with obesity among family members but not among control individuals. Thus re l a- tives of a type 2 diabetic sib pair appear to have inadequate -cell compensation for the insulin resistance associated with obesity even while their glucose tolerance tests remain normal. In contrast -cell compensation (as measured by DI) was intact in the c o n t rol individuals as marked by no significant change in DI with increasing BMI and declining. Indeed when DI was c o m p a red dire c t l y lean family members w e re not significantly diff e rent from obese c o n t rol subjects ( vs ) while obese family members (0.0115) showed a 44% reduction in DI relative to obese cont rol subjects. Our findings might be explained by a d i rect effect of obesity on -cell compensation by the expression of a genetic - c e l l defect in the presence of the insulin re s i s- tance of obesity or by independent eff e c t s of obesity on insulin sensitivity and - c e l l compensation in the context of genetically p redisposed individuals. Dunaif and Finegood (25) found a similar reduction in DI in both lean and obese women with polycystic ovary syndrome. However in contrast to our study of 31 obese contro l subjects and 60 lean control subjects they also re p o rted a small but significant d e c rease in DI among 14 obese subjects c o m p a red with 15 nonobese contro l women. A small effect of obesity on DI was also possible in our control population given that DI was reduced from (95% CI ) to DIABETES CARE VOLUME 23 NUMBE FEBRUARY 2000

5 Elbein Wegner and Kahn ( ). Because the control population was young (mean age 28.7 years) a significant effect on DI might be seen with slightly older or more obese individuals. Considerable data now suggest an interaction between adipocyte mass and insulin secretion mediated either thro u g h leptin (26 28) or the effects of free fatty acids (29) and might provide an explanation for a direct effect of obesity. However in contrast to DI we had no pro b l e m detecting diff e rences in or AIR g l u c o s e w i t h obesity in this population. Thus obesity alone seems unlikely to cause the marked defect in -cell compensation that we find among family members. Ishikawa et al. (16) suggested that unif o rm insulin resistance might explain the diminished insulin response to oral glucose among obese first-degree relatives of a diabetic individual compared with obese cont rol subjects. Our study offers an altern a t i v e explanation. We demonstrated clearly that among family members of a diabetic sib p a i r both increasing BMI and IGT re s u l t e d in significantly decreased insulin sensitivity ( ). Thus the decreased hyperinsulinemic response re p resents impaired compensation for this decrement in insulin sensitivity. We p ropose that a genetic defect in insulin secretion exists that is revealed only with obes i t y / reduced insulin sensitivity. A large study comparing insulin secretion corrected for insulin sensitivity among obese individuals with and without a family history of diabetes is needed to confirm our hypothesis. Several aspects of this study may limit i n t e r p retation of our results. First our population is somewhat unique in re p re s e n t i n g families of nort h e rn European ancestry with a particularly strong propensity for earlier onset type 2 diabetes. Because we used a family-based design our conclusions might be biased by the part i c u l a r families for which we studied multiple individuals. Second our conclusions re s t on the index of -cell compensation DI. In c ross-sectional studies DI is consistently diminished in individuals at higher risk for diabetes (2530) improves with tro g l i t a- zone treatment (3132) and is familial (7). F u rt h e rm o re our obese family members with normal glucose tolerance tests who had a reduced DI also had slightly but significantly higher fasting glucose levels f u rther arguing that the reduced DI is of physiologic importance. Nonetheless the p redictive value of this index for future diabetes has not been demonstrated yet in l o n g - t e rm prospective studies. Finally the A B C Figure 2 Scatterplot of 93 unrelated euglycemic control individuals a g e d 45 years. Figure is analagous to Fig. 1 showing relationship of (A) AIR g l u c o s e (B) and DI (C) with BMI. Ln-transform e d values are shown for all plots where units before transformation were as follows: kg/m 2 for BMI; 10 5 min 1 /[pmol/l] for ; pmol/l for AIR glucose ; 10 1 min 1 for DI. Regression lines are shown on the figure for each relationship based on ln transformations with r 2 value shown in the margin. Significance for each relationship based on simple regression are as follows: versus BMI P 0.001; AIR glucose versus BMI P 0.001; DI versus BMI P = 0.22 (NS). DIABETES CARE VOLUME 23 NUMBE FEBRUARY

6 Reduced insulin secretion with obesity in familial diabetes Table 3 Comparison of 85 normoglycemic family members age c o n t rol population for this study was tested at a diff e rent institution using diff e re n t assays and slightly diff e rent pro t o c o l s. Although DI is theoretically comparable between laboratories we cannot be cert a i n that the relationship between insulin assays is linear in which case DI would not be d i rectly comparable. Consequently we avoided most direct comparisons between populations. While the fall in DI with i n c reasing BMI among family members is robust to this comparison our conclusion that this fall is in part or fully genetic must await additional studies. In summary have the expected relationships to obesity even among members of familial type 2 diabetes k i n d reds at high risk for diabetes. However the relationship of AIR g l u c o s e and i s i m p a i red with increasing BMI in obese family members whereas compensation appears to be appropriate in a contro l population. Our results were robust to multiple methods of analysis both including and excluding family membership as a f a c t o r. Longitudinal studies of these highrisk individuals will help predict longt e rm risk for type 2 diabetes in these subjects and the ability of composite meas u res such as DI to predict future diabetes. 4 5 years by obesity class N o n o b e s e Moderately obese Ve ry obese (BMI 2 7 ) (BMI 27 30) (BMI 3 0 ) P n Age (years) 33.8 ± ± ± 5.2 N S BMI (kg/m 2 ) 23.4 ( ) 33.4 ( ) Fasting glucose (mmol/l) 4.92 ± ± ± Fasting insulin (pmol/l) 23.1 ( ) 37.5 ( ) 72.9 ( ) ( 10 5 m i n 1 /[ p m o l / l ]) ( ) 7.18 ( ) 3.36 ( ) ( p m o l / l ) 141 ( ) 217 ( ) 300 ( ) DI (min 1 ) ( ) ( ) 0.10 ( ) P e rcentile score Lowest decile of perc e n t i l e 14 (26.9) 3 (30.0) 15 (65.2) s c o re Data are n means ± SD means ± SD (95% CIs) or n (%). All comparisons of skewed variables are by Mann-Whitney U test or by Student s t test of natural log transformation of skewed variables. Percentile score is based on comparison with normal unrelated control subjects aged 45 years with varying degrees of obesity (12). Lowest decile of DI compares numbers of individuals falling in the lowest decile of this normal distribution of DI. A c k n o w l e d g m e n t s These studies were s u p p o rted by NIH Grants DK39311 (S.C.E.) and DK17047 (S.E.K.) by the Research Serv i c e of the Department of Veterans Affairs and in part by a Harold Rifkin Family Acquisition (GENNID) Grant from the American Diabetes Association. Pedigree sampling oral glucose tolerance testing and intravenous glucose tolerance testing were perf o rmed at the General Clinical Research Center (GCRC) of the University of Utah and supported by Public Health Serv i c e Grant MO1-RR00064 from the National Center for Research Resources to the University of Utah GCRC. A portion of the data included in this analysis was collected under the GENNID project of the American Diabetes Association and contributed to the GENNID database. The authors wish to thank the members of the GENNID Study Group for their contributions to the protocols and laboratory pro c e- d u res used in this study. We thank Cindy Miles Holly Tuckett and Te resa Maxwell for their valuable contributions to family ascert a i n m e n t and intravenous glucose tolerance testing and the nursing staff of the University of Utah GCRC for their assistance. R e f e re n c e s 1. D e F ronzo RA Bonadonna RC Ferr a n n i n i E: Pathogenesis of NIDDM: a balanced o v e rv i e w. Diabetes Care 15: Wa rram JH Martin BC Krolewski AS: Slow glucose removal rate and hyperinsulinemia p recede the development of type II diabetes in the offspring of diabetic parents. A n n I n t e rn Med 113: M a rtin BC Wa rram JH Rosner B Rich SS Soeldner JS Krolewski AS: Familial clustering of insulin sensitivity. D i a b e t e s 4 1 : B o g a rdus C Lillioja S Nyomba BL Zurlo F S w i n b u rn B Puente E-D Knowler WC Ravussin E Mott DM Bennett PH: Distribution of in vivo insulin action in Pima Indians as a mixture of three normal distributions. D i a b e t e s 38: Sakul H Pratley R Cardon L Ravussin E Mott D Bogardus C: Familiality of physical and metabolic characteristics that pre d i c t the development of non-insulin-dependent diabetes mellitus in Pima Indians. Am J Hum Genet 60: Schumacher MC Hasstedt SJ Hunt SC Williams RR Elbein SC: Major gene eff e c t for insulin levels in familial NIDDM pedig rees. D i a b e t e s 41: Elbein SC Hasstedt SJ Wegner K Kahn SE: Heritability of pancreatic -cell function among nondiabetic members of Caucasian familial type 2 diabetes kindreds. 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