Different pathophysiology of impaired glucose tolerance in first-degree relatives of individuals with type 2 diabetes mellitus

To assess whether an increased genetic predisposition for type 2 diabetes mellitus (T2DM) influences the contributions of insulin resistance and impaired insulin secretion to impaired glucose tolerance (IGT), 437 subjects not known to have T2DM underwent an oral glucose tolerance test and a 3-hour hyperglycemic clamp. Plasma insulin responses and insulin sensitivity were compared between all subjects (unselected for demographic or anthropometric characteristics) who had normal glucose homeostasis and no first-degree T2DM relative (n = 133), IGT with a first-degree T2DM relative (IGT/FH+, n = 74), or IGT without a first-degree T2DM relative (IGT/FH-, n = 50). Compared with those with normal glucose homeostasis, first- and second-phase plasma insulin responses were reduced approximately 45% and 30%, respectively (both P < .001), in IGT/FH+, whereas insulin sensitivity was only approximately 20% reduced (P = .011). In contrast, in IGT/FH-, first-phase plasma insulin responses were only approximately 20% reduced (P = .016), second-phase plasma insulin responses were not reduced, but insulin sensitivity was approximately 40% reduced (P < .001). The IGT/FH+ group differed significantly from the IGT/FH- group by having 25% to 30% lower first-phase plasma insulin responses (P = .026) and 25% to 30% greater insulin sensitivity (P = .027). Adjustment for obesity abolished the differences in insulin resistance but not plasma insulin responses. However, when the IGT groups were stratified into subgroups based on body mass index (BMI), first-phase plasma insulin responses were approximately 30% lower in IGT/FH+ with a BMI of at least 27 kg/m(2) (P = .018) but similar in IGT/FH+ with a BMI less than 27 kg/m(2) compared with the corresponding IGT/FH- subgroups. We conclude that, in IGT, an increased genetic predisposition for T2DM increases the contribution of impaired insulin secretion to its pathophysiology. This effect is enhanced by obesity.

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

OBJECTIVE

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

RESEARCH DESIGN AND METHODS

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

RESULTS

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

CONCLUSIONS

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

Different mechanisms for impaired fasting glucose and impaired postprandial glucose tolerance in humans

OBJECTIVE

To compare the pathophysiology of impaired fasting glucose (IFG) and impaired glucose tolerance (IGT) in a more comprehensive and standardized fashion than has hitherto been done.

RESEARCH DESIGN AND METHODS

We studied 21 individuals with isolated IFG (IFG/normal glucose tolerance [NGT]), 61 individuals with isolated IGT (normal fasting glucose [NFG]/IGT), and 240 healthy control subjects (NFG/NGT) by hyperglycemic clamps to determine first- and second-phase insulin release and insulin sensitivity. Homeostasis model assessment (HOMA) indexes of beta-cell function (HOMA-%B) and insulin resistance (HOMA-IR) were calculated from fasting plasma insulin and glucose concentrations.

RESULTS

Compared with NFG/NGT, IFG/NGT had similar fasting insulin concentrations despite hyperglycemia; therefore, HOMA-IR was increased approximately 30% (P < 0.05), but clamp-determined insulin sensitivity was normal (P > 0.8). HOMA-%B and first-phase insulin responses were reduced approximately 35% (P < 0.002) and approximately 30% (P < 0.02), respectively, but second-phase insulin responses were normal (P > 0.5). NFG/IGT had normal HOMA-IR but approximately 15% decreased clamp-determined insulin sensitivity (P < 0.03). Furthermore, HOMA-%B was normal but both first-phase (P < 0.0003) and second-phase (P < 0.0001) insulin responses were reduced approximately 30%. IFG/NGT differed from NFG/IGT by having approximately 40% lower HOMA-%B (P < 0.012) and approximately 50% greater second-phase insulin responses (P < 0.005).

CONCLUSIONS

Since first-phase insulin responses were similarly reduced in IFG/NGT and NFG/IGT, we conclude that IFG is due to impaired basal insulin secretion and preferential resistance of glucose production to suppression by insulin, as reflected by fasting hyperglycemia despite normal plasma insulin concentrations and increased HOMA-IR, whereas IGT mainly results from reduced second-phase insulin release and peripheral insulin resistance, as reflected by reduced clamp-determined insulin sensitivity.

Disturbances in beta-cell function in impaired fasting glycemia

In a cross-sectional study, we assessed beta-cell function and insulin sensitivity index (ISI) with hyperglycemic clamps (10 mmol/l) in 24 subjects with impaired fasting glycemia (IFG, fasting plasma glucose [FPG] between 6.1 and 7.0 mmol/l), 15 type 2 diabetic subjects (FPG >7.0 mmol/l), and 280 subjects with normal fasting glycemia (NFG, FPG <6.1 mmol/l). First-phase insulin release (0-10 min) was lower in IFG (geometric mean 541 pmol/l.10 min; 95% confidence interval [CI] 416-702 pmol/l.10 min) and in type 2 diabetes (geometric mean 376 pmol/l.10 min; 95% CI 247-572 pmol/l.10 min) than NFG (geometric mean 814 pmol/l.10 min; 95% CI 759-873 pmol/l.10 min) (P < 0.001). Second-phase insulin secretion (140-180 min) was also lower in IFG (geometric mean 251 pmol/l; 95% CI 198-318 pmol/l; P = 0.026) and type 2 diabetes (geometric mean 157 pmol/l; 95% CI 105-235 pmol/l; P < 0.001) than NFG (geometric mean 295 pmol/l; 95% CI 276-315 pmol/l). IFG and type 2 diabetic subjects had a lower ISI (0.15 plus minus 0.02 and 0.16 plus minus 0.02 micromol/kg fat-free mass [FFM]/min/pmol/l, respectively) than NFG (0.24 plus minus 0.01 micromol/kg FFM/min/pmol/l, P < 0.05). We found a stepwise decline in first-phase (and second-phase) secretion in NFG subjects with progressive decline in oral glucose tolerance (P < 0.05). IFG subjects with impaired glucose tolerance (IGT) had lower first-phase secretion than NFG subjects with IGT (P < 0.02), with comparable second-phase secretion and ISI. NFG and IFG subjects with a diabetic glucose tolerance (2-h glucose >11.1 mmol/l) had a lower ISI than their respective IGT counterparts (P < 0.05). We conclude that the early stages of glucose intolerance are associated with disturbances in beta-cell function, while insulin resistance is seen more markedly in later stages.

Relative conributions of beta-cell function and tissue insulin sensitivity to fasting and postglucose-load glycemia

We performed hyperglycemic clamps in 283 nondiabetic Caucasians and, with multiple linear regression, determined the contribution of beta-cell function and tissue insulin sensitivity to variations in glycemia and insulinemia during oral glucose tolerance tests (OGTTs). Impaired glucose tolerance (IGT) subjects had reduced insulin sensitivity (P < .02) and beta-cell function (P < .0001). Normal glucose tolerance (NGT) subjects with first-degree type 2 diabetic relatives had reduced first and second phase insulin secretion (both, P < .05), but normal insulin sensitivity (P = .37). Beta-Cell function and insulin sensitivity accounted for one fourth of the variability in glucose tolerance. Fasting plasma glucose in subjects with NGT (n = 185) was a function of both phases of insulin secretion and of insulin sensitivity (all, P < .05), whereas, in IGT subjects (n = 98), it was a function of first phase insulin secretion and insulin sensitivity (P < .01). Two-hour glycemia was a function of second phase secretion and insulin sensitivity (P < .01). Fasting and 2-hour plasma insulin levels were determined by insulin sensitivity (and glycemia) in NGT subjects (P < .001), but by second phase secretion in IGT (P < .001). We conclude that beta-cell function is reduced in subjects with IGT; glycemia and insulinemia are not regulated by the same mechanisms in IGT and NGT; insulin sensitivity does not contribute to insulinemia in IGT; family history of diabetes influences beta-cell function, but not insulin sensitivity in Caucasians.

Use of the oral glucose tolerance test to assess insulin release and insulin sensitivity

OBJECTIVE

The oral glucose tolerance test (OGTT) has often been used to evaluate apparent insulin release and insulin resistance in various clinical settings. However, because insulin sensitivity and insulin release are interdependent, to what extent they can be predicted from an OGTT is unclear.

RESEARCH DESIGN AND METHODS

We studied insulin sensitivity using the euglycemic-hyperinsulinemic clamp and insulin release using the hyperglycemic clamp in 104 nondiabetic volunteers who had also undergone an OGTT. Demographic parameters (BMI, waist-to-hip ratio, age) and plasma glucose and insulin values from the OGTT were subjected to multiple linear regression to predict the metabolic clearance rate (MCR) of glucose, the insulin sensitivity index (ISI), and first-phase (1st PH) and second-phase (2nd PH) insulin release as measured with the respective clamps.

RESULTS

The equations predicting MCR and ISI contained BMI, insulin (120 min), and glucose (90 min) and were highly correlated with the measured MCR (r = 0.80, P < 0.00005) and ISI (r = 0.79, P < 0.00005). The equations predicting 1st PH and 2nd PH contained insulin (0 and 30 min) and glucose (30 min) and were also highly correlated with the measured 1st PH (r = 0.78, P < 0.00005) and 2nd PH (r = 0.79, P < 0.00005). The parameters predicted by our equations correlated better with the measured parameters than homeostasis model assessment for secretion and resistance, the delta30-min insulin/delta30-min glucose ratio for secretion and insulin (120 min) for insulin resistance taken from the OGTT.

CONCLUSIONS

We thus conclude that predicting insulin sensitivity and insulin release with reasonable accuracy from simple demographic parameters and values obtained during an OGTT is possible. The derived equations should be used in various clinical settings in which the use of clamps or the minimal model would be impractical.

Insulin secretion and insulin sensitivity in people with impaired glucose tolerance

To assess the roles of pancreatic beta-cell (beta-cell) dysfunction and insulin resistance in the pathogenesis of non-insulin dependent diabetes mellitus, we used euglycaemic hyperinsulinaemic and hyperglycaemic clamps to compare insulin secretion and insulin sensitivity in Caucasian individuals of European ancestry with either normal glucose tolerance (NGT) or impaired glucose tolerance (IGT). Both groups were carefully matched for age, gender, obesity and body fat distribution. During the hyperglycaemic clamps, IGT had significantly lower first phase (650 +/- 60 vs 992 +/- 92 pmol l-1, p = 0.001) and second phase (231 +/- 24 vs 326 +/- 21 pmol l-1, p < 0.001) plasma insulin responses while their insulin sensitivity index (0.126 +/- 0.012 mumol kg-1 pM-1) was not significantly different from that of NGT (0.144 +/- 0.012 mumol kg-1 min-1 pM-1), p = 0.69). Similarly, during the euglycaemic hyperinsulinaemic clamps, the insulin sensitivity index of the IGT (0.076 +/- 0.005 mumol kg-1 min-1 pM-1) was not significantly different from that of the NGT (0.086 +/- 0.007 mumol kg-1 min-1 pM-1), p = 0.28. We conclude that since beta-cell dysfunction is already evident in people with impaired glucose tolerance but insulin resistance is not, impaired insulin secretion is most likely the primary genetic factor predisposing to the development of non-insulin dependent diabetes mellitus in Caucasians of European ancestry.

Pancreatic beta-cell dysfunction as the primary genetic lesion in NIDDM. Evidence from studies in normal glucose-tolerant individuals with a first-degree NIDDM relative

OBJECTIVE

To test the hypothesis that insulin resistance precedes impaired insulin secretion in individuals genetically predisposed to non-insulin-dependent diabetes mellitus (NIDDM).

DESIGN

Case-control study.

SETTING

Outpatient facility of clinical research center.

PARTICIPANTS

One hundred volunteers of European ancestry having normal glucose tolerance, 50 with and 50 without a first-degree NIDDM relative, matched for age, sex, and degree of obesity.

MAIN OUTCOME MEASURES

Insulin secretion and insulin sensitivity assessed by hyperglycemic (N = 100) and euglycemic-hyperinsulinemic (N = 62) clamp experiments.

RESULTS

The individuals with a first-degree NIDDM relative had reduced first- and second-phase insulin responses (mean +/- SEM, 939 +/- 68 vs 1209 +/- 82 pmol/L, and 322 +/- 19 vs 407 +/- 24 pmol/L, respectively, P = .001 and .01), but their insulin sensitivity (148 +/- 6 and 92 +/- 6 nmol.kg-1.min-1/pmol.L-1 in hyperglycemic and euglycemic clamp studies) did not differ from that of the control group (126 +/- 5 and 81 +/- 7 nmol.kg-1.min-1/pmol.L-1, in hyperglycemic and euglycemic clamp studies, P = .07 and .24, respectively). In some individuals only first- or only second-phase insulin responses were reduced.

CONCLUSION

In this study population, heterogeneous defects in insulin secretion were demonstrated, while defects in insulin sensitivity were not evident. We therefore conclude that since the earliest defects identified in a group genetically at high risk to develop NIDDM are those related to insulin secretion, defects in insulin secretion rather than insulin sensitivity are likely the major genetic factor predisposing to development of NIDDM.

Glycogen: its mode of formation and contribution to hepatic glucose output in postabsorptive humans

To assess the relative contributions of gluconeogenesis and glycogenolysis to overall hepatic glucose output in postabsorptive normal humans and those of the indirect and direct pathways for glycogen synthesis, we studied six normal volunteers, who had been fasted for 16 h to reduce their hepatic glycogen stores, and then ingested glucose (250 g over 10 h) enriched with [6-3H] glucose to replenish and label their hepatic glycogen. After a 10-h overnight fast, release of the [6-3H] glucose into the circulation was traced with [2-3H] glucose to estimate breakdown of glycogen that had been formed via the direct pathway while gluconeogenesis was simultaneously estimated by incorporation of infused [14C] lactate into plasma glucose. We found that release of [6-3H] glucose into plasma (6.79 +/- 0.69 mumol.kg-1.min-1) accounted for 46 +/- 5% of hepatic glucose output (15.0 +/- 0.7 mumol.kg-1.min-1) while glucose formed from lactate (2.71 +/- 0.28 mumol.kg-1.min-1) accounted for 19 +/- 2% of hepatic glucose output. Since these determinations underestimate direct pathway glycogenolysis and overall gluconeogenesis, a maximal estimate for the contribution of indirect pathway glycogenolysis to hepatic glucose output is obtained by subtracting the sum of direct pathway glycogenolysis and lactate gluconeogenesis from hepatic glucose output. This amounted to a maximum of 36 +/- 5% of hepatic glucose output and 44 +/- 6% of overall glycogenolysis.(ABSTRACT TRUNCATED AT 250 WORDS)

Simultaneous assessment of insulin secretion and insulin sensitivity using a hyperglycemia clamp

A hyperglycemic clamp is an established method to assess insulin secretion and is generally used only for this purpose. To determine whether it could also be used to assess insulin sensitivity, we compared insulin sensitivity indices (ISI) obtained during euglycemic and hyperglycemic clamp experiments in 22 nonobese volunteers (body mass index, 23.9 +/- 0.6 kg/m2) and in 20 obese individuals (body mass index, 30.8 +/- 1.3 kg/m2) matched for age and gender. The ISI values (micromoles per kg.min/pmol) of the obese group assessed during hyperglycemic (0.088 +/- 0.011) and euglycemic (0.050 +/- 0.005) clamp experiments were both significantly lower than the ISI of the nonobese group assessed in hyperglycemic and euglycemic clamp experiments (0.179 +/- 0.024 and 0.096 +/- 0.009, respectively; both P less than 0.01). Although the ISI values obtained with hyperglycemic clamps were consistently greater than those obtained with euglycemic clamp (0.137 +/- 0.016 vs. 0.075 +/- 0.007; P less than 0.001), they were highly correlated (r = 0.84; P less than 0.0001). Moreover, when these indices were converted to clearance rates, thereby correcting for the mass action effects of glucose on glucose disposal, the values obtained with the hyperglycemic clamp (0.0137 +/- 0.0016 mL/kg.min/pmol) were statistically identical to those obtained with the euglycemic clamp (0.0142 +/- 0.0013 mL/kg.min/pmol), as indicated by a regression equation having an intercept of 0 and a slope (1.03) not different from 1. We, therefore, conclude that the hyperglycemic clamp and the euglycemic clamp yield comparable estimates of insulin sensitivity and that, under appropriate conditions, the hyperglycemic clamp technique may be used to assess both insulin sensitivity and insulin secretion in the same individual in a single experiment.