The 30-year controversy on whether type 2 diabetes is a disorder of reduced insulin action or diminished insulin secretion is abating, with most investigators accepting that diminished insulin secretion is a necessary condition for the development of impaired glucose homeostasis [3, 16–18]. The main reason for underestimating the deficit in insulin secretion is failure to recognise the modulating effect of insulin sensitivity on beta cell function [14]. When this was taken into consideration it became evident that subjects at high risk of subsequently developing type 2 diabetes have a diminished insulin-releasing capacity. Thus, reduced first-phase insulin response to i.v. glucose was demonstrated in first-degree relatives of type 2 diabetes [5], women with a history of present or previous gestational diabetes [19, 20], women with polycystic ovary syndrome [21], subjects with cystic fibrosis [22], older subjects [23] and subjects with IGT [24]. These more recent findings provide strong support for our suggestion from four decades ago that subjects with a decreased insulin response to glucose (especially those with a low first-phase response, then labelled prediabetics) are at risk of developing type 2 diabetes when additional factors later in life, such as insulin resistance, demand a major increase in insulin output [25, 26]. The present investigation was initiated to verify this hypothesis in subjects belonging to a population in which these early observations were made.
Twenty-five years of follow-up enabled us to compare insulin responsiveness and sensitivity in subjects who maintained NGT with those who developed IGT or type 2 diabetes. We used simple measures of beta cell function during OGTT, such as incremental plasma insulin at 30 min (ΔI30), alone or related to the corresponding glucose increment (ΔI30/ΔG30, known as the insulinogenic index), which is a better parameter of beta cell sensitivity to glucose [27]. However, normalising the insulinogenic index to the insulin sensitivity of the subject ([ΔI30/ΔG30]/HOMA-IR) provides the best physiological parameter for characterising beta cell responsiveness to glucose [4]. The use of an early time point (30 min) is important, since it is well correlated to first-phase insulin secretion and is probably regulated by similar beta cell mechanisms. For example, the acute insulin response to i.v. glucose correlated with the insulinogenic index of OGTT at 30 min in Pima Indians [28]. We found similar correlations in lean subjects with normal vs reduced first-phase insulin response to the glucose infusion test [29].
The present cross-sectional study confirms the results of Jensen et al. [4] that both insulin secretion and insulin sensitivity decrease with decreasing glucose tolerance. Since neither the insulinogenic index nor the HOMA-IR were normally distributed, it was not possible to evaluate the relative importance of these parameters for deterioration of glucose tolerance from NGT to IGT to type 2 diabetes.
The main interest of this investigation lies in the analysis of predictive factors for glucose intolerance assessed 25 years earlier in a non-obese and non-high-risk population. The subjects were young at recruitment and were only about 55 years old at final OGTT. This may explain why age was not strongly correlated with outcome in the longitudinal study. Furthermore, initial body weight was normal, with very few overweight persons (none with a BMI>30.0 kg/m2 and only 14 of 267 with a BMI>26.0 kg/m2); therefore, BMI was not strongly correlated with outcome in this study. Another explanation, corroborated in the multivariate analyses, is that age and BMI have little or no independent effect on glucose tolerance, but act mainly by decreasing insulin secretion and insulin sensitivity. The heredity of diabetes could be assessed in only 181 subjects. It appeared that about 30% of the subjects had at least one first- or second-degree relative with diabetes. This is the expected prevalence in a Swedish population of similar age [30].
The predictive power of fasting plasma glucose for future diabetes has recently been summarised in a consensus statement published by the International Diabetes Federation [31]: in six studies from different populations without diabetes, the group with IFG accounted for 9–64% of future diabetes cases. Of our subjects, only six had initial fasting plasma glucose in the IFG range (6.1–6.9 mmol/l); only one developed IGT 25 years later. Thus, in our population, fasting glucose was not a strong predictor. In contrast, in all analyses, the K-value was constantly a predictor of final glucose tolerance. To our knowledge there is only one earlier long-term study of the predictive power of the K-value: Warram et al. showed that decreased glucose removal rate was an independent risk factor for developing diabetes among the offspring of diabetic parents [7]. Our study shows that this is also true in subjects recruited from the general population, even when they are overwhelmingly non-obese and lack other diabetes risk factors. The K-value has been described as an integral measure of simultaneous changes in hepatic glucose output and peripheral glucose removal, both of which are modified by insulin release [32]. Indeed, it has been shown by Thorell [33], as well as by us [34], that the K-value is closely correlated with glucose-induced first-phase insulin secretion, while the correlation with insulin sensitivity is weaker [34]. It is therefore not surprising that, in the univariate analyses of the 2-h blood glucose concentrations at the final OGTT, the only significantly associated estimates besides the K-value were ΔI5/ΔG5 and, especially, (ΔI5/ΔG5)/HOMA-IR, both of which are estimates of first-phase insulin response to glucose. Irrespective of whether this was assessed as ΔI5/ΔG5 or corrected for insulin sensitivity ([ΔI5/ΔG5]/HOMA-IR), there was a consistent tendency for lower insulin responses in the group of subjects who developed DGT 25 years later (Table 4). We conclude from these results that the first-phase insulin response is an important predictor of future glucose tolerance, and suggest that in subjects at risk of developing IGT and diabetes, the beta cells have a reduced ability to respond rapidly to acute challenges.
A finding that seems surprising at first glance and in contradiction to the above discussion, is the positive correlation between the 10-min glucose infusion test insulin response (ΔI10) and future glucose intolerance (Tables 3 and 5). What differs between 5 and 10 min during the ‘square-wave’ glucose stimulation of the pancreas during the glucose infusion test? While insulin values at 5 min correspond to first-phase insulin release, the 10 min sample reflects the beginning of second-phase secretion. We have previously demonstrated that second-phase insulin release is a function of so-called time-dependent potentiation (TDP) or glucose priming, which works via a mechanism different from the insulin secretion-initiating effect of the sugar [35, 36]. Of greater relevance to this study, previous work demonstrated that healthy subjects with a low insulin response to glucose or IGT exhibit enhanced TDP [37]. By this mechanism, beta cells try to compensate for the impaired initiatory effect of glucose on insulin secretion and maintain substantial late (second-phase) insulin responses until the advanced stages of type 2 diabetes.
Insulin sensitivity in itself had no predictive value for IGT and type 2 diabetes in the present follow-up, in contrast to several studies in the literature: in Pima Indians [38] and Hispanics [9], not only insulin responsiveness, but also insulin sensitivity was of major significance for development of IGT and type 2 diabetes. Why this difference? Both Pima Indians and Hispanics are populations with a very high prevalence of obesity. The subjects included in our study were of Swedish origin, non-obese and mostly physically fit; thus, in such a ‘low-risk’ population the confounding effect of obesity-related insulin resistance seems to be eliminated, permitting the unravelling of the importance of the underlying beta cell deficiency for the development of IGT. It could be claimed that HOMA-IR is not sensitive enough to disclose subtle alterations in insulin sensitivity. However, comparison of HOMA with the hyperinsulinaemic–euglycaemic clamp, which is the gold standard for measuring insulin sensitivity, revealed high and significant correlation, r≈0.80 [39]. Furthermore, in a previous study in type 2 diabetes patients with similar phenotype, insulin sensitivity determined with hyperinsulinaemic–euglycaemic clamps was normal [40]. Finally, while in vitro insulin sensitivity in muscle biopsies from patients with type 2 diabetes was decreased, it was normalised after long-term preincubation in a medium of normal glucose concentration [41]. Hence, rather than being a primary defect, it seems that the insulin resistance of these patients is secondary to hyperglycaemia, probably as a result of glucose-induced downregulation of glucose transport in muscle [42].
In conclusion, 20% of 267 healthy subjects with normal intravenous glucose tolerance developed decreased oral glucose tolerance 25 years later. Both initial K-value and first-phase insulin response to glucose, corrected for insulin sensitivity ([ΔI5/ΔG5]/HOMA-IR), were consistent strong predictors of glucose tolerance at the end of follow-up. K-value is mainly controlled by first-phase insulin secretion. We therefore believe that a first-phase insulin response that is inadequate for the insulin sensitivity of the subject is an important risk factor for developing IGT and type 2 diabetes. Thus, this study supports the hypothesis that subjects with low insulin response are at risk of developing diabetes [26].