Key findings
In a prospective observational study, we measured c-peptide as a biomarker of endogenous insulin secretion during moderate permissive hyperglycemia in critically ill adult patients with T2DM. Premorbid insulin-requiring diabetes was associated with lower levels of c-peptide and higher plasma creatinine with greater levels of c-peptide. Although, as expected, c-peptide increased in response to a corresponding increase in BGL, early insulin therapy (median 24 units over 24 h) augmented such glucose-stimulated c-peptide secretion independent of the presence of premorbid insulin-requirement, or magnitude of change in BGL. Additionally, a higher insulin dose was associated with a greater c-peptide increase. Finally, we observed somewhat greater c-peptide in patients receiving insulin secretagogues. However, such therapy did not significantly modify the relationship between insulin administration and beta-cell response.
Relationship to previous studies
To date, no study has evaluated early c-peptide secretion during permissive hyperglycemia in critically ill T2DM patients. However, Langouche et al. compared insulin and c-peptide levels between critically ill patients with stress-hyperglycemia randomized to intensive insulin therapy (target BGL 4.4–6.1 mmol/l) or conventional glucose control (target BGL 10–11.1 mmol/l), of whom the majority (approximately 90%) did not have a history of diabetes [
8]. Compared with a cohort of 26 fasted healthy volunteers, c-peptide levels were higher in critically ill patients on ICU admission. Moreover, compared with our cohort of T2DM patients, c-peptide levels on admission and on day 2 were higher in the conventional-control patients in the study by Langouche et al. despite lower blood glucose and similar amount of administered insulin (20–25 units per 24 h). This suggests that our T2DM patients had some degree of beta-cell insufficiency. Furthermore, whereas c-peptide remained elevated in their conventional-control patients during the first week in ICU, c-peptide normalized after achieving normoglycemia in the intensive insulin therapy group. This observation supports our finding that blood glucose change is a major determinant of c-peptide secretion.
The effect of insulin on beta-cell function was previously investigated in volunteers. In healthy volunteers, exogenous insulin administration suppressed c-peptide release during normoglycemia [
12,
13]. This suppressive effect was, however, attenuated during mild hyperglycemia in subjects with non-insulin dependent diabetes [
14]. In contrast, Anderwald et al. found increased c-peptide secretion during exposure to hyperinsulinemic normoglycemia in healthy subjects, whereas c-peptide levels decreased during insulin infusion and persistent normoglycemia in subjects with impaired glucose tolerance or established T2DM [
15].
As these volunteer studies were conducted during strict normoglycemia, their relevance for critically ill T2DM patients with permissive stress-hyperglycemia is limited. More relevant is a recent study measuring endogenous insulin and c-peptide levels during graded hyperglycemia (dextrose infusion to a maximum BGL of 18 mmol/l) following a 4-h isoglycemic clamp with either saline (no insulin, sham) or insulin (2 milliunits/kg/min) in healthy volunteers and in subjects with T2DM [
7]. Pre-exposure to low-dose insulin enhanced hyperglycemia-induced endogenous insulin and c-peptide secretion in both groups. This effect was, however, attenuated in subjects with T2DM. These findings support the notion that exogenous insulin, possibly via autocrine effects, stimulates insulin secretion and that this positive feedback-mechanism is suppressed in patients with stressed beta-cells, such as T2DM patients [
16]. However, in our cohort of patients with acute-on-chronic insulin resistance, we observed a greater percentage increase in c-peptide levels in insulin-treated patients than in patients not receiving insulin during hyperglycemia. This effect persisted after adjusting for the presence of premorbid insulin-requiring diabetes and for the magnitude change in BGL. Although an autocrine insulin effect could potentially explain our finding, we cannot rule out that attenuated glucotoxicity (decreased BGL) during insulin infusion contributed to improved beta-cell function [
17].
In addition to hyperglycemia, impaired kidney function is known to increase plasma c-peptide levels as approximately half of circulating c-peptide is cleared via the kidneys [
18]. Our data, showing an independent association between plasma creatinine and c-peptide levels on admission, support this notion.
Emerging data suggest that c-peptide is not an inactive by-product of insulin secretion but may in fact play an important role during systemic inflammatory stress. For example, treatment with c-peptide after the induction of endotoxic shock in mice attenuated the systemic inflammatory response and improved survival compared with vehicle [
6]. In addition, treatment with c-peptide during resuscitation for hemorrhagic shock in rats ameliorated hypotension, blunted the systemic inflammatory response and reduced neutrophil infiltration in the lung tissue [
19]. Whether c-peptide has similar beneficial effects in critically ill humans is yet to be determined. However, other studies suggest that an elevated c-peptide level is independently associated with micro- and macrovascular complications in T2DM patients [
20]. Accumulation of c-peptide in atherogenic plaques and chemotactic effects of c-peptide on monocytes may be involved in the pathophysiology of such complications [
21].
Implications of study findings
Our findings imply that hyperglycemia stimulates endogenous c-peptide (and therefore insulin) secretion even in critically ill patients with acute-on-chronic insulin resistance. Furthermore, they imply that early exogenous insulin therapy has no suppressive effect on the ability of beta-cells to secrete c-peptide (and therefore insulin) during stress-hyperglycemia. On the contrary, in our cohort, the independent association between insulin therapy and an increase in c-peptide during hyperglycemia supports a previous hypothesis [
7] that exogenous insulin may stimulate beta-cell function (beta-cell recruitment). Such recruitment would logically help deliver more insulin into the portal vein and restore a more physiological splanchnic effect of insulin. Moreover, our findings imply that kidney function needs to be considered when interpreting c-peptide levels. We failed to demonstrate a significant association between insulin secretagogue therapy and beta-cell function in our study cohort. However, only eight patients received such therapy and, consequently, we acknowledge that a lack of effect could be a type 2 error. The effect of insulin sensitizers and secretagogues on beta-cell function and glycemic control needs to be explored in future trials.
Study strengths and limitations
Our study has several strengths. To our knowledge, we are the first to explore early c-peptide levels in a heterogeneous cohort of type 2 diabetic patients with critical illness. Second, we collected detailed information on premorbid diabetic treatment and glycemic control, acute glycemic control, illness severity, and acute kidney function and were therefore able to assess the independent relationship between these variables and c-peptide levels. Third, we analyzed c-peptide levels over two consecutive days in the same laboratory using the same platform to understand changes in response to intervention. Fourth, clinicians were blinded to the c-peptide results, which allowed us to make an unbiased assessment of the relationship between c-peptide and insulin administration in ICU. Finally, our diabetic patients were studied during moderate permissive hyperglycemia, which has two important implications. The first is that it has the potential to trigger greater insulin release allowing us to more clearly assess beta-cell functional reserve. The second is that our liberal glucose target reduced the proportion of patients requiring exogenous insulin therapy, which allowed us assess the impact of insulin therapy in these patients. Yet, we acknowledge that permissive hyperglycemia is not standard practice in most centers. The observed effect of insulin on c-peptide may be less pronounced or even absent in patients receiving a “tighter” glucose control protocol.
Our study also has several limitations. It is a single-center study, decreasing the generalizability of our findings to other centers. However, our ICU has all the typical features of an academic ICU within a tertiary hospital in a developed country, suggesting a degree of external validity. We did not measure insulin levels. However, c-peptide levels more accurately reflect endogenous insulin production during insulin therapy [
22]. Yet, we believe that both c-peptide and insulin levels should be reported in future studies to better understand the physiology of beta-cells under these circumstances. We only assessed c-peptide levels during the first 2 days in ICU. Therefore, we cannot draw any conclusions about longer-term beta-cell function during persistent hyperglycemia in ICU. The presence of an incretin effect induced by enteral nutrition, an important physiological trigger of beta-cell stimulation [
23], was not explored. However, this effect was likely negligible in our cohorts; only one patient in the non-insulin group and no patients in the insulin group received enteral nutrition on day 1. We did not use a reference method, such as a normoglycemic hyperinsulinemic clamp technique, to quantify insulin resistance. However, such clamp techniques are not feasible in the rapidly changing ICU environment and may in themselves affect endogenous insulin secretion. We did not include healthy controls or critically ill patients without diabetes. However, permissive hyperglycemia in nondiabetic patients would represent a significant deviation from our unit’s practice and was therefore not possible. Finally, a proportion of our patients that had received early insulin therapy had significantly greater BGLs during the study period than patients not requiring insulin. This may have enhanced the c-peptide response in the insulin group. However, our findings persisted after adjusting for blood glucose or blood glucose change in multivariable analyses.