Insulin Resistance Testing

Insulin Resistance Testing and Pre-diabetes

Insulin resistance is a condition in which insulin becomes less effective at controlling blood glucose. As a result, β-cells in the pancreas release more insulin to help maintain glucose control. Studies have shown that insulin resistance increases stress on the pancreas which can cause blood glucose to become chronically elevated, eventually leading to type 2 diabetes. Insulin resistance usually has no symptoms but it can be diagnosed using a fasting intact proinsulin test.

Type 2 diabetes is achieving notoriety as one of the scourges of the developed and developing world.  The number of people in the UK alone who have been diagnosed with diabetes is over 4.5 million and it is estimated that nearly 1.3 million people are living with Type 2 diabetes but remain, as yet, undiagnosed (Diabetes UK). Worldwide there are almost 600 million suffering with the disease (World Health Organisation). Currently, diabetes accounts for about 5% of all deaths globally each year and, without intervention, this is likely to increase by more than 50% over the next 10 years.

Type 2 diabetes is essentially a disease of insulin resistance – a condition in which insulin becomes less effective at controlling blood glucose. As a consequence blood glucose may become chronically elevated, leading to many of the adverse health effects associated with type 2 diabetes.

Intact Proinsulin as an Early Indicator of Type 2 Diabetes

A formal diagnosis of type 2 diabetes relies primarily on the demonstration of glucose intolerance. However, in many patients the disease may be well advanced at the time the diagnosis is made. Such patients are already at an increased risk of cardiovascular disease and may require therapeutic intervention. Early detection of the disease provides an opportunity to initiate changes in lifestyle (e.g. diet and exercise) that can prevent or delay the onset of disease symptoms and in doing so impact significantly on patient life quality – as well as reducing treatment costs.

A 4.5 year population-based longitudinal study published in 1999 demonstrated an association between elevated fasting proinsulin concentrations at the beginning of the study and a subsequent progression to diabetes. This predictive value of proinsulin measurement has been confirmed in other studies (2,3) and may derive from its identification of a primary manifestation of type 2 diabetes – namely insulin resistance. A raised fasting proinsulin concentration has been confirmed as a highly specific marker of insulin resistance (4).

Further reading

INSULIN PROCESSING

Insulin is released from the β-cells of the Islets of Langerhans in the pancreas in response to the rise in blood glucose that follows a carbohydrate containing meal. It is synthesised initially as a high molecular weight precursor, proinsulin. This molecule is cleaved at two specific sites to yield two short polypeptide chains linked by two disulphide bridges (Figure 1). Under normal circumstances, only trace amounts of “intact” proinsulin or the intermediate metabolites of processing (“split” proinsulins) survive to be released into peripheral blood. When the pancreas is subjected to repeated or increased stimulation by glucose, as occurs in insulin resistance, this molecular processing becomes less efficient and increasing proportions of intact and split proinsulins appear in peripheral blood.

EARLY DETECTION OF TYPE 2 DIABETES

Insulin is released from the β-cells of the Islets of Langerhans in the pancreas in response to the rise in blood glucose that follows a carbohydrate containing meal. It is synthesised initially as a high molecular weight precursor, proinsulin. This molecule is cleaved at two specific sites to yield two short polypeptide chains linked by two disulphide bridges (Figure 1). Under normal circumstances, only trace amounts of “intact” proinsulin or the intermediate metabolites of processing (“split” proinsulins) survive to be released into peripheral blood. When the pancreas is subjected to repeated or increased stimulation by glucose, as occurs in insulin resistance, this molecular processing becomes less efficient and increasing proportions of intact and split proinsulins appear in peripheral blood.

While the risk of becoming diabetic risk can be calculated on the basis of lifestyle markers, it has not yet been possible to identify at-risk subjects on the basis of biochemical abnormalities. Classically, a diagnosis of diabetes is made on the basis of a fasting plasma glucose >7.0 mmol/l and/or a plasma glucose >11.1 mmol/l two hours after a 75g oral glucose load.

 

Early in 2010, the American Diabetes Association (ADA) recommended the use of glycosylated haemoglobin measurement (HbA1c) both for the diagnosis of type 2 diabetes (HbA1c ≥ 6.5%) and also as a potential screening test for the early diabetic subject (HbA1c 5.7-6.4%). Data produced within the subsequent year have done little to support this latter proposal, one study reporting that an HbA1c within the range 5.7-6.4% detected only 23% of at-risk individuals whereas 59% were found to have impaired glucose tolerance (1). The main attraction of HbA1c as a screening test is that it precludes the inconvenience of obtaining fasting blood samples.

 

In the last 20 years, several studies have suggested that changes in pancreatic β-cell function may indicate a pre-diabetic situation. In 1999, Wareham et al (2) published data on a 4.5 year populationbased longitudinal study in which they demonstrated an association between elevated fasting proinsulin concentrations at the beginning of the study and a subsequent progression to diabetes. This predictive value of proinsulin measurement has been confirmed in other studies (3, 4).

INSULIN RESISTANCE

A quantitative measure of insulin resistance can be helpful in designing appropriate therapy and predicting outcome in patients with type 2 diabetes. The accepted methods for determining insulin resistance, such as the euglycaemic clamp or intravenous glucose tolerance test are expensive and time consuming and so inappropriate for routine investigation. A homeostatic model analysis (HOMA), based on the relationship between fasting insulin and fasting glucose, has been widely used in epidemiological studies as an index of insulin resistance. Overall, the HOMA score correlates well with euglycaemic clamp results, though it is recognised to be of relatively little value in the individual subject because of its low specificity (5).

 

Recent studies have confirmed that circulating proinsulin is a highly specific marker of insulin resistance (6). Earlier work in Japan had demonstrated that while the sulphonylurea glyclazide was effective in improving glycaemic control in diabetics, a thiazolidinedione (pioglitazone) was equally effective but, in addition, produced a significant reduction in proinsulin levels (7). It is logical to conclude that proinsulin measurement in individual diabetic patients provides a reliable index of insulin resistance. In addition to its role as an early indicator of the disease, it may also provide an important marker for selecting and monitoring an appropriate therapeutic regime.

REFERENCES

1. Lorenzo C et al. A1C between 5.7 and 6.4% as a marker for identifying pre-diabetes, insulin sensitivity and secretion, and cardiovascular risk factors. Diabetes Care 2010; 33: 2104-2109.

2. Wareham NJ et al. Fasting proinsulin concentrations predict the development of type 2 diabetes. Diabetes Care 1999; 22: 262-270.

3. Hanley AJ et al. Increased proinsulin levels and decreased acute insulin response independently predict the incidence of type 2 diabetes in the insulin resistance atherosclerosis study. Diabetes 2002; 51: 1263-1270.

4. Zethelius B et al. Insulin resistance, impaired early insulin response and insulin propeptides as predictors of the development of type 2 diabetes: a population-based, 7-year follow-up study in 70-year-old men. Diabetes Care 2004; 27 1433-1438.

5. Bonora E et al. Homeostasis model assessment closely mirrors the glucose clamp technique in the assessment of insulin sensitivity. Diabetes Care 2000; 23: 57-63.

6. Pfützner A et al. IRIS II study: intact proinsulin is confirmed as a highly specific indicator for insulin resistance in a large cross-sectional study design. Diabetes Technology & Therapeutics 2005; 7: 478-486. 7. Kubo K. Effect of pioglitazone on blood proinsulin levels in patients with type 2 diabetes mellitus. Endocrine Journal 2002; 49: 323-328.