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Prevent or delay Type 2 diabetes

By Dr Greg Fulcher, Endocrinologist, Royal North Shore Hospital, NSW.

Impaired glucose tolerance
While publicity concerning the prevalence and significance of type 2 diabetes mellitus (T2DM) as a key health priority has increased over the past few years, relatively little emphasis has been placed upon the group of patients whose glucose levels, although not meeting the criteria for diabetes, are nonetheless too high to be considered normal.

This group includes patients whose fasting plasma glucose levels are ≥ 6.1mmol/L but < 7.0mmol/L (impaired fasting glucose [IFG]), or patients whose 2-hour glucose level following a 75g glucose load is ≥ 7.8mmol/L but <11.1mmol/L (impaired glucose tolerance [IGT]). Over 16% of the Australian population over the age of 25 years have either IGT or IFG.

Clinical significance of IFG and IGT
In the late 1970s IGT was considered to represent early T2DM, since it defined a patient who, on oral glucose tolerance testing (OGTT) had a non-diabetic fasting plasma glucose (FPG) and a 2-hour glucose value that was between those of normal glucose tolerance and T2DM.

By definition, these patients did not develop microvascular complications but did progress to T2DM at a rate of between 2% and 14% per year. This increased diabetic risk was considered to be the major implication of the diagnosis.

More recently, a close association between ‘non-diabetic’ abnormalities of glucose control and the presence and development of cardiovascular disease (CVD) has been described.

As a result it is now accepted that the degree of abnormal glucose homeostasis required to produce macrovascular events is far less than that at which microvascular disease occurs. In fact, a continuum of risk of dysglycaemia appears to exist from early insulin resistance through IFG/IGT to the diabetic state. This form of macrovascular disease has been called dysglycaemic macroangiopathy.

Recently Coutinho et al determined that the relative risks of CVD events associated with a FPG of 6.1mmol/L and a 2-hour post-challenge glucose of 7.8mmol/L were 1.33 and 1.58 respectively.

Furthermore, data from the Honolulu Heart Study revealed that CVD risk was distributed across a continuum of post-challenge glucose levels, making it likely that any degree of post-challenge hyperglycaemia is associated with the development of premature CVD.

Screening for IFG/IGT
While these conditions are common, many patients will be undiagnosed. Screening at-risk patients with an OGTT will be necessary to confirm the diagnosis. At-risk patients include those who:
• are over 45 yrs,
• are overweight (particularly those with central adiposity), or
• have high blood pressure, known cardiovascular disease, a family history of diabetes or past history of gestational diabetes mellitus.
In addition, T2DM or IGT cannot be excluded in a patient with a random glucose of >5.5mmol/L and <11.1mmol/L. In such patients an OGTT should be performed to establish a diagnosis.
Patients with IFG/IGT must be counselled that they are at increased risk of CVD, and strict control of all cardiovascular risk factors should be a priority.

Management of IFG/IGT
Patients with IFG/IGT must be counselled that they are at increased risk of CVD, and strict control of all cardiovascular risk factors (blood pressure, lipids, smoking) should be a priority. Dietary and exercise advice should be given and aspirin therapy should be considered.

Patients should be counselled about the increased risk of progression to diabetes, and advised of the results of recent diabetes prevention studies that have shown that lifestyle change (diet together with at least 150 minutes of exercise per week to achieve a weight loss of 7%) can delay or prevent progression to T2DM.

Conclusion

In conclusion, the diagnosis and management of IFG/IGT is a challenge for all primary care physicians.

Patients should not be discharged from care with the reassurance they do not have diabetes; instead close monitoring for the development of both T2DM and macrovascular disease should be integral to the care plan for these patients.