Managing Impaired Glucose Tolerance in Primary Care

Impaired glucose tolerance is defined as a fasting plasma glucose concentration of less than 7.0 mmol/l with a two-hour oral glucose tolerance test value of 7.8 to 11.1 mmol/l. Impaired glucose tolerance, typically characterised by hyperglycaemia and insulin resistance, is considered to be a stage in the development of type 2 diabetes mellitus and a risk factor for cardiovascular disease.¹ Impaired fasting glycaemia is defined as a fasting glucose of 6.1 to 6.9 mmol/l.

Studies in the United Kingdom have reported the prevalence of impaired glucose tolerance in the 35-65 year age group to be about 17%.²

Risk Factors

• Obesity
• First-degree relative with diabetes
• Certain ethnic groups, e.g. asian, afro-caribbean³
• Hypertension⁴

• Patients with impaired glucose tolerance are usually asymptomatic.
• Features of related risk factors for cardiovascular disease may be present, even with a mild degree of hyperglycemia. They include hypertension, obesity, dyslipidaemia, and macrovascular disease such as stroke, coronary disease, or peripheral vascular disease.

Oral glucose tolerance test:

• Overnight fast of 8-14 hours; only water may be drunk. Smoking should also be avoided during this time.
• Adults: drink 75 g anhydrous glucose dissolved in 250-300 ml water over 5 minutes. For children, the glucose load is 1.75 g per kg body weight.
• Blood glucose is measured before the drink and after 30, 60, 90 and 120 minutes.

• Several cardiovascular findings are more prevalent, including hypertension, raised serum cholesterol, angina, abnormal heart findings, and medical history of arteriosclerosis and stroke.⁵
• Impaired fasting glycaemia is defined as a fasting plasma glucose of 6.1 mmol/l or greater but less than 7.0 mmol/l. There is an increased risk of development of diabetes but less evidence of an associated risk of cardiovascular disease.

• Increasing evidence indicates that intervention can favourably influence the clinical course of impaired glucose tolerance, with some studies showing a 58% reduction in progression to diabetes.⁶
• The metabolic syndrome, (defined by impaired fasting glucose, large waist circumference, and high triglycerides), efficiently identifies subjects likely to have impaired glucose tolerance on oral glucose tolerance test, and thus be appropriate for diabetes prevention interventions.⁷
• Annual screening of people with impaired glucose tolerance by fasting blood glucose is currently recommended. However there is soon due to be guidance from the National Screening Committee.

Non-drug

• It has been shown that the risk of progression from impaired glucose tolerance to type 2 diabetes mellitus can be reduced by lifestyle interventions.⁸
• The advice is essentially the same as diet and exercise advice in diabetes:
  • Weight reduction, if overweight
  • Reduction in total intake of fat, and intake of saturated fat
  • Increasing intake of dietary fibre
  • Increasing physical activity

Drugs

• Reversal of drug-related iatrogenic causation of glucose intolerance. Whenever possible, substitute agent(s) that do not have an adverse effect on glucose tolerance, or reduce the dosage of the offending drug, e.g. replacing a thiazide diuretic when treating hypertension, minimising use of corticosteroids.
• Several clinical trials have found that lifestyle modification is the most effective strategy to prevent progression to type 2 diabetes.⁹ Both metformin and acarbose have been demonstrated to decrease the development of type 2 diabetes.
• Ongoing clinical trials are evaluating other agents, including ACE inhibitors, angiotensin receptor antagonists and thiazolidinediones, to prevent both type 2 diabetes and cardiovascular events. In combination with lifestyle modification, these therapies may provide effective prevention of type 2 diabetes and its consequences in high-risk patients.¹⁰
• It has been shown that ACE inhibitors may have a role in preventing diabetes in high risk individuals.¹¹

• 20-50% of people with impaired glucose tolerance will progress to type 2 diabetes within 10 years of diagnosis.¹²
• In addition, people with impaired glucose tolerance are known to be at significantly increased risk of cardiovascular disease, which may present before the onset of diabetes. Rates of cardiovascular risk factors are intermediate between those with normal glucose tolerance and those with diabetes. Impaired fasting glycaemia has not been shown to be a risk factor for cardiovascular disease.¹³
• Baseline plasma glucose is the most consistent predictor of progression to diabetes.
• Impaired glucose tolerance has not been clearly associated with microvascular complications, e.g. nephropathy, retinopathy or neuropathy.

• Avoid being overweight and eat a healthy diet, with high fibre, low fat and lots of fruit and vegetables.
• Encourage regular physical activity.
• Avoid alcohol abuse.