FACT SHEET: IMPAIRED GLUCOSE TOLERANCE (IGT)

What is IGT?

Impaired Glucose Tolerance (IGT) is the name given to define blood glucose levels that are higher than normal, but below the level of a person with diabetes. IGT is a combination of:

• Impaired secretion of insulin 
• Reduced insulin sensitivity (insulin resistance)

In people with IGT, the rise in blood glucose that occurs after consuming 75g glucose is greater than normal, although not as great as in people with type 2 diabetes.  Fasting blood glucose levels are normal or moderately raised. IGT carries a high risk of progressing to type 2 diabetes, leading to it being referred to as ‘pre-diabetes’ by, among others, the American Diabetes Association.

 

The loss of early insulin secretion in IGT and type 2 diabetes is the result of malfunctioning of the pancreatic ß-cells.  In the long-term, high blood glucose levels are toxic to ß-cells, leading to further deterioration in ß-cell function and worsening blood glucose control.

IGT is often associated with a cluster of inter-related cardiovascular risk factors known as the Metabolic Syndrome, Insulin Resistance Syndrome or Syndrome X. These are:

• High blood pressure (hypertension)
• High LDL-cholesterol (also called "bad" cholesterol)
• Low HDL-cholesterol (also called "good" cholesterol)

Diagnosis

 IGT is diagnosed when:

• Plasma glucose, two hours after consuming 75g glucose, appears to be superior to 7.8mmol/l (normal level) but remains inferior to 11.1mmol/l (diabetes level). The level of plasma glucose is measured by means of an Oral Glucose Tolerance Test (OGTT).
• Fasting plasma glucose is less than 7.0mmol/l, a level above normal but below the threshold for diagnosis of diabetes.

Some people who develop IGT will revert to normal glucose tolerance. Others will remain in a state of IGT.  However, once IGT has developed, the body’s insulin secretion and sensitivity tend to continue to decline, ultimately resulting in type 2 diabetes.

Prevention

People who have IGT are rarely treated because the condition is rarely diagnosed. IGT can be prevented by increased physical activity, maintaining a healthy weight, and following a healthy, balanced diet. Weight loss and increased physical activity can reduce insulin resistance and therefore make the insulin produced more effective at controlling blood glucose. 

Given the high risk of type 2 diabetes associated with IGT, people with IGT should be screened for diabetes at regular intervals. 

Facts on IGT

• About 40-50% of people with IGT will develop type 2 diabetes (accompanied by increased risk of cardiovascular disease and microvascular complications) within ten years.
• Progressing to type 2 diabetes is not inevitable, and approximately 30% of individuals with IGT will return to normal glucose tolerance.
• Cardiovascular complications associated with type 2 diabetes (e.g. increased atherosclerosis) begin to develop well before type 2 diabetes is diagnosed. By that time, macrovascular damage may already be well advanced.
• In 2003 it was estimated that 314 million people (8.2% of the adult population) had IGT.  By 2025, the number of people with IGT is projected to increase to 472 million (9.0% of the adult population).
• The South-East Asia Region currently has the highest number of people with IGT (93 million) and the highest prevalence rate (13.2% of the adult population). The Western Pacific Region is the next highest in terms of number (approximately 78 million people) but its prevalence rate of 5.7% is the lowest. By 2025 it is estimated that the South-East Asian Region will continue to have the highest prevalence rate with 13.5%, followed by the European Region with 10.9%.
• The Prevalence of IGT is more than double that of diabetes in the African and South-East Asian Regions. In the Eastern Mediterranean and Middle East, and the North American Regions the prevalence of IGT is slightly lower than that of diabetes.
• The third National Health and Nutrition Examination Survey (NHANES) carried out in 1988-1994 in the USA tested 2,844 adults with no history of diabetes for IGT (using WHO diagnostic criteria). 15.6% of the survey’s participants were found to have IGT. The results also found that:
  • the prevalence of IGT increased with age
  • there was no significant difference between men and women
  • certain ethnic groups had a higher prevalence of IGT (Mexican Americans, African Americans)
  • the prevalence of IGT in Europe was similar to that in the USA
• The DECODE (Diabetes Epidemiology: Collaborative analysis Of Diagnostic criteria in Europe) study, involving 25,000 participants, showed that raised blood glucose two hours after consuming 75g glucose is an independent risk factor for premature death. The study also found that:
  • People with type 2 diabetes were more than twice as likely to die during the follow-up period than people with normal blood glucose control.
  • People with IGT were 50% more likely to die of cardiovascular complications during follow-up than people with normal blood glucose control. However, as there were four times as many people with IGT as with diabetes, there were more premature deaths attributable to IGT than to diabetes.

References

1. "Pre-diabetes"; American Diabetes Association (2002); link

2. "Definition, Diagnosis and Classification of Diabetes Mellitus and its Complications";  Report of a WHO consultation,  Part 1: Diagnosis and Classification of Diabetes Mellitus; World Health Organisation (1999).

3. Byrne MM, Sturis J, Sobel RJ, Polonsky KS; Elevated plasma glucose 2-h postchallenge predicts defects in ß-cell function. American Journal of Physiology 1996; 270: E572-9.

4. Alberti KGMM. The clinical implications of impaired glucose tolerance. Diabetic Medicine 1996; 13: 927-37.

5. Harris MI, Flegal KM, Cowie CC, Eberhardt MS, Goldstein DE, Little RR, Wiedmeyer H-M, Byrd-Holt DD. Prevalence of diabetes, impaired fasting glucose and impaired glucose tolerance in US adults. The Third National Health and Nutrition Examination Survey, 1988-1994. Diabetes Care 1999; 21: 518-528.

6. Donnelly R, Emslie-Smith AM, Gardner ID, Morris AD. Vascular complications of diabetes. British Medical Journal 2000; 320: 1062-6. 

7. The DECODE study group, on behalf of the European Diabetes Epidemiology Group. Glucose tolerance and mortality: comparison of WHO and American Diabetic Association diagnostic criteria. Lancet 1999; 354: 617-62.

8. Reaven GM. Role of insulin resistance in human disease. Diabetes 1988; 37: 1595-1607. 

9. Diabetes Prevention Program Research Group.  Reduction in the incidence of type 2 diabetes with lifestyle intervention or metformin.  New England Journal of Medicine 2002; 346: 393-403.

10. Tuomilehto J, Lindstrom J, Eriksson JG, Valle TT, Hamalainen H, Ilanne-Parikka P, Keinanen-Kiukaanniemi S, Laakso M, Louheranta A, Rastas M, Salminen V, Uusitupa M. Prevention of type 2 diabetes mellitus by changes in lifestyle among subjects with impaired glucose tolerance. The New England Journal of Medicine 2001; 344: 1343-50.