The Two Types of Prediabetes
The two types of prediabetes are Impaired Fasting Glucose (IFG) and Impaired Glucose Tolerance (IGT). These represent distinct metabolic abnormalities that precede the development of type 2 diabetes and carry increased risk for cardiovascular disease and mortality 1.
Definitions and Diagnostic Criteria
Impaired Fasting Glucose (IFG)
- Defined by fasting plasma glucose (FPG) levels between specific thresholds:
- Requires fasting for at least 8 hours
- Primarily reflects hepatic insulin resistance and impaired first-phase insulin secretion
Impaired Glucose Tolerance (IGT)
- Defined by 2-hour plasma glucose levels during a 75g oral glucose tolerance test (OGTT)
- Diagnostic range: 7.8-11.0 mmol/L (140-199 mg/dL) 1
- Reflects peripheral (muscle) insulin resistance and impaired second-phase insulin secretion
Key Differences Between IFG and IGT
| Characteristic | Impaired Fasting Glucose (IFG) | Impaired Glucose Tolerance (IGT) |
|---|---|---|
| Diagnostic test | Fasting plasma glucose | 2-hour OGTT |
| Primary site of insulin resistance | Liver | Skeletal muscle |
| Insulin secretion defect | Early/first-phase | Late/second-phase |
| Risk of progression to diabetes | Moderate | Higher than IFG alone |
| Evidence for intervention | Less robust | Stronger evidence base |
Clinical Significance
Risk stratification: Both IFG and IGT are associated with increased risk of developing type 2 diabetes, but the risk is highest when both conditions coexist 1, 2.
Cardiovascular risk: Both types of prediabetes are associated with increased risk of cardiovascular events and mortality, with an excess absolute risk of 8.75 per 10,000 person-years for cardiovascular disease 2.
Intervention efficacy: The efficacy of interventions for primary prevention of type 2 diabetes has been demonstrated mainly among individuals with IGT with or without elevated fasting glucose, not for those with isolated IFG 1.
Screening approach: The ADA recommends screening for prediabetes in adults who are overweight or obese (BMI ≥25 kg/m² or ≥23 kg/m² in Asian Americans) and have one or more additional risk factors 1.
Management Implications
Lifestyle modification: Intensive lifestyle changes including caloric restriction, increased physical activity (≥150 min/week), and weight loss are the cornerstone of treatment for both types of prediabetes 2, 3.
Pharmacotherapy: Metformin is most beneficial for individuals with:
- IGT
- Higher baseline glucose levels (FPG ≥110 mg/dL or HbA1c ≥6.0%)
- BMI ≥35 kg/m²
- Age <60 years
- History of gestational diabetes 2
Clinical Pitfalls to Avoid
Diagnostic confusion: Don't rely solely on HbA1c for diagnosis, as it may miss some cases of IGT that would be detected by OGTT 1.
Underestimating risk: Both types of prediabetes represent a continuum of risk, and even glucose levels below current diagnostic thresholds may be associated with increased cardiovascular risk 4.
Inadequate follow-up: Individuals with prediabetes should be tested yearly for progression to diabetes 1.
Overlooking cardiovascular risk management: Treatment should address both glycemic control and cardiovascular risk factors, including hypertension and hyperlipidemia 3.
By understanding the distinct pathophysiology and clinical implications of IFG and IGT, clinicians can better target interventions to prevent or delay the progression to type 2 diabetes and reduce associated cardiovascular complications.