What is the difference between elevated fasting glucose and glucose intolerance?

Difference Between Elevated Fasting Glucose and Glucose Intolerance

Elevated fasting glucose (impaired fasting glucose/IFG) and glucose intolerance (impaired glucose tolerance/IGT) are distinct metabolic abnormalities that reflect different physiological defects in glucose regulation, though both are intermediate states between normal glucose metabolism and diabetes.

Definitions and Diagnostic Criteria

• Impaired Fasting Glucose (IFG): Defined as fasting plasma glucose levels between 100-125 mg/dL (5.6-6.9 mmol/L) after at least 8 hours of fasting 1, 2
• Impaired Glucose Tolerance (IGT): Defined as 2-hour plasma glucose levels between 140-199 mg/dL (7.8-11.0 mmol/L) during a 75g oral glucose tolerance test (OGTT) 1, 3

Key Physiological Differences

Impaired Fasting Glucose (IFG)

• Primarily characterized by hepatic insulin resistance and defects in early insulin secretion 4, 5
• Associated with increased hepatic glucose output in the fasting state 6
• First-phase insulin secretion is typically reduced 5
• Normal or near-normal muscle insulin sensitivity 4
• Often shows increased GLP-1 levels during OGTT 5

Impaired Glucose Tolerance (IGT)

• Primarily characterized by peripheral (muscle) insulin resistance 4, 5
• Both early and late-phase insulin secretion are impaired 4
• Decreased GIP levels during OGTT 5
• Marked muscle insulin resistance with only mild hepatic insulin resistance 4

Epidemiological Differences

• Prevalence: In most populations, IGT is more common than IFG 6
• Gender distribution: IFG is substantially more common in men, while IGT is slightly more common in women 6
• Age patterns: IFG prevalence tends to plateau in middle age, whereas IGT prevalence continues to rise into old age 6
• Concordance: Only about 20-30% of people with IGT also have IFG, and approximately 50% or less of people with IFG have IGT 6

Clinical Implications

• Both conditions increase the risk of developing type 2 diabetes, with the highest risk in those who have both IFG and IGT 6
• Both are associated with cardiovascular risk factors such as hypertension and dyslipidemia 6
• IGT appears to be more strongly associated with cardiovascular disease outcomes than IFG 6
• Both conditions show elevated glucagon levels, indicating alpha cell dysfunction 5

Screening and Prevention

• The USPSTF recommends screening for abnormal blood glucose in adults aged 40-70 years who are overweight or obese 1
• Lifestyle interventions (weight loss and physical activity) have been proven effective in preventing progression to diabetes, particularly in those with IGT 6
• Pharmacological interventions (metformin, acarbose) can delay or prevent diabetes but are less effective than lifestyle changes 3, 6

Practical Considerations

• Fasting plasma glucose testing requires at least 8 hours of no caloric intake 2
• Diagnosis should be confirmed by repeat testing on a different day 2
• The oral glucose tolerance test is more sensitive but less convenient than fasting glucose for identifying prediabetes 1
• When screening, consider using fasting plasma glucose as the initial test due to convenience and cost-effectiveness 2

Understanding these differences is crucial for appropriate risk stratification and targeted preventive interventions in individuals with prediabetes.