Normal Blood Glucose and Insulin Response to OGTT in Non-Diabetic Adults
Blood Glucose Response
In a non-diabetic adult undergoing a 75-gram oral glucose tolerance test, the fasting plasma glucose should be less than 100 mg/dL (5.6 mmol/L), and the 2-hour post-load glucose should be less than 140 mg/dL (7.8 mmol/L). 1
Glucose Trajectory During OGTT
- Fasting (baseline): Normal glucose is <100 mg/dL (5.6 mmol/L) 1
- 2-hour post-load: Normal glucose is <140 mg/dL (7.8 mmol/L) 1, 2
- Intermediate values (prediabetes): 2-hour glucose of 140-199 mg/dL (7.8-11.0 mmol/L) indicates impaired glucose tolerance (IGT) 1, 3
- Diabetic range: 2-hour glucose ≥200 mg/dL (11.1 mmol/L) indicates diabetes 1
The glucose curve typically peaks between 30-60 minutes after the glucose load and then declines, returning toward baseline by 2 hours in individuals with normal glucose tolerance. 4
Insulin Response Pattern
Normal insulin response in non-diabetic adults shows a robust early-phase insulin secretion within the first 30 minutes after glucose ingestion, followed by sustained but lower insulin levels through the 2-hour period. 5
Key Insulin Response Characteristics
Early insulin response (0-30 minutes): The insulinogenic index (ratio of increment of plasma insulin in μU/mL to increment of blood glucose in mg/dL at 30 minutes) should be ≥0.5 in non-diabetic individuals 5
- An insulinogenic index <0.5 indicates impaired insulin secretion and is characteristic of established diabetes 5
Insulin levels correlate with glucose levels: In non-diabetic individuals, insulin levels rise proportionally with glucose elevation, creating a characteristic relationship between glucose and insulin 5
Insulin suppresses lipolysis: Normal insulin response effectively suppresses nonesterified fatty acids (NEFAs), with clearance rates of approximately 11.9 μmol/L/min in individuals with normal glucose tolerance 6
- Impaired suppression of NEFAs (clearance rates <7.6 μmol/L/min) indicates insulin resistance 6
Clinical Context and Interpretation
Important Caveats
Fasting glucose poorly predicts postprandial values: The correlation coefficient between fasting plasma glucose and 2-hour post-load glucose is only 0.50-0.70, meaning fasting values alone cannot reliably predict OGTT results 4
Age-related changes: Elderly individuals (>65 years) may show isolated postprandial hyperglycemia with normal fasting glucose, often associated with elevated fasting NEFAs and impaired NEFA suppression 6
Test standardization: The OGTT requires an 8-hour fast and uses 75 grams of anhydrous glucose dissolved in water 1, 2, 7
Diagnostic Thresholds Summary
The American Diabetes Association criteria define three categories based on 2-hour OGTT values: 1, 2
- Normal glucose tolerance: <140 mg/dL (7.8 mmol/L)
- Impaired glucose tolerance (prediabetes): 140-199 mg/dL (7.8-11.0 mmol/L)
- Diabetes: ≥200 mg/dL (11.1 mmol/L)
Any abnormal result should be confirmed by repeat testing on a different day unless unequivocal hyperglycemia with symptoms is present. 1, 7