How to Prepare for a Two-Hour Oral Glucose Tolerance Test
You must fast for 8-14 hours before the test and consume at least 150 grams of carbohydrate daily for the 3 days preceding the test. 1, 2
Pre-Test Dietary Requirements
Three Days Before the Test
- Consume at least 150 grams of carbohydrate per day for the 3 days leading up to your OGTT 3, 4
- Maintain your usual mixed diet—do not restrict carbohydrates 3
- Critical pitfall: Carbohydrate restriction or low-carb dieting before the test can falsely elevate glucose levels, mimicking early diabetes even in healthy individuals 4
The physiological basis for this requirement is well-established: inadequate carbohydrate intake causes beta cell adaptation and altered peripheral glucose uptake, which can produce falsely abnormal results that mimic the loss of first-phase insulin release seen in early diabetes 4.
The Night Before
- Fast for 8-14 hours (typically overnight) 1, 2
- Ensure your last meal contains at least 50 grams of carbohydrate 4
- No caloric intake during the fasting period—only water is permitted 5, 2
Day of the Test
Morning Preparation
- Arrive fasting (no food or caloric beverages since the night before)
- The test should be performed in the morning 1, 6
- You will drink 75 grams of glucose dissolved in 250-300 mL of water over approximately 5 minutes 1, 6
During the Test
- Blood will be drawn before you drink the glucose solution (fasting sample)
- A second blood sample will be taken exactly 2 hours after you begin drinking the glucose solution 1, 6
- Timing starts from when you begin drinking, not when you finish 1
Important Considerations
What NOT to Do
- Do not fast or restrict carbohydrates in the days before the test 3, 4
- Do not eat within 2 hours of the test (research shows eating within 2 hours can reduce screen-positive rates by half, potentially missing diagnoses) 7
- Avoid stress, recent illness, or unusual physical activity if possible, as these can affect results 2
Special Circumstances
If you have conditions affecting red blood cell turnover (sickle cell disease, recent blood loss, anemia, pregnancy in second/third trimester), inform your healthcare provider, as the OGTT may not be the appropriate test for you 8, 3.
The most common error is inadequate carbohydrate intake before the test, which can lead to false-positive results and unnecessary anxiety, additional testing, and potential misdiagnosis 4.