What should I eat before an OGTT test?

December 30, 2025 •

4 min read

Studies have shown that a person's dietary intake in the days leading up to an oral glucose tolerance test (OGTT) can significantly influence the results. Following specific dietary guidelines is crucial, so understanding exactly what you should eat before an OGTT test is a key part of proper preparation for accurate results.

Quick Summary

Successful preparation for an OGTT involves maintaining a specific carbohydrate intake for several days, followed by a mandatory fasting period of 8 to 12 hours. Normal eating patterns should be followed until the fast begins, with only water permitted during the fasting and testing period.

Key Points

Fasting is Required: Do not eat or drink anything except water for at least 8 to 12 hours before your OGTT.
Maintain Normal Carb Intake: In the 3 days leading up to the test, eat a diet with adequate carbohydrates to avoid skewed results.
Avoid Specific Items: Do not consume caffeine, nicotine, or chew gum on the day of the test as they can interfere with results.
Inform Your Doctor: Certain medications can affect the test. Tell your doctor about all prescriptions and supplements you take beforehand.
Stay Relaxed and Still: During the test, you must remain seated quietly in the testing center to ensure accurate measurements.
Bring Post-Test Snacks: Plan to bring a snack to eat after your final blood draw, as the sugary drink and fasting can cause lightheadedness.
Gestational vs. Non-Pregnant: Testing procedures and fasting times can differ for gestational diabetes screenings; always follow your doctor's specific instructions.

Understanding the Oral Glucose Tolerance Test (OGTT)

An Oral Glucose Tolerance Test (OGTT) is a crucial diagnostic tool used to measure your body's response to sugar (glucose). It is commonly used to screen for and diagnose prediabetes, type 2 diabetes, and gestational diabetes. The test involves a fasting blood draw, followed by consuming a sugary drink, and then subsequent blood draws at timed intervals to track how well your body processes the glucose. Proper dietary preparation is critical for the test to yield reliable and accurate results.

Dietary Preparation in the Days Before the Test

Unlike some blood tests that require no special preparation, the OGTT has a specific dietary recommendation for the 1 to 3 days leading up to the test. During this time, it's essential to eat a normal, well-balanced diet that includes a generous amount of carbohydrates. This is to ensure your body's glucose-processing system is functioning as it normally would. Avoiding major dietary changes, such as restricting carbohydrate intake or dieting, can affect your body's metabolic response and lead to misleading test results. Your body needs to be accustomed to its normal carbohydrate load to be accurately challenged during the test.

Carbohydrate-rich foods to include:

Whole grains, such as whole-wheat bread, oatmeal, and brown rice.
Fruits, including berries, apples, and bananas.
Starchy vegetables, like potatoes, corn, and beans.
Milk, yogurt, and other dairy products.
Pasta and cereal.

The Final Fasting Period

The most critical part of the dietary preparation is the fasting period immediately before the OGTT. You must fast for a minimum of 8 hours and up to 12 hours before your first blood sample is taken. The timing is important, as fasting for too long (over 14 hours) may also produce abnormal results.

During your fast, you should:

Refrain from all food and caloric drinks.
Only consume small sips of plain water.
Avoid caffeine from coffee, tea, and soda.
Do not smoke, chew gum (including sugar-free), or eat hard candies or mints.
Avoid strenuous exercise, as this can affect glucose levels.

What to Avoid Before the Test

In addition to the general fasting rules, there are specific items to avoid to prevent skewed results:

Sugary Foods and Drinks: This includes soda, fruit juice, pastries, and candy bars, especially the morning of a one-hour glucose screen (a variant for pregnancy).
Caffeine: It's best to avoid coffee and caffeinated tea on the morning of the test, as some studies suggest it may affect glucose metabolism.
Certain Medications: Inform your doctor of all medications and supplements you are taking. Some drugs, such as corticosteroids and diuretics, can affect glucose levels. Your doctor may advise you to stop taking them temporarily before the test.
Unusual Dieting: Restricting carbohydrates dramatically in the days leading up to the test can cause unreliable results. Stick to your normal, high-carb diet as instructed.

Comparison of Pre-Test Dietary Phases


Feature	3 Days Prior to Test	8-12 Hours Before Test	During the OGTT (After glucose drink)
Food Intake	Normal, high-carbohydrate diet (approx. 150g per day)	No food or caloric drinks whatsoever	No food or drinks (except small sips of water)
Fluid Intake	Normal fluid intake	Water only, small sips	Water only, small sips
Activity	Normal activity level	Avoid strenuous exercise	Remain seated and rest
Smoking/Gum	OK	No smoking or chewing gum	No smoking or chewing gum
Caffeine	OK	Best to avoid	Not permitted

Practical Tips for Test Day

Since the OGTT can take up to 2 to 3 hours, especially for a gestational diabetes screening, it's wise to prepare for a long wait.

Bring a book, magazine, or other quiet activity to keep yourself occupied during the test, as you will need to remain seated in the collection center.
Wear comfortable clothing and be prepared for multiple blood draws.
Bring a snack or meal with you to eat immediately after the final blood draw to help your blood sugar normalize.
If you experience any symptoms like nausea, dizziness, or lightheadedness, inform the medical staff immediately. These are possible side effects due to the fasting and the sugary drink.

Conclusion

Properly preparing for an OGTT test is a simple yet vital step to ensure the accuracy of your results. By maintaining your usual carbohydrate intake in the days beforehand and strictly adhering to the 8 to 12-hour fasting period, you provide your healthcare provider with the most reliable data possible. Always communicate any questions or concerns about your medications or health conditions with your doctor well in advance of your test. Following these instructions will help achieve a reliable diagnosis and lead to the right treatment plan if necessary.

For more detailed information on glucose tolerance tests, consult the Mayo Clinic's guide on the topic: Glucose tolerance test - Mayo Clinic.

Frequently Asked Questions

Yes, you are typically allowed to drink plain water in small sips during the fasting period. However, all other drinks, including coffee, tea, and juice, are prohibited.

You must fast for at least 8 hours before the test. Fasting overnight is common, with the last meal being eaten the evening before. Do not fast for more than 12-14 hours, as this can also affect results.

If you accidentally eat or drink anything other than plain water during the fasting period, you must contact your doctor or the lab immediately. The test will need to be rescheduled, as the results would be inaccurate.

No, strenuous exercise should be avoided in the 1 to 3 days leading up to the test, and especially on the morning of the test itself. Physical activity can impact your blood glucose levels and compromise the accuracy of the results.

Yes, chewing gum, even sugar-free gum, can affect test results by stimulating digestion and insulin release. You must not chew gum during the fasting period or the test itself.

You should eat a diet with a normal amount of carbohydrates for at least 3 days before the test. Severely restricting carbohydrates or dieting can lead to unreliable results.

No, caffeine should be avoided during the fasting period before the test, as it can potentially influence your glucose metabolism and affect the accuracy of the results.