Preparing for a glucose tolerance test takes about three days of attention to your diet, followed by an eight-hour fast before the test itself. The preparation matters because eating too few carbohydrates, exercising heavily, or breaking your fast can all skew the results and potentially lead to a misdiagnosis. Here’s exactly what to do in the days and hours leading up to the test.
Eat Normally for Three Days Before
For the three days before your test, eat at least 150 grams of carbohydrates per day. That’s roughly the amount in a normal, unrestricted diet: a couple of servings of bread or pasta, some fruit, rice, or potatoes at each meal. The point is to avoid low-carb or keto-style eating in the days leading up to the test, because restricting carbs can artificially raise your blood sugar response to the glucose drink and produce a falsely abnormal result.
You don’t need to eat more than usual or load up on sugar. Just eat your regular meals and don’t cut carbs. If you’ve been following a low-carb diet, talk to whoever ordered your test about when to resume normal eating beforehand.
Keep Exercise Consistent
A hard workout within 24 hours of the test can deplete stored sugar in your muscles and change how your body processes the glucose drink. Research published in the Journal of the International Society of Sports Nutrition found that a bout of exercise intense enough to significantly deplete muscle glycogen, performed within 24 hours before the test, altered both blood sugar and insulin responses. Light walking or your normal daily movement is fine, but skip intense gym sessions, long runs, or heavy lifting the day before.
Fast for Eight Hours
You’ll need to stop eating and drinking eight hours before the test. Most people schedule morning appointments and use sleep as most of the fasting window. If your test is at 8 a.m., stop eating by midnight. Water is the one exception for certain versions of the test (more on the pregnancy screening below), but the safest approach is to ask your provider’s office exactly what’s allowed during your fast.
Don’t smoke on the morning of the test. Smoking can affect blood sugar levels, and you’ll be asked to avoid it until the last blood draw is complete.
What Happens During the Test
The standard oral glucose tolerance test starts with a fasting blood draw. Then you’ll drink a solution of 75 grams of glucose dissolved in about 250 to 300 milliliters of water, roughly the size of a large glass. It tastes very sweet, similar to flat soda. You’ll need to finish it within a few minutes.
After drinking the solution, you sit or lie down and wait. You can’t eat, drink, or smoke during the waiting period. A second blood draw happens at the two-hour mark. Some versions of the test also include a one-hour draw. The whole appointment typically takes two to three hours, so bring something to read or watch.
Your doctor uses the two-hour blood sugar reading to classify your result. A level under 140 mg/dL is normal. Between 140 and 199 mg/dL indicates prediabetes. A reading of 200 mg/dL or higher points to diabetes. The two-hour glucose test catches more cases of prediabetes and diabetes than a fasting blood sugar test alone, which is one reason your doctor may have ordered it.
Pregnancy Testing Is Slightly Different
If you’re pregnant, glucose testing typically happens in two stages. The first is a one-hour screening test, and the good news is that you can eat and drink normally before it. You’ll drink a smaller glucose solution containing 50 grams of glucose, then have your blood drawn after one hour.
If that screening comes back elevated, you’ll be scheduled for the three-hour diagnostic test. This one does require an eight-hour fast beforehand, and you should avoid eating, drinking (except water), smoking, and vigorous exercise during those eight hours. The three-hour test uses a larger glucose dose and involves blood draws at one, two, and three hours after drinking the solution.
Medications and Illness
Some medications can raise or lower blood sugar and interfere with results. Steroids, certain blood pressure medications, and some psychiatric medications are common examples. Don’t stop any medication on your own, but let the ordering provider know everything you’re taking so they can decide whether to adjust the timing or interpret results accordingly.
If you’re sick with a cold, flu, infection, or any acute illness, the test should be postponed. Your body releases stress hormones during illness that temporarily raise blood sugar, which can produce a falsely high result. The same applies to periods of significant physical or emotional stress. Wait until you’re feeling well and back to your normal routine.
Side Effects of the Glucose Drink
The most common reactions are nausea and occasionally vomiting. Some people also feel dizzy, weak, or shaky during the waiting period, especially closer to the end. These symptoms are usually mild and resolve quickly once you eat after the test. Bring a snack for immediately after your last blood draw so you can get something in your stomach right away.
Reactive low blood sugar, where your body overshoots its insulin response and your levels dip too low, can happen but is uncommon in people who haven’t had weight-loss surgery. If you feel sweaty, trembling, or faint during the test, let the staff know immediately. They’re equipped to handle it.
Quick Prep Checklist
- 3 days before: Eat at least 150 grams of carbohydrates daily. No low-carb dieting.
- 24 hours before: Skip intense exercise. Light activity is fine.
- 8 hours before: Stop eating and drinking. No smoking.
- Morning of: Take only the medications your provider approved. Arrive with something to do for two to three hours.
- After the test: Eat a balanced meal or snack as soon as your last blood draw is finished.