What you eat in the days before a glucose test matters just as much as the fasting period itself. Eating too few carbohydrates beforehand can actually skew your results and trigger a false positive, while the wrong choices the night before can leave you feeling terrible during the test. The specifics depend on which type of glucose test you’re taking, so here’s what to know for each one.
Which Test You’re Taking Changes the Rules
There are a few different glucose tests, and each has different prep requirements. The one-hour glucose challenge test (sometimes called a glucola screen) is the initial screening most pregnant people get around 24 to 28 weeks. Fasting is not required for this test. You can eat normally beforehand, though many providers suggest avoiding a sugar-heavy meal right before drinking the glucose solution.
The three-hour glucose tolerance test (OGTT) is different. This is the follow-up test given when the one-hour screen comes back elevated, and it’s also used outside of pregnancy to diagnose type 2 diabetes or prediabetes. For this test, you need to fast for a full eight hours beforehand. Most people schedule it first thing in the morning and stop eating after dinner the night before.
A standard fasting blood glucose test also requires eight hours without food. The prep advice below applies to both the OGTT and fasting glucose tests.
What to Eat in the Three Days Before
This is the part most people miss. If you’ve been eating low-carb, doing keto, or just cutting back on bread and pasta, your body may respond abnormally to the glucose drink, producing a falsely high reading. The reason is straightforward: when your body hasn’t been processing much glucose, it’s temporarily less efficient at clearing sugar from your blood. That inefficiency shows up on the test as impaired glucose tolerance, even if you don’t actually have a problem.
Clinical guidelines call for eating at least 150 grams of carbohydrates per day for the three days leading up to the test. That’s not a huge amount. For reference, 150 grams of carbs looks roughly like two cups of cooked rice, two slices of bread, a banana, and a serving of pasta over the course of a full day. If you normally eat this way, you don’t need to change anything. But if you’ve been restricting carbs, you’ll want to deliberately add them back in. A case report published in the Journal of the Endocrine Society documented a patient whose OGTT came back abnormal after a period of low carbohydrate intake. When she repeated the test after eating more than 150 grams of carbs daily for three days, her results were completely normal.
Stick with whole-food carbohydrate sources: whole grains, potatoes, rice, fruit, beans, and oatmeal. You’re not trying to load up on sugar. You’re trying to make sure your body’s insulin response is warmed up and functioning normally for the test.
Your Last Meal the Night Before
For any glucose test that requires fasting, your dinner the night before is the last food you’ll eat for a while. Aim for a balanced meal that includes a good portion of complex carbohydrates, some protein, and healthy fat. Think grilled chicken with rice and roasted vegetables, or pasta with meat sauce and a side salad. The carbohydrates keep you on track with the 150-gram daily goal, while the protein and fat help you feel full longer through the fasting window.
Finish eating by midnight at the latest. After that, do not eat or drink anything other than plain water. Skip gum, cough drops, and hard candy too, as these contain small amounts of sugar that can affect your results.
What You Can and Cannot Drink While Fasting
Plain water is the only safe choice during the fasting window, and you should drink plenty of it. Staying well hydrated makes your blood draw easier and helps you feel less lightheaded during the test. Avoid flavored water, sparkling water, and tea.
Black coffee is a common question, and the answer is no. Even without cream or sugar, coffee contains caffeine that can influence blood sugar metabolism. It’s also a diuretic, meaning it pulls water from your body and can affect lab values. Have your coffee after the test is done.
Skip the Hard Workout
Vigorous exercise in the hours before a glucose test can lower your blood sugar and interfere with results. Avoid intense workouts, long runs, or heavy lifting during the eight-hour fasting window. Light activity like walking to your car or moving around your house is fine. Once you arrive at the lab, expect to sit quietly for the duration of the test, since even moderate activity between blood draws can change your readings.
If You’re Taking the One-Hour Screen
Since fasting isn’t required for the one-hour glucose challenge, you have more flexibility. That said, what you eat that morning still matters for how you feel. A breakfast heavy in refined sugar (donuts, sugary cereal, juice) right before drinking the glucose solution can make nausea worse and may push your numbers higher than they’d otherwise be.
A better approach is a meal with protein and moderate carbohydrates a couple of hours before your appointment. Eggs with toast, oatmeal with peanut butter, or yogurt with fruit all work well. Some providers give more specific instructions, so follow whatever your office tells you. The key is to eat a normal, balanced meal rather than either skipping food entirely or going heavy on sweets.
Medications and Supplements to Flag
Certain medications and supplements can interfere with glucose readings. High-dose vitamin C and high-dose acetaminophen (the active ingredient in Tylenol) can both cause falsely elevated glucose levels on some monitors. If you’re taking any of these regularly, let your provider know before the test so they can advise whether to pause them temporarily.
What to Bring for After the Test
The glucose drink is extremely sweet, and many people feel shaky, nauseous, or lightheaded afterward, especially with the three-hour test. Pack a protein-rich snack to eat as soon as you’re done: a handful of nuts, cheese and crackers, a protein bar, or a peanut butter sandwich. Pairing protein with a moderate amount of carbohydrates helps stabilize your blood sugar and counteracts the crash that often follows. Bring a water bottle too, since you’ll want to rehydrate after the fasting period and the blood draws.