What you eat before a glucose test depends entirely on which test you’re taking. Some glucose tests require complete fasting, meaning no breakfast at all. Others allow you to eat normally but recommend avoiding sugary foods. Knowing which test you have scheduled is the first step to getting accurate results.
Which Glucose Test Are You Taking?
There are three common glucose tests, and each has different rules about eating beforehand.
The one-hour glucose screening is the most common test during pregnancy. It does not require fasting, so you can eat breakfast. The fasting blood glucose test requires you to skip food entirely for at least eight hours before your blood draw. The three-hour oral glucose tolerance test (OGTT), which is typically ordered if you fail the one-hour screen, also requires a full eight-hour fast. If your provider’s office didn’t specify which test you’re getting, call and ask before planning your morning meal.
If You Can Eat: Best Breakfast Before a One-Hour Screen
For the one-hour glucose screening, you’ll drink a sugary solution at your appointment and have your blood drawn an hour later. Because you’re already consuming a large dose of sugar for the test, what you eat beforehand can push your blood sugar higher than it would otherwise be. Most providers recommend eating a meal that’s low in sugar and built around protein and complex carbohydrates.
Good options include:
- Eggs with vegetables: scrambled eggs or an omelet with broccoli or tomatoes
- Whole grain toast with peanut butter or avocado
- Plain Greek yogurt with berries
- Oatmeal with nuts and sliced apple
The common thread is protein, healthy fat, and slow-digesting carbohydrates. These foods release glucose gradually rather than spiking your blood sugar all at once. Avoid pancakes, donuts, sugary cereal, fruit juice, pastries, and sweetened coffee drinks. These are exactly the foods that can tip a borderline result into a failing score, potentially sending you to the longer three-hour test unnecessarily.
Eat your breakfast at least an hour or two before your appointment if possible, and keep the portion moderate. A normal-sized meal is fine. You don’t need to undereat.
If You’re Fasting: What the Rules Actually Mean
For a fasting blood glucose test or a three-hour OGTT, you cannot eat anything for at least eight hours before your blood draw. That means no breakfast. Most people schedule these tests first thing in the morning and stop eating after dinner the night before.
Water is generally permitted during the fast, and staying hydrated can actually make the blood draw easier. Black coffee is a gray area. One small study found that black coffee did not change fasting glucose levels compared to water alone, but many labs and providers still ask you to avoid it. Follow whatever instructions your specific provider gave you. If they didn’t mention coffee, stick with water to be safe.
What to Eat the Night Before Matters Too
This is the part most people miss. What you eat in the days leading up to a glucose tolerance test can significantly affect your results, and not in the way you might expect. Eating very few carbohydrates before the test can actually cause a false positive, making it look like you have impaired glucose tolerance when you don’t.
Research published in the Journal of the Endocrine Society documented this clearly. When people ate a low-carbohydrate dinner the night before an OGTT (fewer than 50 grams of carbs), 38% of men and 25% of women showed impaired glucose tolerance on the test, even when their total daily carb intake was above 150 grams. The explanation is straightforward: when your body hasn’t been processing carbohydrates regularly, it becomes temporarily less efficient at clearing glucose from the blood. A sudden large dose of sugar during the test then produces an exaggerated spike.
The World Health Organization has long recommended eating at least 150 grams of carbohydrates per day for the three days before an OGTT. That’s roughly the amount in a normal diet that includes grains, fruit, and starchy vegetables at each meal. The dinner immediately before your fast matters most. Aim for at least 50 grams of carbohydrates at that meal, which is about a cup of cooked pasta or rice plus a piece of fruit.
If you’ve been following a low-carb or keto diet, this is especially important. A case study in the same journal showed a patient whose OGTT came back abnormal after following a low-carb diet, then returned completely normal after eating adequate carbohydrates for three days before retesting. Don’t change your eating pattern to try to “game” the test in either direction. Eat normally so the test reflects how your body actually handles sugar in your everyday life.
What to Avoid Before Any Glucose Test
Beyond food choices, a few other things can interfere with results. Sugary drinks, including fruit juice and soda, are the biggest culprit for false highs on the one-hour screen. Energy drinks are also worth skipping the morning of your test.
Some medications can affect blood sugar readings, including certain blood pressure medications, corticosteroids, and even high-dose acetaminophen (Tylenol). If you take any of these regularly, your provider likely already knows, but it’s worth mentioning at your appointment if you’re concerned about an unexpected result. Don’t stop any prescribed medication before a test without asking your provider first.
Stress and poor sleep can also raise blood sugar temporarily. You can’t always control these, but if you had an unusually rough night, it’s worth noting when you discuss your results.
A Quick Reference by Test Type
- One-hour glucose screen: Eat a balanced, low-sugar breakfast with protein and complex carbs. Skip the donuts and juice.
- Fasting glucose test: No food for eight hours. Water is fine. Schedule it early and eat a normal dinner the night before.
- Three-hour OGTT: No food for eight hours. Eat at least 150 grams of carbs daily for the three days before, and make sure your last dinner includes a solid portion of carbohydrates.
The goal with any glucose test is to get an accurate reading, not the lowest possible number. Eating normally in the days before your test, then following the specific fasting or meal instructions for your test type, gives you the most reliable result.