There are several types of blood glucose, or blood sugar, tests. In general, they measure things like fasting blood sugar, average blood sugar over time and blood sugar in response to a sugary glucose drink.
Your doctor may advise you to get a blood glucose test if you have risk factors for diabetes, which can include:
- Being overweight
- Being physically inactive
- Having a family history of diabetes
- Being pregnant (around 7% of pregnant people develop gestational diabetes)¹
What you should (or shouldn’t) eat before a glucose test depends on the type of test.
The most important consideration is whether or not the glucose test requires you to fast in advance. Following this advice is crucial to ensure accurate results and an accurate diagnosis.
But even if your test doesn’t require fasting, how you eat in the weeks and months ahead of a glucose test can affect your results — and more importantly, your health!
Here’s what you should and shouldn’t eat when preparing for various glucose tests.
What are the different types of glucose tests?
Fasting glucose test
What is it? This is a standard screening test for diabetes that measures the level of sugar, or glucose, in your blood after an overnight fast.
Results of 99 milligrams per deciliter (mg/dL) are considered normal, 100-125 mg/dL suggests prediabetes, and 126 mg/dL suggests diabetes.²
Fasting required? Yes. Don’t eat or drink anything except small amounts of water eight to 12 hours before the test.
What is it? A hemoglobin A1C (HbA1c) test is another diabetes screening test that provides an average measure of blood glucose levels over the past two to three months. An HbA1c test is recommended for adults over 45 and anyone with risk factors for diabetes.
A result of below 5.7% is normal, 5.7-6.4% suggests prediabetes, and above 6.5% suggests diabetes.³
Fasting required? No. But HbA1c is often measured at the same time as fasting blood glucose, so you may end up fasting anyway — this is the case with imaware’s Prediabetes/Diabetes Screening Test.
Glucose screening test (or glucose challenge test)
What is it? This is a gestational diabetes screening test performed at 24 to 28 weeks of pregnancy. First, you drink a very sweet liquid, which contains 50 g of sugar. After an hour, your blood will be drawn and measured to see how well your body processes the sugar.⁴ ⁵
Levels above 140 mg/dL are considered high and require a subsequent oral glucose tolerance test to confirm or rule out gestational diabetes.⁶
Fasting required? No, but check with your doctor to make sure you’re doing the glucose screening test and not the oral glucose tolerance test (OGTT), which does require fasting. In some cases, like if you have a family history of diabetes, you may “skip” straight to the more stringent OGTT.
If indeed you’re taking the glucose challenge test, then you don’t need to fast or change your diet first.⁷ Dr. Ford Brewer is a preventive and occupational medicine specialist who says that there are some commonly-held — and incorrect — assumptions around eating in the days before a glucose challenge test. While some websites may advise adopting a low-carb or light diet leading up to the challenge, he notes that dietary changes take weeks or months to take effect. To put it simply, no matter what you may hear elsewhere, a low carb or low sugar diet won’t improve your test results, and it won’t create false high or low numbers either.
Oral glucose tolerance test (OGTT)
A three-hour oral glucose tolerance test, or OGTT, is required to diagnose gestational diabetes in pregnant individuals who have failed a glucose screening test. First, your fasting blood glucose is measured to establish a baseline; then, you drink a liquid with 100 g of sugar and your blood glucose is measured every 60 minutes for the next three hours.
Glucose levels above 180 mg/dL, 155 mg/dL, and 140 mg/dL, respectively, may suggest gestational diabetes.⁸
Glucose tolerance tests are also used in non-pregnant people to identify abnormalities in the body’s response to glucose, and they can be used to help diagnose prediabetes and type 2 diabetes more effectively than a fasting blood glucose test.⁹
Fasting required? Yes. Don’t eat or drink anything eight to 14 hours prior to the test, except for small amounts of water.¹⁰
Can I test for glucose at home?
Yes, there are several home diabetes tests and devices you can use to measure blood glucose levels. For example, diabetes screening test kits typically measure fasting blood glucose and HbA1c from a small blood sample obtained with a lancet. The sample is mailed to a lab, where results are interpreted by a physician.
Additionally, you can use a blood glucose monitor or continuous glucose monitor to measure real-time blood sugar levels throughout the day. Depending on the test or what you want to measure, you may or may not need to fast.
These are relatively accurate forms of blood sugar testing, but in-person tests are necessary to diagnose prediabetes, diabetes and gestational diabetes.
What to eat (and avoid) before a glucose test
It’s always a good idea to check in with your doctor before any screening test — exact instructions may vary and your physician can guide you on when to fast, and for how long.
Whatever the instructions, it’s important to follow them if you want accurate results.
The information above is often all the dietary advice you’ll get before taking a glucose test — and that’s because there’s not a whole lot you can do in a couple days to decrease your glucose levels in any significant way.
However, if you know you have a glucose test, then consistently loading your diet with healthy, nutrient-dense foods for several weeks or months in advance could improve your results.
What does this look like?
Ideally, you should be eating a balanced diet of fiber-rich complex carbohydrates (including veggies and whole grains) and lean proteins and healthy fats. Not only is this combo naturally low in sugar, but fiber, protein and fat all help buffer the rise in blood sugar you experience after a carb-containing meal,¹¹ keeping your glucose levels in check.
Foods to eat
- Non-starchy vegetables: Carrots, Brussels sprouts, asparagus, zucchini, eggplant, broccoli, snow peas, cucumbers, celery, spinach, kale and lettuce are good sources of fiber and nutrients, relatively low in carbs, and have a minimal impact on blood sugar. At most meals, consider filling half of your plate with non-starchy vegetables.¹²
- Starchy vegetables: Sweet potatoes, white potatoes, corn, green peas and beets are good sources of nutrition and fiber, but higher in carbohydrates than non-starchy veggies. Consider filing a quarter of your plate with starchy vegetables.¹³
- Whole grains: Quinoa, brown rice, oats, barley, whole grain pasta and whole wheat bread are complex carbohydrates and good sources of dietary fiber. Enjoy these in similar quantities to starchy vegetables.
- Fruits: Fruits with edible seeds and skins such as raspberries, blackberries, blueberries, strawberries, apples, pears and peaches are among the highest fiber options,¹⁴ making them a particularly healthy sweet treat.
- Legumes: Chickpeas, lentils, kidney beans, black beans and edamame are great sources of dietary fiber and protein.
- Seafood: Salmon, sardines, mackerel, tuna and other seafoods are low in carbohydrates, high in protein and often high in healthy fats like omega-3s.
- Lean meats: All meats are great sources of protein and low in carbs, but lean meats like skinless chicken, lean cuts of beef and pork and skinless turkey may be the best choices for curbing diabetes risk, as they contain fewer saturated fats.
- Healthy fats: Nuts, seeds, avocados, olives, olive oil and avocado oil are all beneficial sources of healthy, unsaturated fats.
Foods to avoid
- Sugary foods (and drinks): Things like ice cream, cake, candy, cookies, donuts and sugary drinks such as soda, sports drinks, energy drinks, sugary cocktails, fruit juice, and even all-fruit smoothies contain high concentrations of sugar that can spike blood sugar levels. Consume these in small quantities within the context of a healthy diet.
- Refined grains (or refined carbs): White bread, sugary breakfast cereals, crackers, white pasta, white rice, corn grits and white flour are all considered refined grain products. This means they’ve been processed so the whole grain is no longer intact, and typically, this type of processing removes fiber, vitamins and minerals.¹⁵ With less fiber, these foods are more likely to spike blood sugar.
What happens if I fail a glucose test?
If your blood sugar falls outside the healthy or ideal target range, your doctor may be able to tell you if you have prediabetes, diabetes, or gestational diabetes.
Depending on the glucose test you’ve taken, you may need additional testing to confirm these results. For example, if your glucose screening test during pregnancy is elevated, you will go back for an oral glucose tolerance test to confirm or rule out gestational diabetes.¹⁶
Keep in mind, gestational diabetes usually goes away after pregnancy. However, having it may increase your future risk of developing type 2 diabetes,¹⁷ so adopting healthy dietary and lifestyle practices to keep blood glucose levels in check is extra important.
Your doctor will advise you on how to manage your blood glucose levels, potentially through medication, dietary interventions, lifestyle changes, or a combination of all three. It can be motivating to know that it may be possible to reverse prediabetes or diabetes.
The first thing you need to do is check if you should be fasting before your glucose test. If you have several weeks or months before your glucose test, the best strategy for improving your results is to consistently eat balanced meals featuring complex carbs, lean proteins and healthy fats.