Is it better to fast before a glucose test

American Diabetes Association Professional Practice Committee. 2. Classification and diagnosis of diabetes: standards of medical care in diabetes-2022. Diabetes Care. 2022;45(Suppl 1):S17-S38. PMID: 34964875pubmed.ncbi.nlm.nih.gov/34964875/.

Committee on Practice Bulletins--Obstetrics. Practice Bulletin No. 190: Gestational diabetes mellitus. Obstet Gynecol. 2018;131(2):e49-e64. PMID: 29370047pubmed.ncbi.nlm.nih.gov/29370047/.

Landon MB, Catalano PM, Gabbe SG. Diabetes mellitus complicating pregnancy. In: Landon MB, Galan HL, Jauniaux ERM, et al, eds. Gabbe's Obstetrics: Normal and Problem Pregnancies. 8th ed. Philadelphia, PA: Elsevier; 2021:chap 45.

Metzger BE. Diabetes mellitus and pregnancy. In: Jameson JL, De Groot LJ, de Kretser DM, et al, eds. Endocrinology: Adult and Pediatric. 7th ed. Philadelphia, PA: Elsevier Saunders; 2016:chap 45.

Moore TR, Hauguel-De Mouzon S, Catalono P. Diabetes in pregnancy. In: Resnik R, Lockwood CJ, Moore TR, Greene MF, Copel JA, Silver RM, eds. Creasy and Resnik's Maternal-Fetal Medicine: Principles and Practice. 8th ed. Philadelphia, PA: Elsevier; 2019:chap 59.

Last reviewed on: 4/19/2022

Reviewed by: John D. Jacobson, MD, Department of Obstetrics and Gynecology, Loma Linda University School of Medicine, Loma Linda, CA. Also reviewed by David C. Dugdale, MD, Medical Director, Brenda Conaway, Editorial Director, and the A.D.A.M. Editorial team.

It depends on the type of blood test you're having. The healthcare professional arranging your test will tell you if you need to do anything to prepare for it.

You can eat and drink as normal before some blood tests. But if you're having a "fasting blood test", you will be told not to eat or drink anything (other than water) beforehand. You may also be told not to smoke before your test.

Page last reviewed: 25 February 2021
Next review due: 25 February 2024

You might wonder what you should eat and what to avoid before a glucose test so you don’t get an abnormal result.

We’ll cover the ins and outs of glucose tests in this article, including what to eat and avoid before you have one done.

What is a glucose test?

A glucose test is when the amount of glucose (sugar) in your bloodstream is measured using a blood sample. To perform a venous glucose test, a small amount of your blood will be collected into a vial to be tested. 

A finger stick test is another type of glucose test and only uses a small drop of blood. Finger stick glucose tests are what you do if you check your blood sugar using a glucometer at home.

In many cases, venous blood draws are preferred when performing glucose tests because they can run other lab tests on the same sample. Hemoglobin A1c tests (a 3-6 month blood sugar average) are also done with venous blood draws, not finger stick tests.

Who might get a glucose test?

Your healthcare provider will likely order a glucose test to screen you for blood sugar problems, such as type 1 diabetes, type 2 diabetes, and prediabetes.

A glucose test is standard during pregnancy to screen for gestational diabetes. Gestational diabetes is when you develop diabetes during pregnancy. It usually goes away after you deliver your baby.

Gestational diabetes screenings typically occur at the end of the second trimester or the beginning of the third trimester (24-28 weeks).

You might also have a glucose test if you’ve been experiencing symptoms of low blood sugar (hypoglycemia), such as fainting and dizziness. 

Types of glucose tests

There are several different types of glucose tests. Each glucose test provides unique information and is useful in different ways.

For example, if you’re trying to figure out how much insulin you need to inject before a meal, checking your blood sugar after eating can help assess how effective your current dose is. 

If your healthcare provider wants to determine if your prediabetes is progressing into diabetes, you’ll likely have a hemoglobin A1c test.

Here are some of the most common blood glucose tests:

Fasting

Your blood glucose levels are measured when you haven’t eaten or drank anything besides water for at least 8-12 hours. Fasting glucose tests are ideal for identifying insulin resistance and/or diagnosing diabetes since your diet doesn’t have as much of an impact on the result.

Random/non-fasting

Your blood glucose is checked randomly or when you haven’t been fasting. If your blood glucose is above 200 mg/dL at any given time, it is indicative of diabetes.

Post-prandial

Checking your blood sugar within 1-2 hours of eating a meal helps identify how your diet impacts your blood glucose.

Glucose tolerance test

After drinking a specific sugary drink (Glucola, etc.), your blood sugar levels are measured one- to three hours afterward to assess your glucose tolerance, or how well your body can lower your blood sugar.

Hemoglobin A1c test

This test measures the average percentage of your red blood cells that had a sugar molecule attached to them over the past 60-90 days. If you have diabetes, you should have an A1c test at least every six months, and more often if your blood sugar is uncontrolled.

How a glucose test works

A glucose test measures the amount of glucose in your bloodstream at a given time. There is always glucose present in your blood, but it becomes problematic if they are consistently too high or too low.

A normal fasting blood sugar level for people without diabetes is less than 100 mg/dL and less than 140 mg/dL within two hours of starting a meal.

For people with diabetes, fasting blood glucose should be within 80-130 mg/dL and less than 180 mg/dL within two hours of eating a meal, though many people might have stricter post-meal blood sugar goals than that.

What to eat before a glucose test

If your healthcare provider has ordered a fasting blood sugar test, you shouldn’t eat or drink anything besides water for at least eight hours before the test. 

Otherwise, you might consider eating some of the following foods before a glucose test:

Protein-rich foods

Protein is a type of macronutrient along with carbohydrates and fat. Unlike carbohydrates, protein doesn’t significantly impact blood glucose levels.

Some examples of protein-rich foods are:

• Meat (beef, poultry, fish, etc.)
• Eggs
• Cheese
• Nuts/nut butter
• Tofu

Non-starchy vegetables

Non-starchy vegetables are very low in carbohydrates, so they don’t raise your blood sugar levels very much. 

A few examples of non-starchy vegetables include:

• Artichoke
• Asparagus
• Beets
• Brussels sprouts
• Broccoli
• Cabbage (green, bok choy, Chinese)
• Carrots
• Cauliflower
• Green beans
• Mushrooms
• Peppers
• Salad greens/lettuce
• Tomatoes

Whole grains

Whole grains contain more fiber than refined grains (more on those in the next section). Fiber-rich foods can help prevent blood sugar spikes because fiber takes longer to digest, which means fiber-rich foods are converted to glucose more slowly.

Bear in mind that whole grains can still raise your blood glucose levels because they are a carbohydrate. They are a better choice than refined carbs, though.

Some examples of whole grains are:

• Whole wheat bread/crackers/pasta, etc.
• Oatmeal
• Brown rice

What you shouldn’t eat before a glucose test

If you’re not going to have a fasting blood glucose test, it’s fine if you eat beforehand. However, eating certain foods can cause a blood sugar spike and might alter the accuracy of your results. Try to stay away from the following foods before a glucose test:

Sugary drinks

Sugar-sweetened beverages like soda, sweetened tea, flavored coffee, and fruit-flavored drinks are one of the worst things to have before a blood glucose test. Sugary drinks are quickly emptied from your stomach and are converted into glucose in your bloodstream, which can lead to your blood sugar skyrocketing. 

Drinking sugar-free or “diet” drinks is a safer alternative, though water is the ideal choice to have before a glucose test.

Refined carbohydrates

Refined carbohydrates are lower in fiber and protein than whole grains, which makes them raise your blood sugar more quickly and significantly. 

Try to stay away from refined carbs like:

• White bread 
• White rice
• Any other grain made with refined grains (muffins, bagels, pasta, etc.)

Starchy vegetables

Unlike non-starchy vegetables, starchy vegetables contain significantly more carbohydrates that can raise your blood glucose. Avoid starchy veggies like potatoes and corn before a glucose test.

Sweetened foods/desserts

Sugary treats are among the foods to avoid in your diet before a glucose test. Added sugar is found in many processed foods, which contribute the majority of the added sugar in a typical American diet (1).

You’ll need to look at the nutrition facts and ingredient labels of processed foods to determine if it’s high in sugar/carbohydrates. 

Some of the more obvious sources of added sugar are in foods such as:

• Ice cream
• Cake/pies
• Cookies
• Sweetened cereals

Fruit juice

Even though the sugar in fruit juice is considered natural (as long as it’s 100% fruit juice), it can still raise your blood sugar quickly. Avoid fruit juices before a glucose test, or at least drink them several hours before your scheduled test to allow your body time to process the glucose. 

All types of popular fruit juices fall into this category, including:

• Apple juice
• Orange juice
• Cranberry juice
• Grape juice
• Grapefruit juice
• 100% juice blends

Other things to know before a glucose test 

Many institutions recommend eating at least 150 grams of carbohydrates per day for the three days preceding a blood glucose test and 50 grams of carbs with the last meal before a fasting test (2). If you temporarily eat a very low-carbohydrate diet, the results might not indicate your true glucose tolerance.

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How can I pass a glucose test?

The important thing to remember is that a glucose test is often meant to detect chronic health issues like type 2 diabetes. If you work hard to get a normal result (such as not eating even when the test is meant to be non-fasting, restricting carbs for the few days before the test, etc.), you might have a false negative result. 

While this might seem appealing, it can be dangerous if you have undiagnosed diabetes, increasing your risk of complications the longer it goes undiagnosed.

The best way to pass a glucose test is to consistently lead a healthy lifestyle. That means eating a healthy diet low in added sugars, being physically active, and taking care of your overall health.

If you’ve already adopted a healthy lifestyle, then following the tips listed above (in terms of what to eat and avoid) can help you pass a glucose test.

If you don’t pass the glucose screening

So, what happens if you don’t pass the glucose test? If you don’t pass a glucose screening, your healthcare provider will likely order more follow-up lab testing. A hemoglobin A1c is often the next test ordered after a failed fasting or non-fasting glucose test.

You’ll probably be encouraged to make lifestyle changes and/or take medications if you fail a glucose screening. If you’re diagnosed with diabetes for the first time, a visit with a Registered Dietitian and/or Diabetes Educator can help answer your questions about making lifestyle changes and following your care plan.

If you fail your one-hour glucose tolerance test during pregnancy, you’ll likely undergo a three-hour glucose tolerance test. This test involves drinking a sugary drink and having your glucose levels tested at 1-, 2-, and 3-hour intervals afterward. 

If you fail the 3-hour glucose test, you’ll be diagnosed with gestational diabetes and will receive guidance from your healthcare provider on how to treat it during pregnancy. Many women with gestational diabetes successfully manage it through diet, activity level, and checking their blood sugar levels at home.

Conclusion

Before a non-fasting glucose test, you should avoid high-sugar foods and drinks, as well as simple/refined carbohydrates. Good foods to eat before a glucose test include non-starchy vegetables, protein, and water.

Does fasting before glucose test help?

Is it better to fast before 1 hour glucose test?

Does fasting affect glucose test?

Can eating before glucose test affect results?