Coffee Before a Glucose Test? When It’s Ok to Drink Coffee!

Posted April 16, 2021 by Michael Chamberlain - See Editorial Guidelines

Many factors can affect a glucose test, much of which are beyond the control of what we eat and drink. But in order to get the most accurate test, we must follow some strict guidelines. Coffee before a glucose test is something we can advise on regarding when it is and isn’t ok, and the reason behind them.

But first, here’s a quick summary to get us started, then we’ll get into the details.

Coffee before glucose test? Coffee is NOT recommended before a fasting glucose test. Coffee interferes with blood test results as it contains caffeine and soluble plant matter. Coffee is also a natural diuretic resulting in difficulty finding a vein. However, it’s ok to drink coffee before a random glucose test.

With the summary in mind, we can start by investigating what a glucose test is to more easily help you understand whether or not you’re able to drink a cup of coffee before. 

What is a glucose test? 

Coffee Before a Glucose Test

A blood glucose test measures the levels the sugar in your blood.

The hormone insulin helps to transport glucose from your bloodstream into your cells. Too much or too little glucose in the blood can be a sign of a serious medical condition.

High blood glucose levels, also called hyperglycemia, may be a sign of diabetes.

Diabetes is a disorder that can cause heart disease, blindness, kidney failure, and other complications. 

Low blood glucose levels or hypoglycemia can also lead to major health problems, including brain damage, if not treated right away.

There are two types of glucose tests:

  • Fasting glucose test 
  • Random glucose test 

Fasting tests are more common because they provide more accurate results and are easier to interpret. Some of the other terms you will encounter with blood sugar testing are: 

  • SMBG – Self-monitoring of blood glucose 
  • FPG – Fasting plasma glucose 
  • FBS Fasting blood sugar 
  • GCT Glucose challenge test 
  • OGTT Oral glucose tolerance test

Here are the general guidelines to follow prior to a glucose test.

Fast before your glucose test. Do NOT eat or drink anything except plain water for 8 hours prior to the test. Do NOT drink coffee, tea, any soda, or other beverages. Do NOT smoke, do NOT chew gum, or exercise.

Why do I need a glucose test? 

Your health care provider or clinician may decide you need a blood glucose test if you have symptoms of high glucose levels, which can indicate diabetes.

Symptoms of high blood glucose levels include:

  • Feeling extra thirsty 
  • Blurred vision
  • More frequent urination
  • Fatigue
  • Wounds that are slow to heal

You may also need a blood glucose test if you have certain risk factors for diabetes. These include:

  • Being overweight
  • A family history of diabetes
  • High blood pressure
  • Lack of exercise
  • Heart disease

Now that you know what a glucose test is and why you might need it, let’s discover how coffee affects your body and, more importantly, your glucose levels and whether it’s safe to drink coffee before a glucose test. 

How coffee affects your glucose levels 

Caffeine raises levels of certain stress hormones, like epinephrine, which we commonly call adrenaline. 

Epinephrine can prevent your cells from processing as much sugar. It may also keep your body from making as much insulin.

Coffee is also a diuretic, which means that it will increase how much you visit the bathroom. This can cause you to become dehydrated. 

The less hydrated you are, the more concentrated the sugar in your blood becomes, increasing your blood sugar level.

This means that you may have an inaccurate reading.

Coffee before a glucose test can affect the results, especially if you add cream and sugar. Added sugar and fat from the cream can make your test results inaccurate.

Can I drink black coffee before a glucose test? 

Not every type of blood test requires you to fast beforehand. But certain common blood tests do require fasting. These include fasting glucose and sometimes triglyceride tests.

Even if you drink it black, coffee can interfere with fasting glucose test blood results. This is because it contains caffeine and soluble plant matter, which might affect your test results.

Coffee Before a Glucose Test

However, most doctors state that it is okay to drink black coffee before a glucose test.

Can I drink coffee before a gestational diabetes test? 

If you have 1 or more risk factors for gestational diabetes, you will be offered a screening test.

The screening test is called an oral glucose tolerance test or OGTT, which takes about 2 hours. It involves having a blood test in the morning when you have not had any food or drink for 8 to 10 hours. 

It’s recommended you don’t drink coffee, but plain water only. However, do check with your doctor. 

According to a recent study by experts on caffeine and insulin resistance, some important results were discovered. 

It was found that in the third trimester of pregnancy, any coffee that is drunk before a 2-hour glucose tolerance test decreased the insulin sensitivity index by 18% in women with gestational diabetes

The above-mentioned study on the effects of caffeine found that caffeine intake is associated with acute impaired glucose tolerance, secondary to increased insulin resistance.

The insulin resistance that occurs after acute caffeine ingestion may be more pronounced in pre-diabetic, non-pregnant adults. 

Other studies have also demonstrated increased insulin resistance after caffeine administration in obese and diabetic adults.

Here’s more information on how to combat insulin resistance.

What else should I avoid before taking a glucose test? 

It’s recommended by health professionals that you don’t eat or drink anything except for water for at least 8 hours before the test. 

Also, make sure you avoid: 

  • Tea 
  • Diet or regular soda 
  • Smoking 
  • Regular or sugar-free gum 
  • Exercise 

You may want to schedule a fasting glucose test first thing in the morning, so you don’t have to fast during the day.

What happens if I do drink coffee before my glucose test? 

If you don’t fast before a glucose test, the results may not be accurate. 

If you forget and eat or drink something, call your provider and check whether the test is still ok to be carried out.

Some tests can be analyzed with a note that it was performed when the patient wasn’t fasting. However, this may give variable results.

Always be honest if you’ve had any food or beverage other than water, even if you did have a cup of coffee before your glucose test, or a snack, or even a meal. 

The important piece of advice is to tell the technician who is performing the test. They should then make a note of this point to review the results with the food intake as a variable.

If, however, fasting is an absolute must for meaningful results, they will reschedule the test.

Final thoughts

We hope this has answered your questions regarding coffee before a glucose test. As always, consult with your medical practitioner if you’re in any doubt or have further questions.