What can you eat or drink when intermittent fasting, and what breaks a fast

Alternate-day fasting is where you eat about 25% of your normal calories, typically no more than 500, every other day.

Fasting for a Blood Test

If your health care provider has told you to fast before a blood test, it means you should not eat or drink anything, except water, for several hours before your test. While you’re fasting for a blood test, you also should not:

How long do I have to fast before the test?

You usually need to fast for 8 to 12 hours before a test, but your provider will tell you exactly how long to fast. For certain tests, you may be asked not to drink alcohol for a longer period of time.

If your blood test requires fasting, you can usually schedule the test early in the morning. That way, most of your fasting time will be while you are sleeping.

Why do I need to fast before my blood test?

When you eat and drink, your bloodstream absorbs sugar, fat, protein, and other nutrients in those foods and beverages. That could affect the results of certain blood tests and lead to the wrong diagnosis.

What types of blood tests require fasting?

Some examples of common blood tests that usually require fasting are:

• Blood sugar (glucose) tests. These tests measure the level of blood glucose in a sample of your blood. They are used to help diagnose and monitor diabetes, prediabetes, and gestational diabetes (diabetes that starts during pregnancy). Not all blood glucose tests require fasting, so ask your provider if you need to fast for your test.
• Cholesterol levels tests (lipid panel). These tests measure the level of fats, called lipids, in your bloodstream, including cholesterol and triglycerides. The test results help show your risk of getting heart disease.
• Basic metabolic panel (BMP). This is a group of tests that measures several natural chemicals in your blood. The results give your provider information about how well your organs are working, including your heart, kidneys, and liver.

Other types of tests may require fasting, so always ask your provider if you aren’t sure whether you should fast before your test.

Can I drink anything besides water during a fast?

No. Juice, coffee, soda, and other beverages can get in your bloodstream and affect your results. But you can drink water. It’s good to drink water before a blood test. It helps keep more fluid in your veins, which can make it easier to get a sample of your blood.

Can I continue taking medicine during a fast?

Ask your provider whether it’s okay to take your usual prescription medicines and/or over-the-counter medicines. But don’t stop taking your medicines unless your provider tells you to stop.

Let your provider know about any vitamins and supplements that you take, too. That way you’ll be sure to avoid taking anything that could affect your test results.

What if I make a mistake and have something to eat or drink besides water during my fast?

Tell your provider if you eat or drink before your test. You may need to reschedule the test for another time when you can fast to make sure you get accurate results.

When can I eat and drink normally again?

You’ll be able to eat as soon as your test is over. You may want to bring a snack with you, so you can eat right away.

Is there anything else I need to know about fasting before a blood test?

If you don’t know whether you should fast for your blood test, ask your provider at least a day before your test so you’ll have time enough time to fast.

If you fast before a test when you don’t need to, that may affect your results, too. That’s why it’s important to find out for sure whether you need to fast.

Always ask your provider any questions you have about how to prepare for your test. Taking the right steps before a blood test helps make sure your results will be accurate.

References

1. Allina Health [Internet]. Minneapolis: Allina Health; Fasting for a Blood Test; [cited 2022 Jul 14]; [about 1 screen]. Available from: https://www.allinahealth.org/-/media/allina-health/files/15008fastingpt.pdf
2. Centers for Disease Control and Prevention [Internet]. Atlanta: U.S. Department of Health and Human Services; Testing for Diabetes; [updated 2021 Aug 10; cited 2022 Jul 14]; [about 3 screens]. Available from: https://www.cdc.gov/diabetes/diabetes-testing/
3. Cleveland Clinic: Health Essentials/Family Medicine [Internet]. Cleveland (OH): Cleveland Clinic; c2022.What You Should Know About Fasting Before a Blood Test; 2022 Feb 16[cited 2022 Jul 25]; [about 6 screens]. Available from: https://health.clevelandclinic.org/fasting-before-blood-test/
4. DeWaters AL, Mejia D, Thomas J, Elwood B, Bowen ME. Patient Preparation for Outpatient Blood Work and the Impact of Surreptitious Fasting on Diagnoses of Diabetes and Prediabetes. Mayo Clin Proc Innov Qual Outcomes. 2020 Jun 20 [cited 2022 Jul 25];4(4):349-356. doi: 10.1016/j.mayocpiqo.2020.04.002. PMID: 32793862; PMCID: PMC7411170. Available from: https://www.ncbi.nlm.nih.gov/pmc/articles/PMC7411170/
5. National Heart, Lung, and Blood Institute [Internet]. Bethesda (MD): U.S. Department of Health and Human Services; Blood Tests; [updated 2022 Mar 24; cited 2022 Jul 25]; [about 8 screens]. Available from: https://www.nhlbi.nih.gov/health-topics/blood-tests
6. Quest Diagnostics [Internet]. Quest Diagnostics; c2022. Fasting for Lab Tests; [cited 2022 Jul 14]; [about 3 screens]. Available from: https://www.questdiagnostics.com/patients/get-tested/prepare/fasting
7. Testing.com [Internet]. Seattle (WA): OneCare Media; c2022. Lipid Panel; [modified 2021 Nov 9; cited 2022 Jul 14]; [about 5 screens]. Available from: https://www.testing.com/tests/lipid-panel/
8. Testing.com [Internet]. Seattle (WA): OneCare Media; c2022. Test Preparation: Your Role; [modified 2021 Jun 6; cited 2022 Jul 14]; [about 14 screens]. Available from: https://www.testing.com/articles/laboratory-test-preparation/
9. University of Rochester Medical Center [Internet]. Rochester (NY): University of Rochester Medical Center; c2022. Health Encyclopedia: Cholesterol in the Blood; [cited 2022 Jul 14]; [about 6 screens]. Available from: https://www.urmc.rochester.edu/encyclopedia/content.aspx?contenttypeid=85&contentid=P00220

Related Medical Tests

• Basic Metabolic Panel (BMP)
• Blood Glucose Test
• Cholesterol Levels
• How to Cope with Medical Test Anxiety
• How to Prepare for a Lab Test
• How to Prepare Your Child for a Lab Test
• How to Understand Your Lab Results
• Lipase Tests
• Triglycerides Test
• What You Need to Know About Blood Testing

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What can you eat or drink when intermittent fasting, and what breaks a fast?

Intermittent fasting involves alternating between periods of eating and fasting. But is all food and drink off limits during a fast? What about your morning cup of coffee or tea? And what exactly breaks a fast?

There are several different types of intermittent fasting (IF), and what’s allowed during a fast depends on which method you choose and what your goals are.

Generally speaking, while fasting with time-restricted intermittent eating, you don’t eat any food at all and only drink beverages with very few calories, such as water or unsweetened coffee and tea without milk.

When you’re not fasting, it’s important to eat a healthy and varied diet that’s good for your body.

Many people follow IF because they want to improve their health. During an extended fast, your body switches from burning sugar to burning fat for energy, and some scientists believe this may be beneficial.

Our studies show that how you respond to the foods you eat is unique to you. What works for another person isn’t necessarily the right approach for you.

With the ZOE program, you get personalized nutrition advice to help you find the best foods for your body.

Take our free quiz to learn more about how you can achieve your long-term health goals, and read on to discover what you can eat or drink while intermittent fasting.

Types of intermittent fasting

There are several different approaches to IF, but they all focus on when you eat rather than what you eat.

Time-restricted eating involves alternating between eating and fasting windows. This is the most common approach, since it simply extends the period of fasting that you already go through when you sleep.

With 16/8 time-restricted eating, you can eat what you choose during an 8-hour window and fast for the other 16 hours. For example, you could eat between 9 a.m. and 5 p.m. and then fast from 5 p.m. until 9 a.m. the next day.

You can repeat this cycle daily or as often as you’d like throughout the week. There are also 18/6 and 20/4 versions of time-restricted eating.

In a survey of ZOE participants who also do IF:

• 46% said they practiced the 16/8 method
• 21% followed an 18/6 pattern
• 8% followed a 20/4 method, which involves a 20-hour fasting window

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Other forms of IF include twice-a-week fasting and alternate-day fasting, known as the 5:2 method.

With the 5:2 method, you generally eat no more than 500–800 calories on 2 days of the week. The days you fast are up to you, but you should keep at least a single eating day between them.

Alternate-day fasting is where you eat about 25% of your normal calories, typically no more than 500, every other day.

On non-fasting days with these methods, you eat a balanced, healthy diet.

What can you eat or drink during fasting periods?

Do you really have to avoid all food and drink during your fasting windows? It depends on which method you’re following.

With alternate-day fasting and the 5:2 method, you consume a reduced number of calories on fasting days.

If you follow time-restricted eating, the fasting period generally means consuming no food and only certain kinds of drink.

Here is why:

Your body breaks down the food you eat into sugar, or glucose, for energy. When food — and therefore glucose — is absent for an extended period of time, your body instead breaks down fat to release another energy source called ketones.

This is called the metabolic switch, and research suggests that alternating between using glucose and ketones for energy — or eating and fasting — could be beneficial.

Technically speaking, eating or drinking any calories breaks a fast. That means it’s generally considered OK to drink black, unsweetened coffee or tea, as well as water.

However, some of the potential benefits of fasting periods may have as much to do with avoiding certain metabolic responses, such as a rise in blood sugar levels. So what does that mean when it comes to adding milk to your morning beverage?

While everyone responds to food differently, ZOE’s data suggest that, for most people, the amount of milk you have in your tea or coffee is not enough to lead to blood sugar spikes.

Avoid artificial sweeteners if you’re in the fasting period, as some research suggests that they can affect your blood sugar levels.

What should you eat while following IF?

There are technically no restrictions on what foods you can eat when you’re not fasting.

Based on our research, we know that food quality is important to ensure a healthy gut, stabilize blood sugar and fat responses, and avoid dietary inflammation.

Eating a healthy, varied diet is great for your overall health. Incorporating a wide range of plants in your day will feed the beneficial bacteria that live in your gut and help to make sure your body gets the nutrients it needs.

High-quality sources of fiber and protein can help you feel fuller longer.

• whole grains
• fruits
• vegetables
• lean protein, such as poultry, fish, nuts, seeds, and legumes like lentils and peas

What you choose to eat can also depend on what you’re trying to achieve through intermittent fasting.

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Understand how your body responds to food with ZOE

Eating for heart health

For example, if your aim is to improve your heart health, consider including plenty of healthy fats and fiber, and monitor your salt intake. Some options include:

• vegetable oils such as canola, olive, or sunflower
• brown rice, whole wheat bread, and other whole grains
• whole fruits and vegetables
• fish rich in omega-3 fatty acids and other protein sources such as beans and legumes
• salt-free seasonings such as herbs and spices

Eating to lower inflammation

If your goal is to lower inflammation, focus on controlling unhealthy blood fat and sugar responses by choosing:

• whole foods that contain fiber, such as whole grains, fruit, and vegetables
• healthy omega-3 fats, such as fish, nuts, and seeds
• foods with anti-inflammatory properties that are high in bioactive molecules, such as polyphenols, found in colorful fruits and vegetables

ZOE’s PREDICT program, the largest nutrition study of its kind in the world, looks at how the foods that you eat can cause inflammation in your body.

Following on from our studies, the ZOE at-home test analyzes your blood sugar and blood fat responses along with the bacteria that are in your gut and uses the latest science to help you find the best foods for your body.

Take a free quiz to find out what ZOE’s personalized nutrition program can do for you.

Summary

Intermittent fasting is an approach to eating that focuses on when you eat rather than on what you eat.

There are different types of IF, but time-restricted eating, specifically the 16/8 method, is the most popular.

With time-restricted intermittent eating, you don’t eat any food while fasting and only consume drinks with very few calories, such as water or black, unsweetened coffee and tea.

During the periods when you do eat, try to follow a healthy diet rich in whole grains, fruits, vegetables, and lean protein.

ZOE’s research shows that there is no one-size-fits all way of eating. Personalized nutrition advice based on your unique metabolism and gut microbiome can help you find the foods that work best for you and your long-term health goals.

Find out more about the ZOE program.