Eating before and after a blood test

Eating before and after a blood test

The information presented here does not constitute medical advice, and is not intended to be a substitute for a consultation with your medical doctor. Always verify with your health test provider about the specific preparations needed for your urine test or blood draw.

One of the benefits of doing regular blood tests is to track the impact diet, supplements and lifestyle changes have on your health.

As a result, it is best to keep what you consume and how you live your life close to the usual in the weeks before your blood draw.

Nevertheless, there are a number of blood and urine tests that may require you to give up certain foods, or even fast (not eat or drink anything other than water) in the hours leading up to the test.

This article explores the types of blood and urine tests that require fasting, why fasting is needed, and what to do if you make a mistake.

Basic or comprehensive metabolic panel

Metabolic panels test for blood sugar, electrolyte balance, and kidney function.

Typically, you will be asked to fast for 10-12 hours before a comprehensive metabolic panel.

Vitamin B12 test

This test measures the levels of Vitamin B12 (Cobalamin) in the blood. Vitamin B12 plays a role in the normal functioning of the brain and nervous system, and the formation of red blood cells.

Typically, you will be asked to fast for 6-8 hours before a Vitamin B12 test.

Iron blood test

An iron blood test measures the levels of the mineral iron in the blood. This helps identify conditions that are caused by a lack of iron in the blood, such as anemia.

Iron is contained in some types of food and can be absorbed very quickly into the blood. Eating before the iron blood test can thus show inflated levels of iron.

Typically, you will be asked to fast from the morning of the iron test.

You should also avoid taking multivitamin or iron supplements for 24 hours before the iron blood test.

Fasting blood sugar test

A fasting blood glucose test measures the level of sugar in the blood, and can help diagnose diabetes.

Typically, you will be asked to fast for 8-10 hours before a fasting blood glucose test.

Some patient groups with diabetes, such as pregnant women, may be required to have their glucose levels checked both while fasting and after meals.

For random, timed, and post-meal glucose tests, follow your doctor's instructions.

Blood cholesterol tests

Blood cholesterol tests, also known as lipid profiles, assess the quantities of fats in the blood. Elevated cholesterol levels can lead to an increased risk of certain health conditions.

The amounts of these fats will increase if a person has recently eaten food.

Typically, you will be asked to fast for 9-12 hours before a cholesterol test.

New guidelines suggest that fasting is not necessary before cholesterol and triglyceride tests. However, you will still need to refrain from drinking alcohol for 24 hours before the test.

Liver function panel

A liver function panel includes several tests that help diagnose liver disease:

  • alanine transaminase (ALT)
  • aspartate aminotransferase (AST)
  • alkaline phosphatase (ALP)
  • gamma-glutamyl transferase (GGT)

Typically, you will be asked to refrain from alcohol or smoking for 24 hours before a liver function panel.

Renal function panel

The renal function panel tests the functioning of your kidneys.

Typically, you will be asked to fast for 8-12 hours before the test panel.

Other things to avoid

Chewing gum

Chewing gum, even if sugar-free, can speed up digestion, and should be avoided before a fasting test.


Exercise can speed up digestion and should be avoided before a fasting test.


Even if a blood tests requires fasting, it will usually only be for a maximum of 24 hours.

If you drink plenty of water, and eat a balanced meal prior to your fasting period and again after your blood sample has been taken, it shouldn’t be much of a challenge. In fact, it will be beneficial to your health independent of the lab test that follows.

Make sure to meet all preparation criteria from your laboratory or doctor, and if you make a mistake and consume food or supplements during your fasting period, make sure to let the test lab technician know as soon as you arrive for your blood or urine test.

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