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What Is Fasting and Do I Need to Fast?

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When your healthcare provider orders certain tests, you may be told to fast for several hours before the test or overnight. Here’s what you need to know to comply with your doctor’s instructions.

What Is Fasting?

Fasting is when you consume no food or drinks, with the exception of water, for a set period of time. It’s important to drink plenty of water, not just because you’re not eating, but because it’s easier to draw blood from your veins when you’re well hydrated.

Why Fasting Is Necessary?

The nutrients in the food and beverages you consume are absorbed into your blood stream and could impact factors measured by certain tests. Tests to determine your lipid levels (cholesterol, triglycerides and HDL) usually require fasting. You also will be told to fast before glucose testing. Other tests may also require fasting. If you don’t fast, or fast for a shorter time than prescribed, your tests could give inaccurate results, meaning you’ll likely have to repeat the test. If you think fasting is going to be a problem for you, please discuss it with your healthcare provider.

How Long Must I Fast?

Generally, before a lipid or a glucose tolerance test, you’ll be told to fast for eight hours. However, your healthcare provider may advise you differently. Always follow his/her instructions to the letter.

Continue to Take Medications

Be sure to take your usual medications unless told otherwise by your healthcare provider. If you’re taking vitamin/mineral supplements, ask whether you should continue to take those as well.

Breaking Your Fast

As soon as you’ve had your blood drawn, you’ll probably want to eat and drink something. It’s a good idea to bring along a snack so you can do so as soon as possible. Schedule your blood test for the early morning to minimize the length of time you’ll have to go without food.