Diet Soda and Fasting for Blood Tests

When you need a blood test, it's important that you don't do anything that could cause inaccuracies. Drinking diet soda with artificial sweetener before a blood test might skew your test results. Whether your tests are routine or for a particular condition, that's a risk you shouldn't take.

Fasting blood tests typically require a small sample from a vein.
Credit: prakobphoto/iStock/GettyImages

Why Is Fasting Required?

No one likes to have to go without food for an extended period of time and, hopefully, doctors request it only when it's absolutely necessary. Fasting before some types of blood tests is routine, because nutrients in food and beverages enter your bloodstream and may alter levels of the specific substance your test is designed to measure.

For example, according to Harvard Health Publishing, a blood glucose test may be used to detect diabetes or to monitor the effectiveness of treatments for people who have diabetes. Because foods are converted to glucose and enter the bloodstream, eating too close to the test could give false high results. Triglycerides are also measured after fasting because their levels remain elevated after a meal.

Diet Soda and Fasting

While fasting, you can still drink water and unsweetened coffee and tea because these do not affect blood sugar. But drinking diet sodas with artificial sweetener before a blood test can affect your results. Ingredients in these beverages vary widely as do their effects on blood sugar.

According to UC Davis, artificial sweeteners including acesulfame K, aspartame, saccharine, sucralose, stevia and neotame do not affect blood sugar.

Sugar alcohols provide some carbohydrate that will affect blood sugar. These include:

  • Isomalt
  • Lactitol
  • Maltitol
  • Mannitol
  • Sorbitol
  • Xylitol

However, just because some of the sweeteners on this list don't affect blood sugar doesn't mean you can drink diet sodas containing them before your test. The drinks may also contain other ingredients that can affect your blood sugar. If your doctor says nothing but water and unsweetened tea and coffee, it's a good idea to follow these orders so your test results are as accurate as possible.

Read more: Normal Sugar Glucose Levels in Men and Women

Other Testing Precautions

If you made a mistake and drank a diet soda during the time you were supposed to be fasting, give your doctor's office a call. Your doctor may recommend rescheduling the test just to be on the safe side.

According to U.S. National Library of Medicine, there are a few other things you shouldn't do in the eight to 12 hours before a fasting blood test:

  • Exercise
  • Chew gum
  • Smoke

In general, fasting blood tests are scheduled for first thing in the morning. So you have dinner the night before and then skip breakfast. If you smoke, abstain during this period. If you're a morning exerciser, skip your workout on the day of your test. And hold off on the gum chewing until afterwards. You can resume your regular habits immediately after the test.

You will likely be very hungry, so bring a snack with you to your appointment. Rather than grabbing a sugary coffee drink or sugar-laden granola bar on the way to work, pack a snack containing complex carbohydrates that digest and absorb slowly into your bloodstream. An apple and some nut butter, hummus and carrot sticks, oatmeal with fresh berries and hard-boiled eggs with whole-grain bread are all healthy choices that will help you regain your energy after fasting.

Read more: List of Good Carbohydrates to Eat

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