You have an annual physical on the calendar and a nagging question: should you skip breakfast? The honest answer is that the visit itself does not decide it. Whether you need to fast depends on whether your provider orders blood tests that require it. This guide explains how fasting works, which tests usually require it, and how to get a clear answer before the appointment so you do not arrive hungry for no reason, or arrive fed and have to reschedule.
This article is general information, not medical advice. Your provider's instructions always come first.
The short answer
An annual physical, also called a routine checkup or health checkup, is a preventive visit. It can include a physical exam, screening tests, vaccines and counseling about lifestyle. Screening tests are the part that may involve blood work, and it is the blood work, not the physical itself, that may require fasting.
MedlinePlus, the consumer health site of the U.S. National Library of Medicine, also gives a clear rule of thumb: you should not fast if your provider has not told you to. It notes that fasting for some blood tests may itself affect your results. If you are unsure, ask.
What "fasting" actually means
When a provider asks you to fast before a blood test, it means no food and no drinks other than plain water for several hours beforehand. According to MedlinePlus, you should also avoid chewing gum, smoking and exercising during the fast.
The usual window is 8 to 12 hours before the test. Your provider will tell you the exact length. For some tests you may be asked not to drink alcohol for a longer period.
MedlinePlus suggests scheduling the test early in the morning, so that most of your fasting time is spent sleeping.
Why fasting matters for some tests
Everything you eat and drink delivers sugar, fat, protein and other nutrients to your bloodstream. Those nutrients can change the levels of the substances a lab is trying to measure. Fasting keeps food and drink from skewing the results, so the test is more accurate.
Blood tests that commonly require fasting
MedlinePlus lists several common examples:
- Blood glucose (blood sugar) tests. These help diagnose and monitor diabetes and prediabetes. Not every glucose test requires fasting, so this is worth confirming.
- Cholesterol levels (lipid panel). These measure fats such as cholesterol and triglycerides in your blood and help estimate heart disease risk.
- Basic metabolic panel (BMP). A group of tests that gives information about how organs such as the heart, kidneys and liver are working.
MedlinePlus also lists liver function tests and a renal (kidney) function panel as tests you may sometimes be asked to fast for. For example, some liver function tests only require fasting if they are ordered as part of a comprehensive metabolic panel.
Whether your own tests require fasting is a question for your provider.
Parts of the visit that are not blood tests
Much of a checkup is not a lab test at all. The physical exam, vaccine updates and a conversation about your diet, exercise and other habits are part of what MedlinePlus describes as a health checkup, and none of them is a blood test. MedlinePlus says you should not fast unless your provider tells you to, so confirm with the office rather than assuming.
One related detail: MedlinePlus's blood pressure guidance says not to take a blood pressure reading if you have had a meal, caffeine or tobacco in the past 30 minutes. So it may be worth skipping coffee right before you walk in, even on a day when you are not fasting. If you are not sure how to time your morning, the office can tell you.
What you can and cannot drink while fasting
- Plain water is allowed. In fact, MedlinePlus notes that staying well hydrated can make the blood draw a little easier, because water helps keep more fluid in your veins.
- Coffee, juice and soda are not allowed. These can enter your bloodstream and affect results.
- Be careful with flavored water. Lemon-flavored or flavored carbonated water may contain sugars or artificial sweeteners, so plain water is the safer choice.
If you drink black coffee every morning, plan around it. Coffee is on the "no" list for a fasting test.
Medicines, vitamins and supplements
Ask your provider whether it is okay to take your usual prescription and over-the-counter medicines before the test. For some tests, your provider may ask you to stop a medicine, but MedlinePlus is clear that you should not stop taking your medicines unless your provider tells you to.
Also mention any vitamins and supplements you take. Some can affect test results, and your provider can tell you what to do about them.
What if you eat by mistake?
It happens: a handful of crackers, a sip of juice, a mint. MedlinePlus says to 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, so that your results are accurate.
A simple pre-visit checklist
- Ask when you book. Ask the office whether blood work is planned and whether it requires fasting. MedlinePlus suggests contacting your provider at least a full day before the test if you have questions, so there is time to prepare.
- Get the instructions in writing. MedlinePlus says your provider will give you a copy of the instructions if they ask you to fast. Keep it handy.
- Know your start time. If the fast is 8 to 12 hours, count back from your appointment and decide when your last meal should be.
- Schedule early if you can. A morning slot means most of the fast happens overnight.
- Plan for after. You can eat as soon as the test is done. Feeling light-headed after a fasting blood test is common, so a snack in your bag is a good idea.
- Bring your lists. MedlinePlus recommends bringing a list of questions and a list of the medicines, vitamins and supplements you take, and reviewing your family health history beforehand.
Questions worth asking your provider
- Are you ordering blood work at this visit?
- Which tests need a fast, and for how many hours?
- Can I take my regular medicines that morning?
- Are there tests I can have done without fasting?
- Who do I call if I accidentally eat or drink something?
Why it is worth keeping the appointment at all
It is tempting to skip a checkup when you feel fine. MedlinePlus points out that high blood pressure usually has no symptoms, so having it checked regularly is the way to find it, and that high blood sugar and high cholesterol may not have any symptoms in the early stages. A simple blood test can check for the latter two. Regular visits are also a chance to update vaccinations, talk about a healthy lifestyle and maintain a relationship with a provider in case you become ill.
MedlinePlus says checkup recommendations are based on sex and age, so talk with your provider about how often you should have checkups.
Key takeaways
- Fasting depends on the blood tests ordered, not on the fact that it is an annual physical.
- When required, fasting usually means 8 to 12 hours with nothing but plain water.
- Do not fast unless your provider tells you to.
- Do not stop your medicines unless your provider tells you to.
- If you eat or drink by mistake, tell your provider. You may need to reschedule so your results are accurate.
This article offers general health information and is not a substitute for professional medical advice, diagnosis or treatment. Follow the instructions your own provider gives you.
Sources
- MedlinePlus (U.S. National Library of Medicine): Fasting for a Blood Test. https://medlineplus.gov/lab-tests/fasting-for-a-blood-test/
- MedlinePlus: Health Checkup (topic page). https://medlineplus.gov/healthcheckup.html
- MedlinePlus Medical Encyclopedia: Physical exam frequency. https://medlineplus.gov/ency/article/002125.htm
- MedlinePlus Medical Encyclopedia: Blood pressure measurement. https://medlineplus.gov/ency/article/007490.htm