Fasting Before Blood Work: What Actually Requires It (and What Doesn't)
Fasting is over-prescribed for blood work. Here's a clear breakdown of what actually needs an empty stomach, what doesn't, and how to prepare without guessing.
Why some blood tests require fasting
Food raises your blood sugar within minutes and your blood fats (triglycerides) for several hours after eating. For tests that measure those things directly, eating shortly before the draw would give a false picture of your baseline levels.
For the vast majority of other tests — thyroid, ferritin, kidney, liver, most hormones, most vitamins — recent food does not meaningfully change the result.
Which tests require fasting
Fasting glucose — 8 hours minimum.
Fasting insulin and HOMA-IR — 10–12 hours.
Full lipid panel with triglycerides — 8–12 hours.
Some specialty metabolic tests — check with your clinician.
HbA1c (long-term blood sugar), TSH, ferritin, vitamin D, vitamin B12, CBC, liver, kidney and most hormones do NOT require fasting.
What you can drink and take while fasting
Water is fine — being well hydrated actually makes the blood draw easier. Black coffee or plain tea (no milk, no sugar, no sweetener) is generally accepted, though very heavy coffee drinkers should moderate.
Prescribed medications should almost always be continued on schedule unless a clinician has told you otherwise. If you're diabetic and on insulin or sulfonylureas, ask the clinic before adjusting anything to avoid hypoglycaemia.
Frequently asked questions
What if I forgot and had breakfast?+
For fasting-only tests, we'll usually reschedule that portion. For non-fasting tests we can still proceed. Just tell us — an honest disclosure protects the accuracy of your results.
Can I chew gum while fasting?+
Best avoided. Sugar-free gum still triggers digestive responses that can affect glucose and insulin.