You need to fast for 8 to 12 hours before a fasting blood sugar test. That means no food or drinks other than water during that window. Most people schedule their blood draw first thing in the morning and stop eating after dinner the night before, which makes hitting that 8-to-12-hour range straightforward.
Why 8 to 12 Hours
After you eat, your blood sugar rises and your body releases insulin to bring it back down. For most people, insulin and blood glucose return to baseline within about two hours of a meal. The fasting window extends well beyond that two-hour mark to ensure your blood sugar reflects your body’s resting state, not something you recently ate.
The 8-hour minimum gives your body enough time to fully process your last meal and stabilize. The 12-hour upper limit exists because fasting too long can also affect your results. When you go without food for an extended period, your liver starts producing its own glucose to keep your brain and organs fueled. If you fast significantly beyond 12 hours, that process can push your reading higher than it would be under normal conditions, potentially giving a misleading result.
What You Can and Can’t Have
Water is always fine during the fasting period, and staying hydrated actually helps. Dehydration can make your veins harder to find for a blood draw and may slightly concentrate your blood, so sipping water throughout the evening and morning is a good idea.
Skip everything else: coffee (even black), tea, juice, soda, gum, and mints. Some of these contain sugars or other compounds that can interfere with results. If you’re unsure about a specific item, the safest approach is to stick with plain water only.
Medications and Supplements
Most prescribed medications are fine to take during your fast, but this isn’t universal. Some medicines and supplements can affect blood sugar readings. Ask your provider ahead of time whether you should take your usual medications on the morning of the test or hold off until after your blood is drawn. Don’t stop any prescription medication on your own without being told to do so.
Vitamins and supplements deserve the same attention. Let your provider know what you take so they can flag anything that might skew the results.
What the Results Mean
Fasting blood sugar is measured in milligrams per deciliter (mg/dL). The American Diabetes Association uses these cutoffs:
- Normal: below 100 mg/dL
- Prediabetes: 100 to 125 mg/dL
- Diabetes: 126 mg/dL or higher
A single reading of 126 mg/dL or above doesn’t automatically mean a diabetes diagnosis. Providers typically confirm with a second test on a different day or use an additional test, like a two-hour glucose tolerance test or an A1C, before making a formal diagnosis. Prediabetes results are worth paying attention to because they signal that your blood sugar regulation is trending in the wrong direction, often years before diabetes develops.
Fasting for a Glucose Tolerance Test
If your provider orders a glucose tolerance test instead of (or in addition to) a standard fasting blood sugar, the fasting rules change slightly depending on the type.
For a standard two-hour oral glucose tolerance test, you fast for at least 8 hours, have your blood drawn, then drink a sugary solution and sit for two hours while your blood is drawn again at intervals. A result below 140 mg/dL at the two-hour mark is normal, 140 to 199 mg/dL indicates prediabetes, and 200 mg/dL or higher points to diabetes.
Pregnancy screening works a bit differently. The initial gestational diabetes screen typically does not require fasting at all. You drink a solution containing 50 grams of sugar and have your blood drawn one hour later. If that result comes back elevated, a follow-up three-hour test is ordered, and that one does require an 8-hour fast beforehand. Some providers use a single-step two-hour test with 75 grams of sugar, which also requires fasting.
Practical Tips for Your Fast
The easiest strategy is to schedule your blood draw as early in the morning as possible. If your appointment is at 8 a.m., you can eat dinner at 8 p.m. the night before and comfortably hit the 12-hour mark. Eating a balanced dinner with protein, fat, and fiber (rather than a heavy carb-loaded meal) helps keep your blood sugar steady overnight.
Avoid intense exercise the morning of your test. A hard workout can temporarily change your blood sugar levels in either direction, which could affect accuracy. Light activity like walking is fine. If you accidentally ate or drank something during your fasting window, let the lab know before they draw your blood. They may suggest rescheduling rather than producing a result that could be misinterpreted.