What to eat before a glucose test depends on which test is being performed. Fasting glucose and the oral glucose tolerance test (OGTT) require 8 to 12 hours of water-only fasting; the 1-hour gestational challenge does not require fasting; and A1C requires no preparation at all. The most common pre-test mistake is over-restricting carbohydrate in the days before an OGTT, which can produce a falsely high result.
Fasting Rules by Test Type
| Test | Fasting Required | Hours | Day-Before Diet |
|---|---|---|---|
| Fasting plasma glucose | Yes | 8 to 12 hr | Normal eating |
| Oral glucose tolerance test (75 g) | Yes | 8 to 12 hr | Normal carbs (150+ g/day for 3 days) |
| Gestational 1-hr glucose challenge | No | — | Normal eating |
| Gestational 3-hr OGTT (after failed 1-hr) | Yes | 8 to 12 hr | Normal carbs (150+ g/day for 3 days) |
| Hemoglobin A1C | No | — | Normal eating |
| Random plasma glucose | No | — | Note last meal time |
The Day Before
For fasting glucose and any OGTT, the goal the day before is to eat normally. That specifically means:
- Continue your usual carbohydrate intake. Aim for at least 150 grams of carbohydrate per day for the 3 days before an OGTT.
- Eat dinner at your usual time, then begin the fast 8 to 12 hours before your scheduled draw.
- Avoid alcohol — even modest amounts the evening before can affect liver glucose handling.
- Avoid heavy or unusually high-fat meals that may slow gastric emptying and skew morning fasting glucose.
- Stay well hydrated; you can drink water freely until the draw.
The Morning Of
During the fasting window before fasting glucose or an OGTT:
- Water is allowed. Coffee, tea, juice, soda, sports drinks, flavored water, and energy drinks are not.
- No food, including mints, cough drops, and gum.
- No smoking or vaping (nicotine raises glucose).
- No vigorous exercise the morning of the test.
- Take routine medications only if your clinician has told you to (some diabetes medications are held the morning of testing — confirm with the ordering clinician).
Why Carbohydrate Loading Matters Before an OGTT
This is the most-missed rule. People preparing for an OGTT often try to “do well” by eating low-carb in the days before, assuming a clean diet will give a better result. The opposite happens. When you have been eating low-carb, your pancreas downregulates the rapid first-phase insulin response. When you then drink the 75 gram glucose load, the delayed insulin response causes glucose to rise higher and stay elevated, mimicking impaired glucose tolerance even in someone with normal metabolic health.
The American Diabetes Association recommends at least 150 grams of carbohydrate per day for 3 days before an OGTT. Examples that easily reach 150 grams include normal portions of bread, rice, pasta, fruit, dairy, and starchy vegetables.
What to Bring
- Photo ID and insurance card.
- The lab order or e-order reference.
- A bottle of water.
- A snack with carbohydrate and protein for after the final draw — especially after a 2-hour or 3-hour OGTT, since the glucose load can leave you shaky as it clears.
- A book or device for the wait between draws (an OGTT visit can take 2 to 3 hours).
The Glucose Drink Itself
The OGTT begins with a fasting blood draw, after which you drink a measured glucose solution (75 grams for non-pregnant adults, 50 grams for the gestational 1-hour challenge, 100 grams for the gestational 3-hour test in some protocols). The drink is sweet, syrupy, and best swallowed within 5 minutes. Vomiting after the drink invalidates the test and requires rescheduling. If you are nervous about tolerating the drink, sit down for a few minutes before swallowing and sip cold water between gulps.
What Can Skew Results
Even with proper fasting and carb intake, several factors can shift glucose readings:
- Acute illness or infection: Reschedule if you are sick.
- Recent surgery or trauma: Wait until you are recovered.
- Steroid medications: Discuss timing with your clinician.
- Severe stress or sleep deprivation.
- Long fast (over 14 hours): Can paradoxically raise fasting glucose through counter-regulatory hormones.
If any of these apply, reschedule the test rather than risk an uninterpretable result. Background context is in our detection of prediabetes guide.
What “Normal” Looks Like
For reference, after appropriate prep, the standard result thresholds are:
- Fasting glucose below 100 mg/dL is normal; 100 to 125 mg/dL is impaired (prediabetes); 126 mg/dL or higher meets diabetes criteria when confirmed.
- 2-hour OGTT below 140 mg/dL is normal; 140 to 199 mg/dL is impaired; 200 mg/dL or higher meets diabetes criteria.
For fuller context on these cutoffs and what comes after a borderline reading, see our A1C levels overview.
Common Questions
- Can I brush my teeth? Yes, just do not swallow toothpaste.
- Can I take my blood pressure pill? Most non-diabetes medications can be taken with water; confirm with your clinician for medications that affect glucose.
- What if I accidentally ate something? Reschedule. A non-fasting “fasting” draw is not interpretable.
- Can I chew sugar-free gum? No. Even sugar-free gum can stimulate digestive responses that affect the result.
The Bottom Line
Eat normally the day before — including normal carbohydrate intake — and fast on water only for 8 to 12 hours before fasting glucose or an OGTT. Avoid coffee, gum, alcohol, exercise, and tobacco the morning of the test. The single most preventable mistake is going low-carb in the days before, which can falsely flag impaired glucose tolerance. When in doubt, ask the lab or your clinician before the appointment.
