How to Prepare for a PET Scan (Diet, Diabetes, Exercise)

Quick answer: for the most common PET scan (FDG PET-CT), you usually need to fast for 4 to 6 hours, drinking only plain water; many centers also ask for a low-carbohydrate, high-protein diet for 24 hours before. Avoid strenuous exercise for 24 hours, stay warm, and arrive on time. If you have diabetes, your blood sugar must be in range on the day (often under 200 mg/dL), so you will get a specific plan for your medications. Always follow your own center’s instructions; they vary.

In PET, preparation is not a formality. I have had to reschedule scans because of a sugary coffee on the way in, a morning run or a high blood sugar reading. Here is why each step matters and how to get it right.

Why preparation matters for PET

The most common PET tracer, FDG, is a form of glucose (sugar). It goes wherever the body is using sugar: active tumors, but also muscles you have exercised, brown fat when you are cold, and the whole body when blood sugar and insulin are high after a meal. Good preparation keeps the tracer where it gives useful information, so the radiologist can see what matters. Poor preparation can make the scan hard to read, and it may have to be repeated.

The day before

Diet. Many centers ask you to follow a low-carbohydrate, high-protein diet for 24 hours before the scan.

Generally fine	Generally avoid
Meat, poultry, fish and seafood (not breaded or fried)	Sugar, sweets, candy, desserts
Eggs, cheese, nuts	Bread, pasta, rice, cereals, potatoes
Low-carb vegetables: lettuce, spinach, broccoli, cauliflower, green beans, mushrooms, peppers	Fruit and fruit juice, starchy vegetables, beans
Butter, oil, mayonnaise	Soda, sweetened drinks, alcohol
Water	Some centers also ask you to avoid caffeine

Exercise. Avoid strenuous exercise and physical activity for 24 hours before the scan: running, weightlifting, cycling, yoga, heavy yard work. Exercised muscles take up tracer and can hide or mimic disease.

Stay warm. Cold activates brown fat, which also takes up tracer. Many centers ask you to stay warm for 24 hours before, and European guidelines advise avoiding cold in the hour before the scan.

Water. Drink plenty of water, unless your doctor has told you to limit fluids.

The day of the scan

Fast for 4 to 6 hours, or as your center instructs. That includes gum, breath mints and cough drops.
Drink only plain water. Water is encouraged: it helps clear tracer that is not needed and improves image quality. Do not drink coffee with sugar or milk, juice or soda.
Medications: unless told otherwise, take your usual non-diabetes medications with water.
Wear warm, comfortable clothing without metal: no zippers, underwire, jewelry or hairpins.
Arrive on time. The tracer loses its activity quickly, and late arrival can mean rescheduling.
Bring: your medication list, previous scans if done elsewhere, and your glucose meter if you have diabetes.

If you have diabetes

Blood sugar is the most important factor for a good FDG PET scan in people with diabetes. High glucose competes with the tracer, and insulin pushes tracer into muscles instead of the areas being studied.

Your blood sugar will be checked before the injection. Many protocols require it to be under 200 mg/dL; some centers use lower cut-offs. If it is out of range, the scan may be rescheduled.
Ask for a specific plan when you book. The timing of your last insulin dose and diabetes pills depends on the type of insulin and on your appointment time, and the rules are different for each type. Do not change your medications without instructions from your doctor or the PET center.
Morning appointments are often easier for people with diabetes, because the fast fits overnight. Ask for one if possible.
Metformin can increase tracer uptake in the bowel, but guidelines say it does not necessarily have to be stopped. Ask your center.
Watch for low blood sugar during the fast and the low-carb diet. If you feel shaky, sweaty or confused, treat it and tell the team; your safety comes first.

New joint guidelines for people with type 1 and type 2 diabetes were published in March 2026, so protocols are being updated. Your center’s instructions take priority.

From the scanner room: [TU EXPERIENCIA: cómo gestionáis en la práctica a los pacientes diabéticos (cita por la mañana, control de glucemia al llegar, qué pasa si la glucosa está alta) y qué errores ves con más frecuencia.]

Other situations

Breastfeeding: tell the team. They may advise pumping and storing milk ahead of time and pausing for a period after the scan.
Pregnancy: always tell the team if you are or might be pregnant.
Claustrophobia or pain: if you need medication to lie still, ask your doctor in advance and arrange a ride home.
Non-FDG PET scans (such as PSMA PET for prostate cancer) often have different, simpler preparation. Follow the instructions for your specific scan. See [link: PSMA PET Scan for Prostate Cancer: A Patient Guide].

After the scan

You can usually eat and return to normal activities. Keep drinking water and empty your bladder often to help clear the tracer. Some centers advise avoiding prolonged close contact with young children and pregnant women for several hours. See [link: Am I Radioactive After a PET Scan? Precautions for Family and Pets].

Your PET preparation checklist

The day before

Low-carb, high-protein meals; no sugar
No strenuous exercise
Stay warm
Drink plenty of water
If diabetic: confirm your medication plan with the center

The day of the scan

Fast for the time your center specifies (usually 4 to 6 hours)
Only plain water: no gum, mints or sweetened drinks
Warm clothes without metal
Bring medication list, glucose meter and previous scans
Arrive on time

Tips from the technologist

[TU EXPERIENCIA: 3-5 consejos propios. Ideas: llevar abrigo aunque haga calor, no subir escaleras corriendo para llegar a tiempo, ir en coche o transporte en vez de andar mucho, no masticar chicle por los nervios.]

Frequently asked questions

Can I drink coffee before a PET scan?

Not with sugar or milk, because you need to fast. Some centers ask you to avoid all caffeine for 24 hours. Plain water is always fine.

Can I brush my teeth before a PET scan?

Usually yes, without swallowing. Avoid gum and mints.

What happens if I eat before a PET scan?

Tell the team. Eating raises your blood sugar and insulin and can change where the tracer goes. The scan may need to be rescheduled.

Why can’t I exercise before a PET scan?

Exercised muscles take up the tracer, which can hide abnormal areas or look like disease.

Why do I need to stay warm?

Cold activates brown fat, which takes up the tracer and can make images harder to read.

What should I wear to a PET scan?

Warm, comfortable clothes without metal. The scanner and waiting rooms can be cool.

For the full day, see [link: PET Scan: What to Expect From Start to Finish].

This article is for general information and does not replace advice from your doctor or PET center. Do not change diabetes medications without medical instructions.