For a standard FDG PET scan, you should eat nothing for breakfast on the morning of your appointment. Most imaging centers require you to fast for at least four to six hours before the radiotracer injection, and since scans are often scheduled in the morning, that typically means skipping breakfast entirely. The reason comes down to how the tracer works: it mimics sugar, so any food that raises your blood glucose or triggers an insulin response will interfere with the scan’s ability to detect what your doctors are looking for. There are, however, a few things you can consume, and the rules shift depending on the type of PET scan you’re having.
Why Food Interferes With an FDG PET Scan
The tracer used in most PET scans is a molecule called 18F-FDG, which is essentially a radioactive form of glucose. Your body treats it like sugar, pulling it into cells that are hungry for energy. Cancer cells, areas of infection, and inflamed tissue all burn through glucose faster than normal tissue, so they light up on the scan. The problem is that FDG and real glucose compete for the same entry points into cells. If you eat breakfast and your blood sugar rises, that extra glucose crowds out the tracer, and the tissues your doctor needs to see end up absorbing less of it.1Journal of Nuclear Medicine Technology. 18F-FDG PET and PET/CT Patient Preparation: A Review of the Literature – Section: Glucose Levels, Insulin, and Antidiabetic Medication
It gets worse. When blood sugar climbs, your body releases insulin to bring it back down. Insulin drives glucose (and therefore the tracer) into muscle and fat tissue at a much higher rate than normal. The result is that your muscles and heart soak up the FDG that should have gone to a tumor or infection site, washing out the contrast between abnormal tissue and everything else. In patients with high blood glucose, studies show significantly lower tracer uptake in tumors and the brain, while the liver absorbs more than usual.2PubMed Central. FDG imaging with long-axial field-of-view PET/CT in patients with high blood glucose-a matched pair analysis – Section: Results
The relationship between blood sugar and image quality is not a gentle slope. Brain uptake, for instance, drops in a steep, almost exponential fashion as glucose rises, not in the steady, linear way you might expect from simple competition between two molecules.3PubMed Central. Effects of blood glucose level on 18F-FDG uptake for PET/CT in normal organs: A systematic review – Section: Results Even a modest rise from a light breakfast can meaningfully degrade the scan.
What You Can Have Before the Scan
Plain water is not only allowed but encouraged. Drinking water before and after your FDG injection helps flush the tracer through your kidneys and bladder, reducing radiation exposure to those organs and cutting down on a bright bladder signal that can obscure nearby structures.4Physical Sciences, Life Science and Engineering. Effect of Drinking Water on Image Quality FDG PET/CT Scan Test and How Can Water Reduce the Radiation In Human Body – Section: Abstract Most facilities will hand you a bottle of water after the injection and ask you to keep sipping.
Black coffee, with no sugar, cream, or milk, is permitted at many centers. Caffeine on its own does not raise blood glucose or trigger a significant insulin response, which is what matters for FDG imaging. That said, some facilities still ask you to skip it, particularly if the scan involves your heart or abdomen, because caffeine can slightly alter blood flow patterns. The safest approach is to check with your imaging center ahead of time. If they give you the green light, keep it plain: black, unsweetened, no flavored creamers, no “just a splash” of anything.
Artificially sweetened beverages fall into a gray area. While non-caloric sweeteners do not contain glucose, some research in animal models suggests certain low-calorie sweeteners can alter insulin sensitivity and glucose uptake in organs like the liver, brain, and gut.5SpringerLink / European Journal of Nuclear Medicine and Molecular Imaging. Low-calorie sweeteners augment tissue-specific insulin sensitivity in a large animal model of obesity Whether this effect is large enough to matter in humans during a single PET scan is unclear, but the precaution is simple: just drink water or plain black coffee and remove the uncertainty entirely.
The Exception for Cardiac PET Scans
If your PET scan is specifically looking at inflammation in the heart, such as suspected sarcoidosis or an infected heart valve, the preparation rules flip. For these scans, your doctor actually wants the heart muscle to stop using glucose so that any FDG uptake in the heart clearly points to disease rather than normal heart metabolism. The way to accomplish this is to push the heart into burning fat for fuel instead of sugar, and the tool for that is diet.
The standard protocol calls for eating a high-fat, very-low-carbohydrate diet for the two meals before your scan, followed by a fast. Think along the lines of eggs cooked in butter, bacon, cheese, avocado, nuts, and olive oil, while avoiding bread, fruit, juice, cereal, pasta, and anything sweet. A review of the available literature supports at least two high-fat, no-carbohydrate meals followed by a four-to-twelve-hour fast before the scan.6PubMed Central. Patient preparation for cardiac fluorine-18 fluorodeoxyglucose positron emission tomography imaging of inflammation – Section: Abstract
This diet genuinely works. In one study, patients who followed a very-high-fat, low-carbohydrate preparation had an average heart tracer uptake roughly half that of patients who simply fasted overnight.7PubMed. Suppression of myocardial 18F-FDG uptake by preparing patients with a high-fat, low-carbohydrate diet – Section: Results Another study found that nearly all diet-compliant patients achieved low enough cardiac uptake for a readable scan, compared with only about two-thirds of controls.8PubMed. Suppression of myocardial 18F-FDG uptake with a preparatory “Atkins-style” low-carbohydrate diet – Section: Results The dietary approach also suppresses uptake in brown fat and skeletal muscle, which helps clean up the overall image.9PubMed. Preparation methods prior to PET/CT scanning that decrease uptake of 18F-FDG by myocardium, brown adipose tissue, and skeletal muscle – Section: Conclusion
So if you’re scheduled for a cardiac FDG PET, your “breakfast” the day before the scan might be scrambled eggs with cheese and sausage, and you would then fast overnight before the morning appointment. Your center will give you specific meal instructions. Follow them closely, because half-measures tend to produce unreadable scans that need to be repeated.
What Happens If Your Blood Sugar Is Too High
Before you receive the injection, a technologist will check your blood glucose with a finger stick. Most facilities set a cutoff somewhere around 150 to 200 mg/dL. One study of patients with bloodstream infections used a threshold of 200 mg/dL (11 mmol/L) before allowing the scan to proceed.10Journal of Nuclear Medicine. Importance of Blood Glucose Management Before 18F-FDG PET/CT in 322 Patients with Bacteremia of Unknown Origin – Section: Materials and Methods Cutoff values vary by institution and by what the scan is looking for, but the point is the same: if your sugar is above the limit, you will be sent home and rescheduled.
This is not rare. A four-year review at one hospital found that out-of-range blood glucose was responsible for 130 out of 846 missed PET appointments, making it one of the most common reasons for cancellation. Incorrect preparation in general accounted for another 143 missed slots.11Journal of Nuclear Medicine. Analyzing common reasons and demographics among missed PET appointments – Section: Results Each cancellation wastes the radiotracer (which is produced daily and cannot be stored), delays your results, and adds stress. Getting the preparation right matters.
Managing Diabetes Around a PET Scan
If you have diabetes, the fasting rules still apply, but they require more coordination. The concern is keeping your blood sugar low enough for a good scan without sending it dangerously low from skipping meals while still on medication. Most centers will ask you to follow specific instructions about your insulin or oral medications on the morning of the scan. A common approach for people on insulin is to take a reduced dose or skip the short-acting dose that covers breakfast, since you’re not eating. For those on oral medications like metformin, the guidance varies by drug and by center.
The key is to call the imaging center or your prescribing doctor a few days before your scan, not the morning of. They can tell you exactly which medications to take, which to skip, and what time to take them. If your fasting glucose tends to run high even without food, let them know. Some centers have protocols for managing blood sugar on-site if it’s above their threshold, while others prefer to reschedule. Coming in prepared avoids the frustration of being turned away.
Things That Seem Harmless but Are Not
A few things that wouldn’t cross your mind as “eating” can still interfere with a PET scan. Chewing gum is a classic example. The repetitive jaw motion activates muscles in your face and tongue, and those muscles absorb FDG just like any active muscle would. Case reports have documented very high tracer uptake in the tongue from gum chewing alone, which can confuse the scan if the area of interest is your head or neck.12PubMed. Chewing gum causing a very high FDG uptake of the tongue on PET scan Even sugar-free gum is a problem here, because the issue is the chewing itself, not the sugar content.
Vigorous exercise in the day or two before your scan also increases muscle uptake of FDG. A hard workout the morning of the scan, or even a brisk walk to the imaging center, can cause your leg and core muscles to light up and obscure the areas your doctor is interested in. Most centers recommend avoiding strenuous physical activity for 24 hours before the appointment and arriving by car or wheelchair if possible.
Parenteral nutrition and IV dextrose are relevant for hospitalized patients. If you’re receiving nutrition through a feeding tube or an IV line that contains sugar, those need to be stopped well in advance of the scan. Guidelines for FDG PET preparation explicitly call for cessation of tube feeding, IV dextrose, and parenteral nutrition, limiting oral intake to plain water only.13Rheumatology Advances in Practice. Characterising polymyalgia rheumatica on whole-body 18F-FDG PET/CT: an atlas – Section: Pitfalls to consider
Staying Warm Before the Scan
This one surprises most people: being cold before and during the tracer uptake phase can mess up your scan. Your body has pockets of brown fat, mostly around your neck, shoulders, and spine, that burn energy to generate heat when you’re chilly. When those fat deposits activate, they gobble up FDG and create bright spots on the scan that can look confusingly similar to lymph nodes or other abnormalities.
Research in young lymphoma patients found that the frequency of brown fat activation depended heavily on the outdoor temperature and was significantly reduced by keeping patients warm during the uptake period.14Scientific Reports. Prevention of activated brown adipose tissue on 18F-FDG-PET scans of young lymphoma patients: results of an ancillary study within the EuroNet-PHL-C2 trial – Section: Results Earlier work confirmed that controlling the room temperature before and during the injection could significantly cut brown fat uptake in the neck and paravertebral areas.15PubMed. Reduction of brown fat 2-deoxy-2-[F-18]fluoro-D-glucose uptake by controlling environmental temperature prior to positron emission tomography scan – Section: Conclusion The practical takeaway: dress warmly on the way to your appointment, especially in cold weather, and don’t hesitate to ask for a blanket in the waiting area. Younger and leaner patients tend to have more active brown fat, so this matters more if you fit that profile.
When the Rules Are Different Entirely
Everything above applies to FDG PET scans, which are by far the most common type. But PET scanning can use other tracers that work through completely different mechanisms and may not require fasting at all. If you’re having a PSMA PET scan for prostate cancer, for instance, the tracer targets a protein on cancer cells rather than sugar metabolism, so your blood glucose level is irrelevant and the dietary prep is different.
That said, even non-FDG tracers can have their own food-related quirks. A study of PSMA-ligand uptake in salivary glands found a small but statistically significant increase in parotid gland uptake after eating, likely from the glands activating while processing food.16PubMed Central. The effect of eating on the uptake of PSMA ligands in the salivary glands – Section: Results The researchers recommended avoiding food around the time of tracer injection for therapeutic PSMA applications, where excess salivary gland uptake could mean unnecessary radiation to those glands. For diagnostic PSMA scans, the effect was small enough to be clinically unimportant in most cases.
If you’re unsure which tracer your scan uses, your scheduling paperwork will usually say, or you can call the nuclear medicine department directly. They deal with this question constantly and would rather spend two minutes on the phone than have you show up incorrectly prepared.
A Practical Morning Checklist
Since the rules vary by scan type, here’s what the preparation typically looks like in practice:
- Standard FDG PET: No food for at least 4 to 6 hours (many centers say 6). Drink plenty of plain water. Black unsweetened coffee may be allowed but confirm in advance. No gum, no candy, no mints.
- Cardiac FDG PET: Two high-fat, zero-carb meals the day before, then fast overnight. Eggs, cheese, nuts, and butter are typical choices. No bread, fruit, or sugar.
- Non-FDG PET: Follow the specific instructions from your center. Fasting may not be required.
For all types, avoid strenuous exercise for 24 hours, dress warmly, and arrive calmly. If you take medications for diabetes, blood pressure, or other conditions, get specific guidance from your doctor or imaging center at least a day or two in advance. Bring your medication list to the appointment.
Dealing With Scan-Day Anxiety
It’s worth mentioning that the fasting itself, combined with the stress of waiting for what might be an important result, can make the morning of a PET scan genuinely unpleasant. Research on cancer patients undergoing PET/CT found that anxiety levels before the scan averaged around 6.4 on a 10-point scale and dropped to about 5.7 afterward, suggesting that a good chunk of the stress is about the procedure and waiting rather than the results alone.17PubMed Central. Oncological Patient Anxiety in Imaging Studies: the PET/CT Example – Section: Discussion
Knowing what to expect tends to help. The injection itself is a standard IV, similar to having blood drawn. After the injection, you’ll sit quietly in a dimly lit room for about 45 to 60 minutes while the tracer distributes through your body. The scan itself takes around 20 to 30 minutes and is painless. Bringing a book, podcast, or playlist for the waiting period is a good idea. And if the main source of your anxiety is not knowing what you can eat or drink, hopefully the answer is now clear: water freely, potentially black coffee, and nothing else until the scan is done.