What Can I Eat Before a PET Scan?

For a standard oncologic PET scan, the short answer is: nothing. You need to fast for at least four to six hours before the radiotracer is injected, with plain water as the only thing you should consume during that window. The reason comes down to blood sugar and how the tracer works, but the practical reality is more nuanced than “just skip breakfast,” especially if you have diabetes, take metformin, or are scheduled for a cardiac PET scan rather than a cancer-focused one.

Why Fasting Matters for PET Scans

A PET scan uses a sugar-based radiotracer called FDG (fluorodeoxyglucose), which is essentially a glucose molecule tagged with a tiny amount of radioactive fluorine. Your cells absorb it the same way they absorb regular sugar. Cancer cells, inflamed tissue, and other metabolically active areas take up more of it, which is how the scan spots problems. But here is the catch: if you eat before the scan, your blood sugar rises, and your body releases insulin. That insulin drives FDG into your muscles and heart instead of letting it concentrate in the areas your doctor actually needs to see.

Eating triggers insulin release, which activates glucose transporters on muscle and heart cells. Those transporters pull in FDG indiscriminately, creating bright spots on the scan that have nothing to do with disease.¹ The result is a scan where your skeletal muscles light up while actual tumors or inflammatory lesions get harder to detect because less tracer reaches them.

One case report illustrates this vividly: a patient whose blood glucose was measured at 153 mg/dL before the scan showed intense tracer buildup throughout their skeletal muscles, making the images far less useful for diagnosis.² That is essentially what your scan looks like when you eat beforehand: muscles glowing, tumors hiding.

How Blood Sugar Levels Affect the Scan

The relationship between blood glucose and scan quality is not a gentle slope. Research tracking tracer uptake in the brain at different blood glucose levels found that things stay relatively normal up to about 110 mg/dL. After that, the picture deteriorates fast. At glucose levels between 121 and 140 mg/dL, brain tracer uptake dropped by roughly 35%. Between 141 and 160 mg/dL, it fell by about half. Above 200 mg/dL, the reduction exceeded 65%.³ This is why most imaging centers will check your blood sugar with a finger stick before injecting the tracer. If it is too high, they may reschedule.

The typical cutoff varies by facility. Many centers use 150 mg/dL as a go/no-go threshold, though some are stricter at 120 mg/dL and others allow up to 200 mg/dL. An expert panel recently noted that guidelines differ on the exact serum glucose threshold for proceeding with the scan, and recommended that each facility establish its own standardized protocol.⁴ If your center has not told you their number, ask when you schedule.

What You Can and Cannot Drink

Plain water is not just allowed, it is encouraged. Staying hydrated helps the tracer distribute evenly and helps your kidneys clear the radioactive sugar after the scan. Most centers will ask you to drink several glasses of water in the hours before your appointment.

What you cannot drink is anything with calories or sugar: no juice, no soda, no milk, no smoothies, no sports drinks. These will spike your blood sugar just like food. Black coffee and unsweetened tea fall into a gray area. Some facilities allow them; others do not. The concern is not about sugar but about caffeine and its potential to activate brown fat tissue, which can create confusing bright spots on the scan. In animal studies, nicotine increased brown fat tracer uptake nearly eightfold, and ephedrine increased it nearly fourfold, while caffeine produced only a slight, non-significant increase.⁵ So caffeine alone is probably a minor player, but many centers still ban it to be safe. Sugar-free gum, mints, and diet sodas should also be avoided because even some artificial sweeteners can trigger a small insulin response in certain people.

The Cardiac PET Exception

If you are having a cardiac PET scan, particularly one looking for inflammation in the heart muscle from conditions like sarcoidosis, the dietary instructions flip in an unexpected way. Instead of simply fasting, your doctor may tell you to eat specific foods before the scan.

The goal with a cardiac inflammation scan is the opposite of the usual goal. You want the normal heart muscle to stop absorbing the tracer so that only inflamed areas light up. To accomplish this, you need to shift your body’s fuel source from sugar to fat. That means eating high-fat, very-low-carbohydrate meals in the day or two before the scan. Think butter, olive oil, cheese, eggs, avocado, and meat with no bread, pasta, fruit, or anything starchy.

The evidence behind this is solid. A high-fat, low-carbohydrate meal eaten three to six hours before the tracer injection suppresses normal heart muscle uptake.⁶ This dietary approach also helps reduce tracer uptake in brown fat and skeletal muscle.⁷ A review of the available research supports using a high-fat, no-carbohydrate diet for at least two meals before the scan, followed by a four-to-twelve-hour fast before the tracer injection.⁸

Some protocols go further. For suspected cardiac sarcoidosis, a prolonged 72-hour high-fat, high-protein, very-low-carbohydrate diet has been shown to successfully suppress normal heart muscle uptake, potentially allowing more accurate detection of active disease.⁹ Other centers use a standardized ketogenic preparation where two meals are replaced with a carbohydrate-free formula, followed by at least 12 hours of fasting.¹⁰ The specifics will come from your cardiologist or nuclear medicine team, but the principle is always the same: starve the heart of sugar so it switches to burning fat.

If You Have Diabetes

Diabetes complicates PET scan preparation because you are balancing two competing needs: keeping your blood sugar controlled enough for a readable scan while avoiding a dangerous low from fasting. The approach depends on what type of diabetes you have and what medications you take.

For people on insulin, the general recommendation is to hold short-acting insulin for four to six hours before the scan.¹ Long-acting or basal insulin is usually continued as normal. If you use an insulin pump, an expert panel supports keeping it running in basal mode and avoiding correctional boluses immediately before imaging.⁴ The reason for avoiding a bolus of fast-acting insulin close to the scan is the same reason you avoid eating: insulin drives the tracer into your muscles and away from the areas being investigated.

For cardiac PET scans in people with diabetes, some protocols call for more aggressive insulin adjustments. One center’s approach for non-insulin-dependent diabetics was to hold insulin entirely for 12 to 24 hours before the scan, while insulin-dependent patients were instructed to take half their usual dose during that same period.¹⁰ These are aggressive adjustments that carry real hypoglycemia risk, so they should always be managed in close coordination with your prescribing physician.

The practical advice: schedule your PET scan early in the morning if you can. That way, your overnight fast does most of the work, you take your long-acting insulin as usual the night before, skip breakfast and your morning short-acting dose, and arrive with blood sugar that has had the best chance of staying in a reasonable range.

Metformin and Bowel Uptake

If you take metformin for type 2 diabetes, you have a separate issue to deal with. Metformin increases glucose uptake in the lining of the intestines, which means the bowel can light up brightly on a PET scan and potentially mask or mimic disease in the abdomen.¹¹

Research has quantified just how much metformin skews the picture. In one study, metformin use was a strong independent predictor of increased bowel tracer uptake, with the pattern described as intense, diffuse, and continuous along the bowel wall, especially in the colon.¹² The effect is strong enough that stopping metformin before the scan is widely recommended. A randomized study found that stopping for 48 hours produced significantly lower bowel uptake than either continuing the drug or stopping for just 24 hours.¹³

Your imaging center should give you specific instructions about when to stop metformin. If they do not mention it, bring it up. This is one of the most common medication interactions with PET scans, and it is entirely fixable with a brief pause in the drug. Just check with the doctor managing your diabetes before making any changes.

Why You Should Skip the Gym

Exercise is another thing that can ruin a PET scan, and the effect lasts longer than most people expect. When you work out, your muscles consume a lot of glucose to repair and recover. For the same reason that eating causes muscle uptake, recent exercise does too: your muscles remain hungry for sugar well after you finish your workout and will happily gobble up the tracer.

Guidelines typically require avoiding strenuous exercise for 24 hours before the scan. But even that window may not be enough in every case. One documented case involved a 15-year-old who had vigorous exercise four days before the scan and still showed intense tracer uptake throughout his skeletal muscles.¹⁴ That is an extreme example, and most people will be fine after a day of rest, but it shows how persistent the effect can be after particularly intense workouts. Play it safe: take it easy for at least a full day before the scan. A gentle walk is fine. A CrossFit session is not.

Nicotine, Cold Weather, and Brown Fat

Brown adipose tissue, often called brown fat, is a type of fat that burns calories to generate heat. When it activates, it absorbs glucose rapidly, which means it can produce bright spots on a PET scan that look like abnormal tissue. Several things can trigger this activation, and some of them are under your control.

Nicotine is a major one. In animal research, nicotine increased brown fat tracer uptake nearly eightfold compared to controls. The combination of nicotine and ephedrine (a stimulant found in some decongestants and supplements) pushed it even higher, to a twelvefold increase.⁵ If you smoke or vape, avoid nicotine for as long as you can before the scan. Most centers recommend at least the morning of the scan, though longer is better.

Cold temperature is the other big trigger. A large retrospective analysis of over 15,000 PET scans found that brown fat activation was significantly more common on colder days, in younger patients, in women, and in people with lower body mass index.¹⁵ You cannot control the weather, but you can dress warmly on your way to the appointment. Some imaging centers keep their waiting areas deliberately warm and may offer blankets during the uptake period for this reason.

A Practical Timeline for the Day Before and Day Of

Pulling all of this together, here is what a typical preparation looks like for a standard oncologic PET scan:

Two days before: If you take metformin, ask your doctor about stopping it 48 hours ahead of the scan.
Day before: Avoid strenuous exercise entirely. Eat normally, but consider having your last meal relatively early in the evening so your fast is as long as possible.
Morning of: No food. No sugary or caloric drinks. Drink plenty of plain water. Skip your morning short-acting insulin if applicable, but take your basal insulin as directed. Do not smoke or vape. Dress warmly if it is cold outside.
At the center: Your blood glucose will be checked. If it is within the acceptable range, you will receive the tracer injection and then rest quietly for about an hour while it distributes through your body. The scan itself usually takes 20 to 40 minutes.

For a cardiac PET scan looking for inflammation, the timeline starts earlier. You may need to eat two or more high-fat, zero-carbohydrate meals the day before and then fast overnight, depending on the specific protocol your center uses. The key is following their instructions precisely, because the dietary manipulation is the entire foundation of that scan’s ability to detect disease in the heart muscle.

What Happens If You Accidentally Eat

If you eat something by mistake on the morning of your scan, tell the staff when you arrive. Do not try to hide it. The most likely outcome is that they will postpone the scan by a few hours or reschedule you for another day. While that is inconvenient, it is far better than going ahead with a compromised scan that either misses something or produces false positives that lead to unnecessary worry or follow-up testing.

The degree of impact depends on what and how much you ate. A small sip of sweetened coffee an hour before arrival is different from a full breakfast of pancakes and orange juice. Your nuclear medicine team can evaluate whether it makes sense to wait or reschedule based on your blood glucose at check-in. The case report described earlier, where a patient’s muscles dominated the scan after eating, is a reminder that even a single lapse can substantially degrade image quality.² The tracer is expensive, the scanner time is limited, and most importantly, you want your results to be reliable.

Chewing Gum, Cough Drops, and Other Small Things People Worry About

People often wonder whether tiny incidental things can throw off the scan. Sugar-free gum, throat lozenges, toothpaste (if swallowed), or even lip balm with sugar in it come up frequently. The honest answer is that most of these are unlikely to move your blood sugar enough to matter on their own. But imaging centers tend to be conservative, and their “nothing by mouth except water” rule exists precisely so patients do not have to guess which small things are okay and which are not. The simplest approach is to treat the fast literally: water only, no exceptions, and save the gum for after.

One thing that genuinely does not matter is body position during the uptake period. Some patients worry about whether they should sit or lie down while waiting. Research looking at tracer uptake in floor-of-mouth muscles found no significant differences between patients who sat versus those who lay supine during the uptake phase.¹⁶ So sit comfortably in the recliner, rest quietly, and do not stress about your posture. The things that matter for scan quality are the ones already covered: fasting, blood sugar, medications, exercise, and staying warm.

1
PubMed Central. Optimal Management of Insulin in Patients undergoing 18F-Fluourodeoxyglucose Positron Emission Tomography Scans
2
PubMed Central. Do Not Eat Before a Positron Emission Tomography-Computed Tomography (PET/CT) Scan: A Case Report
3
Journal of Nuclear Medicine Technology. Assessing the Effect of Various Blood Glucose Levels on 18F-FDG Activity in the Brain, Liver, and Blood Pool
4
PubMed. Advances in Diabetes Medications and Patient Preparation for Oncologic FDG PET: AJR Expert Panel Narrative Review
5
Journal of Nuclear Medicine. Effect of Nicotine and Ephedrine on the Accumulation of 18F-FDG in Brown Adipose Tissue
6
PubMed. Suppression of myocardial 18F-FDG uptake by preparing patients with a high-fat, low-carbohydrate diet
7
PubMed. Preparation methods prior to PET/CT scanning that decrease uptake of 18F-FDG by myocardium, brown adipose tissue, and skeletal muscle
8
PubMed Central. Patient preparation for cardiac fluorine-18 fluorodeoxyglucose positron emission tomography imaging of inflammation
9
Clinical Nuclear Medicine. Suppression of Myocardial 18F-FDG Uptake Through Prolonged High-Fat, High-Protein, and Very-Low-Carbohydrate Diet Before FDG-PET/CT for Evaluation of Patients With Suspected Cardiac Sarcoidosis
10
European Heart Journal – Imaging Methods and Practice. Standardized ketogenic dietary preparation for metabolic PET imaging in suspected and known cardiac sarcoidosis
11
PubMed Central. Effect of metformin on 18F-fluorodeoxyglucose uptake and positron emission tomographic imaging
12
PubMed. High and typical 18F-FDG bowel uptake in patients treated with metformin
13
PubMed. Metformin Discontinuation prior to FDG PET/CT: A Randomized Controlled Study to Compare 24- and 48-hour Bowel Activity
14
PubMed. Long-lasting FDG uptake in the muscles after strenuous exercise
15
PubMed Central. Factors influencing brown fat activation in FDG PET/CT: a retrospective analysis of 15,000+ cases
16
PubMed Central. Physiologic [18F]fluorodeoxyglucose uptake of floor of mouth muscles in PET/CT imaging: a problem of body position during FDG uptake?