The honest answer depends on a question most people skip: why are you fasting in the first place? A fast for weight management, a fast for gut rest, and a fast before a blood draw are three different physiological states with three different thresholds. A scoop of pre-workout that is completely irrelevant to one can genuinely interfere with another.
So instead of a yes or no, the useful approach is to read the label and ask what each ingredient does to insulin, to protein synthesis signaling, and to the calorie ledger.
Three Definitions of Breaking a Fast
Most disagreement online comes from people using the same word for different goals.
- Calorie restriction fasting. The goal is an energy deficit across the day. The threshold is calories, full stop. Anything under roughly 10 to 20 calories is noise against a daily target of 2,000 or more.
- Metabolic fasting. The goal is keeping insulin low and fat oxidation high. The threshold is anything that provokes a meaningful insulin response, which mostly means carbohydrate and to a lesser degree protein.
- Autophagy or cellular fasting. The goal is nutrient sensing pathways staying quiet. This is the strictest definition, because amino acids alone can signal nutrient availability even with almost no calories attached. It is also the definition with the weakest human evidence, since most autophagy timing data comes from animal and cell models rather than people. Treat confident claims about exact autophagy windows in humans with skepticism.
Pick your definition first. The rest of this article becomes straightforward once you have.
Ingredient by Ingredient
| Ingredient | Typical dose | Calories | Breaks a fast? |
|---|---|---|---|
| Caffeine anhydrous | 150 to 350 mg | 0 | No, under any definition |
| Beta-alanine | 2 to 3.2 g | Trace | Effectively no |
| L-citrulline or citrulline malate | 6 to 8 g | Trace to ~10 | No for calories, debatable for autophagy |
| Creatine monohydrate | 3 to 5 g | 0 | No |
| BCAAs or EAAs | 5 to 10 g | 20 to 40 | Yes for strict fasting, no for calorie fasting |
| Dextrose, maltodextrin, cluster dextrin | 15 to 30 g | 60 to 120 | Yes, clearly |
| Sucralose, stevia, acesulfame K | Milligrams | ~0 | Almost certainly no |
| Natural flavors, silica, citric acid | Trace | ~0 | No |
Caffeine Is the Easy One
Caffeine contains no calories and does not raise insulin. If anything it nudges physiology in the same direction fasting does, increasing circulating catecholamines and supporting fat mobilization. It is also the ingredient with the strongest performance evidence in the tub, which the NIH Office of Dietary Supplements reflects in its review of ergogenic aids.
The catch is tolerance, not fasting. Training fasted already elevates sympathetic drive, and stacking 300 mg of caffeine on top of that on an empty stomach makes jitteriness, nausea, and elevated heart rate more likely. The FDA cites 400 mg per day as the amount not generally associated with negative effects in healthy adults, and fasted training is a reasonable time to sit well below that ceiling. If caffeine on an empty stomach reliably wrecks you, a lower stimulant formula is a better fix than skipping pre-workout entirely, and the same logic applies to ingredients like beta-alanine that people drop for tolerability reasons.
Amino Acids Are Where It Gets Interesting
This is the real dividing line. Free amino acids, particularly leucine, signal nutrient availability to the mTOR pathway. That is exactly the signal a strict autophagy focused fast is trying to avoid. So a pre-workout containing 5 grams of EAAs technically ends that kind of fast even though it contributes only about 20 calories.
For a calorie focused or insulin focused fast, that same 20 calories is meaningless. Note also that the practical case for fasted BCAA use is weak in the first place: if you eat adequate protein across the day, added BCAAs add little, and whole protein at your next meal does the job better. Background on the underlying metabolic shifts during food restriction is summarized in the StatPearls chapter on the physiology of fasting.
Sweeteners: Mostly a Non-Issue
Non-nutritive sweeteners are the most argued about item and the least likely to matter. The concern traces to a small study by Pepino and colleagues in Diabetes Care, which found that sucralose consumed before a glucose load modestly altered glycemic and insulin responses in people with obesity. That is a real finding, but it involved sucralose paired with a large glucose drink, not sucralose alone in water. Evidence that a few milligrams of sucralose on its own meaningfully raises insulin in fasted humans is thin.
Practical read: sweeteners do not break a fast in any way that will change your body composition. If artificial sweeteners upset your stomach or trigger appetite, that is a reason to avoid them, and it is a different reason.
The Ingredient That Actually Breaks It
Carbohydrate. Any formula with dextrose, maltodextrin, highly branched cyclic dextrin, or a fruit juice powder base carries real calories and a real insulin response. If your tub lists 20 grams of carbs per scoop, you are not fasted after drinking it under any definition. Check the nutrition panel rather than the marketing copy, because plenty of “intra-workout” and “pump” formulas are carbohydrate products with a stimulant label.
Who This Actually Affects
- Most people doing 16:8 for fat loss. Essentially no one in this group is affected by a zero calorie pre-workout. Drink it and stop thinking about it.
- People fasting for autophagy or longevity goals. Choose a formula with no amino acids. Caffeine, creatine, and citrulline are the safer picks.
- People with reflux or a sensitive stomach. The problem is usually caffeine and acidity on an empty stomach, not the fast itself.
- Anyone fasting before medical bloodwork. Skip all supplements and follow your provider’s instructions. Caffeine can affect several measured values.
- People chasing a specific insulin protocol under medical supervision. Bring the label to your clinician rather than relying on general guidance.
A Simple Decision Rule
- Read the supplement facts panel and find total calories per scoop.
- Under 10 calories with no amino acids listed? Fine for every practical fasting goal.
- Contains BCAAs or EAAs? Fine unless you are specifically chasing autophagy.
- Contains any carbohydrate source above a gram or two? You are no longer fasted.
One last note on timing. Fasted training does not require pre-workout, and pre-workout does not require fasting. Both are tools, and the daily consistency of your training and protein intake matters far more than the hour you take a scoop. That is the same principle behind taking creatine every day regardless of whether you train: saturation and habit beat clever timing almost every time.
References
- National Institutes of Health, Office of Dietary Supplements. Dietary Supplements for Exercise and Athletic Performance: Fact Sheet for Health Professionals.
- U.S. Food and Drug Administration. Spilling the Beans: How Much Caffeine is Too Much?
- Rui L. et al. Physiology, Fasting. StatPearls. NCBI Bookshelf.
- Pepino MY, Tiemann CD, Patterson BW, Wice BM, Klein S. Sucralose affects glycemic and hormonal responses to an oral glucose load. Diabetes Care. 2013.
- National Institute of Diabetes and Digestive and Kidney Diseases. Insulin Resistance and Prediabetes.
