Do energy drinks break a fast?
Regular energy drinks break a fast — roughly 27 g of sugar and 110 calories per can. Sugar-free versions probably do not, on the same evidence as diet soda. Separately from fasting: energy drinks have measurable cardiovascular effects in adolescents, and a fasted stomach is not the ideal place to put a large caffeine dose.
| Fasting for | Verdict | Why |
|---|---|---|
| Weight loss | Depends | Regular versions are roughly 110 kcal per can and break a fast. Sugar-free versions are zero-calorie and do not. |
| Autophagy | Depends | Sugar-containing versions clearly break it. Sugar-free versions containing BCAAs or taurine are a genuine grey area, and several fitness-branded drinks do contain amino acids. |
| Blood work | Breaks it | Not permitted in any version. |
| Ketosis | Depends | Regular versions end ketosis. Sugar-free versions do not contain meaningful carbohydrate. |
| Gut rest | Breaks it | Acid, carbonation and a large caffeine dose. Not rest. |
Why
For regular energy drinks the mechanism is uninteresting — it is a sugary soft drink with caffeine, and the sugar settles it. The sugar-free case follows the diet soda evidence, with one addition that matters for fasting specifically: many fitness-positioned energy drinks contain added amino acids, most often taurine and sometimes BCAAs. Taurine is not a strong mTOR signal, but BCAAs are, and an entry that says "sugar-free energy drinks do not break a fast" without reading the ingredient panel is giving the wrong answer to anyone fasting for autophagy. The wider clinical picture is worth stating plainly: clinical analysis in adolescents found measurable physiological effects from energy drink consumption, and those effects are not attenuated by the drink being sugar-free.
Approximately 110 kcal and 27 g sugar per 250 ml can; 0 to 10 kcal for sugar-free.
The exceptions
This is usually where the real question lives.
- Check for BCAAs or amino acids on sugar-free versions — several contain them, and they break an autophagy fast.
- A large caffeine dose on an empty stomach hits harder. Nausea and palpitations are common reports.
- Some "zero sugar" energy drinks contain juice and are not actually zero carbohydrate.
- If you have a heart condition, this is a question for your doctor rather than a fasting site.
Sources
- Physiological Impacts of Energy Drink Consumption: A Clinical Analysis in Adolescents — Nutrients, 2024PMID 39064773
- The Effect of Non-Nutritive Sweetened Beverages on Postprandial Glycemic and Endocrine Responses: A Systematic Review and Network Meta-Analysis — Nutrients, 2023PMID 36839408
- Branched Chain Amino Acids — Annu Rev Physiol, 2019PMID 30485760
- FoodData Central — USDA Agricultural Research Service, 2026
No clinician reviews this site. Sources on this entry were verified by the author, who is not a doctor or a dietitian — what that means.
Last updated 2026-08-15.
Related
Fasting Index sells no supplements and takes no affiliate revenue. General information only, not medical advice.