Whether you should you train on an empty stomach or after eating, is a question that sparks plenty of debate. Supporters of fasted workouts often claim it’s the best way to maximise fat burning and support weight loss. On the flip side, advocates of eating before exercise argue that having fuel in the tank helps you perform at your best for longer. Both positions make sense at first glance, but the reality is more nuanced once you consider how the human body works.
Fasted training and weight loss
A key point to understand is that burning fat during exercise is not the same as losing body fat over time.
When you exercise after several hours without food – like first thing in the morning – your body has lower carbohydrate stores (glycogen) to use for energy. With less carbohydrate available, your body draws more on fat stores to fuel your workout. This means a greater proportion of the energy you burn during that session comes from fat (Viera et al., 2016).
Some research even suggests that exercising before breakfast can increase fat use across the whole day, boosting 24-hour fat oxidation (Hacket & Hagstrom, 2017). That said, over weeks and months, studies consistently find no meaningful difference in body-fat loss between fasted and fed training when total calorie intake and exercise are the same (Schoenfeld et al., 2014).
In other words, while fasted exercise can shift your body’s fuel use toward fat in the short term, long-term fat loss relies more on maintaining a calorie deficit and exercising consistently than on whether you train fasted or fed. If weight loss is your goal, fuelling up can help you push harder and burn more calories overall.
Exercise performance
Research has consistently shown that eating before endurance or high-intensity exercise lasting more than an hour can improve performance and delay fatigue (Ramos-Campo et al., 2024). Consuming a small carb-rich snack – like a banana, toast, or a sports drink – 30–60 minutes beforehand, helps maintain blood glucose and provides readily available energy for your muscles. Additionally, evidence indicates that including a small amount of protein with carbohydrates may further support muscle function during longer sessions (Stearns RL et al., 2010).
Interestingly, regularly training in a fasted state can trigger metabolic and muscular adaptations that might give you an edge when you later compete after eating (Aird TP et al., 2018). Fasted sessions teach your body to work efficiently in tougher conditions where fuel is limited – a skill that can pay off when you switch to being well-fuelled. While it’s generally not recommended to start a race or strength event without eating, the fasted workouts you’ve completed beforehand can help you perform better once energy is on your side.
In short, fasted exercise may offer metabolic benefits, but for most athletes, being fuelled before long or intense workouts is associated with better performance and a lower perception of effort.
Key Takeaways
- Fasted exercise burns more fat during the session but doesn’t necessarily lead to greater long-term fat loss.
- Calorie /kilojoule balance and consistent exercise are more important for body composition than whether you train fasted or fed.
- Eating carbohydrate-rich foods before long or high-intensity workouts helps maintain energy, delay fatigue, and support muscle function.
- Fasted training can still be useful for metabolic adaptations and improving your body’s efficiency under limited fuel conditions.
- Balance is key: incorporate both fed and fasted sessions depending on your goals and how your body responds.
References
- Vieira AF, et al. Effects of aerobic exercise performed in fasted v. fed state on fat and carbohydrate metabolism in adults: a systematic review and meta-analysis. British Journal of Nutrition. 2016;116(7):1153-1164. https://pubmed.ncbi.nlm.nih.gov/27609363/
- Hackett D and Hagstrom AD. Effect of Overnight Fasted Exercise on Weight Loss and Body Composition: A Systematic Review and Meta-Analysis. Journal of Functional Morphology and Kinesiology. 2017; 2(4):43. https://www.mdpi.com/2411-5142/2/4/43
- Schoenfeld, B.J., Aragon, A.A., Wilborn, C.D. et al. Body composition changes associated with fasted versus non-fasted aerobic exercise. J Int Soc Sports Nutr 11, 54 (2014). https://doi.org/10.1186/s12970-014-0054-7
- Ramos-Campo DJ et al. The ergogenic effects of acute carbohydrate feeding on endurance performance: a systematic review, meta-analysis and meta-regression. Crit Rev Food Sci Nutr. 2024;64(30):11196-11205. https://pubmed.ncbi.nlm.nih.gov/37449467/
- Stearns RL et al. Effects of ingesting protein in combination with carbohydrate during exercise on endurance performance: a systematic review with meta-analysis. J Strength Cond Res. 2010 Aug;24(8):2192-202. https://pubmed.ncbi.nlm.nih.gov/20683237/
- Aird TP et al. Effects of fasted vs fed-state exercise on performance and post-exercise metabolism: A systematic review and meta-analysis. Scandinavian Journal of Medicine & Science in Sports. 2018 May;28(5):1476-1493. https://pubmed.ncbi.nlm.nih.gov/29315892/
