TL;DR
Performance should be similar between fed and fasted state, except for high-duration aerobic exercise (where eating low GI carbohydrate is advantageous).
There appears to be more muscle adaptation and free-fatty acid utilization when exercizing fasted.
Should we exercise fasted? Should we eat before? What? How much?
These are questions I’ve seen asked around here a lot, and decided to search the literature. I guess this can be a controversial topic, as there are many strong opinions out there in the fitness community. There should be a lot of individual variation and each of you may feel better exercising fasted or after eating, and it’s probably fine either way.
First, I’ll present the conclusions of a 2018 systematic review by Aird et al:
Effects of fasted vs fed-state exercise on performance and post-exercise metabolism: A systematic review and meta-analysis.
Performance
There appears to be a beneficial effect of eating before a prolonged (>60min) aerobic activity (mainly using low/moderate GI carbohydrate meals – a quick list for you, here).
For short duration aerobic activity, 57% of the studies found no difference between fed or fasted state.
There’s less evidence available for anaerobic exercise. One study found benefit of a carbohydrate meal before anaerobic exercise (improved time to exhaustion). Three studies found no difference before HIIT.
Metabolism
Fasted exercise mobilizes and promotes free-fatty acid utilization.
Regarding muscle adaptation, here’s the authors conclusion:
Some studies on overweight, inactive individuals, showed less differences between fasted and fed state exercise.
Other recent studies
After the publication of the review above, there have been many more studies. I’ll just present the conclusions of a few.
Other review, by Wallis et al 2019 concludes:
Kondo et al, 2019 found that transient hypoglycemia after preexercise CHO ingestion occurs in some, but not all, subjects and subjects with enhanced insulin responses seem to be more prone to transient hypoglycemia in the fasted condition.
Kotopoulea-Nikolaidi et al, 2019 tried to evaluate psychophysiological responses and found the following:
Conclusion
There’s much evidence that benefits exercising fasted for metabolic reasons. I assume there should be a lot of individual variations, and it’s not clear if these differences will be persistent for long periods of training or if they’re noticeable in practice.
There’s some research with protein intake instead of carbohydrates, but it’s still very little.
It appears to be an increasing interest in this topic, so I hope there’s more and more information appearing in the near future.
What do you guys prefer? Can you see any noticeable difference in performance either way?
References
Aird, T. P., Davies, R. W., & Carson, B. P. (2018). 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, 28(5), 1476–1493.
Kondo S, Tanisawa K, Suzuki K, Terada S, Higuchi M. Preexercise Carbohydrate Ingestion and Transient Hypoglycemia: Fasting versus Feeding. Med Sci Sports Exerc. 2018;51(1):168-173.
Kotopoulea-Nikolaidi M, Watkins E, Giannopoulou I. Effects of High Carbohydrate vs. High Protein Pre-exercise Feedings on Psychophysiological Responses to High Intensity Interval Exercise in Overweight Perimenopausal Women. Front Nutr. 2019;5:141. Published 2019 Jan 22. doi:10.3389/fnut.2018.00141
Wallis, G., & Gonzalez, J. (2019). Is exercise best served on an empty stomach? Proceedings of the Nutrition Society, 78(1), 110-117. doi:10.1017/S0029665118002574