Fasted exercise is training while you haven’t had food for a while, like going for a jog after waking up. Many clueless Instagram influencers are big proponents of fasted exercise and love making extreme claims about it. Fortunately, you’ve stumbled onto this science-based article, where the truth will be revealed to you.
But enough small talk, let’s see what the research clearly says about all things fasted cardio and fasted strength training.
Fasted cardio is more commonly performed than fasted strength training. It’s often done in an attempt to enhance fat loss. Fasted cardio bunnies often over-exaggerate claims about how fasted cardio burns more fat than fed cardio.
This is technically true, but not practically relevant. Fasted cardio can burn more fat than fed cardio, but burning more fat doesn’t mean you lose more fat off your body.
Here’s why.
This is why looking at a short-term bout of fasted vs fed cardio is shortsighted. Your long-term energy balance (total calories in vs calories out) is what matters, not whether or not food is in your stomach when jog.
This has consistently played out in all research comparing fed vs fasted cardio while total daily calories are matched (3,4,5). The end result is always similar weight loss and body composition changes. Sorry fasted cardio bunnies, nothing magically superior about hungry running.
That being said, fasted cardio can have some merit in specific conditions:
I don’t really know many people who strength train fasted, but let’s go over it anyways. As you could probably predict by now, fasted strength training doesn’t have any body composition advantages either (3,4,18). Again, energy balance is king.
Furthermore, fed strength training has been shown to improve glucose control and metabolic markers better than fasted strength training in type 2 diabetic patients (20). This debunks the myth that fasted lifting improves nutrient partitioning better than fed strength training.
So now that you understand fasted cardio and fasted strength training doesn’t boost your metabolism nor enhance body composition when all else is equal, a reasonable question is does fasted training impact exercise performance?
For cardio, fasted workouts seem to be fine if it’s low intensity and well under 1 hour, but longer bouts are compromised (19,29).
And the research for how it impacts strength training performance is quite mixed. Some studies show fasted strength training performance makes no difference (12,13,14) while others show lifting fed boosts performance (15,16,17).
A systematic review on ramadan fasting’s effect on exercise performance in sport’s athletes, found fasting didn’t impact endurance or strength performance, but did compromise power and sprint performance (28). Keep in mind, this is done in sports athletes, not advanced strength trainees, so it’s safe to say that advanced lifters will likely have their performance compromised.
From the body of research I discussed above, fasted exercise doesn’t cause muscle loss when calories are matched. The body composition changes were generally the same in the short term assuming exercise is within an hour.
However, keep in mind, fasted training is likely suboptimal over the long term especially if taken to extremes. Fasting is never more anabolic than eating. Your body will produce additional growth hormone to compensate and minimize muscle breakdown, but compromise IGF-1 (a very anabolic hormone) production in the process (21).
Physiologically speaking, you can only fast for so long before losing muscle even if you increase anabolic signaling through strength training (30).
Eventually protein breakdown increases and anabolic signaling diminishes cause well, you got no food (22,23). This occurs as early as 15 hours, but isn’t too impactful. However, this will likely affect lean individuals more as they’re more prone to muscle loss especially those who lift weights
This is why long fasts (even with exercise) beyond about 16-20 hours are not advised for those looking to maximize muscle growth, especially in lean lifters (29).
Fasted exercise doesn’t have any of the benefits often claimed. However, the potential drawbacks of fasted exercise like performance/muscle loss is largely overstated. Performance/muscle loss may only be relevant to lean advanced lifters.
If you prefer to workout fasted or if it helps you naturally eat less for fat loss, I can see some practical reasons why someone would want to do it, especially if it’s merely light cardio. That being said, I don’t recommend it for advanced lifters looking to maximize results because at the end of the day, while you have little to lose from fasted exercise, you have nothing to gain. It’s a low risk, no reward style of training.
Edinburgh, Robert M, et al. “Skipping Breakfast Before Exercise Creates a More Negative 24-Hour Energy Balance: A Randomized Controlled Trial in Healthy Physically Active Young Men.” The Journal of Nutrition, Oxford University Press, 1 Aug. 2019, www.ncbi.nlm.nih.gov/pmc/articles/PMC6675614/.
Van Proeyen, Karen, et al. “Training in the Fasted State Improves Glucose Tolerance during Fat-Rich Diet.” The Journal of Physiology, Blackwell Science Inc, 1 Nov. 2010, www.ncbi.nlm.nih.gov/pmc/articles/PMC3002457/.
B. Sadeghirad, S. Motaghipisheh, et al. “Effects of Ramadan Fasting on Physical Performance: A Systematic Review with Meta-Analysis.” Sports Medicine, Springer International Publishing, 1 Jan. 1970, link.springer.com/article/10.1007%2Fs40279-020-01257-0.
Bernardino, 1California State University San. “Fasted Versus Nonfasted Aerobic Exercise on Body… : Strength & Conditioning Journal.” LWW, journals.lww.com/nsca-scj/Abstract/9000/Fasted_Versus_Nonfasted_Aerobic_Exercise_on_Body.99259.aspx.
Yasuda. “Breakfast before Resistance Exercise Lessens Urinary Markers of Muscle Protein Breakdown in Young Men: A Crossover Trial.” Nutrition (Burbank, Los Angeles County, Calif.), U.S. National Library of Medicine, pubmed.ncbi.nlm.nih.gov/33418493/.
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