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3 Things You Should Know About Fasted Exercise

By Alexander VanHouten, Master Trainer & Life Time Education Specialist

Should you eat before or after you work out?

Great question. And with the intermittent fasting research that is growing[i], one that will likely become more and more common.

Though there are a number of benefits worth discussing just on the fasting front, (including but not limited to improved cognition, lowered cancer risk, better hormone and lipid regulation, and even spiritual enlightenment[ii]) I won’t be covering those today.

Here, I want to talk about fasting, specifically in relation to exercise so that whether you are incorporating a fast, considering 6am workouts, or were just plain unable to eat before the gym today, you have some helpful guidelines to continue to get the most out of your workout. No matter how empty your stomach is.

Fasted Exercise Can Help You Burn More Fat But Might Not Help You Tone

If you type “fasted exercise” into google, (go ahead, I’ll wait) you’ll get a number of hits, all with a flavor that goes something like “if you’re going to exercise, an empty stomach will help you burn more fat”.

What’s harder to find are the important caveats to this statement “Burn More Fat” is a buzzword that once read, gym-goers stop reading further. Presumably to go do that thing that burns more fat.

Before you become another proverbial lemming running to blindly jump off the cliff that promises to help you achieve your hopes and dreams of unprecedented fat loss, please keep reading.

A vast meta-analysis has been performed on a large number of studies and the results are in. Fasted exercise does, indeed, increase fat percentage burned during activity. However, it only seems to do so consistently in low intensity cardiovascular exercise activities.[iii]

That is, results are mixed or negative for fasted interval training and weight training. And more importantly, in deconditioned individuals especially, carbohydrate burn (and therefore muscles burned through gluconeogenesis) actually increased with fasted exercise.

Another important caveat to note is that once you dig through the very long study, you’ll find that the conditioned individuals who performed fasted exercise with increased fat burn likely burned fat that was INSIDE their muscles (Intramuscular Triglycerides, not subcutaneous triglycerides). In other words, they may have lost fat but that fat loss did not help them appear more “toned”.

So, fasted exercise CAN help you burn more fat, but it’s going to be most helpful in this department if it’s done at a low intensity by a “conditioned” (think regularly exercises at a moderate intensity for 1 hour, 4 days/week for the last 6 months) individual.

Fasted Exercise Can Decrease Performance Output, But Might Improve Grit and Recovery

You heard me right. And the confusion deepens.

Except in long-distance endurance events, (for which there are mixed results) fasting for more than eight hours seems to reduce performance, especially with regard to strength and power.[iv][v] For this reason, a fasted state could make your strength and hypertrophy lifts, agility drills and power work less adaptive by limiting your ability to perform during the exercise.

But before you say, “well I can’t workout today…Alex said if I hadn’t eaten, it was a waste of time” I’ve got two things for you to consider.

One study found that in a single workout, fasting prior[vi]seemed to boost the anabolic effect of the post-workout meal. That is, individuals who lifted weights in a fasted state, then consumed protein and carbohydrates, boosted recovery and muscle growth as a result. This effect is further compounded if the individual supplements BCAA’s prior to the workout in a fasted state. [vii] The next step is a long term study to find if this is good for gym-goers overall, but it’s an interesting finding nonetheless.

Another athlete suggests that training in a fasted state every once in awhile might teach them to perform while in a less than optimal state, thereby strengthening their mental focus and resolve (grit) for the real thing. That way, when the real event comes around and they are in an optimal state, they might be able to perform at higher levels.[viii]

What about the long-term?

Fasted Exercise Can Lower Your Metabolism

Many goals that I encounter in the club have short term result outcomes that people want to maintain over the long run.

“I want to lose 20 pounds by the new year”. Awesome. Do you want to keep it off after the new year? “Uhhh, yeah! Why would I ever want to gain that back?”

So it’s important to look at the results of fasted exercise through the lens of long-term effects. Unfortunately, there are not many of these studies out there. Though, there are enough to draw an important conclusion.

Too much fasted exercise will lower your metabolism in the long run.

A small study (but THEY included WOMEN! A very under-represented population in fasted research thus far) showed that 22 days of alternating fasts with exercise caused a 5% reduction in metabolic output.[ix]

BUT even more telling is the fact that EVERY WINNER on the hit show BIGGEST LOSER has a lower resting metabolism today (and sadly all but one has gained their weight back).[x] If there are any perfect case studies of the effects of long-term calorie-deprived exercise, these unfortunate individuals are it.

When you consider that the human body is the ultimate efficiency machine, you realize that doing too much of your exercise in a fasted state will ultimately condition your body to adapt to putting out work while reducing expenditures elsewhere.

Like a smart business cutting costs, your body looks for opportunities to cut corners and not all of them are good. Lowered thyroid function, lowered sex hormones, lessened muscle mass, weaker digestive function, less healthy skin and nails, less dense cardiac tissue…all are pivots your body can make to preserve energy in response to exercise in a depleted state. All are bad juju for those of us who want to live a self-actualized lifestyle.

How to Incorporate Fasted Exercise

So there is good and not-so-good in fasted exercise. How should you incorporate it, if at all?

First of all, don’t overcomplicate it. For most of us, it’s always better to exercise than not, regardless of when you last ate. You were made to move and you’ve got to keep that habit alive. But if you’re going to be smart about the fasting thing, consider this:

  1. Try low-intensity fasted cardio (1 out of your 2-5 cardio sessions per week) and try fasted weight training (again 1 out of your 3-5 lifting sessions per week) to see how they affect you. As long as fasted exercise is only a small portion of what you do, (less than 30% of your total exercise time) you will not create the lower metabolism adaptation that you don’t want, and you may improve your fat burn, recovery, and grit.
  2. Steer away from high-intensity cardio, power training or heavy strength days on an empty stomach. And if you can’t help it, (for whatever reason) do a scoop or two of BCAA’s before the workout. Then follow it up with a well-balanced meal as soon as you can.

Whether you’re fasting on purpose to help you with your health and fitness goals, waking up from an early alarm or stumbling into the gym after a long day without a snack, now you can be smart about your fasted exercise regimen.

Time to make it happen. I’ll see you on the fitness floor!



[iii] Vieira AF, Costa RR, Macedo RC, Coconcelli L, Kruel LF. Effects of aerobic exercise performed in fasted v. fed state on fat and carbohydrate metabolism in adults: a systematic review and meta-analysis.

Br J Nutr. 2016 Oct;116(7):1153-1164.



[v]Chtourou H, Hammouda O, Souissi H, Chamari K, Chaouachi A, Souissi N.

The effect of ramadan fasting on physical performances, mood state and perceived exertion in young footballers.

Asian J Sports Med. 2011 Sep;2(3):177-85.


[vi] Deldicque, L., De Bock, K., Maris, M. et al. Eur J Appl Physiol (2010) 108: 791. doi:10.1007/s00421-009-1289-x.

[vii] Håkan K. R. Karlsson, Per-Anders Nilsson, Johnny Nilsson, Alexander V. Chibalin, Juleen R. Zierath, Eva Blomstrand.Branched-chain amino acids increase p70S6k phosphorylation in human skeletal muscle after resistance exercise. American Journal of Physiology – Endocrinology and Metabolism Published 10 June 2004 Vol. 287 no. 1, E1-E7 DOI: 10.1152/ajpendo.00430.2003.



[ix] Alternate-day fasting in nonobese subjects: effects on body weight, body composition, and energy metabolism.

Heilbronn LK, Smith SR, Martin CK, Anton SD, Ravussin E.

Am J Clin Nutr. 2005 Jan;81(1):69-73.



The posts on this blog are not intended to suggest or recommend the diagnosis, treatment, cure, or prevention of any disease, nor to substitute for medical treatment, nor to be an alternative to medical advice. The use of the suggestions and recommendations on this blog post is at the choice and risk of the reader.
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