We’re all familiar with the amazing feeling of blood coursing through our veins during a good workout, loaded with energy, and absolutely crushing our lift.

But without that quick supply of glucose from a loaded pre-workout meal, what’s your workout going to be like?

Slow? Exhausting? The best you’ve ever had?

The effect fasting will have on your workout varies greatly between people, but as a general rule, it can elicit some pretty positive benefits when exercise is done at just the right time.

If you’ve started intermittent fasting and maybe you’re a morning workout kind of person but aren’t sure how fasting will affect your performance, this article is for you.

We want you to understand the ins and outs of fasting and exercise, and when is the best time to hit the gym is if you’re following an intermittent fasting protocol.

Benefits Of Fasting For Physical Performance

If you do a quick internet search on the benefits of fasting, you’ll find no end to the lists. Whether it’s cardiovascular health, weight management, mental clarity, chronic disease prevention, or anything else, there’s pretty much no reason anyone wouldn’t want to do it.

Except for the fact that you have to go anywhere from 12 to 24+ hours without food… and if you’re a foodie, you’ll know how difficult that can be.

But aside from many of the long-term health benefits, what about the benefits of fasting on your athletic performance? There are two main ones you’ll find:

1. It Increases Fat Oxidation

Training in a fasted state forces the body to use energy more efficiently since it doesn’t have access to an abundance of glucose floating around the bloodstream, especially if you’ve been fasting for less than 12 hours. This means it can help to decrease your reliance on glucose and give you a bit of metabolic flexibility.

But what fasted training also does is activate the sympathetic nervous system—the part of the nervous system responsible for the fight-or-flight (stress) response.

Activation of the SNS increases heart rate and respiration rate, decreases non-essential functions (digestion, reproduction, etc.), and increases muscle tension, all in the effort of preparing the body for a fight.

Interestingly though, as much as this seems like it’s not beneficial for exercise, it also increases lipolysis, or fat breakdown, to supply the body with energy 1.

And since there’s little glycogen or glucose available (depleted from fasting), the body is forced to oxidize fat for energy. Fat oxidation = fat burn.

Long story short, training in a fasted state essentially helps your body to become metabolically flexible, whereby it’s able to use either carbohydrates or fat to supply energy depending on availability rather than relying only on carbohydrates.

2. Decreases Insulin Resistance

The whole theory behind eating small meals every few hours needs to be put to bed.

You’ll see athletes doing it all the time to keep their energy levels up during training or daily activities.

Still, the problem with constant glucose spikes is that you’re also getting regular insulin spikes—remember, insulin is required to shuttle glucose into cells.

Over time, this constant circulation of glucose and insulin in the bloodstream causes the body stress.

And to make matters worse, chronically high insulin levels (i.e. insulin always in the bloodstream) can reduce your body’s sensitivity to insulin, leading to a vicious, never-ending cycle of glucose-insulin accumulation.

Think of it this way; it’s like a friend continually poking at you for what seems like hours on end.

In the beginning, you may respond, but as they keep prodding and prying, you eventually start to ignore them.

The same thing happens with insulin knocking on the cells doors, always wanting to get glucose in; eventually, the cells stop responding, and this action sets the stage for pre-diabetes.

On another note, much like how your muscles need time to recover after a challenging workout, your digestive system also needs time to recover so it can work effectively and efficiently when the next stimulus comes along.

Fasting, however, provides that much-needed downtime for your digestive and endocrine systems, along with dimming the inflammatory response 2.

But the thing with fasted workouts is that while your digestive organs get a break, training may cause your body’s physiological systems to go into overdrive.

As a result, your glycogen storage becomes depleted, but there’s some evidence to suggest that fasting causes a dramatic increase in growth hormone secretion 3, 4.

The effect this has on strength, however, is still up for debate.

After all, how your body responds to glucose is key to nutrient uptake and the subsequent effects on utilization.

Fasting and Strength Training

When it comes to fasting and strength training, there isn’t much evidence to support enhanced muscle growth or any sort of entry into some super anabolic state compared to strength training in a fed state.

However, where you do pick up some ground is that training in a fasted state—assuming adequate protein intake during the fed state post-workout—doesn’t alter muscle mass.

And while training in a fasted state does speed up the rate of muscle breakdown 5, the actual process of muscle protein synthesis is what’s keeping you afloat.

Catabolism isn’t a fast enough process that you’re going to lose substantial amounts of muscle mass during a single fasted workout or even multiple ones.

Research shows that acute fasted exercise does not increase the expression of FOXO3 or activation of MURF1, a key transcription factor in catabolic processes and a component of the muscular ubiquitin-proteasome system, respectively, after 2-6 h post-training, provided that substantial carbohydrates and protein are consumed after training 6.

You may not gain muscle, but the good news here is that you likely won’t lose any.

If you’re fasting and plan on doing any sort of resistance training, you need to consider two points:

1. What Was Your Last Meal?

Protein and vegetables? Unfortunately, that’s not going to cut it for a morning lift, especially not at a high intensity or if you’re planning to go heavy.

If you’re planning to hit the gym at any point during a fasted state and expect to hit your max (really you shouldn’t be trying to max out in a fasted state anyway), you need adequate fuel in your body from the night before.

That means heavy carbs and protein to carry you through. At this stage, your glycogen stores are at full capacity, and you will still have some remaining to pull from when needed.

However, if your glycogen stores weren’t topped up the night before and are substantially more depleted from an overnight fast, both your stamina and your strength are going to suffer. If you’re not eating sufficient carbs the night before, save your lift for a fed state and hit the cardio machines.

2. How Long Have You Been Fasting?

If you’ve been fasting for 12 hours and plan to hit the weights, chances are you’re probably going to have enough glucose still circulating (via glycogen breakdown) to get a pretty good workout without damaging existing muscle tissue.

If you’ve been fasting anywhere from 16 to 20 hours, you may notice a slight decline in performance.

Studies show that short-term alternate day fasting—anywhere between 20 and 28 hours—had no influence on whole-body protein metabolism in lean, healthy men.

They suggested that protein catabolism started on the third day of fasting (48h+), while energy during the first 2–3 days of fasting was derived mainly from glycogen and fat metabolism 7.

Fasting and Endurance

Endurance activity is generally where fasting seems to elicit the most favorable effects.

Like we said before, the lack of available fuel in the form of glucose means your body’s energy systems must be more efficient and switch over to an alternate fuel supply; fat is one of the densest sources of clean energy.

There are limited studies available on the effects of fasting on endurance performance.

Still, of those that find some benefit, results suggest that the minimal increase in performance may be attributed to preferential use of lipids over carbohydrates, which supply more energy per gram compared to carbs and protein.

Carbohydrate metabolism, on the other hand, was associated with muscle fatigue and hyperventilation 7.

Interestingly, though, one specific study found that during low-intensity endurance exercises, adipose tissue lipolysis and muscle fat oxidation rates increased compared to a fed state and protein degradation was downregulated 6.

As such, endurance training with low glycogen availability can cause more beneficial adaptations and performance compared to performing endurance training sessions with replenished glycogen stores 8.

When To Exercise During Fasting

With all of that said, when you should train during fasting will depend on the type of workout you’re doing and the results you’re looking to achieve.

Based on available research, fasted strength training is best done directly following an overnight fast before you break your fast (typically before breakfast) or consider it during your eating window (non-fasted) if you’re looking to put on muscle or build strength.

Endurance (aerobic) exercise, on the other hand, can be done at any point during your fast, but it’s important to realize that your performance and risk of fatigue will probably hinge on how fat-adapted (easily able to utilize fat for fuel) your body is.

References

  1. J Achten, AE Jeukendrup. Optimizing fat oxidation through exercise and diet. Nutrition. 2004; 20(7-8): 716-727.
  2. EF Sutton, R Beyl, KS Early, WT Cefalu, E Ravussin, CM Peterson. Early Time-Restricted Feeding Improves Insulin Sensitivity, Blood Pressure, and Oxidative Stress Even without Weight Loss in Men with Prediabetes. Cell Metab. 2018; 27(6): 1212-1221.
  3. MR Blackman, JD Sorkin, T Münzer, et al. Growth hormone and sex steroid administration in healthy aged women and men: a randomized controlled trial. JAMA. 2002; 288(18): 2282-2292.
  4. ML Hartman, JD Veldhuis, ML Johnson, et al. Augmented growth hormone (GH) secretory burst frequency and amplitude mediate enhanced GH secretion during a two-day fast in normal men. J Clin Endocrinol Metab. 1992; 74(4): 757-765.
  5. HT Pitkanen, T Nykanen, J Knuutinen, et al. Free amino acid pool and muscle protein balance after resistance exercise. Med Sci Sports Exerc. 2003; 35(5): 784-792.
  6. N Vicente-Salar, A Urdampilleta Otegui, E Roche Collado. Endurance Training in Fasting Conditions: Biological Adaptations and Body Weight Management. Nutr Hosp. 2015; 32(6): 2409-2420.
  7. H Zouhal, A Saeidi, A Salhi, et al. Exercise Training and Fasting: Current Insights. Open Access J Sports Med. 2020; 11: 1-28.
  8. P Knuima, MT Hopman, M Mensink. Glycogen availability and skeletal muscle adaptations with endurance and resistance exercise. Nutr Metab (Lond). 2015; 12: 59.