Fasting & Performance
If a person chooses to fast or not, it might be interesting to find out how fasting can impact performance in exercise. In this article, we will discuss what the evidence shows as it relates to fasting and performance in both long duration exercise and high intensity, short duration exercise; not only that, we will also understand the physiological reasoning for this scientific conclusion. Let us dive right in.
What is fasting?
We are going to include this section, because it is important to define considering there could be multiple definitions of “fasting”. Technically, the moment we stop consuming food, we are, from that point, fasting. Still, that isn’t useful. So, we will discuss a typical intermittent fasting style and a more extreme fasting style. The former, is a 16 hour fast and 8 hour feeding window. The latter, is a 24 hour fast.
Fasting impact on Performance?
As promised, we will break this up into the two main types of exercise.
Let’s discuss how resistance training, sprinting, and other short burst-high octane activities fare when a person is deprived of nutrition.
It seems that when a person fasts for a length of time (like 16 hours), and then consumes a meal a few hours prior to exercise, there is no impact on performance . However, there is little data on if a person were to fast for an extended period of time and train fasted, but based on a study looking, not at gym performance, but at body composition changes with exercise in a fasted state and a fed state, there was no difference seen . So, while we cannot directly tell if fasting has an impact on performance, one might imagine that if fasting did have an impact on performance during a fasted state, that this would eventually reflect in decreases in fat free mass (FFM) after weeks of sub-optimal performance. We can, at least, say that the difference is not significant. So, based on the current literature, I believe it is safe to err on the side of training while fasting (during or after the fasting state) has no significant impact on performance (assuming, of course, all other variables – like calories, macronutrients, etc. - remain the same).
However, what happens when, if, a person needs to fast for 24 hours or longer?
In cases like that, it seems that fasting may have a detrimental impact on high intensity performance .
Lower Intensity/Long Duration
So, we have an idea of how fasting impacts high intensity sessions of activity that may only last a few seconds or a few minutes, but how does fasting impact steady state activity like running, biking, soccer, rowing, and other such activities.
Well, the evidence is a bit clearer when we look at fasting and endurance exercise, and if we take our first condition, a 16 hour fast, it turns out that fasting does negatively impact performance . Now, if a person fasts for 16 hours and trains within that fasting time, especially near the end, there is a detriment to performance. However, this may not be the case if a person consumes a meal after the fasting period and then exercises some time afterward .
On the other hand, a 24+ hour fast seems to dramatically impact performance for the worse with significant decreases in time at which one can continue to train at a given constant intensity .
Understanding the Physiology
While the mechanisms by which these results are explained are a bit controversial, we will give a plausible explanation for why fasting does or does not impact performance – this may change as more evidence emerges in each of the above conditions.
That said, high intensity exercise is typically associated with high glycolytic (aka, glucose use) metabolism . So, then, how does fasting not affect the muscle’s ability to create force? Well, it does, actually. It just so happens that if a person fasts, then trains, they will see a greater decrease in glucose/glycogen in their muscles post workout, but there is likely still sufficient glycogen to fulfill all the requirements in exercise (20-40% drop in glycogen) . However, since the body is eventually fed in the 24 hour period (prior or after exercise), the body will recover said glycogen sooner or later. Essentially, the fast is not long enough to lead to so low glycogen levels that the body cannot perform optimally. I would guess, however, if a person were to go through another workout later in the day, their performance would suffer (although this may be true, even in normal feeding conditions). The bottom line is, so long as you consume the appropriate amount of nutrition, your body has enough stored glucose (glycogen) to get you through a single bout of high intensity activity (assuming that activity does not last for hours on end).
In the case of fasting longer than 24 hours, we likely see a situation in which the body has released more glycogen, from the liver, to maintain blood glucose levels and getting to a point at which it can no longer supply enough glucose/glycogen for intense activity as it could at 16 hours of fasting . There is, essentially, a minimum threshold that is exceeded when fasting too long.
Then, if this is true, why not see a similar effect when discussing endurance exercise?
We do, actually. The same effect is seen, and endurance exercise even has the benefit of being able to rely on fatty acids (aka, fat use) to supply energy and decrease its reliance on glycogen for energy . However, while this is a protective effect and is possible through human metabolism via the aerobic metabolic system, it is not a sufficient protective effect. While less glycogen is used, if time is strictly equated with high intensity exercise, when we endurance exercise, time is never truly equated for in the real world; so, although endurance exercise uses less glycogen, it uses it far longer than high intensity exercise, so exhaustion sets in and is more common to see in timed endurance training. This impact is more severe the longer one fasts prior to exercise.
Interestingly, however, if time is not a factor and distance is measured, then fasting can be beneficial to exercise as it, over time, can lead to the body adapting to rely more on fat oxidation rather than glycogen, which can allow a person to maintain a slower pace for an infinite time . Exceptions apply, as individual variability through genetics and lifestyle can swing metabolism to be more fat adapted or glycogen adapted.
I should note, however, that fasting can also be done for religious reasons, and can also abstain from water. Abstaining from water for 16 hours or more may also play a role in performance in both conditions, especially endurance. Also, keep in mind that if a person is trying to lose weight, they will be in a calorie deficit and therefor will have lower glycogen levels, and this may lead to detriment in training if a person compounds that with fasting. So, there are many variables to consider (exercise type, exercise modality, nutrition, fasting type, training time relative to fast, metabolism, muscle fiber type, and more), but in general, fasting is seen as neutral or detrimental to exercise performance unless that exercise performance is measured in distance, as opposed to time, in which case fasting may give a slight edge.
Where does that leave us, then? It leaves us with a mild conclusion that there is likely no noticeable impact of fasting and training when it relates to high intensity exercise like lifting, if a person still reaches their nutrition numbers by the end of the day – this applies for training within the fasting window and within the feeding window, post-fast. However, I am making an educated guess that if a person were to train twice in the same day, one might see a significant decrease in performance that could be attributed to the fast. Now, it likely has little effect on endurance exercise, either, unless that endurance exercise is based on time and goes on for hours, in which case the slowly depleting glycogen stores in the muscle will eventually run out and a premature fatigue may set in. However, if distance is your goal, without consideration for time, then fasting will make things more difficult, but not impossible. Overall, fasting is not beneficial to training, but in most cases, it will not hurt your efforts.
Writer: Nicolas Verhoeven
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