Fasted Training—A Nuanced View

A three-part article that takes a deep dive into this practice and comes up with some surprising and hopefully valuable findings.

Part I: Definition, Description, Effects

I was recently informed that someone had altered an article I wrote years ago on another website. This caused me to have a look at that article. First, let me state that I DID NOT WRITE THAT ARTICLE despite my name showing as the author.

There is a warning in the article stating that the information contained in it is “outdated, incorrect, and potentially harmful.” It says that “new scientific and experiential studies have been done that directly and definitively contradict the information provided in this article.”

I have had several people contact me asking for an explanation of what has changed.

Any training method can be harmful if it is done incorrectly. Fasted training is often done incorrectly because people do not understand it. Rather than making dogmatic proclamations like in that article’s warning I am hoping the following, more nuanced, explanation will help athletes understand how, when, and, most importantly, when not to use fasted training.

The information I will present in this article comes from decades of experience coaching beginner to world-class athletes and a review of the science concerning its use with athletes. I propose to explain fasted training, how you might want to use it and how you should not use it. As with so much in the nutrition sphere, it is easy to find studies extolling the exact opposite effects of the same nutrition strategy. I intend to inform the reader and allow him or her to make their own decision. I make no excuse for this article going into the weeds at times. That is needed to provide some scientific context. I do try to summarize the germane information into an actionable strategy.

If fasted training is so dangerous and incorrect, why have I and many other coaches seen such great results using it. We’ve implemented it with many amateurs who can only put in a few hours of training in a week as well as top athletes like Tom Evans, winner of the 2023 Western States Endurance Run, or professional alpinist David Goettler or 8000-meter peak veteran Adrian Ballinger?

If fasted training is so dangerous and incorrect, why do so many Tour de France riders include it as part of their training? Why do some of the best triathletes in the world use fasted training? Through personal correspondence with a number of the top coaches in these sports, I can confirm this as one of the tools in the coaches’ toolbox. Top athletes cannot afford to do anything “harmful” to their performance.

Contrary to the dire warnings quoted above we and many other coaches have used fasted training to good effect. We routinely see athletes who have done some fasted training improve their speed at the “crossover point: (the intensity where 50% of the energy needed to support the exercise comes from fat and 50% from carbohydrates). This is verified in the laboratory when they take a Metabolic Efficiency Test (MET). In many case we see this 50/50 speed improve by more than 20-30%. These athletes are improving their performance with the judicious, controlled use of fasted training. In the end, performance is the ultimate test of a particular training method’s efficacy.

Blanket statements like those made in the warning in that article (that I didn’t write) overlook the nuance needed for even a superficial understanding of any complex subject. Fasted training is a very complex subject that involves your body’s endocrine (hormone) system, metabolism, and nervous system, not to mention its psychological effects.

Definition:

Fasted training means training having not eaten for 4-12 hours. Doing so means you will start that training session with low muscle and/or liver glycogen stores. It is a strategic training method where carbohydrate fueling is done either later in the session or after, but not before training. To be effective and not harmful this method should only be used with aerobic base training (Zone 1 and Zone 2).

A typical approach to fasted training is to exercise first thing in the morning before you have eaten. Probably 10-12 hours have passed since you last ate. Doing fasted training later in the day requires more planning and discipline to avoid eating 3-4 hours before exercise.

Fasted Training IS NOT

a calorie reduction or restriction strategy

for losing fat (may be a side effect)

for losing weight (may be a side effect)

If you choose to implement fasted training for any of these reasons, you are making a mistake, and it will not give the positive results it is capable of.

First, the warnings:

Fasted training is not for everyone. It has more potential risk for women. It should not be used for all training sessions. It should not be used for high-intensity sessions or where high performance is needed or desired (like a race or when you want to hang with a fitter partner or group). It will slow recovery. I’ll discuss each of these points in more detail.

Fasted Training Effects:

While there are many follow on effects of fasted training, the main reasons athletes use it are:

Increasing the rate your muscles can use fat for fuel
Enable your muscles to use more fat at higher intensities, sparing your limited glycogen stores
Increase your aerobic capacity by increasing the mitochondrial density in the working muscles.

Fat Adaptation:

There are a couple of significant benefits of being able to derive more of your energy for exercise from fat. First, even the leanest elite marathon runner has virtually unlimited intra-muscle fat storage. That’s fat in the muscles, close to the mitochondria, not the subcutaneous fat you can see or pinch.

The second, strongly related to the next point (aerobic capacity), is that, when fat adapted, you can continue to use a significant proportion of fat for fuel even at high intensities.

In this study which looked at fat vs carbohydrate usage differences between recreationally trained and well trained runners, Stephen Seiler et al. concluded that: “despite similar RPE, blood lactate and carbohydrate oxidation rates, the better performance by the Well Trained group was explained by their nearly threefold higher rates of fat oxidation at high intensity.”

Which leads us to the next point…

Aerobic Capacity:

If you’re a bit rusty on the basics of metabolism and how it acts as a recycling plant for the energy molecule ATP, you should re-read this article. When the Adenosine Tri-Phosphate molecule bonds are broken, energy is released that powers muscle contraction. This process results in the end product molecules Adenosine Di-phosphate (ADP) + a phosphate molecule and Adenosine Mono-phosphate (AMP) + 2 phosphate molecules. The energy (primarily fat or carbohydrates) in your food is used to recombine these back into ATP to repeat the whole process. Hence, the recycling plant analogy.

During exercise, ATP turnover can increase more than 100 times over resting. Managing cellular ATP levels to meet exercise demands while maintaining cellular homeostasis is a complex process involving interactions of many cell functions. Because of the increased ATP turnover rate, AMP and ADP levels fluctuate. When AMP/ATP and ADP/ATP ratios fall outside the cell’s homeostatic range, an enzyme called AMPk is produced.

AMPk triggers a signaling cascade leading to the up-regulating several genes responsible for various aerobic adaptations in the affected muscle cells. One of the genes, PGC1 , is responsible for the increase in mitochondrial mass in cells. Mitochondria are the cell’s aerobic powerhouses. They are where ATP (the fuel for muscle contraction) is recycled.

Glucose (stored as glycogen in the liver and muscles) is an essential fuel for metabolism. As muscle glycogen depletes, those ADP/ATP and AMP/ATP ratios get out of whack, producing AMPk.

Muscle glycogen stores, even for well-endurance-trained athletes, top out at about 1500Kcal. Liver glycogen stores generally max out at about 500Kcal. An athlete can burn through 500 to 800 kcal of glycogen in an hour at moderate intensity.

You do not need to completely deplete glycogen stores to zero to cause the AMPk pathway to fire up. You just need to upset the AMP/ ATP and ADP/ATP ratio for the cell to start the AMPk signaling cascade.

The main goal of all endurance training is to increase the mitochondrial mass of the active muscle cells, thereby increasing those cells’ aerobic capacity.

One of the most potent signals for your muscles to increase their mitochondrial mass is muscle glycogen depletion during training.

More resources on the metabolic effects of fasted training:

Armed with this basic understanding of the metabolic effects of fasted exercise, you can see that we can jump-start the AMPk signaling pathway by starting the training session with those AMP/ATP and ADP/ATP ratios already slightly out of whack, thereby enhancing the aerobic training effect.

Positive effects

Glycogen-sparing/Fat adaptation
Elevated levels of Human Growth Hormone (HGH)
Increased Insulin sensitivity (corrected typo- was “decreased” ….sorry)
Lowered Tri-glycerol levels
Increased mitochondrial mass/aerobic capacity
Potential increased fat use at high-intensity
Elevates cortisol levels, which increases gluconeogenesis (more on this later)

Negative effects

Slower recovery after training.
Elevated levels of the stress hormone cortisol can have a catabolic effect (more on this later).
If mis/overused, it can result in Relative Energy Deficiency (REDS) or Overtraining Syndrome (OTS)
Can disturb female hormones (read more below)
Potential to cannibalize lean muscle for fuel, especially for women

Part II: The Cortisol Rabbit Hole

As you can see above, Cortisol is on both the positive and negative lists. As in much of life, two seemingly opposing facts can be true simultaneously.

Elevated Cortisol seems to be the leading cause of concern among those who warn against fasted training. An oversimplified and incomplete presentation in scientific studies and the popular fitness press has led to the idea that elevated Cortisol is simply a “bad” thing.

Cortisol is a member of the glucocorticoid family of hormones. It’s released from the adrenal glands in response to stress, be it physical or psychological. Studies performed by Hans Selye in the early 20^th century formed the foundation of our current understanding of the body’s response (adaptation or maladaptation) to stress.

Some recent studies and much of the popular sports press have focused on Cortisol’s catabolic (breaking down of protein structures) effect [11] whereby protein and triglycerides (fat) are converted to glycogen in the liver via process known as gluconeogenesis. It would have been an evolutionary dead end for Cortisol to only have adverse effects. Actually, it plays critical parts in several essential positive metabolic functions [1,2,3,5], not to mention its role as an anti-inflammatory and immunosuppressant. A family of corticosteroid anti-inflammatory drugs are designed to fight significant inflammation when your body’s cortisol response is insufficient. Some studies show that the catabolic process brought on by elevated Cortisol may contribute to the anabolic rebuilding of protein structures as an adaptation to training [2,3,5,7].

The Testosterone/Cortisol Balance

Despite the focus on cortisol as the problem with stress it seems the real issue is when the ratio of testosterone and cortisol declines. That’s what several researchers have found. This can occur when either the anabolic hormone testosterone levels decline or when the catabolic hormone cortisol levels increase. We’ll refer to this as the T/C ratio. Make the denominator larger and/or the numerator smaller and the ratio declines.

In a 1980s a landmark study by Adlercruetz et al [11] examined the T/C ratio in young male army recruits doing vigorous physical exercise. Some of their findings summarized here were:

“Moderate physical exercise does not cause any changes in the plasma levels of the catabolic hormone cortisol and the anabolic hormone testosterone compared with the concentrations during a control day.”
“Fit compared to unfit men tended to have smaller mean decreases in plasma testosterone and free testosterone index during the day both during a control day and during a day with submaximal marching exercise.”
“Intense physical exercise invariably leads to an increase in plasma cortisol and a decrease in plasma testosterone compared with the concentrations during a control day.”

They suggested a decrease in the ratio of free testosterone to cortisol of 30% indicated that an athlete was in an “over-reaching” state and due for a recovery period. Over-reaching, as you may recall from some of my other writings, is a normal temporary state of fatigue in athletes that resolves in a few days to a week and results in “super compensation” or increase in fitness.

Some later research misconstrued this 30% decline in the T/C ratio as an indication of the dangerous long term maladaptive state of overtraining rather than the short term over reaching that positively benefits fitness. This idea that a 30% change from resting state in the T/C ratio was going to cause maladaptation seems to have caught on and may be at the root of the current concerns about elevated cortisol.

In this study [10] the authors summarized their findings this way:

Regrettably in recent years, some researchers in the exercise sciences, as well as some sports enthusiasts, have proposed that cortisol has a counter-productive role in exercise and can lead to a mal-adaptation to the exercise training process (i.e., its catabolic nature) [4]. Most certainly cortisol has catabolic actions in the human body; however, in many respects these actions can be beneficial and productive in the response to the stress of exercise and exercise training [5]. The view by some exercise specialists that increases in cortisol can lead to a predominance of catabolism in the body, which results in undesirable aspects within the adaptation of athletes in sports training, is an over-simplification of the hormonal actions of cortisol. This simplified and incomplete notion regarding the role-action of cortisol during exercise training has even resulted in the development of nutritional-pharmaceutical supplements and dietary strategies which attempt to suppress cortisol levels at rest and in response to exercise [5, 6, 7]. Such actions may in fact actually compromise the ability of select physiological systems to respond and adapt to the stress of exercise.

In this study, the authors concluded:

Classical studies of H. Selye on the general adaptation syndrome evidenced the involvement of the adrenal cortex in adaptation processes. Accordingly, Cortisol has been nominated as the adaptation hormone. However, during the past 15 – 20 years, several researchers in exercise physiology and sports medicine have had the opinion that the decreased ratio of testosterone/cortisol indicates a predominance of catabolism that is undesirable for adaptation and improvement of performance in athletes. In their opinion, an increased cortisol concentration is “guilty” of association with maladaptation… Actually, in a number of studies, the decreased ratio was associated with improved performance of athletes. In high-level rowers, the cortisol response to all-out exercise increased in conjunction with improved performance during a training year.

As with any disturbance to the body’s endocrine system’s homeostasis, there are many follow-on effects to increased cortisol levels; some mentioned above are positive, and some listed below are potentially harmful.

Stressful exercise often causes a transient increase in cortisol over resting levels along with a decrease in free testosterone levels. But it is that same physical stress that causes the various adaptations to training to occur. Stress-free training is not going to do much good for increasing your fitness. Transient increases in cortisol and decreases in testosterone go with the territory of being an athlete and may be mitigated with several strategies we’ll discuss later.

However, the point that seems to be overlooked by those who consider cortisol to be “harmful” is that it is chronic reduction in the testosterone/cortisol ratio that can lead to several stress-related problems, such as:

Missed or irregular periods of women
Infertility in both sexes
Over Training Syndrome
Increased adipose fat
Decreased muscle mass
Failure to adapt to training stimuli

Some of you are old enough to recall the Floyd Landis doping scandal that broke in the late 2000s. Landis “won” the Tour de France cycling race but was soon after stripped of his title when he tested positive for excessively high testosterone levels. After denying the charge for some time he eventually came clean and implicated Lance Armstrong and several others in a massive doping scandal.

We often associate the use of exogenous testosterone with the domain of body building and weight lifting where it helps increase muscle mass. Could it be that its effectiveness as a banned performance enhancing drug (PED) in endurance sports comes from the fact that it increases the numerator of the T/C ratio, thereby allowing the denominator, cortisol, to also rise to higher levels before crossing that negative 30% threshold? With elevated testosterone levels the athlete would be able to handle considerably more training stress with their concomitant improved adaptations, ultimately leading to improved performance. Just a thought…

Fasting

Intermittent fasting and fasted training both increase cortisol levels. Many studies show fasting has positive benefits for life span [8,9]. Clearly then, the elevated cortisol isn’t all “bad” in that context. Could there be some positive effects of fasted training that also outweigh the potentially harmful effects of fasted training?

Practicality

Hormone testing needed to assess and monitor both testosterone and cortisol levels is completely out of reach of all but the most well-funded athlete or those involved in a study. The tests require sending a sample to a lab, and they are quite expensive. The time delay and cost render this a moot point. We are left with only being able to discuss this issue in general terms and make only qualitative recommendations.

What kind of training reduces the testosterone/cortisol (T/C) ratio the most?
How long does this reduction last?
What can you do to lower cortisol levels?

Following are some notes related to these three questions

The most significant reduction in the testosterone/cortisol ratio comes from high-intensity aerobic workouts done at 90-100% of VO2 max. An example would be an interval session of 4x5min in Zone 4. Anyone who has ever done this kind of training can attest to its stressful nature, and that stress can be felt for a day or two afterward. This type of training also is a potent stimulus and has been used for nearly a hundred years by successful male and female athletes. It would stand to reason that the positive training adaptations outweigh the adverse effects of temporarily decreased testosterone/cortisol ratio. This is another area where misuse of this training can quickly lead to an over-trained athlete. This type of training done in a fasted state is a recipe for maladaptation.
Long-duration aerobic steady-state work done at about 50-70% of VO2 max is another exercise form that will increase cortisol levels and reduce testosterone. This spans what we call Zone 1 and Zone 2. Studies indicate that 60-90 minutes at this intensity is a sort of threshold beyond which the T/C ratio begins to fall.The duration may depend on factors like training status and genetics. Anyone who has done a 6-hour run in the mountains will know that even though the intensity was purely aerobic, they will feel the stress from this run for a few days. Like the interval training discussed above, there is a long history of this type of training yielding powerful effects on endurance performance. So the benefits seem to outweigh the risks of the transient low T/C ratio this kind of training elicits. All this assumes that there is adequate recovery and fueling between these stressful sessions. Doing the first 60-90 minutes fasted and then beginning to fuel with carbs would significantly reduce post-exercise cortisol while minimally affecting the benefits of starting these sessions fasted.
Muscular Endurance: I could not find studies including the kind of ME sessions we frequently use. But I suspect those would fall into either category one or two above and transiently jack up cortisol. Do not do your ME sessions in a fasted state, AND replenish carbs and protein within 30 minutes.
Strength, Speed, and Power sessions with adequate recovery between sets (1-3 minutes) seem to have a negligible impact on reducing the T/C ratio.
The most exciting thing I found in my research was that exercising below 50% of VO2 max lowers cortisol to BELOW resting levels. WOW! This answers one of my biggest questions: Why are recovery sessions so beneficial and restorative? Now I think I know. At least, this makes a ton of sense to me. It may explain why the “cool down” after a stressful workout is so important. The low-intensity aerobic work during the cool down allows many systems to return to homeostasis. Could cortisol levels also be lowered during this cool down? Seems likely. Doing longer recovery-level sessions may have a similar but more profound effect of reducing Cortisol and its adverse effects.

How Long do elevated cortisol or reduced testosterone levels last?

I could not find any quantitative info on this. In general, well-trained athletes recover to resting levels faster [6]. Younger athletes recover faster as well. Age is probably the main reason we seniors can’t handle the training loads we did when we were younger. It is suspected that there is also a genetic component involved. This is borne out in my observations of elite athletes with a prodigious capacity for hard work and a remarkable ability to recover quickly. But this is kind of a chicken-and-egg question.

I suspect there is a shortage of information about how long these rises in cortisol levels last because there are so many variables. Studies comparing the time course of the transient cortisol spikes would need to control for, at the very least, age, training history, type of training, intensity of that training, and duration.

This seems to be the central question to answer when it comes to deciding if fasted training does more harm than good. One of the main arguments against fasted workouts in the morning is that since cortisol levels follow a daily pattern that peaks in the morning, it is thought that piling training, not to mention fasted training, on that already elevated cortisol will dramatically raise cortisol levels. While that makes sense and is undoubtedly true, what is less clear is how long those levels stay elevated and what steps could be taken to mitigate the rise and shorten its time course.

As far as I can tell, we do not have an answer to these questions.

What can you do to lower cortisol levels?

Engage in only very light aerobic exercise below 50% of VO2max. That’s probably not acceptable to most people reading this who are seeking to improve their athletic performance
Reduce the stress in your daily life. Professional athletes get good at this.
Meditate, relax, sleep, and eat enough healthy calories.
Don’t skip the cool downs after hard training.
Don’t treat your recovery sessions like fitness-building workouts. Keep their intensity below 50% of VO2 max or at the low end of Z 1 or below.
Eat within 30 minutes of finishing all training sessions.

Special Considerations for Women

For women, things get much more complicated when it comes to fasted training. I admit I am out of my depth regarding special considerations for women doing fasted training. As a result, I have spent a lot of time trying to learn more. I’m going to try to summarize what I have learned. The following generalizations mean “most” women or “most” men. All of us fall somewhere along a spectrum of responses to stress, and only generalizations are safe to make.

T/C Ratio

One big difference from men and problem for women is that the T/C ratio discussed above is probably much, much more sensitive to increased cortisol in women than in men.

Normal free testosterone levels in men run from 300-1000 ng/dl (nanograms/deciliter).

Normal free testosterone levels in women run from 15-70 ng/dl

You don’t need to break out the calculator to see that the numerator in the T/C ratio for men is between 4.28 and 67 times larger than that same numerator in the T/C ratio for women.

If we pick a middle of the testosterone level range for both men (650) and women (42) the middle range T/C ratio for women will be more than 15 times (650/42) as sensitive to increases in cortisol as will the T/C ratio for men. Because there is so little research on this as it relates to women I can’t quote any studies but if we assume that women’s cortisol responds to stress similarly to men’s, women are at a vastly greater risk of lowering this T/C ratio into the danger zone.

Fat Utilization and Storage

Women are better at metabolizing fat than men, especially in long-duration, low-intensity exercise. During the second half of the menstrual cycle (luteal phase), the hormones estrogen and progesterone are elevated. Estrogen increases fat oxidation rates, and progesterone decreases glycogen storage (availability). In this phase, then, women are already getting a boost in fat metabolism. Overall, they will probably see much less fat adaptation than men.

Women have evolved to store and hold on to more fat than men. The presumed reason is that it enhances the survivability of offspring. Consequently, the added stress of fasted training can flip a metabolic “food scarcity” switch, causing them to add adipose fat.

Catabolic Affects

Women are likelier to break down protein structures for fuel in a low glycogen (fasted) state. Similarly to the previous point, when in a fasted state, women are more likely to use more protein for fuel during exercise than men.

Warnings

You should not engage in fasted training if you are missing or having irregular periods.

If you are trying to get pregnant or are pregnant, you should not engage in fasted training.

If you have any history or susceptibility to disordered eating or have been diagnosed with an eating disorder, fasted training is a bad idea for you.

However…

I read many resources about the risks of women doing fasted training. In almost all of them, fasted training is conflated with fasting and Relative Energy Deficiency in Sports (RED-S) or Low Energy Availability (LEA), which means you are not getting enough calories in your daily food intake. That is not what fasted training is. Fasted training means shifting your eating window an hour or two later in the morning while maintaining the overall caloric intake you need.

I am not universally suggesting women should or should not use fasted training. Like any training tool, it deserves careful consideration and application. It may well be that the balance of risk/reward for women leans too much to the risk side.

Gender Differences in Metabolism

https://www.womenshealthmag.com/uk/fitness/a41499257/fed-state-exercise

https://pubmed.ncbi.nlm.nih.gov/18787373

Part III: Proper Implementation

Gradualness:

Just as with any training stimulus, our bodies do not respond well to shocks imposed by sudden changes. If you choose to experiment with fasted training, ease into it. Even pro athletes use fasted training in only one or two sessions a week. And they typically do it in the early base phase when intensity is deliberately kept low. Start with no more than a one-hour fasted Zone 1-2 session in a week to see how it affects your recovery and energy level throughout the day. Doing too much too soon will almost surely backfire and could exhaust you.

Refuel

Get carbohydrates (best if coupled with protein) on board within 30 minutes of finishing any fasted session. If the session is longer than an hour, you should consider beginning to fuel part way through the session. This method has been standard among Evoke coaches for years. I was interested to hear that even professional cyclists use the technique during multi-hour training rides. Failure to refuel adequately will also cause this valuable tool to have harmful effects.

Low Intensity

Use fasted training only for Zone 1 and 2 sessions. This is probably the most common mistake people make. Fat should contribute about 50% or more of the total energy requirements during these low-intensity training sessions. Aerobic capacity and fat adaptation benefit mainly from long-duration, low-intensity work. Doing fasted high-intensity training where glycogen becomes the dominant energy source will diminish the training effect of that session because you need a high glycolytic turnover to power high-intensity training. Doing fasted Zone 3, 4, or 5 workouts will deeply deplete glycogen, delaying recovery and making you feel generally low energy.

Who Might Benefit

Low-Volume Trainers:

We have seen the most significant benefit of including some fasted training into an athlete’s plan for low-volume trainers. These are the amateurs with family and work responsibilities who may struggle to find time to get in more than 3 to 4 hours of Zone 1 or Zone 2 training sessions in a week. Low-volume trainers usually have plenty of time between training sessions to refuel and restore their glycogen stores, so recovery is less of an issue for them.

They stand to benefit the most from fasted training sessions because, unlike a high-volume trainer, they are not accumulating enough volume in Z1 and Z2 to promote aerobic adaptations significantly. Fasted training for this population can, to some extent, help compensate for their lack of aerobic volume. Don’t take this to mean that it is a shortcut to aerobic fitness. Many other adaptations occur with a high volume of training that can’t be duplicated with fasted training.

High Altitude Mountaineers:

We have seen substantial gains with this group when using fasted training. Because carbohydrates take less oxygen to metabolize, they are the preferred fuel at high altitudes. However, the mere act of eating at 7000 meters and above can be very challenging. While moving at high altitude, it can be downright dangerous, not to mention uncomfortable, to get any calories if you have to remove mittens to open a package of gel or a bar or take off a pack to fish out a frozen snack. Even in camp, your appetite will be suppressed, and getting enough calories is a frequent problem. Being well-fat-adapted can help provide more calories from intra-muscular fat stores.

Who May Not Benefit

High Volume Trainers:

Athletes doing a high volume (>10-12 hours of Z1-2/week) of aerobic training will, much of the time, start training where muscle (and possibly liver) glycogen is in a somewhat depleted state even when not doing fasted training. They will likely see smaller gains and a higher risk of poor recovery. We usually only use fasted training with elites during the early base period when low intensity and lower volumes allow it.

These folks will have less time between training sessions to refuel and restore muscle glycogen levels. Their recovery and, hence, their training quality, are more likely to be compromised. If this pattern persists, they can push themselves into an overtraining situation. This is the only potentially harmful outcome of fasted training. This “harmful” outcome is only possible if fasted training is misused and misunderstood.

High-Intensity Trainers:

The training for any athlete intensifies as the goal event approaches. This may mean adding faster running into the program for the ultra-runner. This would be in the thick of the ME weighted pack carries for the mountaineer. In both these cases, being well-fueled for all training sessions will become the priority, and fasted training should be dropped.

Exceptions Are Not The Rule

Many of you will have heard of some elite athletes doing extremely long and/or high-intensity training sessions in a fasted state or with minimal feeding. You may recall my podcast with Kilian, where we reviewed some extraordinary multi-hour things he has pulled off while eating almost nothing. I have seen similar things accomplished by other elite athletes. While it would be a mistake to try to replicate these feats unless you, too, have a decade or two of high-volume training, it does question the blanket statements made about fasted training being dangerous. How could these people perform so well if fasted exercise was so harmful?

Summary

Some do’s and don’ts of fasted training:

It is not a calorie-restriction dietary regimen. You eat the same number of calories. You just shift their intake timing a bit.
Do not do every session in a fasted state. Start with one low-intensity session per week.
It is not a weight loss diet. Trying to lose weight can mean not getting the calories you need.
It is not a fat-loss diet. The fat you use for exercise is invisible. It’s in the muscles. The subcutaneous fat you’d like to lose is the last fat to go when in a calorie deficit.
Do not do high-intensity workouts in a fasted state. This CAN be “harmful.” Only do low-intensity (Zone 1&2) training in a fasted state
Women should approach fasted training with caution. Apply the same do’s and don’ts listed above but I would highly recommend working with a coach or nutritionist with experience with this training and fueling approach since the health risk is greater if you misuse fasted training.
Eat carbs immediately following a fasted session.

Some more resources for those who want to do even deeper diving:

Further Resources