Fasted Training/Training Low – Why It’s More About How You Finish Than Start
For any athlete looking to improve performance, the prospect of using a nutrition strategy that can help you get more out of your training without actually doing more training is appealing. This is particularly true as most athletes have limits on the time they have available to train and want to maximize the responses they get from each training session.
One such strategy that has received considerable attention in recent years from scientific research, professional athletes, and the media alike is fasted/low glycogen training. Who remembers when the internet had a meltdown over Chris Froome’s 2016 Tour De France rest day breakfast?
This nutrition strategy effectively involves completing carefully selected training sessions commenced with limited carbohydrate availability (through various forms of carbohydrate restriction) to increase the metabolic stress of the session and subsequently drive specific training adaptations that may help improve performance. These adaptations are associated with improving fat metabolism, particularly the increase in mitochondria, the engine of the muscle cells, which is one of the most important adaptations for enhancing endurance performance.
Of note is that the idea behind this strategy is to complete only carefully selected sessions with low carbohydrate availability, not all sessions, as you might do if you adopted a low carbohydrate/ketogenic diet. We know that adopting a low carbohydrate approach chronically (i.e. over weeks and months) may significantly enhance your body's ability to use fat as a fuel, but this will come at the expense of its ability to store and use carbohydrates and, in most situations, negatively impact an athlete’s high-end performance. This is a form of carbohydrate periodisation when the fuelling for each session is adjusted based on the individual's needs, with the aim of developing an ability to use both fat and carbohydrate efficiently.
Train Low Methods
Practically, there are several different strategies that an athlete can use to train low during carefully selected training sessions…
Fasted Training — This effectively involves training first thing in the morning after an overnight fast (or 6hours+ since the last meal); by doing so, the athlete trains with low liver glycogen availability, particularly as this becomes depleted overnight. Providing the session is over 45-60 minutes in duration, this can increase the metabolic stress of the session but to a lesser extent than the other strategies we cover below. Athletes often adopt this strategy when it is practically challenging to eat and comfortably digest a meal pre-exercise (i.e., training first thing in the morning).
Training Twice Per Day — This is common for many athletes with demanding training schedules. Depending on the individual training session demands, the first session typically depletes glycogen stores, lowering carbohydrate availability. The limited recovery time between sessions means that the second session commences with low carbohydrate availability, often even if athletes have tried to refeed between sessions.
Sleep Low — Almost identical to training twice per day, but the glycogen-depleting session is performed in the afternoon/evening, with limited opportunity to refeed before an overnight fast, meaning that the session completed the following morning is commenced with low carbohydrate availability. Factor in 8 hours of sleep, and this can leave an athlete in a state of low carbohydrate and energy availability for a significant amount of time (12 hours+).
Low CHO Diet — The nuclear option is simply to reduce the overall carbohydrate intake within the diet. This relatively quick reduction in carbohydrate availability has a number of negative impacts on performance.
Restricting CHO Intake During A Session — This is a strategy that many athletes inadvertently implement through poor fuelling practices and challenges with the logistics of ensuring fuel is available for the ride. This effectively means that after 60-90 minutes of riding at a moderate to high intensity, the session is completed with a background of low carbohydrate availability.
Adverse Side Effects Of Training Low
Training in this manner typically comes with several negative side effects…
Protein Breakdown — In the absence of sufficient glycogen, the body will manufacture additional carbohydrates to fuel organs like the brain through the breakdown of protein (potentially muscle protein); over time, this can lead to reductions in muscle mass, which, over time, can negatively influence performance.
Training Quality — Training with low carbohydrate availability can negatively impact an athlete’s exercise capacity and typically leads to a reduction in the power an individual is able to produce or at least an increase in perception of effort. If the fuelling around the session is not properly managed, this can lead to a reduction in training quality, which can potentially negatively impact the gains in training over time. While there are some strategies we can use to help manage this (i.e., the use of carbohydrate mouth rinsing and caffeine), you can nearly always expect the session to be of a lower overall quality.
Recovery Time — Training with low carbohydrate availability increases the stress of the session, which can then lead to it taking more significant amounts of time to bounce back from the session, potentially negatively impacting the quality of subsequent training sessions.
Low Energy Availability — For athletes training regularly, it can often be a challenge to consume adequate energy to match daily energy expenditures. As such, training with low-carbohydrate availability can push this further and result in athletes creating even more significant energy deficits, driving low energy availability, which over time can have significant negative effects on both an athlete’s health and performance.
Does It Improve Performance?
Much of the research to date has evaluated the impact on the short-term (i.e., a few hours after exercise) responses of muscle to these strategies, particularly the molecular signaling and gene expression responses, but these responses are very complex. Of particular significance is that short-term molecular responses don’t necessarily translate into long-term adaptations that make an athlete faster.
To date, several studies have examined the impact of some of these strategies when implemented over the course of a few weeks as part of a bigger training block. A meta-analysis by Geji and Bebo., (2021) reviewed the results of nine of these studies, and while a small number of the studies it included have shown performance improvements, this was their conclusion…
“The subsequent meta-analysis demonstrated no overall effect of CHO periodization on endurance performance compared to control endurance training with normal (high) CHO availability. Based on the available literature, we, therefore, conclude that periodised CHO restriction does not per se enhance performance in endurance-trained athletes”.
Now, long-term training studies, even of a couple of weeks in duration, are incredibly difficult and expensive to complete, and that’s before you factor in how difficult performance is to measure meaningfully in a lab. There are a huge number of factors that are very difficult to control, and as such, whilst we haven’t seen convincing evidence of a significant performance effect, the absence of evidence doesn’t necessarily mean an absence of effect, but certainly the evidence we have at the minute, isn’t convincing that this is a shortcut to high performance.
Is Fasted Training Better For Fat Loss?
It’s not uncommon for athletes to look to improve body composition (i.e., lose fat mass to improve an athlete's power-to-weight ratio), and fasted training often seems to be a go-to strategy for many athletes and often coaches. Quite simply, there is nothing magical about training low/fasted for fat loss. If you were to consume the same diet on two days (in terms of total energy intake and macronutrients), but on one day you completed a training session fasted and another fuelled (simply by adjusting the time of intake of different meals), there would be zero difference in terms of fat loss, as has been shown in a meta-analysis by Hackett and Hagstrom (2017).
To achieve fat loss, we need a calorie deficit. While there is nothing magical about fasted training for fat loss, some of the train-low strategies outlined above may help athletes create an energy deficit to drive fat loss while also helping to manage training quality.
One of the few performance studies that saw an improvement in performance in a group of trained triathletes adopting three weeks of sleep-low training, compared to a matched group completing all sessions with high carbohydrate availability, also saw around 1kg in weight loss in the sleep low group (despite being on a controlled diet), with no change seen in the control group. It’s possible that the restriction of carbohydrates between the two sessions, which effectively translated to 12+ hours with minimal total kcal intake during that time, made it difficult for the athletes to consume enough to compensate after the low glycogen session was completed and, therefore, created a deficit.
In practice, if you’re an athlete looking to lose weight and completing low-intensity sessions with limited carbohydrates available doesn’t negatively impact the session quality, it could potentially be used as a tool (there are many alternatives, too) to help drive an energy deficit and support fat loss.
Why It Might Be More About How You Finish Than How You Start?
While much of the research has focused on using techniques pre-exercise in which to drive low carbohydrate availability during a session, we now know that this may not actually be necessary to maximise the molecular response required to improve fat metabolism and stimulate the growth of new mitochondria.
The Glycogen Threshold Hypothesis was developed in a review of the train low research by Impey et al., (2018). When pooling the results from multiple studies, they noticed that much of the enhanced cell signaling and gene expression that may lead to preferential adaptations occurred within a relatively narrow range of muscle glycogen values. Glycogen is measured in millimoles per kilogram of muscle, dry weight. A well-trained athlete, when fully loaded, would have around 700 mmol/kg dw, and the threshold in which enhanced adaptations are reported to occur is around 300–100 mmol/kg dw. While not every study to date supports the theory, it at least suggests that the degree of glycogen depletion or at least the glycogen level in the muscle when commencing exercise plays a role in helping to maximise the adaptations that occur.
In effect, if you start the training session or at least finish the session having depleted around 2/3rds of your glycogen, you are likely to get many of these signaling responses upregulated. Therefore ensuring that the fuel intake is approproate for the session itself (i.e. not to much and not to little), is a key way not only to ‘fuel the work required’ but to help maximise the adaptive responses.
A few days into writing this article, another meta-analysis was published by Diaz-Lara et al. (2024), which reviewed the cell signaling responses to low carbohydrate availability training. Of particular interest was that they showed when the train low trial of the study saw a marked reduction in glycogen stores relative to the control trial (where all training was done with carbohydrate intake), there was greater metabolic gene expression (which is part of the process for driving training adaptations).
“We propose that larger differences in muscle glycogen concentration between high- and low-CHO conditions (> 200 mmol kg dw−1) may be required to enhance acute cell signaling or gene expression”.
As such, providing you get your glycogen stored at low levels in a session, you are likely to see these increases in cell signaling.
Interestingly, we have seen studies such as this one by Team Ineos/SiS Nutritionist Marc Fell, in which even when participants fuelled at close to the current guidelines for competition, i.e. 8grams of carbohydrates per kilogram body weight the day before and a further 2grams of carbohydrates per kilogram body weight at breakfast, before commencing a demanding exercise task (180 min cycling at lactate threshold followed by an exercise capacity test (150% lactate threshold) while consuming either 0g, 45g or 90g of carbohydrates an hour,. In this trial, they saw big reductions in glycogen stores from high levels at ∼700 mmol kg DW to quite low levels at 250 mmol kg DW, and irrespective of the fuelling strategy, glycogen was still depleted, and no difference was seen in cell signaling pathways. As such, if it’s going to be a hard ride, make sure you fuel it well, as carbohydrate restriction won’t offer any additional benefit if the ride is already going to create a massive amount of metabolic stress.
What Are The Practical Implications?
In effect, yes, there can be some benefits to completing some training with limited carbohydrate availability. However, it has to be carefully managed to ensure it is beneficial and not to the detriment of the athlete. If an athlete already has a high training load or is completing a heavy session, it is likely that they are training low for much of their sessions already, and therefore, a focus around better fuelling may the more appropriate to help maximise the benefits of the session. In contrast, if it’s a short session, then training low might be an efficient way to maximize the stress of the session.
Generally, training low is likely to only be of benefit for individuals who have already maximised what they are able to do training-wise in terms of volume and are looking for ways which create further stress of drive adaptations. It is simply an efficient way to bring about adaptations. This particularly applies if you have limited time to train and already have good fuelling practices.
As a nutritionist, it is a strategy that I have rarely used with athletes, I tend to focus more around helping support an athlete periodise their daily carbohydrate intakes to match their individual needs better (i.e. fuelling for the work required), which will likely achieve the similar results without the requirement for acute carbohydrate restriction during a session.
Thanks for reading, and please feel free to ask a question below.
Ben
References
Burke, L. M., Hawley, J. A., Jeukendrup, A., Morton, J. P., Stellingwerff, T., & Maughan, R. J. (2018). Toward a Common Understanding of Diet-Exercise Strategies to Manipulate Fuel Availability for Training and Competition Preparation in Endurance Sport. International journal of sport nutrition and exercise metabolism, 28(5), 451–463. https://doi.org/10.1123/ijsnem.2018-0289
Gejl, K. D., & Nybo, L. (2021). Performance effects of periodized carbohydrate restriction in endurance trained athletes - a systematic review and meta-analysis. Journal of the International Society of Sports Nutrition, 18(1), 37. https://doi.org/10.1186/s12970-021-00435-3
Hackett, Daniel, and Amanda D. Hagstrom. 2017. "Effect of Overnight Fasted Exercise on Weight Loss and Body Composition: A Systematic Review and Meta-Analysis" Journal of Functional Morphology and Kinesiology 2(4), 43. https://doi.org/10.3390/jfmk2040043
Marquet, L. A., Brisswalter, J., Louis, J., Tiollier, E., Burke, L. M., Hawley, J. A., & Hausswirth, C. (2016). Enhanced Endurance Performance by Periodization of Carbohydrate Intake: "Sleep Low" Strategy. Medicine and science in sports and exercise, 48(4), 663–672. https://doi.org/10.1249/MSS.0000000000000823
Impey, S. G., Hearris, M. A., Hammond, K. M., Bartlett, J. D., Louis, J., Close, G. L., & Morton, J. P. (2018). Fuel for the Work Required: A Theoretical Framework for Carbohydrate Periodization and the Glycogen Threshold Hypothesis. Sports medicine (Auckland, N.Z.), 48(5), 1031–1048. https://doi.org/10.1007/s40279-018-0867-7
Diaz-Lara, J., Prieto-Bellver, G., Guadalupe-Grau, A., & Bishop, D. J. (2024). Responses to Exercise with Low Carbohydrate Availability on Muscle Glycogen and Cell Signaling: A Systematic Review and Meta-analysis. Sports medicine (Auckland, N.Z.), 10.1007/s40279-024-02119-9. Advance online publication. https://doi.org/10.1007/s40279-024-02119-9
Fell, J. M., Hearris, M. A., Ellis, D. G., Moran, J. E. P., Jevons, E. F. P., Owens, D. J., Strauss, J. A., Cocks, M., Louis, J. B., Shepherd, S. O., & Morton, J. P. (2021). Carbohydrate improves exercise capacity but does not affect subcellular lipid droplet morphology, AMPK and p53 signalling in human skeletal muscle. The Journal of physiology, 599(11), 2823–2849. https://doi.org/10.1113/JP281127