The word “diet” carries with it a pretty heavy load of debate and opposing opinions nowadays. I’ll keep it short, avoid starting arguments, and say that there is A LOT of misinformation out there related to dietary and nutrition advice being given by individuals not credentialed or educated enough to be giving the advice in the first place. Additionally, there really is NO ONE PERFECT DIET for all athletes. Some athletes thrive on a vegetarian or vegan diet, while others thrive and perform well eating meat and dairy. Others seem to perform well on a low carbohydrate diet whereas others don’t. The research investigating dietary manipulation strategies to enhance athletic performance is a very new and constantly developing field of research. Furthermore, the direction that nutrition research is heading is not towards finding the “perfect” diet for athletic performance, but rather towards finding the best strategies of manipulating nutrient timing and quantity to maximize training and racing performance while maximizing long-term adaptation.
With that being said, there are a few dietary manipulation strategies that are gaining more and more attention due to recent research. The manipulation of carbohydrate intake and timing is one of those areas, particularly the reduction of carbohydrate intake before training sessions to promote fasted training. Fasted training is typically executed by performing a low-intensity exercise session first thing in the morning without eating anything, thereby placing oneself in a fasted state during exercise.
What is the science behind fasted training?
There is some solid evidence emerging showing that training in a fasted state can enhance the aerobic stimulus that is placed on the body from the training session (1). In more technical terms, performing fasted, low-intensity training sessions seems to enhance the metabolic adaptations that take place at a cellular level (i.e., within the mitochondria) allowing the body to be better equipped to metabolize fat as fuel during subsequent training sessions (1,2). How does this happen? Well, training in a fasted state creates a shift towards a reliance on fat metabolism during the training session. Whereas, if you ate a meal prior to training, particularly one high in carbohydrates, the insulin response that would ensue from that meal would send the body through a cascade of events that would force it to rely more heavily on carbohydrates (i.e., glucose specifically) as fuel, blunting the adaptations mentioned above related to the enhanced aerobic stimulus.
Why would enhancing the aerobic stimulus placed on the body matter? Well, fasted training can be a simple way of improving your body’s ability to metabolize fat as fuel without compromising the body’s ability to metabolize and utilize carbohydrate as fuel. This is extremely important for endurance athletes as they need the capacity to effectively metabolize fat during lower-intensity exercise and carbohydrate at higher-intensity exercise. Most athletes that eat a carbohydrate-rich diet are already great at metabolizing carbohydrate as fuel, however, the potential benefits of enhancing one’s capacity to metabolize fat without compromising the ability to metabolize carbohydrates (as is often the case with radical dietary shifts that promote very low carbohydrate intake) is what is really important about fasted training. Although there isn’t as much research demonstrating how these metabolic alterations translate to potential performance improvements for fasted training specifically (2), research examining the effects of “sleep-low” training (performing a training session in the evening without consuming carbohydrates afterwards, followed by sleeping, followed by fasted low-intensity exercise the following morning) has shown improvements in various measures of cycling performance, running performance, and body composition (1). These types of training/dietary manipulations have some similarities, and it is possible fasted training has similar benefits, but more research is needed to demonstrate these benefits.
It could be speculated, however, that improving one’s ability to metabolize fat during exercise can shift the body’s reliance away from carbohydrate and towards fat when training or racing at lower intensities, which may help improve one’s endurance and aerobic efficiency due to less reliance on carbohydrates at lower intensity exercise. This may also help athlete’s spare precious carbohydrate stores in the body when racing long distances (e.g., Ironman racing, ultra-marathons, etc.) and help prolong the duration that one can go without running out of stored carbohydrate in the body (i.e., known as glycogen stores).
What does this mean for YOU as an endurance athlete or coach?
So, although the research investigating fasted training is relatively new and still emerging, in my opinion it might be worth a try as it carries with it the potential benefits of enhanced fat metabolism capacity and a pretty low risk of any unwanted negative effects on performance that might come with a complete dietary overhaul. For example, some athletes might not respond well to a shift from a high carbohydrate diet to a low carbohydrate diet, actually performing worse in training sessions and races because they have now taught the body to be really good at metabolizing fat as fuel at the expense of hindering the body’s ability to use carbohydrates as fuel. Fasted training is a simple way of enhancing the aerobic stimulus placed on an athlete’s system. However, here are the “rules” you’ll want to make sure you follow if employing it into your training routine:
Perform fasted training on an empty stomach first thing in the morning. You can drink fluids, such as water or coffee, as long as these fluids do not contain calories as this would place you out of a fasted state.
There is technically the possibility of doing fasted training sessions in the evening if you eat in the morning and then avoid food for 6-8 hours, but this is usually logistically and practically not possible. For example, you may need to work during the day and will want to be well-fueled so that you can be productive at work. You may also have a morning training session and will want to remain well-fueled post-session to promote adequate recovery.
ONLY do fasted training 1-3x/week for LOW-INENSITY (i.e., zone 1 recovery or zone 2 aerobic) sessions lasting less than 60 minutes.
Doing hard, intense workouts in a fasted state impairs your ability to use carbohydrates as fuel and will hurt your ability to hold higher intensities in a hard session.
Keep in mind that doing an easy session fasted does potentially increase the total stress of the session when compared to doing it in the fasted state, so be sure to recover appropriately afterwards and get in a good post-session recovery meal with proteins and carbohydrates.
ONLY MEN should do fasted training… yes only men, not women. Women, so far, seem to respond better to fed-state training than they do when implementing fasted training (1). Women also carry a higher level of “metabolic stress” with them, for lack of a better phrase, than men do on a regular basis with their hormonal ebbs and flows due to their menstrual cycle. This could be one reason why women don’t respond as well to fasted training as fasted training is a greater stress on the body than fed-state training, but more research is needed to confirm this.
This does not mean that women are not allowed to give fasted training a try. If you are a woman and you do want to give fasted training a try, just pay attention to how you feel during and afterwards, but more importantly, see how it impacts your weekly training. If you feel like the fasted sessions do not feel good and take a long time to recover from, then avoid them. Additionally, if you start implementing fasted training sessions and you feel like other training sessions begin to suffer during the week, it may be smart to avoid the fasted training altogether. The slight benefits that fasted training has is not worth the potential negatives of prolonged recovery if you do not respond well to it.
There you have it! Fasted training might be worth a try, if you are a man, and are looking to enhance the training stimulus you get from easy low-intensity training sessions lasting less than 60 minutes.
1. Earnest CP, Rothschild J, Harnish CR, Naderi A. Metabolic adaptations to endurance training and nutrition strategies influencing performance. Research in Sports Medicine. 2019 Apr 3;27(2):134-46.
2. Aird TP, Davies RW, Carson BP. Effects of fasted vs fed‐state exercise on performance and post‐exercise metabolism: A systematic review and meta‐analysis. Scandinavian journal of medicine & science in sports. 2018 May;28(5):1476-93.
Happy training and racing!
-Ryan Eckert, MS, CSCS
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