Fasted exercise training is a technique used by many endurance athletes. This post is going to unpack the potential benefits of fasted exercise training and examine if you should be incorporating it into your training regimen. Please note that I will be talking about fasted exercise training in the context of endurance sports. Therefore, the information in this blog may not apply to athletes training for other events such as power events, like weight lifting.
The point of fasted exercise training is to force the body to exercise while on an empty tank. It’s essentially exercise with limited carbohydrate availability. As I’ve mentioned before, carbohydrates are the most readily available fuel source during exercise. Carbohydrates are stored in the form of glycogen in the muscle and liver, but your body can only store a limited amount. When you exercise after fasting, these carbohydrate stores have become depleted and you’re running on an “empty tank.”
Training in a fasted stated might lead to some metabolic adaptation. This is because the muscles’ carbohydrate stores determine different signaling events that occur in response to exercise. For instance, for those interested in physiology, training with low carbohydrate stores has shown to increase maximal mitochondrial enzyme activity and content, increase rates of fat oxidation and perhaps, improved exercise intensity. What this essentially means is that training with low carbohydrate stores teaches the muscle to adapt to burning fat and boosts the body’s capacity for stored carbohydrate.
However, although the metabolic advantages of training with low carbohydrate availability has been shown, this metabolic adaptation has not been proven to translate to performance benefits. The main reason for the lack of performance benefits is that the metabolic advantages are counteracted by reduction in training intensity and quality. As I’m sure anyone who has tried exercising after not eating for a long time has experienced, you feel awful, and as a result, you’re often not able to train as hard.
Some athletes, myself included, still incorporate fasted exercise into their training. Because it’s hard to run fast in a fasted state, I only do this for easy runs in which my speed or intensity doesn’t matter. Therefore, I am completing a mixture of workouts in a carbohydrate-depleted state and other workouts with normal to high carbohydrate availability. My speed sessions are always practiced with normal carbohydrate stores because my training intensity and quality matter. My long runs are also always practiced with normal or high carbohydrate levels because I like to treat long runs like a dress rehearsal for the actual race and practice my fueling strategies to ensure that they will work on race-day.
When I do fasted exercise training, it’s after an overnight fast of about 10 hours, and then I run immediately in the morning, before breakfast. It takes about an hour of fasted running to initiate fat burning. However, for those wanting to incorporate this into their training, they will want to build up to this amount of time. Remember, as soon as you are finished running, it’s important to immediately refuel. Regardless of whether you just trained fasted or not, it is extremely important to refuel within the 30-minute window of completing exercise. It’s also important to note that you should not be doing fasted exercise training as your event approaches. On race day, your body’s carbohydrate stores should be fully stocked. The idea is to train low but compete high.
I hope this information provides some helpful insight into the pros and cons of fasted exercise training. It certainly is a new and exciting topic. However, more research is definitely needed into this area to learn how to best incorporate it into a training cycle for optimal performance benefits.