The Science-Backed Benefits of Running on Empty
As run clubs continue their global domination, athletes are becoming more and more interested in the mechanics of the sport and the best ways to train and improve their pace. They’re also starting to explore the ideal diets to fuel their training, and for some runners, the food that works the best is no food at all.
“Fasted cardio” and “low-glycogen” training are two buzzworthy concepts in the running world. While they share some overlap, each has its own definition, benefits, and drawbacks. Both strategies are worth exploring, especially if you're looking to experiment with performance and fat-adaptation in the short term.
With the help of an expert below, we’ll explain what it means to run on empty, why you might consider trying it, and how to know if it isn’t the right plan for you.
What Does It Mean to Run Fasted?
Like many fitness terms, “fasted” doesn’t necessarily have a fixed meaning. “When people speak of fasting, they typically refer to an overnight fast of 10-12 hours,” says Professor Richard J. Bloomer, Dean of the College of Health Sciences at the University of Memphis and an expert in fasting. However, Bloomer points out that some studies consider a “fasted state” to be only four to five hours without food.
Oftentimes, people use the term “fasted state” to refer to the fact that their glycogen stores are low. When that’s the case, the body may switch to burning a higher percentage of fat compared to carbs. However, fasted cardio and low-glycogen training aren’t the same thing.
What Is Low-Glycogen Training?
A “fasted state” isn’t necessarily “low-glycogen training,” another term that’s common in the running space. “Someone could be very well fed, with muscles saturated with glycogen, but yet train first thing in the morning in a fasted state,” says Bloomer. “The fact that they are fasted does not imply that glycogen stores are low (other than liver glycogen).”
People wanting to train in a low-glycogen state need to restrict carbohydrate intake, Bloomer adds, and that’s typically accomplished through some version of a ketogenic diet, which severely limits the amount of carbs you eat. Some athletes may choose to adhere to that lifestyle year-round, or they may use it only during certain training windows, like in the lead-up to a race.
Related: This Disturbing Side Effect of Marathon Running Caught Scientists Off Guard
Benefits of Running Without Fuel
Fat Oxidation
When you run without fuel, your body is forced to burn fat instead of carbohydrates for energy. Over time, it becomes more efficient at burning fat—a process known as “fat oxidation.” The advantages of this process are backed by research. In one small 2015 study, 10 men exercised before breakfast, and their fat oxidation over 24 hours increased. In another small 2017 study, nine women also showed elevated levels of fat oxidation.
Reduces Gastric Discomfort
Fasted cardio also tends to reduce the risk of gastric discomfort, making it especially appealing for runners with sensitive stomachs who find it hard to train after eating. If that sounds like you, it may be helpful to at least experiment with a fasted state and/or low-glycogen training to better understand what foods you can train on and which ones you should avoid.
Convenience
Running fasted is also just super convenient. Simply wake up, grab your shoes, and go for a run without having to worry about breakfast (at least, not until you get back home). Afterward, shoot for lean proteins and healthy carbs.
Who Shouldn’t Do Fasted Cardio?
This style of training isn’t for everyone, says Bloomer.
“From a health risk perspective, those with uncontrolled blood sugar should be cautious if using this approach. However, most people will have the ability to regulate blood sugar with typical workouts, despite being fasted.” If this is a concern of yours, Bloomer recommends having a glucometer on hand to measure your glucose levels, along with some carbohydrates in case you need a quick boost.
Similarly, make sure that the exercise you’re doing is truly cardio and not something like high-intensity interval training. For short bursts of high-intensity exercise, your body relies on glucose (carbohydrates) for fuel. If your glycogen stores are depleted, you’ll almost certainly notice a drop-off in performance.
In addition, some people “simply do not feel good when they exercise fasted.” This group will likely get less out of their workouts when there’s nothing in the tank, so Bloomer suggests that they eat what they feel comfortable with.
That being said, your body may be more adaptable than you realize. If you’re interested in the idea of fasted cardio or low-glycogen training, you can start by running after a relatively small amount of time without any food (fasted) or just without carbs (low-glycogen)—say, four hours. Gradually, you can increase that window and see how your body reacts.
Whatever you decide, though, make sure that it’s practical on race day. The last thing you want to do is make a major adjustment to your diet a few hours before the starting gun goes off.