On the surface, it seems to make sense. Exercise first thing in the morning on an empty stomach and your body should burn more fat. After all, without food intake for eight to 12 hours, you’re in a fasted state. With glycogen stores depleted and low morning insulin levels, your body has to turn to other energy sources, like fat, to power through your workout.
Fasted cardio is a technique that has been around for years in the bodybuilding community, and one that’s gaining popularity among regular gym goers and even endurance athletes. But will forgoing your morning oatmeal really lead to a leaner body and better athletic performance?
Here’s the skinny on sweating it out sans food and whether or not it is suitable for your body and fitness goals.
The Fast Track to Fat Loss?
“Some studies have found that exercising in a fasted state can burn almost 20 percent more fat compared to exercising with fuel in the tank.”
Un-fueled workouts are a common protocol among bodybuilders, especially in preparation for competition when they need to lean out as much as possible before they walk on stage. Some celebrities also turn to this technique to get ready for their close-up, whether that’s a photo shoot or red carpet event.
“Is fasted cardio a good way to burn fat? The answer is yes and the answer is also no,” says Jay Cardiello, a NSCA and ISSA-certified strength and conditioning specialist. The fitness expert who has worked with celebrities such as Jennifer Lopez, Curtis “50 Cent” Jackson and Sofia Vergara, says, “It’s effective, but it’s not sustainable.”
Some studies have found that exercising in a fasted state can burn almost 20 percent more fat compared to exercising with fuel in the tank. Why? Once we eat, insulin (which regulates the breakdown of fat) increases in our body. According to some research, higher insulin levels have been shown to suppress fat metabolism by up to 22 percent.
However, research has demonstrated that fasted cardio does not increase fat burning over a 24-hour period. While your muscles adapt to using more fat when you exercise, you don’t actually lose more fat overall on the days that you exercise versus days that you don’t. Another study suggests that ingesting carbs before working out increases the post-exercise “afterburn” effect more than the fasted state. That means more calories burned throughout the day, not just during your sweat session.
But First, Safety
If you’re curious about running on empty, be sure not to go it alone. Cardiello has employed the fasted cardio method with clients for a short period of time and within a controlled environment. He advises that fasted cardio “shouldn’t be used unless you are being looked after by a nutritionist, professional strength training coach and a medical professional.” For example, 50 Cent used fasted cardio to prepare for an album cover photo shoot. Cardiello closely monitored the rapper’s fitness and nutrition throughout this period to ensure that he was healthy and safe.
Glycogen-Depleted Workouts: The New Face of Fasted Cardio
Beyond bodybuilding and celeb weight loss, endurance athletes have started to turn to “glycogen-depleted workouts,” too. To prevent “bonking” during a race, athletes hope their bodies adapt to low blood-sugar levels by burning fat. Occasional early morning exercise without fuel has been shown to increase VO2 max and the concentration of glycogen found in muscles when at rest.
While fasted runs can adapt the body to more efficiently burn fat as fuel, they didn’t help athletes lose weight or improve their aerobic conditioning or endurance during the studies. Plus, runners still need to practice fueling during workouts to properly prepare for taking in nutrition on race day — particularly if you’re going the 13.1-distance or more.
Not So Fast
“For most people that small change won’t translate into body fat changes and performance benefits over the long-term.”
While fasted cardio may turn your body into an efficient fat-burning machine, Lauren Antonucci, registered dietitian nutritionist and certified sports dietitian, and owner of Nutrition Energy, cautions against this technique. “Can I force my body to burn a higher percentage of fat than carbs? Yes, I can. But I think that the more important question is will I see a benefit as an athlete? So far, the answer is no,” says Antonucci. “For most people that small change won’t translate into body fat changes and performance benefits over the long-term.”
When you wake up in the morning, your body is in a catabolic state due to high levels of cortisol — the hormone associated with stress. That means that your body is primed to break down molecules for energy. But if you’re in this state too long, it can lead to degradation of body tissue, including muscle, and a decrease in overall health.
“If you don’t eat, you’re not going to have the energy to train at full capacity,” says Cardiello. “If you’re on empty, you’ll go into a major breakdown phase after the workout.” In fact, research has shown that hitting the weights without fuel can lead to the breakdown of muscle.
“Recovery from these types of workouts can be very long and not as enjoyable. I’d rather fuel myself optimally and give myself the best chance of recovery,” Antonucci says. “The return on investment [for proper fueling] will make it back to you in how you feel during your workout, how you recover, and over the long-term.”
To Eat Or Not to Eat
If eating before your early morning session at the track or gym doesn’t work with your schedule or stomach, that’s fine. Just pay attention to potential side effects such as dizziness or nausea.
And if you’re truly interested in improving athletic performance or losing fat, make sure you’re asking the right question — not are you eating before workouts, but what are you eating throughout the day?
“Check in on your diet and reduce your calories. Make some dietary changes overall,” says Antonucci. According to Cardiello, “Feeding the whole system will help increase the intensity level during a workout, thus recruiting more muscle activation and boosting metabolism for the whole day.”
Originally published August 2014. Updated March 2017.