Many people within the fitness community practice something called fasted cardio to help achieve their goals. Known for its association with quicker weight loss, fasted exercise has become quite popular.

So what is all the hype?

Ahead, discover the potential benefits of fasted cardio and whether fasted weight lifting offers the same.

What Is Fasted Cardio?

Fasted cardio is exactly what it sounds like, where someone performs cardiovascular or aerobic exercise in a fasted state. Otherwise known as morning cardio, proponents often perform it upon waking and before their first meal (or post-workout meal).

While there is no set guideline for the length of the prior fast, advocates generally aim for at least an eight hour fast. Ten to 12 hour fasts sometimes precede depending on when they last ate the day before.

Benefits of Fasted Cardio

The main potential benefit of fasted cardio centers around quick weight loss, which is often the main motivator for people trying it. In a fasted state, the body relies on stored forms of energy, glycogen (in the liver and muscles) and fat from adipose tissue.

Thus, fans of fasted cardio claim using stored forms of energy leads to more fat burn because it essentially helps tip the balance of burning and consuming calories towards burning more. In theory, this makes some sense.

However, enter skeptics here.

These skeptics point out that depending on the intensity of the exercise, the body will likely utilize glycogen as a fuel source first. In fact, studies show that the average person can store about 15 grams of glycogen per kilogram of body weight. To highlight a practical example, this means a 70 kilogram person could store about 1,050 grams of glycogen, which translates into 4,200 calories of energy. For additional perspective, running a marathon burns about 2,000 to 3,000 calories of energy.

What Does Science Say?

Some research suggests that an 8 to 12 hour fasted cardio session may help burn up to 20 percent more fat. (That is pretty significant!) Yet, other research shows fasted versus fed cardio makes absolutely no difference in overall fat loss. Likely, genetics, personality, hormone balance, and type of workout play a role in whether morning cardio produces desired results.

Another potential benefit caters to those who experience GI distress when eating before a cardio workout. A number of people feel nauseous or make frequent bathroom trips if they eat before running or other intense cardio. Thus, fasted cardio offers a way to sidestep this nuisance.

Finally, incorporating some fasted cardio sessions can help build mental toughness. Learning how to push through a workout even when it feels difficult can translate into more grit during competitions. However, this may not be necessary or beneficial for a recreational athlete.

Risks of Fasted Cardio

There are possibly more risks than benefits of fasted cardio. These include exhausting mental challenges, injury risks, and certain medical conditions.

Mental Challenges

For one, performing intense exercise in a fasted state is very mentally and emotionally challenging. So much so, that it could negate the practice session and negatively affect competition performance down the road.

Injury Risks

Lack of pre-workout fuel can deter concentration, which may lead to higher risk of accidents and injuries. When concentration starts to waver, that is a sign one is about to "hit the wall".

In terms of physical performance, not eating before cardio can decrease output. Meaning, performance is not optimized. Research certainly shows that people can exercise longer and more intensely in a fed state. This not only translates into better exercise performance, but also indicates one could theoretically burn more calories.

Medical Conditions & Histories

Additionally, fasted training is not for everyone. These include people with medical conditions, especially if they cause dizziness or lightheadedness from low blood sugar and pressure (like type 1 diabetes and epilepsy). Those with a history of eating disorders should also avoid fasted cardio. Fasted workouts could be dangerous for the first group and triggering for the second.

What About Other Fasted Exercises?

Does fasted weight lifting offer the same benefits as fasted cardio? According to the research, no, and doing so could negate the benefits of strength training. 

In a fasted state, the body will break down more muscle tissue. This especially occurs when exercise is intense, because the body desires the quickest forms of energy, which in this case can be the amino acids from muscles. This is completely counterproductive to the main goal of building muscle through weight lifting.

The same concept applies to other activities that are intense and powerful in nature like power lifting, sprinting, long-endurance running intervals or fartleks, plyometrics, and some circuit training. These activities are simply too intense to burn fat efficiently, thus relying on glycogen stores and protein (amino acids) when the former runs low.

Moreover, a study revealed that running intensely in a fasted state yielded twice the amount of skeletal muscle protein breakdown as training in a fed state.

The Bottom Line

Fasted cardio is a trendy fitness method that may lead to more fat burn but not necessarily at a better rate than fed exercise. While fasted cardio is generally safe for the majority of people, it does require extra caution to avoid injury, illness and decreased performance potential.

But whereas fasted cardio provides some potential benefit, there is essentially no advantage to performing fasted weight lifting or another powerful exercise. Rather, it is more damaging than helpful.

All in all, if it sounds appealing, give it a try. And if not, it is probably not a loss, as the research is conflicting and generally leans toward it being unnecessary.

References:

Friedrich C. How Much Glycogen Can Your Body Store? Cathe Friedrich. Cathe. Published July 13, 2019. cathe.com/how-much-glycogen-can-your-body-store/.

Lindberg S. Can You Lose Weight Faster by Exercising on an Empty Stomach? Healthline. Updated September 11, 2018. www.healthline.com/health/fitness-exercise/fasted-cardio-when-to-eat-workout.

Mateo A. Is It a Good Idea to Go for a Run on an Empty Stomach? Runner's World. Published February 4, 2021. www.runnersworld.com/nutrition-weight-loss/a30277147/fasted-cardio/.

Moore A. Should You Exercise On An Empty Stomach? Here's What The Experts Say. Mindbodygreen. Published June 30, 2020. www.mindbodygreen.com/articles/what-is-fasted-cardio.