Between the increased popularity of intermittent fasting over the past several years and the logistical need for many runners to head out the door for their daily miles first thing in the morning, it’s not uncommon for runners and other athletes to find themselves running on an empty stomach or doing fasted cardio.
Some morning runners find that running on an empty stomach prevents cramps and feels perfectly fine, whereas other runners find that any fasted cardio sessions feel completely exhausting as if they are truly “running on empty.”
But what does the research say? What are the pros and cons of fasted cardio? Are there weight loss or performance benefits of working out on an empty stomach, or is it better to work out after eating?
Whether you’re currently performing fasted workouts or are wondering if a few fasted cardio sessions per week may accelerate your weight loss, keep reading and decide what is best for you.
In this guide, we’re going to look at:
- What Is Fasted Cardio?
- How To Do Fasted Cardio
- The Benefits of Fasted Cardio
- The Drawbacks of Fasted Cardio
- Should You Exercise On An Empty Stomach?
Let’s get started!
What Is Fasted Cardio?
Fasted cardio refers to performing a cardio workout (one that increases your heart rate) on an empty stomach. Examples of exercises that can fall under the umbrella of fasted cardio include running, cycling, swimming, rowing, HIIT workouts, using a stair-climbing machine or elliptical, among others.
The defining feature of fasted cardio is that the exercise is performed after an extended period without food. Most people do fasted cardio first thing in the morning after they get up but before eating breakfast. This may be anywhere from 7-16 or more hours since the last meal or snack was consumed.
Alternatively, some athletes who practice variable intermittent fasting schedules may perform fasted cardio later in the day. Though there aren’t definitive constraints, a workout may be considered “fasted cardio” if it has been at least four hours since any calories were consumed.
How To Do Fasted Cardio
If you decide fasted cardio makes sense for you, the easiest way to try is by doing an aerobic workout in the morning after you wake up and before you eat. Start with a short, easy workout.
For example, if you normally eat a small snack and run 5-6 miles 30 minutes later, try skipping the snack and doing a 20-30 minute easy jog to start. See how your body feels and responds to exercising in a depleted state before taking on a more intense workout.
5 Benefits of Fasted Cardio
There are several potential benefits of running on an empty stomach, let’s check them out!
#1: It May Increase Fat Burning
Fasted cardio has been shown to increase the relative percentage of fat oxidation, meaning that more of the calories you burn during the workout come from stored body fat.
When you exercise, the body uses stored fuel for energy. Carbohydrates are stored as glycogen, fat is stored as triglycerides in fat tissue, and protein forms structural proteins in muscle.
The body has limited glycogen stores in the liver and skeletal muscles, and these levels deplete overnight during your fast. Therefore, when you do fasted cardio, a greater percentage of the energy to support your workout comes from oxidizing fat. In fact, some studies show you may burn up to 20% more fat when exercising on an empty stomach.
That said, while a fasted workout can increase fat burning, it also increases muscle-burning because the body also turns to metabolizing the protein in muscle for energy as well.
Additionally, it’s important to note that the calorie expenditure does not increase during a fasted workout; rather the substrate, or source, of the calories to burn shifts.
#2: It May Contribute to Greater Weight Loss
Although fasted cardio doesn’t burn more calories, some studies have shown that people who exercise on an empty stomach end up eating fewer calories throughout the day.
Since weight loss results from a caloric deficit, it is possible that the reduced caloric intake in a fasted cardio state can yield more weight loss if it indeed contributes to less energy intake throughout the day.
#3: It Can Reduce Digestive Distress
Some people simply feel better working out on an empty stomach. Runners with sensitive stomachs, for example, often find that running on an empty stomach prevents cramping, gas, diarrhea, and bloating.
For some runners, even small snacks can cause side stitches and the need to quickly find a bathroom mid-run. In these cases, fasted workouts may be preferable.
#4: It May Improve Blood Sugar Control
The most common concern about doing fasted cardio is hypoglycemia, or low blood sugar, but most studies show that it does not cause detrimental decreases in blood sugar, even for those with diabetes.
Some studies have even found positive effects on blood sugar regulation and insulin sensitivity from exercising on an empty stomach. However, if you have diabetes, you should consult your healthcare provider before starting a fasted cardio routine.
#5: It Can Be Energizing
Just as some people feel depleted and exhausted if they try to exercise on an empty stomach, some people find they feel more energized.
Moreover, since fasted cardio usually takes place first thing in the morning, some proponents of working out on an empty stomach say it helps them jumpstart the day, and they finish the workout feeling like they are ready to go.
5 Drawbacks of Fasted Cardio
Fasted cardio isn’t without its drawbacks, including the following:
#1: It Does Not Lead to More Fat Loss
Even though doing fasted cardio may help you burn a higher percentage of calories from fat, it does not seem to lead to greater fat loss or favorable changes in body composition.
According to research, fat loss and body composition changes were the same whether cardio exercise is performed in the fed or fasted state.
#2: It Can Increase Cortisol Levels
Cortisol is one of the primary stress hormones in the body, and chronically elevated cortisol levels have been associated with triggering the body to store more fat, particularly in the abdominal area.
Studies have shown that working out on an empty stomach can increase cortisol levels, as the body perceives fasted cardio as a significant physiological stressor.
#3: It May Cause Hormonal Imbalances
Just as doing fasted cardio can alter normal cortisol secretion, there is also evidence to suggest that exercising in the fasted state can cause hormonal abnormalities. This can potentially increase the risk of injury.
#4: It May Be Detrimental to Muscle Mass
As mentioned, working out in a fasted state can force your body to burn protein in muscle tissue for energy, which is detrimental to strength, athletic performance, health, and metabolic rate.
#5: Performance May Suffer
Arguably the biggest problem with fasted cardio is that most research shows athletic performance suffers when you exercise without adequate fuel. Strength, speed, and intensity levels tend to be significantly better when exercise is performed in a fed state, particularly when adequate carbohydrates are available.
The rate of perceived exertion also tends to be higher with fasted cardio, meaning the workout feels harder than it actually is.
The Pros and Cons of Fasted Cardio: Should You Exercise On An Empty Stomach?
Taken together, these findings suggest that while there are several pros and cons to consider, most people running on an empty stomach or doing some other form of fasted cardio will lead to subpar performance. Over time, if you regularly engage in fasted workouts, you may limit your fitness progress.
At the end of the day, deciding whether or not you should workout on an empty stomach depends on your goals, motivations, and what works for your body.
The primary reason to decide to do fasted workouts would be if you really struggle with GI upset if you exercise after eating. However, most coaches say that the digestive tract is somewhat trainable.
Try very light, easily digestible snacks like rice cakes, a few bites of an energy bar, or a couple of dried dates or apricots two hours before a run. If that seems tolerable, gradually move the window to 90 minutes before the run and see if your body feels better.
For more information about general running nutrition, check our Running Nutrition Guide!