There are a number of dedicated gym-goers who prefer to work out on an empty stomach, perhaps because they like working out early but don’t like breakfast, or they believe working out fasted will burn more fat instead of food energy.

While I am not a proponent, my husband is a steadfast pre-breakfast daily exerciser. There is a modicum of research to support this practice. However, intermittent fasting (IF) is not without potential pitfalls to be aware of before making it a habit.

What Happens When Exercising in a Fasted State

Dr. Valter Longo, Director of USC’s Longevity Institute, believes IF can be problematic because “it allows people to improvise and pick and choose periods of fasting that range from 12 hours to weeks, giving the impression that… some ‘abstention from food’ is similar or equivalent, and all provide health benefits.” It’s not technically fasting if you’re going without food for 24 hours or less. The correct term for this practice is time-restricted feeding (TRF).

An empty stomach triggers hormonal shifts within the body that are conducive to both building muscle and burning fat. Improved insulin sensitivity is one such example. Our bodies release insulin when we eat, facilitating the absorption of nutrients from food. Once broken down into glucose, this sugar is then shuttled out of the bloodstream and into the liver, muscles, and fat cells, to be used as an energy substrate.


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However, overconsumption of calories as well as eating too often can create decreased resistance to insulin’s effects, thereby elevating the risk of heart disease and cancer. By eating less frequently, the body releases insulin less often, heightening the body’s sensitivity to insulin, thereby fostering fat loss and more positive blood flow.

During exercise, the body tends to utilize fats for fueling low- to mid-level exertion, and carbs for high-level exertion. Surprisingly, even athletes with very low body fat have access to relatively large reserves of fat calories, an average of approximately 50,000. Endurance athletes sometimes experiment with time-restricted feeding (TRF) since fasted workouts improve the storage efficiency of glycogen.

Training on an empty stomach makes the body more efficient at glycogen storage. An occasional fasted training session, therefore, may improve the quality of workouts performed after normal fueling. When the body learns to exert energy in the absence of calories, it gets better at performing when normal fueling patterns return.

Glycogen Stores Versus Fat Stores

Liver and muscle glycogen are almost always a limited commodity, only supplying 1,400 – 1,800 calories when fully stocked. In a fasted state, the body conserves glycogen and instead utilizes fat as fuel. Some experts claim that weight training in a fasted state induces the release of growth hormone (GH). Activities that promote the release of GH include sleep, exercise, and fasting.

However, while the release of GH during a fast may have an anabolic effect, this is most likely a result of the body trying to mitigate the catabolic impact of not eating, according to a study at the University of Virginia. The conclusion drawn here was that if one’s goal in the gym includes pushing oneself, working out in a fasted state can backfire and actually lower productivity.

Finding the Middle Ground

A happy medium does exist, as my competition coach taught me. If a client prefers to exercise in a fasted state first thing in the morning, a bedtime meal can help ramp up glycogen stores for use the next day. Knowing that fats will temper digestion, a banana or a sweet potato paired with nut butter scores big points.

Even though I always fueled prior to morning lifting, if the designated body part the next day was going to be back or quads (large muscle groups), I would mix some protein powder in with the peanut butter and couple this blend with the banana or sweet potato at my bedtime meal. This combination is not only delicious but will also yield extra energy for the next day’s workouts.

Prolonging the Burn

A few studies have found that cardiorespiratory exercise performed in a fasted state can burn 20% more fat compared to exercising after healthy fueling. One study showed that 24 hours without food increases GH production in men by 2000%,and 1300% in women. 

However, experts point out that fasted cardio does not increase the body’s capacity to burn fat over the ensuing 24 hours. As such, this does not always lead to sustained weight loss. The effect ends at the conclusion of the fast. Ingesting complex carbs prior to hitting the gym increases the post-exercise “afterburn” effect more than that which one experiences in the fasted state. This translates to extra calories burned throughout the day, and not solely during the workout.


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Cardiac Benefits of Fasting

Research cardiologists at the Intermountain Medical Center Heart Institute are reporting that fasting not only lowers one’s risk of coronary artery disease and diabetes, but also causes significant changes in a person’s blood cholesterol levels. Both diabetes and elevated cholesterol are known risk factors for coronary heart disease.

“Fasting causes hunger or stress. In response, the body releases more cholesterol, allowing it to utilize fat as a source of fuel, instead of glucose. This decreases the number of fat cells in the body,” says Dr. Benjamin D. Horne, PhD, MPH, Director of cardiovascular and genetic epidemiology at the Intermountain Medical Center Heart Institute, and the study’s principal investigator. “This is important because the fewer fat cells a body has, the less likely it will experience insulin resistance, or diabetes.”

The Risks of Fasting and Exercise

Before you jump on the fasted cardio trend, there is a downside to consider. Skipping a meal or two can be profoundly difficult for some people, ineffective for others like bodybuilders, and downright dangerous for those who have wrestled with eating disorders.

While exercising in a fasted or semi-fasted state, the possibility exists that the body will begin catabolizing hard-earned muscle as a fuel source, says Chelsea Amengual, MS, RD, Manager of Fitness Programming & Nutrition at Virtual Health Partners. “Plus, you’re more susceptible to hitting the wall, which means you’ll have less energy and not be able to work out as hard or perform as well,” she adds.

Priya Khorana, EdD, a nutrition educator at Columbia University, believes that intermittent fasting and exercising long-term isn’t ideal. “Your body depletes itself of calories and energy, which could ultimately end up slowing your metabolism.”

Certified personal trainer Lynda Lippin stresses the importance of heeding one’s total consumption of macronutrients the day before exercise as well as after training has been completed. “For example, strength workouts generally require more carbohydrates the day of, while cardio/HIIT workouts can be done on a lower carb day,” she explains.

Dr. Niket Sonpal suggests that the optimal method of combining intermittent fasting and exercise involves timing workouts during eating periods so as to utilize the peak nutrition levels. “And if you do heavy lifting, it’s important for your body to have protein after the workout to aid with regeneration,” he adds. Amengual is a big believer in following up any strength training with the carbohydrates and protein combination, ideally within 30 minutes of finishing the workout.

Safety First

The success of any weight loss/exercise program depends upon the safety of sustaining it over time. A few key tips on proceeding prudently can make all the difference when a client is experimenting with TRF.

 Drink Up!

Sonpal stresses that fasting does not include fluid consumption encouraging fasters to drink more water during periods of restricted feeding. In an effort to maintain electrolyte homeostasis, Sonpal suggests coconut water. “It replenishes electrolytes, is low in calories, and tastes pretty good,” he says. Many sports beverages are laden with sugar and empty calories and are best avoided during TRF.

Stick to Low-Intensity and Shorter Workouts

If a client says he/she is pondering a 24-hour intermittent fast, Lippin suggests sticking to low-intensity workouts (walking, yoga, or Pilates). Working within the parameters of a 16- hour fast/8- hour fuel plan, bear in mind that the majority of the 16-hour fasting window is evening, sleep, and early morning. A client on this program can feel comfortable sticking to his/her regular type of exercise.


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During my foray into TRF, I chose a fueling window of 12 hours per day, allowing me to tap into the benefits while not having to sacrifice any part of my training. According to Dr. Longo, if you eat only for four to six hours a day, “then you start to see gallstone formation [and] increase the chance that you’re [going to] need your gallbladder removed.”

Tune Into Your Body

The most important advice to heed when exercising during IF is to listen to your body. “If you start to feel weak or dizzy, chances are you’re experiencing low blood sugar or are dehydrated,” explains Amengual. If this occurs, she suggests choosing a carbohydrate-electrolyte drink immediately, followed by a well-balanced meal.

While exercising and intermittent fasting or time-restricted feeding may prove highly effective for certain individuals, others may not feel comfortable working out at all during a fast. According to Harvard Health, a full 38% of individuals who experimented with intermittent fasting were unable to sustain the process. If fasting runs the risk of inciting a binge on the weekends or cheat days, this may not be an ideal plan.

Trainers should insist that prior to exercising on any TRF or IF program, clients consult a doctor or healthcare provider, particularly if they have any pre-existing medical conditions. Just as each client has unique goals in the gym, discovering his/her nutritional habits can help foster not only a relationship with open communication, but yet another avenue down which you can travel together as you impart your knowledge on this subject as well as fitness! 

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Cathleen Kronemer

Cathleen Kronemer is an NFPT CEC writer and a member of the NFPT Certification Council Board. Cathleen is an AFAA-Certified Group Exercise Instructor, NSCA-Certified Personal Trainer, ACE-Certified Health Coach, former competitive bodybuilder and freelance writer. She is employed at the Jewish Community Center in St. Louis, MO. Cathleen has been involved in the fitness industry for over three decades. Feel free to contact her at She welcomes your feedback and your comments!