Fasted cardio has become popular among bodybuilders looking to lean down for competition day and celebrities wanting to look red carpet carved.
In theory, it sounds like a great idea — push through a hungry morning workout and you’ll be well on your way to burning fat all day!
But before you give yourself the green light to grab a dozen “well deserved” doughnuts, it’s important to understand whether fasted cardio is right for you and what foods will keep it working for you.
As a nutrition professional and experimenter of fasted cardio, I’m eager to ignite your hungry minds.
So what should you eat after fasted cardio?
Eating after fasted cardio will depend on the type of exercise you do and your fitness goal. Typically, it’s recommended to have a healthy source of protein and carbohydrate after a fasted workout. However, if your goal is to continue using fat as fuel (such as with a keto diet), you’ll need to continue low-carb.
Let’s run through the research on what’s right after running on an empty tummy.
What is Fasted Cardio?
Just like it sounds, fasted cardio is a form of cardio exercise (running, walking, biking) while fasting. This is typically done in the morning since your body has had all night to use stored energy.
The idea behind hitting a cardio workout first thing in the morning (or after about six hours of not eating), is that since glycogen (the storage form of glucose which is your brain and muscles’ primary source of fuel) is used up, your body will burn fat as fuel instead.
In the next sections, we’ll get sciency and explore this in more detail. Feel free to skip ahead if you’re just looking for what to eat after fasted cardio and 5 things to know.
THE ROLE OF GLUCOSE
When you eat, your body turns food into energy. Glucose is your brain and muscles’ primary and preferred fuel source.
According to Kaiser Permanente, your body changes all of the carbohydrate foods (rice, bread, potatoes, corn) into glucose. A portion of the protein foods (eggs, meat, soy) get turned into glucose but most is stored in the liver. Your body turns about ten percent of the fat foods you eat (butter, avocado, oil) into glucose.
Since there are times when food is not available, such as when you sleep, your body ends up storing any extra glucose from food, for later usage. The storage form of glucose is called glycogen.
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THE ROLE OF GLYCOGEN
Glycogen is a readily mobilized storage form of glucose. It’s main role is to maintain glucose levels in your blood during fasted or when there’s strenuous activity. According to research, it differs from fat because it can provide energy in the absence of oxygen, so it can provide energy for anaerobic exercise.
After a night without eating, your body has been breaking down glycogen back to glucose in order to be used as energy to support your basal metabolic functions.
Once your body has used up it’s glycogen stores, either by fasting or exercising, it starts to use fat for energy. It will also break down protein (meaning your muscles) for fuel.
What to Eat After Fasted Cardio
After doing fasted cardio, you’re most likely hungry. And rightfully so since you’re literally running on empty.
The International Society of Sports Nutrition recommends if you fasted before your cardio, you should eat a combination of protein and carbohydrate soon after the workout (within an hour). This is because it can help promote muscle growth.
Excellent protein and carbohydrate containing combos:
If you’re going to exercise again, it’s important to refuel by adding 1.2 gram per kilogram body weight per hour of carbohydrate, in addition to that protein and carbohydrate.
If you’re following a keto diet and trying to use fasted training to kick yourself into ketosis*, limit your carb intake through the rest of the day and your body is more likely to switch to using fat as fuel.
*Please always consult a physician before trying a new diet like the keto diet
Related Article: Lifting Weights While Fasted: Should You Do It?
5 Things to Know Before Giving Fasted Cardio a Go
1. GLYCOGEN DEPLETION: WATER WEIGHT ILLUSION
If you’re new to a low-carb diet or fasted cardio, it’s common to lose a good amount of weight when you start to deplete your glycogen stores. Although you may get excited by the scale’s downward trend or muscles showing through, it’s important to realize that it’s most likely water and not fat loss.
Glycogen is stored in the liver, muscles, and fat cells in a hydrated form, meaning that it’s linked to water. Studies suggest it’s in the ratio of three to four parts of water. So when your body breaks down the glycogen, water weight is shed.
The problem is that when you eat carbs again, your muscles and liver will want to hold onto the glucose and glycogen again, meaning water will come back right where it started. And you’ll regain the water weight.
So in the short term when you do fasted cardio, you may have an initial water weight loss which helps give the appearance of weight loss, but this will most likely return once you eat a meal containing carbohydrates.
Related Article: The Best Pre-Workout For Fasted Cardio
2. NOT ALL CARDIO IS CREATED EQUAL
In general, the best way to do fasted cardio is at a low intensity such as a walk, light jog, or bike. The easier workouts will help your body use fat. Fasting cardio is a great opportunity to practice low-intensity steady-state cardio (LISS).
High-intensity exercises such as sprinting or high-intensity interval training (HIIT) break down these glycogen stores for fuel as well. Meaning you’ll be left feeling very fatigued, weak, sore after the exercise, or even lightheaded.
3. WHEN TO EAT AFTER FASTED CARDIO
According to the International Society of Sports Nutrition (ISSN), if you’re doing fasted cardio, have done a long workout, or are doing a more intense workout, it’s good to eat within an hour to replenish the muscle glycogen.
When it comes to your anabolic window, or the amount of time you have after a training to help refuel and rebuild, studies are open to question and uncertain about exact doses of nutrients.
However, something to be aware of is that if you’re doing fasted cardio, you may be more hungry than usual. Set yourself up for success by stocking up on healthy breakfast options at home. If you have to stop by the store after your fasted cardio, you may end up buying foods that could sabotage your efforts.
4. WHAT ABOUT COFFEE?
But first — coffee. When you’re trying to do fasted cardio, coffee may be the only thing that helps you get out of bed and out the door.
Caffeine is a known fat burner because it aids in mobilization of fat from tissue and can slightly increase metabolic rate. However, it’s important to note that releasing fatty acids doesn’t promote weight loss.
Another thing to consider is that since coffee is acidic, having it on an empty stomach, especially right before a workout, may cause you some stomach upset. If you’re sensitive to the effects of caffeine, drinking on an empty stomach may increase anxiety and stress.
What you add to your morning cup could cancel out the fasting state. Check out: 7 Best Coffee Creamers For Weight Loss (Plus, 3 To Avoid).
5. DANGERS OF FASTED CARDIO
Fasted cardio may be dangerous for certain medical conditions and medications. Always check with your doctor before trying any new fitness or nutrition routine.
When you wake up in the morning, your stress hormone, cortisol is high. Cortisol promotes a catabolic (muscle break down) state. So if you’re in this state for too long — including having chronically high stress — studies show it can lead to muscle breakdown.
Glycogen is also associated with potassium. When you restrict carbs or do fasted cardio and deplete your glycogen stores, potassium levels in your body may drop. This has the potential to be very dangerous if you’re working out intensely or in hot temperatures.
Electrolytes, including potassium, sodium, magnesium, and calcium, need to be kept in a very specific range in our bodies. You need them to conduct nerve impulses (messages) and control muscle movement, including our heart. So if you’re exercising intensely or sweating a lot, you may want to consider a sports drink.
Make sure to listen to your body. If your workouts are leaving you feeling lightheaded or faint, fasted cardio may not be right for you.
Is Fasted Cardio Worthwhile? What the Science Says
Fasted cardio seems like it could be an ideal situation. Once you get over that morning grumps and get that workout in, your body could be burning more fat as fuel throughout the day. However, the research seems to be conflicting and inconclusive.
In older studies, they found that increases in the hormone insulin, after ingesting glucose or fructose, suppressed fat breakdown during exercise. This was conducted on a small sample size of six healthy men who cycled for 60 minutes. Something to note here is that subjects were given very simple versions of carbs instead of whole foods.
In another small sample size study, eight men performed the same moderate level exercise with and without fastings. They found that doing cardio while fasting, does not burn more fat than performing cardio after eating. The fasting before exercise did not enhance lipid usage and as a result, it was recommended to do physical activity after a light meal.
A more extensive comparative study made the strong claim that despite decades of research on the effects of exercise and fat metabolism, there is still no clear understanding of how exercise helps regulate fat mass.
To test the rate of weight loss when comparing a fasted cardio group with a non fasted group, a study used twenty healthy females. Subjects were separated into two groups: fasted training or those that consumed a meal replacement prior to exercise and those who received it after exercise. Subjects lost weight on a lower calorie diet regardless of whether they were fasting or not prior to exercise.
More research is needed when it comes to the post exercise effect of fat burn from fasted cardio. It appears that fasted cardio can improve the muscle’s ability to burn fat during the exercise, but the overall benefits are not significant, unless you’re using it to promote ketosis.
Use your intuition and consider how fasted cardio impacts the way you eat for the rest of the day. Rather than focusing all your energy on whether or not you’re eating before a workout, think about your overall nutrition and how effective your workouts are.
If you do decide that fasted cardio is right for you, aim to refuel within an hour with protein and carbohydrates.
Maybe you end up doing fasted cardio just can’t stomach anything pre-workout and that’s ok too.
The most effective type of exercise and nutrition plan is the type that you can do consistently. So whether rolling out of bed first and “tempting” yourself with breakfast afterwards gets you going, or some nourishment to fuel your workout, do what works best for you. The key is consistency.
About The Author
Lisa is a registered dietitian nutritionist (RDN) with over 15 years of experience in nutrition, fitness, and mental health coaching and education. She studied Foods and Nutrition at San Diego State University and earned a Master of Science in Holistic Nutrition at Hawthorn University.
Having certifications and experience in group exercise, intuitive eating, coaching and psychotherapy, and digestive wellness, she’s enthusiastic about the relationship between the body and mind.
She’s dedicated to helping people understand how to implement healthy habit change, while gaining a deeper understanding of what makes them feel their personal best.