You’ve probably heard the buzz: Want to burn more fat? Just hop on the treadmill before breakfast.

Fasted cardio – doing aerobic exercise on an empty stomach, typically first thing in the morning – has become a popular strategy for those chasing fat loss. Proponents swear it helps the body tap into fat stores more efficiently, while critics argue it’s just another fitness myth dressed up in science-y language.

So what’s the truth? Let’s break down the science behind fasted cardio to understand what it does, what it doesn’t do, and whether it’s worth adding to your routine.

The Science Behind Fasted Cardio: What Happens in the Body

When you wake up after an overnight fast and hit the gym without eating, your body is in a low-insulin state. That means blood sugar and glycogen levels are lower than usual, which encourages your body to turn to stored fat as a fuel source. This process is known as fat oxidation.

Because carbohydrates aren’t readily available, your body ramps up the breakdown of fat into fatty acids, which can be used for energy, especially during low to moderate-intensity exercise like walking or steady-state cardio. This metabolic shift is the core of fasted cardio’s fat-burning claim.

Fat Oxidation vs. Fat Loss

Here’s where it gets more nuanced. Just because your body is burning more fat during the workout doesn’t mean you’re automatically losing more body fat over time.

Several studies have shown that when calorie intake and overall activity levels are the same, there’s no meaningful difference in fat loss between fasted and fed cardio. In other words, whether you burn more fat in the moment doesn’t necessarily impact your results unless it contributes to a consistent calorie deficit.

So while fasted cardio might sound like a fat-burning shortcut, the bigger picture – how much you eat, how much you move, and how consistent you are – matters far more.

Muscle Preservation Considerations

One of the potential downsides of fasted cardio is an increased risk of muscle breakdown, especially during longer or more intense sessions. Without food in your system, your body may not have enough amino acids readily available to support muscle maintenance, particularly if you’re already in a calorie deficit.

That’s why many coaches suggest sipping on branched-chain amino acids (BCAAs) or a small amount of protein before fasted sessions, especially if muscle preservation is a priority. Fasted cardio isn’t inherently bad for muscle, but it does require careful management to avoid unintended losses in lean mass.

When Fasted Cardio Can Be Beneficial

That said, fasted cardio can be a helpful tool in certain situations:

• You prefer working out in the morning and don’t like eating beforehand. Some people just feel better exercising on an empty stomach.
• You’re doing low- to moderate-intensity workouts (like walking, steady-state cycling, or gentle jogging), where muscle breakdown is less of a concern.
• You’re an experienced trainee or bodybuilder in a cutting phase and are carefully managing nutrition, training, and recovery.

It’s also been suggested that fasted cardio might improve metabolic flexibility, or the body’s ability to switch between fuel sources. This benefit is still being researched, but it’s an interesting angle for endurance athletes and those experimenting with different training strategies.

When It Might Do More Harm Than Good

Fasted cardio is not for everyone, and for some, it might do more harm than good.

• Beginners may struggle with energy or performance if they haven’t eaten.
• Anyone with blood sugar sensitivity or low blood pressure may feel dizzy, shaky, or nauseous.
• People focused on performance gains, like sprinting or high-intensity intervals, are generally better off with some fuel in the tank.
• Those with a history of disordered eating may find fasted training reinforces unhealthy patterns around food and exercise.

If any of these describe you, you’re better off focusing on fueling well, training hard, and recovering smarter, not chasing fasted cardio as a fat-loss shortcut.

How to Do Fasted Cardio the Smart Way

Fasted cardio isn’t inherently good or bad. It’s all about context. If you’re curious to try it, approaching it with intention can help you reap potential benefits while avoiding common pitfalls. Here’s how to do it right:

1. Start with Low to Moderate Intensity

Fasted cardio is best paired with steady-state, lower-intensity workouts. Think:

• Walking (especially incline treadmill walking)
• Light jogging
• Easy cycling
• Yoga or bodyweight flows

These types of sessions tap into fat stores without overly taxing your system. High-intensity workouts – like sprint intervals or heavy lifting – require quick-burning fuel (carbs), which your body won’t have in a fasted state. Doing intense training fasted can spike cortisol, increase muscle breakdown, and leave you feeling sluggish.

2. Hydrate Before and During Your Workout

Even if you’re skipping food, don’t skip water. After a night of sleep, you’re already mildly dehydrated, and dehydration can reduce performance, make workouts feel harder, and increase your risk of dizziness.

Try drinking a full glass of water when you wake up, and sip throughout your workout. For longer sessions or hot environments, consider adding electrolytes (especially sodium and potassium) to support fluid balance.

3. Consider a Protein Buffer

If your main concern is maintaining lean muscle while tapping into fat stores, a little strategic fueling can help. Consuming 10–20g of protein (like a small scoop of whey or a few BCAAs) before fasted cardio can blunt muscle breakdown without fully shifting your body out of the fat-burning state.

This is especially helpful for people in a calorie deficit, or anyone doing fasted cardio multiple times a week.

4. Listen to Your Body’s Signals

Your body will tell you if fasted cardio isn’t working for you. Signs it might not be a good fit include:

• Low energy or extreme fatigue
• Dizziness or lightheadedness
• Decreased performance
• Trouble recovering after workouts

If your fasted sessions leave you drained instead of energized, it’s okay to eat before you train. There’s no bonus reward for pushing through when your body’s clearly asking for fuel.

5. Don’t Overdo It

Fasted cardio can be a useful tool, but like any tool, it’s not meant for daily use by everyone. If you’re doing it every day, especially in a calorie deficit, you could be at higher risk for elevated cortisol, muscle loss, or overtraining.

Start with 1–2 sessions a week and adjust based on how your body responds. For many people, it’s most effective as a complement to a larger training and nutrition strategy, not a daily requirement.

6. Keep the Bigger Picture in Mind

Remember: fasted cardio is just one small variable in your training. Total calorie intake, nutrient timing, sleep quality, resistance training, and overall consistency all play a much bigger role in body composition and performance.

Use fasted cardio if it fits your routine and goals, but don’t fall into the trap of thinking it’s a fat-loss magic bullet.

Final Take: Hack or Hype?

Fasted cardio isn’t magic, but it’s not total hype either.

It can help certain individuals burn fat a bit more efficiently in the short term, especially when paired with a dialed-in nutrition and training plan. But for most people, the total amount of movement, the quality of your workouts, your nutrition, and how well you recover will have a much bigger impact on your progress than whether you exercised on an empty stomach.

So, if fasted cardio feels good for you and fits your lifestyle, great. If not? Don’t sweat it. Fed cardio burns calories too.

If you’re looking for a personalized fat-loss strategy that works with your body and goals, not against them, you can work with a coach to build a plan that balances strength, performance, and sustainability (fasted cardio optional).

Written by Emily Greffenius. Reviewed by Meghan Farrell, CPT, BSN