Buzz
Fasted cardio is currently trending. The idea is simple: get moving before you eat, and your body will be forced to burn fat, since there’s no food in your stomach to provide energy. But does exercising on an empty stomach really have any significant impact on your weight or health? The answer might not be what you expect. Let’s examine the science behind the claims.
What exactly is fasted cardio?
Fasted cardio doesn’t involve a rigorous starvation routine. It simply means doing cardio when you haven’t eaten for a while. Most people practice fasted cardio first thing in the morning, before breakfast, as you’ve naturally been fasting through the night. (The term “breakfast” even comes from the idea of ‘breaking’ your ‘fast.’)
Cardio refers to any rhythmic movement you can maintain for extended periods, whether it’s jogging, cycling, swimming, or using gym equipment like a stepper or elliptical machine. So, if you've ever gone for a jog before eating, you’ve already done fasted cardio.
Can fasted cardio aid in fat loss?
In a narrow, technical sense: possibly. But in the broader, real-world context: not really.
Keep in mind, we’re always burning fat constantly, just by being alive. How else do you think your body stays active overnight while you sleep? It taps into some of your fat stores, knowing that you'll eat again during the day to replenish it.
In other words, there’s a distinction between burning fat and losing fat. We burn fat and replace it all day long; burning fat doesn’t mean it’s lost. True fat loss only happens when we don’t eat enough to fully replace the fat we've burned.
In the end, you don’t really need to worry about what fuel your body is using at any given time. If you’re curious about the specifics, your body also stores carbohydrates (known as glycogen), and those carbs are used up during fasted cardio—so the energy isn’t solely coming from fat after all.
What does the research say about fasted cardio?
Researchers have explored whether fasted cardio might trigger fat burning, which could eventually lead to fat loss. However, the findings are clear: fasted cardio does not appear to help people shed pounds.
In this 2014 study, 20 women were put on a calorie-restricted diet (designed to promote weight loss) and did one hour of cardio three mornings a week. Half of the women consumed a shake before their cardio, while the other half had it afterward. Both groups ate the same total number of calories each day.
The results? Both groups lost the same amount of weight. Fasted cardio offered no extra advantage.
It’s unclear whether fasted cardio actually boosts fat burning. A 2018 meta-analysis found that 38% of studies on fasted cardio showed higher levels of free fatty acids in the bloodstream compared to fed cardio, suggesting fat was being burned. But a 2011 study discovered greater fat burning in individuals who ate before their cardio sessions. Given these mixed results, I wouldn't rely on fasted cardio to burn more fat, let alone lose it.
The drawbacks of fasted cardio
The main downside of fasted cardio is that you're likely to perform better during a workout when you're fed. You'll have more energy, be able to push yourself harder, and feel less fatigued. If you find exercise exhausting, a pre-workout meal, snack, or even a sports drink could make a big difference.
Longer workouts especially benefit from eating beforehand. Endurance athletes know that marathons, all-day cycling trips, and extended hikes require a substantial calorie intake, primarily from carbs. If you're working out for more than an hour (or even 90 minutes), you should not only eat beforehand but also bring along fuel to snack on during your workout.
In extreme cases, exercising without food can make you feel dizzy or lightheaded (especially if you’re pregnant or have medical conditions affecting blood sugar levels). It can also lead to ‘hitting the wall’ after a few hours—when your body runs out of stored carbs and can't maintain the effort level you’re aiming for. (This is more of a concern for marathon runners than for casual joggers, so don’t be discouraged if you're just learning how to fuel for a short morning run.)
When fasted cardio might actually work
Despite frequently recommending pre-workout carbs, I personally do a lot of fasted cardio. I don’t do it for fat burning; I do it because I like jogging in the morning and prefer not to take the time to eat beforehand.
Fasted cardio could be a good option if:
You have a sensitive stomach and may feel nauseous if you eat before your run.
You don’t have the time to eat beforehand, and your workout will be relatively short or easy.
You have a limited calorie budget for the day and want to save your carbs for later (perhaps before an evening weightlifting session).
In these cases, skipping the pre-workout meal is fine as long as your cardio session is short or light. Longer sessions still require fueling. If stomach discomfort is your concern, fasting may not be ideal—eating a few hours beforehand would be better. You could even have a big midnight snack if you need to do a long run in the morning.
It might also be helpful to consider small snacks, such as a banana or a sip of sports drink, that can provide some energy without upsetting your stomach.
