Gurgle, gurgle, gurgle… good morning, stomach.

You may have heard about the fat-burning benefits of rolling out of bed and jumping right into a workout, but is this recent workout trend all it’s cracked up to be? While fasted cardio does work for some lifestyles, read this before you try it for your own weight loss journey.

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Fasted cardio is performed when your body is in a fasted state, which means it isn’t digesting food. Basically, it means doing cardio on an empty stomach.

This would normally happen first thing in the morning, after sleeping overnight, but it can also happen later in the day if you practice intermittent fasting.

Compared to nonfasted cardio, fasted cardio is touted as a way to accelerate fat loss. And although that sounds great, its effectiveness has not been fully proven. Let’s dive deeper.

Mostly, yes. If you’re generally healthy, it’s probably just fine to incorporate short or moderate-length steady-state fasted cardio sessions into your routine.

However, if you’ll be exercising for any extended period of time or doing a high intensity workout, fasted cardio can be risky due to potential side effects of low blood sugar or dehydration, such as lightheadedness, dizziness, shaking, or even passing out.

The idea behind fasted cardio is that if you fast overnight and work out first thing, your body is depleted of glucose — its main source of energy — and will instead use stored fat for fuel.

Research is mixed on the effectiveness of this approach.

One review found that, in several studies, fasted exercise led to higher metabolic performance after the workout was complete. However, the same review noted that for prolonged aerobic activity, eating before the workout enhanced performance (1).

While more research is needed to make more concrete claims, fasted cardio has some other potential benefits:

  • If you’re tight on time, fasted cardio saves you from having to prepare, eat, and digest a meal beforehand.
  • If you practice intermittent fasting, fasted cardio allows you to exercise before you eat for the day.
  • If you prefer working out on an empty stomach, fasted cardio could be an effective option, especially if you have a sensitive stomach or feel more energetic without a meal before a workout.

The most important aspect of weight loss is burning more calories than you consume. Research is mixed on whether fasted cardio actually promotes fat loss.

In one study, 20 young females were split into two groups — one group did 1 hour of fasted steady-state cardio, and the other did 1 hour of nonfasted steady-state cardio. Both groups exercised 3 days per week for 4 weeks and followed a diet with a calorie deficit (2).

Researchers found no difference in weight loss or body composition between groups (2).

However, some research does support fasted cardio’s increased fat-burning effects during a workout.

A review of 27 studies published in the British Journal of Nutrition in 2016 concluded that “aerobic exercise performed in the fasted state induces higher fat oxidation than exercise performed in the fed state” (1).

This research illustrates that while fasted cardio may burn more calories than nonfasted cardio during the session itself, the difference it makes to total daily calorie expenditure in a span of 24 hours is trivial.

Non-exercise activity thermogenesis, or NEAT for short, is energy expended on all daily activity that aren’t exercise, eating, or sleeping. This value has been shown to account for about 15% of calories burned in a day, depending on a person’s activity level (3).

In general, if you’re looking to lose weight, increasing your daily movement, whether fasted or not, is still the best plan.

Moving more every day — walking, taking the stairs, getting up from your desk regularly, playing with your kids — will have a larger impact on weight loss than a 30-minute session of fasted cardio.

Although fasted cardio does have some benefits, it’s important to be aware of potential risks.

It could hinder muscle-building

If there are not enough carbohydrates in your system for energy, your body begins a process called gluconeogenesis, which converts protein into fuel (4).

This means there’s less protein left to rebuild muscle. Low intensity steady-state cardio may be better than high intensity exercise in a fasted state — that way, your body relies on free fatty acids for fuel rather than burning carbohydrates (5).

It may hinder performance

Especially if you’re knocking out a moderate or high intensity workout — like HIIT, boot camp, or weight training — on an empty stomach, your energy levels will not be up to par without some fuel beforehand. Also, look out for signs of low blood sugar and dehydration.

Avoid fasted cardio if you have a medical condition that’s affected by low blood sugar or blood pressure or if you’re pregnant.

It’s also best for complete beginners to shy away from fasted cardio — understanding your body well should be the first step on your exercise journey.

If you’re generally healthy, the decision to incorporate fasted cardio is a personal one. If you’ve never tried it before, start slowly.

First, make sure to hydrate before and during the session.

Try a low to moderate intensity steady-state session — like walking, running, biking, or the elliptical — for 10 minutes and see how you feel. If it goes well, work your way up to 30 minutes as time goes on.

Afterward, make sure to fuel up with a balanced meal or snack packed with protein and carbs.

Avoid high intensity work — where your heart is pumping hard — or any session more than an hour long during fasted cardio.

You can incorporate low intensity steady-state fasted cardio on multiple days throughout the week, but make sure you’re taking 1 or 2 rest days too.

Cardio, fasted or not, is great for your body. And while nutrition is key for weight loss, cardio can help you reach a weight loss goal.

While research on its metabolic effects is still inconclusive, fasted cardio may work better for your lifestyle or preferences, so if you’re generally healthy, feel free to give it a go.