Fasted Cardio: Benefits, Drawbacks, and When to Use It

Fasted cardio: holy grail for fat loss or a great way to feel miserable?

Some swear it’s the secret to burning fat faster; others call it overhyped. But what really happens when you work out on an empty tank?

In this article, you’ll find out if exercising on empty really gives you an edge, or if you’re just running on fumes for no reason.

Expect both evidence-based science and personal opinion.

This article is for informational purposes and is not a substitute for professional care.

What Is Fasted Cardio?

Fasted cardio is when you perform aerobic exercise on an empty stomach. Black coffee is OK, but not anything with calories.

For most people, it means doing cardio in the morning before you’ve had your first bite to eat.

You could, of course, do fasted cardio later in the day, too, as long as you don’t eat anything for a number of hours.

That’s a far less common practice, although it works well with intermittent fasting, where you don’t eat your first meal until later in the day, sometimes not before evening.

Cardio on an empty stomach in the afternoon or evening can also reduce how many calories you eat during the day because you don’t have time to compensate for the long daytime fast before it’s time to hit the hay again.¹

Regardless, the “fasted state” means the state your body is in when you’ve gone about 8–12 hours without food. Usually, but not necessarily, after a full night’s sleep.

Why Do Fasted Cardio?

By far the most common reason people do cardio on an empty stomach is to burn more fat.

When you haven’t eaten, you force your body to use fat instead of the energy from food. But, as you’ll learn below, burning more fat during a workout doesn’t automatically mean that you lose more weight or body fat long-term.

Another reason is exercise performance.

If you train without carbs readily available, the body becomes more efficient at using fat for fuel.

There’s a concept in endurance sports called “train low, compete high”; when you then fuel up with carbs before an event, you have both the immediate fuel (carbs) and a body that can easily use fat once the carb energy runs out.

However, for most athletes, fueling your body before long or high-intensity efforts almost always leads to better performance. It allows you to train harder and longer and ultimately get better results than going in fasted.²

For a lifter, only the fat loss question really matters, so that’s what we’re focusing on.

Does Fasted Cardio Work for Fat Loss?

Yes, fasted cardio absolutely works.

Bodybuilders have used it for decades to get contest-lean. Standard practice used to be you got out of bed, took your fat-burners (legal or otherwise), then hit the pavement, treadmill, or exercise bike for an hour or so.

The question is whether it works better than non-fasted cardio.

The short answer is yes for convenience; not really for extra fat loss. Not for the vast majority of people trying to lose weight.

The theory behind fasted cardio is straightforward.

When you’re fasted, your insulin levels are low, and your body has mostly run out of energy from food (glycogen). The idea is to force your body to tap into its fat stores to fuel your cardio, and end up losing more body fat than if you had exercised after a meal.

However, as many studies have found, you don’t fool your body that easily, and it’s very hard to figure out a way around the calories-in minus calories-out equation.

Fasted cardio increases the amount of fat you burn during your workout.³ But the effect on your total fat loss seems to be negligible compared to doing cardio after a meal.

Since the first research review on fasted cardio and fat loss in 2011, plenty of controlled trials, several scientific articles, reviews, and a meta-analysis have reached similar conclusions.^{4 5 6 7 8}

Over weeks to months, doing cardio fasted doesn’t beat fed cardio for losing fat or weight when calories and protein are matched, in everyone from untrained, overweight adults to physique athletes.

The increased fat oxidation doesn’t reliably translate to more fat loss over time because your body compensates later in the day.

Recent work even questions whether acute “fasted = better lipid metabolism” is generally true.⁹

When Does Fasted Cardio Actually Work? (For Stubborn Fat)

There is one time when cardio might make a difference for fat loss. Noticeable fat loss.

That’s when you’re already lean and looking to get super lean or contest-ready.

While calories in vs. calories out determines fat loss, it doesn’t tell the whole story when you get to the end of a cutting diet.

When it’s time to get “stage ready” or peel off that last layer of fat, we’re not just talking about general fat loss anymore.

Speculation vs. Research

Now, the peer-reviewed study Effects of Fasted vs. Fed Aerobic Training on Subcutaneous Adipose Tissue in 6% Body Fat Male Bodybuilders doesn’t exist, so the following is mostly speculation. But it’s speculation based on established principles, applied to a specific, extreme scenario.

I should mention a 2017 review of the training (and steroid) strategies of competitive bodybuilders that took notice of the practice of fasted cardio and concluded that it is “highly advisable to discourage the performance of fasted aerobic exercise in order to prevent negative alterations in metabolism”.¹⁰

However, the studies they reference do not support such dramatics; they rely on very simple mechanics measured in non-bodybuilders and no strength training.

That’s one problem with this kind of research. It uses semi-relevant parameters at best, and other studies use the results to draw black-and-white conclusions that are taken out of context.

Taking the results of a 2-hour measurement in average young men with 15–20% body fat who don’t lift weights, and concluding that competitive bodybuilders at the very extreme end of what humans can accomplish in both muscle mass and leanness would have no benefit from doing something over the course of a 12-week contest prep does not fly with me.

I’m not saying fasted cardio is some kind of magic solution for a bodybuilding cut. I’m sure it’s not, and you can get contest-ready (or ripped in general) without it.

What I am saying is that I don’t rule out that it does make a slight difference. And on the bodybuilding stage, minor differences make all the difference.

Fasted Cardio for Stubborn Fat

Anyway, let’s go back to how you might benefit from fasted cardio if you’re already decently lean.

We’re talking about mobilizing stubborn body fat that remains even though you’re lean everywhere else.

If you’ve ever dieted down to the point of being ripped, you’ve probably had one or more areas that you had to fight, like the last guest at a party who just won’t leave.

That kind of stubborn fat is most often on the lower back and abs for men, and hips and thighs for women. And you aren’t imagining things: this fat is different.

Your fat cells have “go” (Beta-2) and “stop” (Alpha-2) receptors. Beta-receptors react to adrenaline and tell the cell to release fat. Alpha-receptors react to adrenaline and tell the cell to stop releasing fat.¹¹

Stubborn fat is packed with a higher ratio of these “stop” signals.¹²

It’s also harder for your body to get blood to these areas.¹³ Blood is the transport system for hormones (catecholamines like adrenaline) that tell fat to be released and for carrying away the fatty acids once they are mobilized.

These factors, combined with the fact that when you get very lean, your fat cells become more sensitive to insulin, make it hard to get the fatty acids out.¹⁴

Even a tiny insulin spike from a small pre-workout meal can slam the brakes on fat mobilization at this point. But if you do your cardio in a fasted state, your insulin levels are at rock bottom, which takes that fat-storing/fat-retention signal out of the equation.

Then, once you have liberated them, the fatty acids can be moved with the increased blood flow to other parts of the body and oxidized instead of being re-esterified back into your body fat.

In short, to get rid of the last of the stubborn fat, you might benefit from fasted cardio. Once you get the fatty acids out of their hidey-holes (fat cells), they’re easy to burn off.

You coax fatty acids out of the most resistant cells in your body when conditions are most favorable, which might make a meaningful difference.

Meaningful? Maybe, but Not for Everyone

And what is a meaningful difference for someone who is already almost contest lean would be completely unnoticeable in someone with “normal” levels of body fat, not even overweight.

It’s not a trick for the average person trying to lose 20 pounds. For them, adherence to a calorie deficit is 99.9% of the battle.

But if you are already lean and want to get ripped, fasted cardio could be the specialized tool you need for a unique job: attacking a very specific problem (mobilization of the last stubborn fat) that only exists at extreme levels of leanness.

That being said, there are no studies that provide direct proof that fasted cardio produces extra fat loss at stage-lean levels. This is speculation, based on how fat-loss mechanics work under different conditions and practical observations on strength athletes, competitive bodybuilders in particular.

Personal Anecdote Time

I have noticed these benefits myself when getting contest-level lean, with fasted cardio helping at the end of a cut.

I’m talking about getting from 8% body fat down to 5–6%. Unfortunately, I wasn’t able to clone myself at the time, so I have no placebo-controlled proof.

I have dieted down to that body fat level many times, and fasted cardio consistently made it easier compared to when I didn’t utilize it. But of course, other factors might have affected the results at those times. So take personal anecdotes for what they are: anecdotes, not proof.

Who Should and Shouldn’t Do Fasted Cardio?

For healthy people, exercising fasted is perfectly safe. The body does not break down if it doesn’t get food before doing something physical.

That being said, if you have any issues with blood sugar regulation, like hypoglycemia or diabetes, exercising on an empty stomach might not be a good idea. The same goes if you have adrenal issues or hormonal imbalances.

If you have a medical condition that affects any of the above, I always recommend consulting with your doctor first.

Also, if you’re prone to low energy availability (common in endurance athletes, especially women), you want to be cautious with frequent fasted training. Or fasting in general, for that matter.

So, who’s on team Fasted Cardio?

• Some people feel better doing cardio fasted. If you feel sluggish or nauseous if you eat first, if you can’t stomach the thought of food sloshing around while you train, then fasted cardio could be your thing simply for the comfort factor.
• If you prefer training early mornings and are up at the crack of dawn to squeeze in a cardio workout, waiting for a meal to digest will not be practical. Fasted cardio is a great option for morning larks.
• When you need to get just a little leaner, fasted cardio might offer some benefits, helping you lose the last stubborn fat.
• And finally, if you’ve tried fasted cardio and you enjoy it, feel good, and are seeing the results you want, then there’s no compelling reason to stop.

Does Fasted Cardio Burn Muscle?

One of the most common fears about fasted cardio is muscle loss. Won’t you break down muscle tissue if you haven’t eaten?

If you’re on top of your nutrition and training, muscle loss from fasted cardio is highly unlikely to be a real concern.

At least as long as you eat enough protein and lift weights. Both these things protect your muscles and make it far less likely that your body needs to tap into your biceps for fuel.

And your body is a remarkably efficient and intelligent system. It’s not going to go tearing down hard-earned muscle for a jog on an empty stomach.

Protection Mechanisms Are In Place

Even after an overnight fast, your muscles still hold a significant amount of carbs as muscle glycogen (unless you’re doing keto). Your body will use that for fuel before it even considers touching muscle protein in any meaningful way. It’s mainly your liver glycogen that’s depleted overnight.

Also, cardio boosts muscle protein synthesis (MPS). Not like strength training, but significantly. Fasted walking on a treadmill at 40% of VO₂max (a leisurely stroll) increases MPS by 45%.¹⁵

You always break down more muscle than you build when you’re fasted, but exercise, even fasted cardio, makes that balance less negative. And after you eat, MPS overtakes breakdown. By a lot:

So, while you might not want to continue fasting for hours and hours after cardio on an empty stomach, that time you spend on your morning powerwalk will not result in muscle loss.

The Leaner You Are, the Greater the Risk

The risk of muscle loss increases when your sessions get long, hard, and/or you’re very lean and deep in a deficit.

A 30–60 minute walk, jog, or bike ride? No problem.

A 90-minute, high-intensity interval session on an empty stomach as a 220-pound bodybuilder with one-digit body fat? That’s a different story.

The higher the intensity and duration, the greater the energy demand. That makes it more likely your body will start looking for alternative fuel sources, including protein.

If you’re very lean or paranoid about muscle, some protein before or during cardio can blunt protein breakdown without killing fat oxidation.

Some evidence supports “protein-enhanced cardio,” a pre-cardio feeding with up to 40 g of protein. Not only do you reduce protein breakdown, but you also don’t hinder fat oxidation, even though it will raise insulin a bit.¹⁶ Useful if muscle retention is paramount, although it will partially blunt the “pure” fasted effect (which may or may not be relevant for anything).

If you’re prepping for a bodybuilding show and are already extremely lean and on poverty calories, your body is in a much more catabolic (breakdown) state. In this scenario, every little bit counts, and taking precautions makes sense.

And if someone were eating less than, say, 1.2 g/kg/day (0.5 g/lb) of protein, the risk would be higher because they don’t have the same protective buffer. But that’s not you. Right?

Checklist: How to Do Fasted Cardio Right

1. Make sure you’re really in a fasted state (8–12 hours since your last meal)
2. Drink plenty of water before you begin, as you’re slightly dehydrated when you wake.
3. Caffeine (3–6 mg/kg or 1.36–2.72 mg/lb) beforehand is optional but makes you more alert and mobilizes fatty acids, making them available as energy.
4. Choose low to moderate intensity for most fasted sessions.
5. Go for 20–60 minutes; long enough to matter, short enough to still enjoy life.
6. Eat a meal full of protein and carbs (optional) when you’re done (but no rush).
7. Listen to your body; if you consistently feel like crap, fasted cardio may not be the best option for you.

Final Rep

Fasted cardio – love it, hate it, or tolerate it – can be a useful tool for getting into shape, but there are few compelling reasons to force yourself to do it if you don’t want to.

• For the vast majority, it makes zero difference if you do your cardio before breakfast or later in the day.
• For a very small minority (very lean athletes and bodybuilders trying to get shredded), fasted cardio might make a noticeable difference.

Here’s my advice:

• Do you like fasted cardio, feel good, and perform well doing it? Great, stick with it.
• Do you feel weak, dizzy, and hate every second of it? Eat something first. A happy lifter is a consistent lifter.

Whether you’re sweating on an empty stomach or fueled by oatmeal and ambition, that you do it at all is always more important than when you do it.

Last reviewed: 2025-08-13

References

1. Int J Sport Nutr Exerc Metab. 2022 Sep 28;33(1):11-22. Fasting Before Evening Exercise Reduces Net Energy Intake and Increases Fat Oxidation, but Impairs Performance in Healthy Males and Females.
2. Scand J Med Sci Sports. 2018 May;28(5):1476-1493. Effects of fasted vs fed-state exercise on performance and post-exercise metabolism: A systematic review and meta-analysis.
3. Br J Nutr. 2016 Oct;116(7):1153-1164. Effects of aerobic exercise performed in fasted v. fed state on fat and carbohydrate metabolism in adults: a systematic review and meta-analysis.
4. Strength and Conditioning Journal 33(1):p 23-25, February 2011. Does Cardio After an Overnight Fast Maximize Fat Loss?
5. Journal of the International Society of Sports Nutrition volume 11, Article number: 54 (2014). Body composition changes associated with fasted versus non-fasted aerobic exercise.
6. J. Funct. Morphol. Kinesiol. 2017, 2(4), 43. Effect of Overnight Fasted Exercise on Weight Loss and Body Composition: A Systematic Review and Meta-Analysis.
7. Strength and Conditioning Journal 42(5):p 71-78, October 2020. Fasted Versus Nonfasted Aerobic Exercise on Body Composition: Considerations for Physique Athletes.
8. J Sports Med. 2020 Jan 21;11:1–28. Exercise Training and Fasting: Current Insights.
9. Clin Nutr ESPEN. 2025 Apr:66:320-331. The effects of acute bouts of exercise in fasted vs. fed states on glucose and lipid metabolism in healthy adults: A systematic review and meta-analysis of randomized clinical trials.
10. Eur J Transl Myol. 2017 Mar 24;27(1):6247. Nutrition, Pharmacological and Training Strategies Adopted by Six Bodybuilders: Case Report and Critical Review.
11. Journal of Lipid Research, Volume 24, Issue 8, August 1983, Pages 945-966. Adrenergic regulation of adipocyte.
12. Eur J Clin Invest. 1987 Apr;17(2):156-65. Heterogeneous distribution of beta and alpha-2 adrenoceptor binding sites in human fat cells from various fat deposits: functional consequences.
13. Diabetologia, Volume 55, pages 3029–3037, (2012). Marked resistance of femoral adipose tissue blood flow and lipolysis to adrenaline in vivo.
14. J Physiol. 2009 Sep 1;587(Pt 20):4949–4961. Improved insulin sensitivity after weight loss and exercise training is mediated by a reduction in plasma fatty acid mobilization, not enhanced oxidative capacity.
15. J Appl Physiol (1985). 2009 Jun;106(6):2026-39. Human muscle protein synthesis and breakdown during and after exercise.
16. J Int Soc Sports Nutr. 2023 Dec;20(1):2275006. The effect of pre-exercise protein intake on substrate metabolism, energy expenditure, and energy intake: a dose-response study.