Fasted cardio is having a moment. Wake up and get your exercise in before breakfast, so the theory goes, and your body will be forced to burn fat, because there will be no food in your stomach to use for fuel. But does fasting before a workout actually make a difference to your weight, or your health? Not the way you’d think. Let’s take a look at the science.
What is fasted cardio?
“Fasted” cardio doesn’t require some kind of extreme starvation protocol. It just means doing cardio when you haven’t eaten recently. The most common way to do fasted cardio is to work out first thing in the morning, before breakfast, since you’ve been fasting all night. (The word “breakfast” even comes from the fact that the first thing you eat is breaking your fast.)
Cardio refers to any movement you can do rhythmically for minutes or hours on end, like jogging, cycling, swimming, or using gym machines like a stepper or an elliptical. So if you’ve ever gone for a morning jog without having a snack first, you’ve already done fasted cardio.
In a limited, technical sense: maybe. In a real-world, big-picture sense: no.
Remember that we are all burning fat all the time, just by existing. How do you think your body stays alive all night while you are sleeping? It burns through a little bit of your fat stores, knowing that during the day you will eat more food and it will be able to replace that fat.
In other words, there is a difference between burning fat and losing fat. We burn fat and replace it continuously throughout the day; burning fat doesn’t mean losing fat. We only lose fat when we don’t eat enough to replace all the fat that we burned.
Ultimately, you don’t have to pay attention to what fuel your body is using at what time. If you really want to nerd out about the details, your body has carb stores as well (called glycogen), and you burn through those stored carbs when you do fasted cardio—so the energy you use isn’t all coming from fat anyway.
What the science says about fasted cardio
Scientists have tested the possibility that fasted cardio might lead to fat burning, which in turn might lead to fat loss. Unfortunately, the results are pretty clear that fasted cardio does not seem to help people lose weight.
In this 2014 study, 20 women were put on a calorie-restricted diet (which would help them lose weight), and did an hour of cardio three mornings each week. Half the women got a shake before their cardio session, and half got the shake after. Members of each group were eating the same number of calories each day in total.
The results? Both groups lost the same amount of weight. Fasted cardio provided no additional benefit.
It’s not even clear whether fasted cardio increases fat burning. A 2018 meta-analysis found that 38% of studies on fasted cardio found more free fatty acids in the bloodstream when compared to fed cardio; that means that fat was being burned. But one 2011 study actually found that fat burning was greater among people who ate before they did their cardio. Based on these results, I wouldn’t trust fasted cardio to burn more fat, much less lose it.
What do you think so far?
The downsides of fasted cardio
The biggest downside of fasted cardio is that you'll almost always perform better during a workout when you’re fed. You'll be able to push harder, work longer, and feel less tired doing it. If you find exercise to be exhausting, a pre-workout meal, snack, or even a sports drink may change that.
Longer sessions will especially benefit from eating beforehand. It's well-known among endurance athletes that marathons, all-day bike rides, and lengthy hikes benefit from a hefty supply of calories, mainly in the form of carbs. If you’re exercising for more than an hour at a time (some say more than 90 minutes) you should not only eat beforehand, but also bring fuel with you to eat on the go.
At the extreme end, going without food during exercise can leave you felling dizzy or lightheaded (especially if you are pregnant or have medical issues that can affect your blood sugar). It can also mean “hitting the wall” after a few hours—that point you reach where your body doesn’t have enough stored carbs to keep up the level of effort you’re aiming for. (This is more of an issue for marathoners than for casual joggers, so don’t let that scare you off if you’re just figuring out fueling for a short morning run.)
When fasted cardio can make sense
Even though I sing constantly the praises of pre-workout carbs, I do a lot of fasted cardio myself. I don’t do it for fat burning benefits; I do it because I jog in the morning and I don’t want to bother finding the time to eat beforehand.
It may make sense to do cardio on an empty stomach if:
You have a sensitive stomach, and might get nauseous if you eat right before you run.
You don’t have time to eat beforehand, and your workout will be a short or easy one anyway.
You have a limited calorie budget for the day, and want to save your carbs for later (before an evening weightlifting workout, perhaps).
In these cases, I would only skip the pre-workout meal if your cardio session will be a relatively short or easy one. Long sessions still require fueling. If stomach discomfort is the issue, consider eating a meal a few hours beforehand—or even have a filling dinner or midnight snack before a long or hard morning run. It’s also worth figuring out whether there might be small snacks, like a banana or a swig of sports drink, that can give you some fuel before your run without triggering your stomach issues.
