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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 youâd think. Letâs take a look at the science.
âFastedâ cardio doesnât require some kind of extreme starvation protocol. It just means doing cardio when you havenât eaten recently. The most common way to do fasted cardio is to work out first thing in the morning, before breakfast, since youâve been fasting all night. (The word âbreakfastâ 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 youâve ever gone for a morning jog without having a snack first, youâve 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 doesnât mean losing fat. We only lose fat when we donât eat enough to replace all the fat that we burned.
Ultimately, you donât 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 cardioâso the energy you use isnât all coming from fat anyway.
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.
Itâs 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 found that fat burning was greater among people who ate before they did their cardio. Based on these results, I wouldnât trust fasted cardio to burn more fat, much less lose it.
The biggest downside of fasted cardio is that you'll almost always perform better during a workout when youâre 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 youâre 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 âhitting the wallâ after a few hoursâthat point you reach where your body doesnât have enough stored carbs to keep up the level of effort youâre aiming for. (This is more of an issue for marathoners than for casual joggers, so donât let that scare you off if youâre just figuring out fueling for a short morning run.)
Even though I sing constantly the praises of pre-workout carbs, I do a lot of fasted cardio myself. I donât do it for fat burning benefits; I do it because I jog in the morning and I donât want to bother finding the time to eat beforehand.
It may make sense to do cardio on an empty stomach if:
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, fasting isnât as good as eating a few hours beforehandâyou could even have a big midnight snack if you need to do a long run first thing in the morning.
Itâs 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 without triggering your stomach issues.
Full story here:
What is fasted cardio?
âFastedâ cardio doesnât require some kind of extreme starvation protocol. It just means doing cardio when you havenât eaten recently. The most common way to do fasted cardio is to work out first thing in the morning, before breakfast, since youâve been fasting all night. (The word âbreakfastâ 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 youâve ever gone for a morning jog without having a snack first, youâve already done fasted cardio.
Does fasted cardio help you burn fat?
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 doesnât mean losing fat. We only lose fat when we donât eat enough to replace all the fat that we burned.
Ultimately, you donât 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 cardioâso the energy you use isnât 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.
Itâs 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 found that fat burning was greater among people who ate before they did their cardio. Based on these results, I wouldnât trust fasted cardio to burn more fat, much less lose it.
The downsides of fasted cardio
The biggest downside of fasted cardio is that you'll almost always perform better during a workout when youâre 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 youâre 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 âhitting the wallâ after a few hoursâthat point you reach where your body doesnât have enough stored carbs to keep up the level of effort youâre aiming for. (This is more of an issue for marathoners than for casual joggers, so donât let that scare you off if youâre 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 donât do it for fat burning benefits; I do it because I jog in the morning and I donât 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 donât 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, fasting isnât as good as eating a few hours beforehandâyou could even have a big midnight snack if you need to do a long run first thing in the morning.
Itâs 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 without triggering your stomach issues.
Full story here:
![[DK]](/data/avatars/s/57/57999.jpg?1720990206){kind=avatar}
