When Fasted Cardio Really Makes Sense
Fasted cardio is the moment. Wake up and exercise before breakfast, the theory goes, and your body will be forced to burn fat because there won’t be any food in your stomach to use as fuel. But does fasting before exercise really affect your weight or health? Not the way you think. Let’s look at the science.
What is fasted cardio?
Fasted cardio does not require any strict fasting protocol. This simply means doing cardio when you haven’t eaten recently. The most common way to do fasted cardio is to do it 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 to break your fast .)
Cardio is any movement that you can do rhythmically for minutes or hours on end , such as jogging, cycling, swimming, or using exercise machines such as a stepper or elliptical machine. So if you’ve ever gone for a morning run without eating, you’ve already done fasted cardio.
Does fasted cardio help you burn fat?
In a limited, technical sense: possible. In the real world, in a general sense: no.
Remember, we are all constantly burning fat just by existing . How do you think your body stays alive all night while you sleep? It burns some of your fat reserves, knowing that you will eat more food throughout the day and it can 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 does not mean losing fat. We only lose fat when we don’t eat enough to replace all the fat we burned.
Ultimately, you don’t need to pay attention to what fuel your body uses at what time. If you really want to get into the details, your body also has stored carbohydrates (called glycogen), and you burn those stored carbohydrates when you do fasted cardio, so the energy you use isn’t all coming from fat anyway.
What Science Says About Fasted Cardio
Scientists tested the possibility that fasted cardio could lead to fat burning, which in turn could lead to fat loss. Unfortunately, the results are quite clear: Fasted cardio doesn’t seem to help people lose weight.
In this 2014 study, 20 women were put on a calorie-restricted diet (which helped them lose weight) and did an hour of cardio three times a week. Half the women shook themselves before the cardio workout, and half shook them after. Members of each group consumed a total of the same number of calories each day.
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 fasted cardio studies found more free fatty acids in the bloodstream compared to fasted cardio; this means that fat was burned. But one 2011 study found that fat burning occurred faster among people who ate before doing cardio . Based on these results, I wouldn’t trust fasted cardio to burn more fat, much less lose it.
Cons of Fasted Cardio
The biggest downside to fasted cardio is that you will almost always perform better during a workout if you’re fed. You will be able to exert more effort, work longer and be less tired. If you find exercise tiring, eating a meal, snack, or even a sports drink before your workout can change that.
For longer sessions, it is especially helpful to eat beforehand. It is well known among endurance athletes that marathons, all-day cycling and long hikes provide benefits due to the high amount of calories, mostly in the form of carbohydrates. If you’re training for more than an hour at a time (some say more than 90 minutes), you should not only eat beforehand, but also bring fuel to eat on the go .
In extreme cases, not eating during exercise can make you feel dizzy or lightheaded (especially if you’re pregnant or have health problems that could affect your blood sugar). It can also mean that after a few hours you “hit the wall” – the point where your body doesn’t have enough stored carbohydrates to support the level of effort you’re aiming for. (This is more of an issue for marathon runners than casual runners, so don’t let this put you off if you’re just looking for an energy boost for a short morning run.)
When fasted cardio may make sense
Even though I constantly praise pre-workout carbs, I do a lot of fasted cardio myself. I’m not doing this to burn fat; I do this because I run in the morning and don’t want to find time to eat beforehand.
It may make sense to do fasted cardio if:
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You have a sensitive stomach and may feel nauseous if you eat right before a run.
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You don’t have time to eat beforehand, and the workout will be short or easy anyway.
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You’re on a limited calorie budget for the day and want to save your carbs for later (perhaps before your evening weightlifting workout).
In these cases, I would only skip a pre-workout meal if your cardio session is going to be relatively short or light. Long sessions still require fuel. If stomach discomfort is the problem, fasting isn’t as beneficial as eating a few hours before—you might even want to have a big midnight snack if you need to go for a long run first thing in the morning.
It’s also worth finding out if you can have small snacks, such as a banana or a sip of a sports drink, that can give you some energy without causing stomach problems.