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Should you eat before a workout? Does it give you more energy and enhance performance, or does it compromise your ability to burn fat? Let’s dive into the research to determine once and for all: is fasted cardio beneficial for achieving your goals?
Put simply, fasted cardio is just doing a cardio-based workout, such as rowing, running, or HIIT, on an empty stomach. Often this “fasting period” is an 8-12 hour window, commonly overnight for those who prefer to train in the morning. But depending on the speed of your digestive system and the size of your last meal, you could technically be considered “fasted” if you’ve used the glycogen stores in your blood steam and your stomach is empty, even if this means it’s only been a couple of hours since you last ate.
The common denominator: you have no fuel (in the form of glycogen) circulating your blood, so your body has to look for another source of energy to fuel you through your workout… In an ideal world, your body will use fat (instead of glycogen) stores to give you the energy you need. This is the basis for the supposed benefits of fasted cardio; burning body fat as fuel!
In theory, when you switch to using stored body fat as your energy source in your workouts, you’ll burn more fat. This means fasted cardio may help you lose fat faster than fed-state cardio, and achieve your body composition and weight loss goals… theoretically. More on this later.
Have you ever worked out too soon after a big meal, and felt the food sloshing around in your stomach the entire time?
Exercising on a full stomach can cause bloating, cramping, nausea and serious discomfort, so fasted cardio is definitely useful if you’re struggling with this problem! Indigestion, heartburn, nausea and even diarrhoea are much less likely if you’re doing your cardio fasted, than if you hit the gym on a full stomach.
If you tend to hit the gym at the crack of dawn, there’s one additional bonus to fasted cardio sessions: the time you save on whipping up breakfast or a pre-workout snack is extra minutes you can use to catch some shut eye! Every minute of sleep counts right…
This is also handy if you’re short on time, or always find yourself rushing around in the morning. Time is precious!
A significant downside to fasted cardio is the impact it can have on your performance during your cardio sessions.
During higher intensity cardio workouts, your body actually needs carbohydrates and glucose for fuel and quick access to energy. However in an unfed state, your body doesn’t have a ready supply of glucose, meaning your performance is often compromised.
This problem is worsened in weight training, with studies showing lifting weights while fasted results lower energy levels, hindered performance, and lower maximum weight ranges.
Fasted cardio causes fatigue and exhaustion much faster than exercising in a fed state. Given the high intensity and heart rate associated with cardio training, when glucose and carbohydrates aren’t available to use as energy, you’re more likely to feel tired, weak, sore and even potentially lightheaded or nauseous.
The truth is, while carbs get a bad rap, they’re crucial for providing your body with glucose to use as energy to power your cardio sessions. And if you haven’t eaten for an extended amount of time, you have neither carbs nor glucose available for this purpose, so your workout and energy levels will suffer.
While theoretically, as we mentioned, fasted cardio relies on the idea that in the absence of glucose to provide energy, your body will burn its own fat stores to fuel your workout, this is not necessarily the case…
In fact, your body actually doesn’t discriminate in where it’s obtaining its energy from. This means your body can just as easily break down your muscle stores and tissue, liberating stored glucose to use as fuel in your fasted cardio sessions. That means, not only are you not burning your body’s fat stores, you’re actually breaking down your own muscle as well.
In fact, a study showed that one hour of steady state fasted cardio led to double the amount of protein breakdown in muscles, compared to the same hour of cardio performed in a fed state.
And as we know, more muscle mass means you burn more calories at rest during the day… So it’s a lose-lose situation, unfortunately. If you’re looking to build or maintain lean muscle mass, fasted cardio might not be the way forward...
It’s clear that fasted cardio is really only beneficial for those who struggle with stomach discomfort if they’ve eaten too close to a workout, or simply don’t have time before their morning sweat session to devour breakfast or a pre-workout snack. If these behavioural preferences don’t apply to you, then fasted cardio may not deliver on the benefits it promises… So it really comes down to personal preference as to whether you opt for fasted or fed cardio.
While the lack of energy availability in a fasted state can limit your intensity, performance and access to energy, you may also find yourself breaking down your own muscle mass in order to supply your body with much-needed fuel for your cardio workouts. Research has proven the difference in fat burned in a fasted versus non-fasted cardio session is negligible - especially over a 24-hour period following your session - so it’s time to consider whether the risk is worth the (almost non-existent) reward…
If you do decide to go ahead with your fasted workouts, be sure to eat a nourishing meal to replenish your severely depleted glycogen stores and refuel your body post-workout. Be sure to include a source of protein, fat, and most importantly carbohydrates, and hydrate at the same time!