It's true that hitting the gym in a fasted state may allow your body to burn more fat during your workout. But does that mean fasted cardio is an ideal approach to overall fat loss? Here's what the science says about this popular workout tactic.
What is Fasted Cardio?
Fasted cardio is doing any sort of cardio or conditioning workout on an empty stomach. And since it takes roughly four to six hours for you to digest all of your food - depending on what you ate - fasted cardio typically happens in the morning.
Another popular use of fasted cardio occurs during intermittent fasting, where individuals will restrict their eating window for long periods of time and train during this period. This is often followed by an eating window where a large post-workout meal is consumed.
Why Workout Fasted?
So why would one want to workout without fuel to power their gym sesh?
There is a common theory (backed by a small amount of research), that training in a fasted state will force your body to utilize more body fat for energy- thus ramping up your fat burn.
Does it Help You Burn More Fat?
A small meta-analysis study, looking at less than 300 people, concluded that doing cardio in a fasted state can increase your ability to burn fat (1). And this study is often used by fasted cardio advocates to showcase this phenomenon.
Here's the catch!
Looking at the research more closely, even though fat utilization increases during a fasted workout, once you resume eating, your body switches back to carb-burning mode and actually starts using more sugars for energy (5).
And even more interestingly, eating carbs (aka sugars) before the gym causes you to utilize more carbs during training since they are available, but then switching to more of a fat-burning mode after your workout (6).
In other words, it's a wash! Your body uses fuel when you have it and relies on body fat stores when you don't. Working out fasted likely makes no difference for overall daily fat burn or body composition (7,8).
Moreover, burning fat and losing body fat are not exactly the same thing. Burning fat is only referring to your body's ability to use this nutrient as a source of energy and does not override the calorie equation.
Meaning that fat burning only results in fat loss when you are using more energy than you are taking in. Otherwise, your body can just take the food you eat and store more body fat.
Fat loss can only occur in a calorie deficit.
Other Potential Benefits of Fasted Cardio
Besides your ability to use fat for energy, are there any other reasons one might want to workout fasted?
For some people, hitting the gym on an empty stomach is preferred. Eating too soon before a workout can sometimes cause GI distress and isn't always the best pre-workout solution (9).
That's because when you start ramping up the cardio while your digesting food, your body diverts more blood flow and resources to your muscles instead of your digestion system. And this can sometimes cause an upset stomach or nausea (10).
Of course, it depends on what you ate, how much, and how soon before you hit the gym.
Skipping meals is also a way to cut calories. However, this doesn't mean you won't be hungrier later and research suggests how many times you eat a day is not as important as being consistent with your diet.
Outside of personal preference and digestion woes, there isn't much research indicating any other potential benefits of fasted cardio.
Disadvantages of Working Out Fasted
If you are strength training or looking to increase your muscle mass, working out fasted may put you at a disadvantage.
Getting a proper pre-workout meal can provide you a source of fuel to power your workouts, essentially helping you train harder (11,12). Additionally, including a source of protein before the gym may also help protect some of your lean mass (13,14,15).
Moreover, if you are low on energy to begin with, skipping food before a workout can cause you to have low blood sugar, leaving you feeling dizzy, nauseous, and weak. This not only increases your risk of injury but makes your workout feel pretty sucky altogether.
To Eat or Not to Eat Before a Workout?
While fasted cardio likely won't help you burn more fat, it doesn't mean you have to eat before the gym either. This one boils down to personal preference and your fitness goals.
If you enjoy working out on an empty stomach and don't feel like your workouts suffer, keep at it. But if you find having more fuel before training helps you push harder, then make sure you time a quality pre-workout option accordingly.
If you do choose to forgo the food before you train, just make sure you re-fuel and recover with the right post-workout options to help you reach your fitness and nutrition goals.