According to many fitness 'gurus,' the answer is yes. They claim that there are two reasons fasted cardio is superior for fat loss: Since insulin levels are low first thing in the morning, your body burns more fat when you exercise at that time. (After all, elevated insulin levels suppress fat burning.)(Jul 9, 2019
Since insulin levels are low first thing in the morning, your body burns more fat when you exercise at that time. (After all, elevated insulin levels suppress fat burning.)(1)
The overnight fast reduces glycogen levels within your body. As a result, if you train in a fasted state, your body will burn more fat for fuel instead of glycogen.
Both of these reasons sound plausible. And research also shows that if you elevate insulin levels by consuming food, especially carbohydrates, you’ll burn less fat for fuel during exercise.(2)(3)(4)
So that sounds like a win for fasted cardio, correct?
Well, even though you do indeed burn more fat for fuel during a fasted cardio session, this does not imply that you'll lose more fat overall.
That's because research shows that if you burn more of one substrate during a cardio session, you will burn less of it over the next twenty-four hours.(5)
In other words, your body will compensate for the increase in fat burning later on in the day. And, as a result, you’ll burn the same amount of fat over a twenty-four-hour period whether you train fasted or not.
Food provides fuel for energy, so if there’s nothing available in your stomach, it makes sense that your body would tap into its stored energy—mainly fat—to power exercise. The trouble is, the store it seems to go after first is protein, in the form of your hard-earned muscle mass. According to research published in Strength and Conditioning Journal in 2012, protein breakdown can actually double when you do cardio on an empty stomach.
Furthermore, studies have shown that fasted cardio did not burn fat more effectively than cardio performed after a meal. And since workout intensity directly affects how many calories you burn, not eating before a long and intense session could reduce the energy you have to train with and lead to a subpar workout.
But what about high-intensity interval training (HIIT)? That’s supposed to burn fat and spare muscle, right? The truth is, performing it on an empty stomach causes you to break down fat faster than your body can use it for energy, which results—sadly—in the fat being shuttled back into your fat cells!
The Cons of Fasted Cardio
Here's the thing: While your body may turn toward the fat stores in your adipose tissue for energy, it doesn't discriminate where it gets the energy from, says Dr. Trentacosta. That means that your body could break down your muscle tissue for fuel. Ugh.
Vavrek agrees, adding that instead of using fat from your adipose tissue, your body may use the protein that makes up your muscle tissue as fuel. In fact, one study found that one hour of steady cardio in a fasted state resulted in twice the amount of protein breakdown in muscles, compared to non-fasted cardio. The researchers concluded that performing cardiovascular exercise while fasting might not be a good choice for people seeking to gain or maintain muscle mass. (Related: Everything You Need to Know About Burning Fat and Building Muscle)
On the other hand, workouts at a higher heart rate and intensity require carbohydrates for quick energy. Without them, you'll probably feel tired, weak, sore, and even nauseated or lightheaded. (That's the same reason keto-dieters may need to rethink their workout routine while on the high-fat plan.)
Ultimately, whether your body burns fat or breaks down muscle depends on what kind of exercise you're doing, says Jim White, R.D.N., an ACSM exercise physiologist and owner of Jim White Fitness and Nutrition Studios. "The idea is to stay between 50 and 60 percent of your target heart rate, which you can do during a walk, slow run, elliptical jaunt, or yoga class." The easier the workout, the more likely your body will use fat.
Translation: If you're in a fasted state, don't do HIIT, boot camp, or CrossFit classes, says White-and definitely don't strength train. If you lift weights while fasted, you won't have the energy to lift to the best of your ability. At best, you're not maximizing the benefits of your workout. At worst, you could end up getting injured, says White.
That said, whatever the intensity or type of exercise, Vavrek cautions against fasted cardio. "Working out in fasted state is just not your best option for fat loss." The reason: Being un-fueled will limit the intensity you're able to bring to a workout, and high-intensity training has been shown to help you burn more fat and calories in the 24 hours after a HIIT workout than a steady-pace run. This is much to do with the total number of calories burned during HIIT being so high, so your body is going to burn both carbs and fat during these quick, intense workouts. Plus, an older study found that ingesting carbs before working out increases the post-exercise afterburn effect more than the fasted state. (Related: The Science Behind the Afterburn Effect)
That being said, fasted cardio is probably not the best option for most. "Many people will tire too easily or hit a wall in their workouts without fuel. Some may even get dizzy," says Dr. Trentacosta. (That's why Conrad emphasizes the importance of talking with a health care provider before cutting out your pre-workout fuel.)