If you heard the phrase “fasted cardio” and filed it away with every other trendy weight-loss tactic — from cryogenic treatments to eliminating whole food groups from your diet — you weren’t completely off base.
Technically, it can help you lose body fat, just like any other exercise, but….whether or not fasted cardio is a realistic, sustainable approach to losing weight is questionable.
What Is Fasted Cardio?
Assuming you eat dinner around 7 p.m., abstain from midnight snacking, and start your workout around 6:30 a.m., fasted cardio is fairly easy to practice if you work out in the morning. But, beyond a personal preference to skip the cereal and head straight to the gym, why would anyone want to try fasted cardio?
Does Fasted Cardio Burn Fat?
For many, the promise of accelerated weight loss is motivation enough to experiment with fasted cardio. But how exactly does doing cardio on an empty stomach lead to a reduced waistline overall, if it does at all?
In simple terms, weight loss — as well as weight gain and weight maintenance — is determined by one’s caloric intake versus their energy expenditure. If you burn more calories than you consume, you’ll lose weight. Eat more than you use, and your body will store those excess calories as adipose tissue, aka body fat, which can be broken down for fuel.
Now let’s get into the slightly more specific details. In very broad terms, when we exercise, our bodies burn a combination of things for energy.
- First, there’s blood sugar mostly converted from the food we eat.
- Next, there’s glycogen, a carbohydrate stored in the liver and muscle tissue.
- Finally, there’s fat from our body’s store of adipose tissue.
The harder your effort level, the more prone you are to burn blood sugar and glycogen. Less intense efforts are more likely to burn your fat stores. For example, if you jog at a steady rate for six miles, you’re more likely to be burning fat, but if you add some sprints in there, you’ll more likely be using blood sugar and glycogen during those efforts.
Time to bring this full circle: Because fasted cardio is performed when you haven’t fueled up in a while, you have limited blood sugar and glycogen stores, so the body conserves that and is more prone to burn fat for energy.
And here’s a twist: Protein can also work as a fuel. So when you’re burning body fat, you can also break down other tissue, such as muscle, for energy. In other words, if you rely heavily on body fat for fuel, you’re prone to experience muscle breakdown as well.
Does Fasted Cardio Actually Work?
In theory, fasted cardio is the ideal workout for anyone looking to drop a few pounds and manage your weight. In practice, it’s a little more complicated for the everyday exerciser, explains Denis Faye, M.S., Openfit’s executive director of nutrition. “If you’re taking it easy, fasted cardio is fine, but the point of many workouts is to push yourself hard, especially with HIIT, and that’s where fasting can backfire,” he says.
If you’re heading into your morning sweat session without enough blood sugar and glycogen, you may be unable to hit those peaks as hard as you otherwise could and, therefore, you won’t reach your calorie-burning potential (or maybe even make it through your workout). What’s more, you run the risk of hurting muscle gains when the body starts breaking down protein for fuel.
Also, keep in mind that it’s your overall daily calorie consumption that affects fat loss, not just not what you eat (or don’t eat) in the morning. So if you follow up fasted cardio with a hearty lunch and snacks, excess calories will be stored as fat, putting you right back where you started.
Benefits of Fasted Cardio
Caveats aside, there may be some benefits of fasted cardio. Just keep in mind that these benefits should be taken into consideration with your goals and lifestyle.
Fasted Cardio for Endurance Athletes
Fasted cardio may be worthwhile for endurance athletes, like triathletes and distance runners, as it can help train the body to more efficiently tap into fat stores before glycogen has been depleted. (This is particularly beneficial at, say, mile 18 when you’ve definitely burned through the calories from that pre-dawn breakfast sandwich and are all out of energy gels.)
“But keep in mind that fasted state training is just that—training,” Faye says. “When it comes to your actual event, make sure to go into it well fed or you’ll lose a valuable edge.”
Preference for Fasted Cardio
Some people simply prefer fasted cardio to fed cardio. They may feel lighter and quicker before eating, or report improved weight-loss results after starting fasted cardio. As long as you’re staying hydrated and feel good during a workout, it’s generally safe to practice fasted cardio.
Fasted Cardio for Fat Loss
Actually, this one is a non-benefit. Sure, during your workout, you may burn more fat, but once it nets out over the course of the day, fasted cardio does not lead to additional fat loss. The fat you burn in the morning isn’t magically protected from the calories you eat during the rest of the day, so if you eat 3,000 calories but only burn 2,400 calories, those extra 600 still get stored as adipose tissue, regardless of when your workout happened.
What Are the Best Workouts to Do When Fasted?
While any exercise that raises your heart rate is technically cardio, some workouts are better suited to fasted cardio. Pete McCall, M.S., C.S.C.S., author of Smarter Workouts: The Science of Exercise Made Simple recommends doing 30-minute, low-intensity workouts three times a week and opting for activities like walking and easy cycling over running or intense interval training.
“You’re not burning as many net calories, but the calories you are burning you’re burning primarily from fat,” McCall explains. This technique also minimizes protein (muscle) breakdown for fuel by preserving glycogen.