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Should You Be Doing Fasted Cardio?

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Photo: stockvisual / Getty Images

If you're anything like us, your IG feed has a high-volume of ~fitspirational~ belfies, smoothie bowls, and (recently) proud body hair pics. But there's another thing people love talking (nay, bragging) about on their social platforms: fasted cardio workouts.

What Is Fasted Cardio, Exactly?

At the most basic level, fasted cardio involves increasing your heart rate without noshing on a pre-workout meal or snack beforehand. Fasted cardio fanatics claim the practice maximizes your fat-burning potential. But, naturally, you might wonder whether working out on an empty stomach is a good (and safe!) idea or just a trend that sounds legit. *Sigh*.

The Basics of Fasted Cardio Workouts

First things first: How long do you have to go without food for your workout to be considered "fasted"?

Usually, eight to 12 hours, says sports medicine specialist Natasha Trentacosta. M.D., of Cedars-Sinai Kerlan-Jobe Institute in Los Angeles. But for some people, it may be just three to six hours, depending on how fast your digestive system is working and how much food you ate at your last meal. "Once the body has stopped processing and breaking down food, your insulin levels are low and there's no fuel (glycogen) circulating in your blood," says Dr. Trentacosta. As a result, your body has to turn to another source of energy—usually fat—to power you through the workout.

Typically, fasted cardio happens in the morning (after an overnight fast). But a fasted state can also be achieved later in the day (for instance, if you're doing intermittent fasting or skipped lunch), says sports medicine dietitian Kacie Vavrek, M.S., R.D., C.S.S.D., of The Ohio State University Wexner Medical Center.

Bodybuilders have been using fasted cardio as a fat-loss technique for years, and regular gym goers have recently been adopting it as well. But you may have been doing fasted cardio workouts already without realizing it. Technically, any time you head straight to an early-morning workout without eating first, you're doing a fasted workout. (Related: How to Wake Up Early for a Morning Workout, According to Women Who Do It at 4 A.M.)

The Benefits of Fasted Cardio

If your primary goal is to lower your body fat percentage and your go-to workout is low- to moderate- intensity cardio, fasted cardio may offer some perks. "Research does support that you'll burn more fat when you run in the fasted state than when your body does not have circulating nutrients to use for energy," says Dr. Trentacosta. For instance, one small study found that when people ran on a treadmill in a fasted state, they burned 20 percent more fat compared to those who had eaten breakfast.

Why? When you don't have readily available energy from food, your body has to look elsewhere, explains Dr. Trentacosta.

"Fasted cardio may be effective in getting the body to help burn stubborn fat for someone that has been working out regularly for a while," agrees chiropractic doctor and certified strength coach Allen Conrad, B.S., D.C., C.S.C.S. Read: Newbie exercisers shouldn't try it. That's because people who've been working out for a while tend to know their limits and be more in touch with their bodies, he explains.

While running on empty may make you feel sluggish at first, over time, your body will adapt to be more efficient at burning fat for fuel. Conrad says this may be beneficial if you work out for longer than 30 minutes at a time, four or more times a week (like endurance runners or triathlon-ers). In fact, research published in the Journal of Applied Physiology comparing fasted individuals versus fed individuals over the course of six weeks found that, when training at the same intensity, those who consistently trained in a fasted state showed more improvement in their endurance exercise performance compared to those who noshed before training.

Perhaps one of the biggest reasons people work out on an empty stomach is because skipping a pre-workout meal or snack means a few more precious zzz's. The standard recommendation is to wait at least 30 minutes after eating to work out—and that's if you're only having a banana or slice of toast with nut butter (and not, say, a three-egg omelet with bacon). Eating a larger breakfast before hitting the gym in the morning is a fairly obvious recipe for GI distress. The easy fix: Waiting to eat until after your workout. (Related: What to Eat Before Working Out and When to Eat It)

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)

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.

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.)

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)

So Is Fasted Cardio Worth It?

Maybe. The evidence is pretty mixed, so, ultimately, it comes down to your personal preference and goals.

"There are absolutely people who love it. In part, because it's something new, and, in part, because it just works with their body," says White. If you're a morning exerciser and don't like eating before your sweat session, it may be worth giving it a try.

If you do decide to fast, make sure to eat after your workout, he says. His go-to is a PB&J smoothie, but there are tons of post-workout meal recipes that pack the right combo of carbs and protein. Fair warning: You might be hungrier than usual.

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.)

If working out while hangry is not for you, there are plenty of other, more effective ways to burn fat.

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