Is ‘Fasted Cardio’ a Fast Track to Your Fitness Goals? Maybe, Maybe Not

If you’ve ever rolled out of bed, and immediately bee-lined it to the treadmill cup-oh-buzz in hand, then you’ve unknowingly given fasted cardio a try. Working out on an empty stomach isn't always intentional. But fitness professionals say there are reasons to give it a try—particularly if you have endurance or body recomposition goals in mind. 

What is fasted cardio?

The type of exercise and amount of time lapsed since your last nosh matter here. To count as fasted cardio, the workout in question should be a rhythmic, monostructural movement like cycling, running, or rowing, says certified strength and conditioning coach and personal trainer Josh Schlottman. 

And how long do you need to have forgone food? According to John Gardner, NASM-certified trainer and CEO behind fitness platform Kickoff, it’s however long it takes for your stomach to become empty and for your digestive system to be in the "off" position. “Depending on how fast your digestive system works or what the last thing you ate was, your body could be in a fasted state in four to six hours,'' he says. However, the best results typically come from working out after a 12-hour fast. Most commonly: On an empty stomach first thing in the morning. 

Caffeine-cravers, fear not: It is kosher to consume coffee (sans creamers or sugar), pre-workout, and water ahead of a fasted cardio session, according to Schlottman. Actually, it might be better to: One of the biggest road-blocks people run up against with fasted cardio is having low energy levels. “Having some caffeine can help to give you an energy boost to power through your workout,” he says. 

First off: Is fasted cardio safe? 

As long as you’re in sound health, you should be perfectly fine giving fasted cardio a try. However, if you have a pre-existing medical condition, Gardner suggests talking to a healthcare professional. “Fasted cardio can be extremely dangerous for individuals with any medical condition that’s impacted by low-blood sugar,” he says. You should also skip it if you feel light-headed or dizzy when you try exercising on an empty stomach, he adds. 

It might support your body recomposition goals 

If you’re looking to reduce body fat percentage, fasted cardio may be beneficial. When you exercise after eating, your body uses the calories you’ve just consumed as energy to fuel your workout, explains Schlottman. “When you’re in a fasted state, your body doesn't have any quick carbohydrates or other calories that it can easily use as fuel,” he says. Instead, it has to turn to glycogen, which is how carbohydrates get stored away long-term in the muscles and liver. Once your glycogen stores are tapped out, the body turns to fat stores for fuel, he says. 

The result? You burn more fat. One study published in the British Journal of Nutrition found that people who ran on a treadmill in a fasted state burned 20 percent more fat than those who ate first. 

It could increase your stamina

Fasted cardio may also support your endurance goals. That’s because it trains your body to rely less on using quick-burning carbs and sugars for energy, and instead to use fat for fuel, explains Schlottman. Because your body has much larger reserves of fat stores than it does of glycogen, this ability can keep endurance athletes from “bonking” or “hitting the wall.” Indeed, in one study published in the Journal of Applied Physiology individuals who exercised fasted saw more endurance gains than those who exercised at the same intensity in a fed state. 

Keep in mind...  

Beware that burning fat for fuel is a less efficient process, so the same workout at the same intensity will feel harder, sports dietitian Natalie Rizzo, RD, previously told Well+Good. Not everyone can happily work out on an empty stomach. If exercising without eating is so miserable that it keeps you from exercising at all, forget it and just have that pre-workout snack.

Rizzo also warns that everyone's body responds differently, and doing fasted cardio workouts regularly over an extended period of time could lead to vitamin deficiencies, mood fluctuations, and lowered immunity. Be sure to listen to your body, and adjust your strategy if you think fasted cardio workouts are backfiring.

Also, “make sure you eat lots of carbs and a balanced meal post-workout to recharge your energy and give the body the fuel it needs,” says Gardner. Failure to fuel yourself after a fasted cardio session will interfere with your ability to recover properly, he says. Which could undermine any gains you might have otherwise made.