By Lauren Sambataro
In the fitness and wellness world, fasted cardio—and fasting in general—are trendy topics, and have become extremely popular practices over the past few years. You may have read about it, or heard that J.Lo and A-Rod are doing it, or watched your TikTok BFF rave that fasted cardio is the “be-all end-all technique” for burning stubborn fat.
So, what is fasted cardio? What’s the science behind it (hint: it’s scant, and expert opinions are mixed)? Is it safe, and is it right for you?
What is fasted cardio?
If you’ve ever worked out first thing in the morning before breakfast or without grabbing a pre-workout snack, then you’ve essentially done fasted cardio, which is exercising after not eating for a certain amount of time. According to science, a true fasted state starts about 8-12 hours after your last meal. Assuming you’re not sleepwalking to the fridge for a 2 a.m. nosh, those practicing fasted cardio typically work out soon after waking up.
The concept behind fasted cardio is that exercising at around 50 to 60 percent of your target heart rate (think Zones 1 and 2) on an empty stomach will burn stored fat as a fuel source (because your glycogen stores will be depleted) rather than using your breakfast (or banana or protein bar) for energy.
Fasted Cardio = Fat Loss? Sign me up!
Sounds good, right? Who doesn’t want to be a lean, mean, efficient-fat-burning machine? The catch: There’s not a ton of research to back it up. And the research that exists is inconclusive.
The proposed fat-burning advantage is based on lipid oxidation (breaking down of fats) during training. However, this doesn’t take into account that the body is highly adaptable; essentially, it will always find a way to adjust to find the most easily accessible fuel. Plus, our bodies are all different—our metabolism, our health, our age or medications we take, among other things—which means that we burn fat and lose weight differently.
Why fasted cardio could work for you
There is some research to support that fasted exercise may burn more fat. One study, for example, found that exercising in a fasted state may result in a greater reliance on fat as an energy source when there are no circulating nutrients to use, which has potential implications for weight management.
Another older study showed that regular fasted training may contribute to exercise performance and endurance. More recent research has indicated that skipping a pre-workout snack could potentially have some post-exercise metabolic advantages.
Potential downsides to exercising on empty stomach
While your body may turn to fat stores for energy, it could also break down muscle tissue for fuel. You don’t get to choose.
In fact, one study found that one hour of steady-state fasted cardio resulted in twice the amount of protein breakdown in muscles compared to non-fasted cardio—so not the best option if you’re looking to maintain or gain muscle mass. Even if your body does break down fat (meaning fatty acids get released from fat cells and then move throughout the body) it’s not proven whether fasted exercise actually burns that fat. Those fatty acids that are broken down but not oxidized are just returned back to fat cells for storage. Our bodies are crazy, right?
A small 2018 study that looked at exercising in a fasted state found that not eating pre-workout negatively affected intensity and volume of training. Not getting through your workout — or as we coaches like to say, bonking!— happens when depleted glycogen stores cause you to hit a wall.
If you’re prone to overeating, then fasted cardio could pose another pitfall. If you finish a workout starved due to a rebound effect of ghrelin levels, a.k.a the “hunger hormone,” that can lead to binge eating or overeating to compensate for the deficit.
Should you give fasted cardio a try?
Maybe. The science is mixed, so really it comes down to your personal preference, and whether fasting before exercise feels right for you and your body.
Consider your training goals. If you want to get stronger/more powerful, experiment with how it feels to train fed with more or fewer carbohydrates. As with any training modality, consistency is key. One fasted workout isn't going to change your body composition; rather it's what you consistently do over time.
If you want to experiment with fasted cardio, choose moderate- to low-intensity workouts, and don’t exceed 60 minutes without eating. And be sure to drink water to stay hydrated.
The truth is that fasted cardio is likely not the miracle fat-blaster most would hope for. Some people feel great and love it — including myself and coach Dyan— because it just works with our bodies. Keep in mind that contrary to what we’ve previously been told, you don’t need to eat before a workout. In fact, we are evolutionarily designed to go for periods of time without food, including during activity. (Which is probably why men generally do much better with fasting than women as they were the hunter-gatherer-foragers back in the old days.)
If you’re an early morning exerciser and not a breakfast eater, then give it a go. It might take a little time for your body (and mental state!) to acclimate. If after a couple of fasted workouts you’re still not feeling great—you’re weak or lightheaded or nauseous—then stop. I don’t recommend it for newbie exercisers since it can take a while to get to know your body and its limits. Fasted workouts are also contraindicated for cycling women.
Plus, there are many reasons to exercise that have nothing to do with burning fat. Your workouts should make you feel strong and accomplished, help to bust your stress and sleep well. All of which leads to a healthier, happier you. If exercising on an empty stomach takes away the benefits and the joy, then that tells you it’s not the right technique for you.