Fasted cardio and weight lifting are controversial topics. Bro science says that they’re absolutely the way to go for maximum fitness results – refer to millions of Instagram ab pictures. Conventional nutritionists say that the body needs adequate carbs before, during and after any athletic endeavor.

Who’s right? Or is there maybe a middle road?

In this article, we’ll break down what actually happens in your body when exercising in a fasted state and how it can benefit you. We’ll also cover any potential risks. So whether you’re interested in fasting weight loss, muscle gain, or performance, you’ll find all the tips and information you need to get started.

Fasting and Fitness: What You Need to Know:

Fasting for weight loss

Even outside of ‘bro science’ circles, it’s pretty well known that fasted training helps you lose fat.

In a study performed on 20 athletes, the experimental fasting group burned through a special type of fat during exercise, and the carb-fed control group burned through none.[1] These special fat reserves are known as intramyocellular lipids (IMCLs); they exist so that you can perform enough work to get your next meal when you haven’t eaten anything. You can think of IMCLs as a high-octane fuel source just for your muscles.

The cool part is that once you oxidize some of these intramyocellular lipids during fasted cardio or weightlifting, your body has to recruit fat from other parts of the body (think: belly, hips, thighs) to replace the IMCLs. They’re essential for a functioning metabolism. [2]

This process is an evolutionary advantage that helped us hunt and thrive even when we didn’t have donuts and crepes to munch on before work.

“In previous incarnations of the human,” says Mark Sisson, owner of Mark’s Daily Apple, “fasting preceded food procurement.” You didn’t just pop open the fridge for food – “you worked for it,” he said. “You hunted (and gathered) for it. You exerted yourself to fill that empty belly.” [3]

Sisson’s perspective is that it’s actually un-natural to not exercise when you’re fasting.

Other studies have shown that fasting and exercise independently contribute to increased hormone sensitive lipase, an important fat-burning enzyme.[4], [5] Thomas Delauer, a fasting researcher and expert, refers to fasted exercise as a “double whammy” fat burning tool for this reason. [6], [7]

What kind of exercise is best for fasted weight loss?

You might think the best fasted exercise would be some HIIT or a few rounds of sprints to get the workout over quickly. But in reality, high-intensity exercise will decrease the rate at which you burn fat.

According to research, the sweet spot for fat burning happens during mild to moderate aerobic exercise. [8] So anything like jogging, walking, lifting weights, swimming, and even running below an anaerobic threshold is fine – just so long as you keep your exercise under an hour.

You’ll also want to stay away from intense exercises that tip you into an anaerobic, sugar-burning metabolism. (Anything that ends in ‘till exhaustion’ is a no-go when you’re fasting.) This jibes with the anthropological research that depicts our ancestors as walking and jogging for several miles a day in their foraging and hunting patterns, much of which happened in the fasted state.

Fasting for muscle gain

That a person can build muscle from working out in a fasted state is not up for debate. Apart from the successful bro science experiments plastered on social media, several small trials have shown that fasted weight training does not prevent muscle growth compared to fed weight training. [9]

But is there any science that says that we should wait to lift till after we’ve eaten?

A study published in the European Journal of Applied Physiology examined six young men over the course of two experimental gym sessions – one fasted, the other fed. In both weight lifting sessions the lifters were provided a recovery drink made of protein and carbs over a four-hour period. [10] After comparing several metabolic markers, the researchers concluded that “prior fasting may stimulate the anabolic (muscle building) response” to a post-workout shake. And the results were two-fold over the fed group.

Although the results of a small trial aren’t irrefutable, this is compelling evidence that fasted training can be more favorable. And the evidence doesn’t stop there.

Growth hormone is commonly known to increase during a fast. Now, you’re obviously not going to grow muscle while working out in a fasted state just because of the presence of growth hormone alone – you need protein sometime after the workout. But research suggests that growth hormone, by way of regulating IGF-1 levels, prevents muscle from being used as fuel during a workout. [11] Study authors noted that growth hormone was “decisive component of protein conservation” during a fast.

Thomas Delauer agrees with this claim.

“You’re not going to lose as much muscle as you think, if any,” he said on his YouTube channel. “You have high amounts of human growth hormone that stops the breakdown of muscle.” [12]

Sisson says that fitting in a workout during a fast is a smart strategy for preventing muscle loss, whether you’re on a 16-hour intermittent fast or a 5-day prolonged fast. “If you don’t use your muscles during a fast,” he said on his blog, “your body will consider them fair game. [13]

“Those muscles can provide a big dose of amino acids that convert into glucose,” he continued, “and if you’re not using them, you’ll lose them to make glucose.”

How should you train in a fasted state?

Everybody is unique and responds in different ways to fasted weight training. But to start, fasted weight training sessions shouldn’t be ‘balls to the wall’ workouts. You don’t know how your blood sugar will respond to ‘till failure’ workouts on an empty stomach, and there’s a potential risk for acute bouts of hypoglycemia or even syncope passing out. (Just like any workout.)

So start out with sessions lasting under an hour, and keep your perceived exertion below an 8 on a scale to 10.

Dr. Rhonda Patrick, PhD fitness researcher, says that your post workout eating window changes when you’re fasted. [14] “There’s been a lot of research done looking at, you know, ‘is there this anabolic window that you have to take protein to increase muscle growth,’” she said in a podcast interview. [15] “And over the past few years it’s been pretty consensus that there’s a lot longer time you have.

“But if you’re going into a workout fasted,” she said, “you may want to consume protein within an hour of your workout.” Sisson agrees, adding that occasionally delaying your post workout meal can improve your growth hormone response, though it shouldn’t be done regularly.

“There are no hard-and-fast rules to exercising while fasting,” he said. “You know what works for you.”