Does fasted cardio help accelerate fat loss?
On paper it makes perfect sense. Food provides fuel for energy. If nothing is available in your stomach, your body must tap into other energy dense storage systems—like fat molecules—to power exercise. So don’t eat, do cardio, and watch the scale go in the opposite direction.
As much as it might make sense, this fitness practice might be more of an illusion than any David Blaine special. And it could also explain why so many people actually struggle to lose weight.
When you perform high intensity cardio fasted, you break down fat faster than you can use it as energy. This is not a good thing.
Understanding why the, “fasted cardio is great for fat loss” became popular is a matter of separating what science says from what it actually means.
Fat loss—the type that you can see in the mirror—isn’t a light switch. It’s not a process that flips on and off only when you exercise. It’s a combination of many factors. While not directly harmful, research—both new and old—has made it pretty clear that fasting before exercise isn’t doing anything to make the fat loss system more efficient.
That’s not to say that you can’t do fasted cardio and lose fat. The real question (and what you care about for considerations of time and hunger) is much more basic: is fasted cardio undeniably better for weight loss?
To fully understand what’s best for your body, it would help to understand how fat loss occurs. It’s best to view this as a complicated process that can be boiled down into 4 general factors:
Weight Loss Factor #1: Calories burned
To drop weight you must burn calories. But research in the Strength and Conditioning Journal found that fasted cardio does not increase lipolysis—the process of burning fat—compared to when you’ve enjoyed a pre-workout meal.
So while you’re burning calories fasted, the actual process of fat loss is not accelerated if you’re cranking away for 30 to 60 minutes. Not to mention, fat loss is directly linked to workout intensity. When you fast and then try to do a longer workout, the likelihood of you tapping out is increased. Better to eat a little food and push harder, rather than have a sub-par training session.
Weight Loss Factor #2: Protein breakdown
Remember, this isn’t an attempt to become skinny and drop muscle mass. You want to retain lean mass and drop fat. For that to happen, you want to focus on protein retention and limit protein breakdown. But when you exercise fasted, it can double protein breakdown, meaning some of that hard work is eating away at your muscle.
Weight Loss Factor #3: Fat breakdown
Research shows that if you want to really accelerate the fat burning process with cardio, you need to do it at lower intensity. But it’s not just low intensity—it’s like a crawl through the desert. We’re talking about 2 hours slogging away on a slow walk, and even then it’s limited. And who has 2 hours per day just for cardio?
You might assume you’ll just do a HIIT workout and bump up the intensity, while cutting down on time. However, when you perform high intensity cardio fasted, you break down fat faster than you can use it as energy. When that happens, sadly, it the fat is shuttled back into your fat cells.
This is one of the silver bullets to consider as to why fasted cardio might not be the best route.
Weight Loss Factor #4: The Afterburn
The final consideration is EPOC, or exercise post-exercise oxygen consumption. This is what your body keeps burning after you stop training.
It’s the main reason people cite as a reason for lifting weights; you stop training and your body keeps the “metabolic furnace” churning.
This does happen, but eating prior to a workout increases your “afterburn” compared to a fasted state.
The Bottom line
You can do fasted cardio, and you might even “feel” better without food sloshing around in your gut. But doing cardio while fasted will not have undeniable fat loss benefits.
Does it “work” for some people? Of course. But don’t confuse individual situations with scientific realities. If anything, fasted cardio is most likely to make no difference in your speeding up your body transformation goals.
Adam Bornstein is a New York Times bestselling author and the author of You Can’t Screw This Up. He is the founder of Born Fitness, and the co-founder of Arnold’s Pump Club (with Arnold Schwarzenegger) and Pen Name Consulting. An award-winning writer and editor, Bornstein was previously the Chief Nutrition Officer for Ladder, the Fitness and Nutrition editor for Men’s Health, Editorial Director at LIVESTRONG.com, and a columnist for SHAPE, Men’s Fitness, and Muscle & Fitness. He’s also a nutrition and fitness advisor for LeBron James, Cindy Crawford, Lindsey Vonn, and Arnold Schwarzenegger. According to The Huffington Post, Bornstein is “one of the most inspiring sources in all of health and fitness.” His work has been featured in dozens of publications, including The New York Times, Fast Company, ESPN, and GQ, and he’s appeared on Good Morning America, The Today Show, and E! News.