Back in the day, I used to do a lot of fasted cardio, mainly because I believed that working out on an empty stomach would help me lose fat faster.
I’d been reading Body-for-LIFE by Bill Phillips, and fasted cardio was one of the things he recommended. The idea was to skip breakfast, work out on an empty stomach, then leave an hour after your workout before eating anything.
At the time, Phillips claimed that fasted cardio would burn 300% more fat than the same amount of exercise done after eating.
As it turns out, the benefits of fasted cardio aren’t as great as was once believed.
That’s not to say there are no benefits at all. But if you’re doing fasted cardio because you think it’s going to make a big difference to the speed at which you lose fat, chances are you’re going to be disappointed with the results.
Let’s start at the top.
What Is Fasted Cardio?
Fasted cardio refers to aerobic exercise performed in a fasted state, where your body is no longer processing or digesting food. It’s usually done first in the morning before breakfast.
The argument in favor of working out on an empty stomach is that you’ll lose fat faster than if you’d done the same amount of exercise later in the day after a meal.
Because you haven’t eaten anything, blood sugar and insulin levels are low, creating the perfect metabolic environment for fat to be burned.
On the flip side, critics say that fasted cardio increases your risk of muscle loss. Working out while fasting, they say, will just eat away at the muscle you’ve worked so hard to build.
So who’s right? Will working out on an empty stomach help you lose fat faster? Or will it put you on the fast track to muscle loss?
Fasted Cardio: What the Science Says
Most studies show that working out on an empty stomach leads to more fat being burned than the same amount of exercise done after a meal.
In one study, researchers got a group of men to go for a morning run, either before or after breakfast . Skipping breakfast and doing cardio in a fasted state led to a 20 percent increase in the amount of fat burned during the workout.
Rather than looking at the amount of fat burned during exercise, researchers from Japan’s University of Tsukuba went a step further and measured it for an entire day .
For the study, nine young male endurance athletes ran on a treadmill for 100 minutes, either before breakfast or after lunch. On a third occasion, they ran for 50 minutes before breakfast and after lunch.
The total number of calories burned was similar in all three trials. But the number of fat calories burned during the fasted workout was higher than it was during the other two conditions.
Doing cardio before breakfast burned a total of 1142 fat calories, compared with 608 fat calories when the same amount of exercise was done after lunch.
That’s a big difference. Over a 24-hour period, working out while fasting led to almost twice as much fat being burned compared to a day where the exact same workout was done after lunch.
A few years back, a team of Brazilian researchers published a meta-analysis that looked at the effects of fed versus fasted cardio on fat metabolism .
A meta-analysis involves pooling the results from multiple trials on the same subject. Instead of lots of small experiments, you end up with one big experiment, conducted on lots of people. As a result, you’re left with a conclusion that’s more reliable than anything that could have been drawn from each of the smaller studies.
After pooling the results of 27 trials, covering a total of 272 people, they concluded that “aerobic exercise performed in the fasted state induces higher fat oxidation than exercise performed in the fed state.”
Working out while fasting has also been shown to decrease the amount of weight gained during six weeks of overfeeding.
Subjects given a carbohydrate-rich breakfast approximately 90 minutes before working out gained roughly twice as much weight, most of which was fat, compared with those who exercised on an empty stomach .
The same group of researchers report that fasted cardio leads to a number of beneficial metabolic adaptations not seen with exercise in a fed state, most notably a greater increase in the ability of muscle to burn fat for fuel .
There’s even more good news.
Working out on an empty stomach also alters the activity of various genes involved in fat metabolism compared to the same amount of exercise done in a fed state , with the researchers concluding that “feeding before exercise blunts some of the health-related changes induced by exercise training.”
So, there you have it.
More fat is burned when you exercise in a fasted state. Not only that, but working out on an empty stomach increases the amount of fat burned over a 24-hour period, as well as altering the expression of various genes involved in getting you lean, mean and ripped.
If you want to lose fat as fast as humanly possible, fasted cardio is most definitely the way to go.
Or is it?
Not necessarily. And here’s why.
For one, many of these studies have used pre-exercise meals containing carbohydrate.
Had the meals contained no carbohydrate, which can suppress fat oxidation, the results may have been very different. A pre-exercise serving of whey protein, for example, has not been shown to impair fat oxidation during exercise, despite raising insulin levels .
More important, short-term changes in fat metabolism during fasted cardio don’t automatically translate to long-term changes in body composition.
Over time, your body will adjust the rate at which it burns fat and carbs. That’s why it’s important to look at how fasted cardio affects fat loss over a period of weeks and months.
What happens to fat metabolism during a fasted workout, or even for several hours after exercise, doesn’t paint a complete picture.
There are only a handful of studies to look at the long-term effects of fasted cardio on body composition. All show much the same thing. Doing your workouts on an empty stomach is unlikely to have much of an impact on the speed at which fat is lost.
Is Fasted Cardio Really Better for Fat Loss?
When a team of researchers looked at the effect of four weeks of cardio performed in a fed or fasted state, it was only fasted cardio that led to a decrease in body fat percentage . At first glance, this looks to be a ringing endorsement for working out while fasting.
But when you dig into the detail, the results are a lot less impressive.
In fact, the amount of extra fat lost – less than one half of one pound – was very small. Any differences between the groups could have been down to the fact that body fat callipers, which aren’t very accurate, were used to track changes in body composition.
In a follow-up study, 20 young women were assigned to one of two groups: a fasted group that performed 50 minutes of walking or jogging three times a week after an overnight fast, or a group that ate breakfast before exercise .
The meals were provided in the form of a shake that contained 250 calories (40 grams of carbohydrate, 20 grams of protein, and 0.5 grams of fat).
After four weeks, there was no significant difference in the amount of fat lost between the two groups.
In a similar trial, this time using HIIT rather than steady-state cardio, researchers from Canada’s McMaster University looked at the impact of fasted versus fed exercise in two groups of overweight and obese women .
Both groups performed 18 sessions of sprint interval training for six weeks. They also consumed an identical breakfast (an energy bar, yogurt, and orange juice, which provided a total of 439 calories) on training days.
However, while one group ate the meal 60 minutes before training, group two exercised in a fasted state and ate their meal 60 minutes after the workout.
After six weeks, both groups lost an average of 1.3 pounds of fat. But there was no difference in the amount of fat lost between the fed and fasted groups. Women who hit the gym before breakfast lost no more fat than women who exercised one hour after eating.
In short, the beneficial effect of fasted cardio on fat metabolism doesn’t translate into a significant increase in the rate of fat loss over time .
Potential Benefits of Fasted Cardio
This doesn’t mean there are no benefits to fasted cardio, and it may provide an advantage for certain people in certain situations.
Some researchers, for example, think that fasted cardio provides greater metabolic health benefits than fed cardio .
I’ve also seen solid arguments in favor of carbohydrate periodization, where some of your training (i.e. longer, low-intensity steady-state work) is done with a low carbohydrate availability, while more intense workouts are done with a higher carbohydrate availability.
The idea is that purposely doing some of your training in a fasted state, with low glycogen stores (the name for carbohydrate stored in your body) can boost performance when you eventually compete with carbohydrate reserves fully restored [13, 14].
But, that’s more as a way to improve performance for the endurance guys, rather than to drop fat more quickly.
Bodybuilders have been using fasted cardio as part of their pre-contest preparations for decades. In some (but not all) cases, training and nutrition practices that have survived for many years do so because they’re effective.
While fasted cardio doesn’t appear to affect the total amount of fat you lose, it may increase the amount of fat lost from certain areas of your body.
This increase in blood flow has a couple of potential benefits.
For one, hormones that trigger the breakdown of stored fat will have an easier time getting to the fat cells in the first place. Second, once the fat has been mobilized, an improved blood flow makes it easier for the free fatty acids to be shuttled away and oxidized, or burned off, elsewhere.
While working out on an empty stomach is unlikely to help you lose more fat compared to the same amount of exercise performed in a fed state, it may have an effect on regional fat loss, making it easier to get rid of fat from certain areas of your body.
Should You Do Cardio on an Empty Stomach?
So, what’s the bottom line? Should you do cardio on an empty stomach?
If you’re doing it because you think it’s going to help you lose fat faster, then no.
There’s very little research to support the idea that fasted cardio is dramatically more effective for fat loss than the same amount of exercise done after a meal.
But on the flip side, there’s no compelling evidence to suggest that working out before eating increases the risk of muscle loss either.
If you don’t like working out on an empty stomach, and doing so makes it less likely that you’ll make it to the gym on a regular basis, there isn’t much point in switching to morning workouts solely for their fat-burning benefits.
Staying consistent with your training trumps most other things when it comes to getting in shape, and you need to pick a training time that works for you.
Whether or not you work out on an empty stomach or after a meal is a decision that can be left to personal preference. As far as improving your body composition is concerned, there doesn’t seem to be any great harm or benefit to one or the other.
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ABOUT THE AUTHORChristian Finn is the nation’s leading authority on science-based, joint-friendly ways to build muscle. A former "trainer to the trainers," he holds a masters degree in exercise science, and has been featured in or contributed to major media on two continents, including the BBC and Sunday Times in the U.K. and Men’s Health and Men’s Fitness in the U.S.