Ask a group of fitness experts whether you should fast before a cardio workout and you’ll get varying opinions. Some will tell you to fast if you’re trying to lose body fat, while others will tell you that fueling up before a workout is an absolute must.
Why so much disagreement? It’s not as straightforward as it seems. For example, fasted cardio may work well for one person but not another and the intensity and duration of the workout can be a factor. You wouldn’t do a fifteen-mile run without fueling up beforehand but for a 30-minute brisk walk, you probably don’t need to eat a meal or snack. Likewise, for intensity. You don’t necessarily need to eat before a moderate-intensity session but if you’re planning an HIIT routine, munching on a snack beforehand may help you maximize your performance.
What Science Says
A recent study revisited the issue of fasted training. It showed that fasting may be beneficial if you’re working out at a moderate intensity. In this study, researchers at the University of Bath in the United Kingdom asked a group of overweight, healthy males to walk for 60 minutes at 60% of their V02 max. During one session, they fasted beforehand. In another session, they ate a high-carbohydrate, high-calorie diet 2 hours before hitting the road. Before and after the 60-minute walk, researchers took biopsy samples of their fat tissue.
After looking at the fat tissue of the participants, they made some interesting observations. When the participants fasted before their brisk walk, it activated two genes involved in fat breakdown. These genes, PDK4 and hormone sensitive lipase (HSL), are active when your body is breaking down stored fat. When they didn’t fast prior to the walk, these genes were not activated to the same degree.
What does this mean? In a fasted state, your body must switch from burning carbohydrates as fuel to burning more fat. Fat-burning genes are turned on so that this conversion can be made. However, after a meal, particularly a high-carb one, your body has an immediate source of fuel – carbohydrates. As a result, fat-burning genes don’t have to be activated to the same degree since there’s plenty of carbohydrates around to meet energy needs. That combined with the fact that insulin is higher after a high-carb meal makes it harder to mobilize fat stores.
This isn’t the only study showing that fasted cardio burns more fat. A study published in the British Journal of Nutrition showed that fasted subjects burned 20% more fat when they did cardio in the morning without eating than those that ate breakfast beforehand.
The Role of Intensity
One factor that feeds into the fasted cardio equation is exercise intensity. In the most recent study, the participants were working out at 60% of their aerobic capacity, corresponding to moderate-intensity exercise. But what it the participants had done high-intensity exercise, such as an HIIT workout? Would they approach an intense workout with the same vigor and intensity in a carb-depleted state? That’s one reason fitness professionals don’t always recommend exercising in a fasted state. Studies show that high-intensity workouts are more effective for fat loss than exercising at a moderate intensity. This is partially due to the EPOC or afterburn phenomenon where your body must burn more calories to recover from a more intense workout. As a result, you burn more calories for hours after a vigorous workout, like HIIT training.
When you work out at a high intensity, fuel usage shifts strongly toward carbohydrates and away from fat. If your muscles and liver are depleted of glycogen, you don’t have less fuel to feed your muscles during high-intensity exercise. In reality, even after an overnight fast, you still have enough glycogen to propel you through a high-intensity workout but some people still find that their performance suffers when they don’t eat a pre-workout meal or snack.
Is Muscle Loss an Issue?
Another concern is that exercising in a fasted state could make it harder to build or retain muscle tissue. Normally, protein isn’t a significant fuel source for exercising muscle. However, when your body is in an energy depleted state, it uses a higher percentage of protein as fuel than it normally would. This works because our liver can convert the amino acids from protein into glucose. Also, when you wake up in the morning, your cortisol level is at its peak. When your glycogen stores are low and your cortisol is high, your body is more likely to use protein as a fuel source. This will be more of a problem if you’re doing long periods of exercise.
According to the American Council on Exercise, exercising in a fasting state could also lead to fat redistribution. When you wake up in the morning and exercise when your cortisol level is at its peak, you mobilize even more fatty acids and send them into your bloodstream so muscle cells can turn them into ATP and use the ATP as fuel. But, what happens to the fatty acids that entered your bloodstream and weren’t converted to ATP? Those unused fatty acids will return to and be stored in fat tissue. The place that they’re likely to go is to your tummy. That’s probably not where you want it! People who have an elevated cortisol level tend to store fat in the abdominal area.
Finally, a study published in the Journal of the International Society of Sports Nutrition asked two groups of healthy, young women to do moderate-intensity exercise an hour a day three times a week. Some worked out after an overnight fast while the other ate a meal before exercising. By the end of the study, both groups lost weight and body fat with no significant differences between the two groups. So, fasted cardio may burn more fat but won’t necessarily lead to greater fat loss.
The Bottom Line
Fasted cardio turns on key genes involved in fat metabolism. Although this may give additional fat-burning benefits, it’s most appropriate for days when you’re doing moderate-intensity cardio rather than an HIIT routine. Ultimately, intensity matters more and you need to be able to give it your best effort.
If you do fasted cardio in the morning, the morning rise in cortisol and the lack of available carbohydrates can potentially make it harder to build muscle. There’s also the question of what happens to the fatty acids you mobilize during exercise that don’t get turned into ATP. As the American Council on Exercise points out, they could end up around your waist and tummy.
If fasted cardio works for you, do it, but it might best be of use when you’re not doing a high-intensity workout. On those days where you’re taking it a little easier, try skipping breakfast and see how it works for you.
Science Daily. “Lose Fat Faster Before Breakfast”
Science Daily. “To Eat or Not to Eat: That is the Question”
Br J Nutr. 23:1-12, 2013.
Bodybuilding.com. “Is Fasted Cardio The Best For Burning Fat?”
American Council on Exercise. “Does Fasted Cardio Offer Significant Benefits?”
Journal of the International Society of Sports Nutrition201411:54. DOI: 10.1186/s12970-014-0054-7.