Does Fasted Cardio Burn More Fat?

Certified Nutrition Coach & Personal Trainer Analyzes the Data

By: Jeremy Fox, CNC, CPTUpdated: April 16, 2024

Fasted cardio is one of the most polarizing topics surrounding weight loss. Many bodybuilders and athletes swear by its effectiveness for burning fat, while some fitness gurus say the benefits of fasted cardio are a myth!

It’s one of those situations where you probably don’t know who to listen to or what information is correct. As a data-driven engineer and personal trainer, I decided to review the science and determine which is better: fasted or fed cardio.

Fasted Cardio for Fat Loss

What is Fasted Cardio?

Fasted cardio is aerobic exercise performed on an empty stomach, usually first thing in the morning after an overnight fast. However, you can do it at any time of day as long as you haven’t eaten for 10-12 hours.

It’s important to understand that fasting is more of a physiological state than simply having an empty stomach. Technically, a fasted state involves low blood glucose and insulin levels, which occur well after your stomach empties.

When insulin is low, your body is more likely to burn fat stores for energy, which is why people say that fasted cardio results in greater fat loss. With that in mind, let’s review the potential benefits of fasted cardio.

Proposed Benefits of Fasted Cardio

The main reported benefits of fasted cardio are that it increases fatty acid oxidation and lipid metabolism, resulting in a faster rate of fat loss. However, the strength of evidence supporting these claims varies.

  1. Increase fatty acid oxidation (evidence: high)
  2. Lose more body fat and weight (evidence: low)
  3. Improve metabolic function (evidence: medium)

Next, I’ll review the scientific research for each benefit to determine if fasted cardio actually burns more fat.

1. Higher Fat Oxidation

Fatty acid oxidation is the process of breaking down stored fat into smaller units that your body can use for energy. A systematic review of 23 studies found that exercising while fasted results in significantly higher fat oxidation than exercising after eating1.

The reason is that when you haven’t eaten for a while, your body breaks down fat into fatty acids through lipolysis. Then, you can burn those fatty acids for energy. However, when you eat, your body releases insulin, suppressing lipolysis and encouraging the burning of glucose instead of fat.

For example, a study by the University of Texas compared fat oxidation during fed and fasted cardio and found that the rate of fat oxidation was nearly double in the fasted state2. Those results sound promising, but increased fat oxidation doesn’t necessarily mean more significant fat loss.

fasted cardio fat oxidation

Adapted from Horowitz et. al2

2. Burn More Fat

Determining if fasted cardio burns more fat requires measuring changes in body fat and muscle mass. A study in the Journal of the International Society of Sports Nutrition tracked body composition changes and found no significant difference associated with cardiovascular exercise in the fasted vs fed state3.

Similarly, a 2017 meta-analysis found no significant differences in body composition changes between overnight fasted and fed exercise4. However, the authors admitted these findings came from a relatively small sample size and that we should use caution when applying them.

Based on this evidence, creating a calorie deficit seems much more significant in achieving fat loss than the time of your last meal. Still, there may be other potential benefits of a fasted workout.

Fasted Cardio Fat Loss Graph

Adapted from Schoenfeld et al. These results seem to show that fasted cardio resulted in roughly one pound more fat loss and muscle loss in four weeks. However, the authors’ analysis says the difference was not statistically different between fasted and fed groups.

3. Healthy Metabolic Adaptations

Metabolic flexibility refers to the body’s ability to switch between burning carbohydrates and fat for an energy source. Research shows that the fed or fasted state, exercise, and body composition influence metabolic flexibility5.

Exercising while fasting may lead to positive metabolic changes in your skeletal muscles, such as increased insulin sensitivity. Improved insulin function could eventually make your body more adept at burning fat during aerobic workouts, resulting in weight loss.

However, more research is needed to fully establish how exercising in a fasted state impacts metabolic adaptations and other long term effects.

Is Fasted Cardio Better for Fat Loss?

The current research suggests that fasted cardio burns about the same amount of fat as exercising after eating a meal. Therefore, it will not make or break your fat loss progress.

You should focus more on increasing your activity and reducing calorie intake than on workout timing. Whether you implement early morning workouts depends on your preference and fitness goals.

Nevertheless, recent studies indicate that fasted cardio can positively affect insulin sensitivity, which could result in better long-term weight management and overall health. In addition, early morning workouts are a great way to start your day positively and improve your mental health.

Overall, fasted cardio should be viewed as one piece of a fitness plan that includes strength training and a balanced diet.

fasted cardio faster fat loss?

Taking a break during my morning bike ride to let a family of Canadian geese cross the path. @jeremyfoxfitness

Effective Fasted Cardio Workouts

So, let’s assume you want to try fasted cardio. Do you wake up and start running like Forrest Gump? Or is there a way to do it to get the best results?

As it turns out, duration and intensity are important factors.

Duration

According to the Nutrition Research Institute, fat oxidation peaks after about 75 minutes of exercise6. So, there’s no need to go on a cross-country trek.

Conversely, if your cardio session is too short, you won’t burn many calories. For that reason, I recommend at least 30 minutes of fasted cardio for fat-burning.

Heart Rate

Exercise intensity is significant because your body burns more calories per minute as effort increases. But high-intensity workouts also use more energy from glycogen stores.

For that reason, it’s best to target low to medium-intensity or Zone 2 cardio to burn the most calories and fat. That translates to about 50-70% of your max heart rate.

fasted cardio energy usage

To calculate your max heart rate, subtract your age from 220. Then multiply by 0.5 and 0.7 to get your heart rate targets for fasted cardio.

Example:

If you’re 25 years old you would take 220 – 25 = 195. So your max heart rate is about 195 beats per minute.

From there, multiply 195 x 0.5 = 98. Then 195 x 0.7 = 137. In this case, your target heart rate for fat burning is 98 to 137 bpm.

Or find your approximate age in the table below.

Fasted Cardio Heart Rate Zones

What about fasted HIIT cardio?

High-intensity interval training (HIIT) is a great way to burn more calories in a shorter period of time. However, during the workout, you burn more energy from carbs than fat.

Additionally, a study found that fasted HIIT does not increase fat loss significantly compared to fed HIIT7. Despite this, HIIT is still a good option if you are pressed for time or looking to improve your overall fitness.

My Fasted Cardio Experience

I have attained single-digit body fat several times using both fasted cardio and fed cardio. Therefore, I know that regular exercise is the key to achieving your weight loss goals, regardless of when you last ate.

That said, I believe fasted cardio can provide a slight advantage based on my personal experience as a competitive bodybuilder and anecdotal evidence from others in the fitness industry.

However, I recommend dialing in other aspects of your diet and exercise plan instead of relying on fasted cardio to see a difference.

Fasted Cardio Muscle Loss

One concern with fasted workouts is the possibility of losing muscle. This idea stems from the fact that exercise raises cortisol levels and creates a catabolic state associated with muscle breakdown.

Studies comparing fasted vs fed cardio do not show a significant difference in muscle loss. Nevertheless, maintaining lean mass is paramount, so nutrient timing around cardio workouts justifies further consideration.

Researchers at Ball State University found that protein synthesis increased when participants consumed carbohydrates and protein after cardio8. Another study demonstrated similar results using essential amino acids (EAAs) alone9.

Therefore, you could have a protein shake after your cardio workout or supplement with EAA powder to maintain a fasted state while preserving muscle tissue.

Example Fasted Cardio Routine

Here is how I implement fasted cardio in a fat loss program for someone with a typical 9 to 5 work schedule.

Fasted Cardio with Evening Weight Training

Time Routine
6:30 am Wake up and drink water or black coffee
7:00 am 30 mins steady state cardio
7:30 am Drink post-cardio protein shake or EAAs
8:00 am to 5:00 pm Eat high-protein, low-carb meals every few hours
5:30 pm 45-60 mins of resistance training
6:30 pm Post-workout carb/protein shake
8:30 pm Eat high-protein, medium-carb meal
10:30 pm Bedtime

As you can see, you don’t need to overdo it with cardio. A good starting point is 30 minutes a day, 5 days a week (150 minutes weekly). Then, you can gradually ramp it up from there if needed.

Also, don’t waste money on fancy fat-burning supplements, most of which are garbage. Although there is promising research on yohimbine, particularly in combination with fasted cardio.

Of course, it’s necessary to create a negative energy balance by eating fewer calories than you burn each day. Weight training is also beneficial to prevent muscle loss due to calorie restriction.

Remember that there’s no single right way to use fasted cardio. And it doesn’t mean you should never do cardio in the fed state.

The best routine for you depends on your schedule, body type, and goals.

References
1) Vieira, Alexandra Ferreira, et al. “Effects of aerobic exercise performed in fasted v. fed state on fat and carbohydrate metabolism in adults: a systematic review and meta-analysis.” British Journal of Nutrition 116.7 (2016): 1153-1164.
2) Horowitz, Jeffrey F., et al. “Lipolytic suppression following carbohydrate ingestion limits fat oxidation during exercise.” American Journal of Physiology-Endocrinology And Metabolism 273.4 (1997): E768-E775.
3) Schoenfeld, Brad Jon, et al. “Body composition changes associated with fasted versus non-fasted aerobic exercise.” Journal of the International Society of Sports Nutrition 11 (2014): 1-7.
4) Hackett, Daniel, and Amanda D. Hagstrom. “Effect of overnight fasted exercise on weight loss and body composition: A systematic review and meta-analysis.” Journal of Functional Morphology and Kinesiology 2.4 (2017): 43.
5) Aird, Thomas P., Robert W. Davies, and Brian P. Carson. “Effects of fasted vs fed‐state exercise on performance and post‐exercise metabolism: A systematic review and meta‐analysis.” Scandinavian journal of medicine & science in sports 28.5 (2018): 1476-1493.
6) van Loon, Luc JC, et al. “Intramyocellular lipids form an important substrate source during moderate intensity exercise in endurance‐trained males in a fasted state.” The Journal of physiology 553.2 (2003): 611-625.

7) Gillen, Jenna B., et al. “Interval training in the fed or fasted state improves body composition and muscle oxidative capacity in overweight women.” Obesity 21.11 (2013): 2249-2255.
8) Reidy, Paul T., et al. “The effect of feeding during recovery from aerobic exercise on skeletal muscle intracellular signaling.” International journal of sport nutrition and exercise metabolism 24.1 (2014): 70-78.
9) Wolfe, Robert R. “Skeletal muscle protein metabolism and resistance exercise.” The Journal of nutrition 136.2 (2006): 525S-528S.

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By |April 16, 2024|Workouts|Comments Off on Is Fasted Cardio Really Better for Fat Loss?
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