person running on a treadmill

Fasted Cardio: Is It Best for Fat Loss and Retaining Muscle?

The fitness realm goes back and forth on the effectiveness of doing fasted cardio. Data thus far suggests that fasted cardio doesn’t appear to have advantages over cardio in the fed state. In fact, doing large amounts of fasted cardio can lead to more muscle loss and poorer body composition than if you had some fast-acting protein in your system beforehand.

But this doesn’t mean fasted cardio has no merit. For some gym-goers, doing cardio right away in the morning before eating is a great way to start the day. The key is that you balance fasted cardio with proper weight training, diet, and supplementation.

Let’s dive into the science behind fasted cardio to better understand how it can be beneficial and the right way to incorporate it into your lifestyle

Cortisol Importance

Cortisol is a hormone in humans that regulates metabolism, growth, immunity, and cognitive performance. Physiologically, it is the most potent stress hormone in humans.

Cortisol’s role is significant in regards to muscular atrophy due to its catabolic nature. Catabolic hormones, unlike anabolic hormones, serve to break down tissues for energy production.

Nevertheless, cortisol is essential to sustain the body. In fact, it’s a large part of the reason you spring out of bed in the morning when your alarm goes off. Cortisol is also what keeps you going when you’re exhausted and deprived of nourishment.

For example, during phases of under-eating, or while doing fasted cardio, cortisol initiates a process called gluconeogenesis. This process helps maintain normal blood glucose levels by using amino acids instead of sugar. Unfortunately for gym-goers looking to build muscle, this comes at the price of breaking down skeletal muscle proteins in order to use amino acids for energy.

Naturally, cortisol is often touted as a “bad” hormone by bodybuilders, primarily because it arises from stress and is largely catabolic. However, cortisol is far from bad, and can, in fact, be quite useful, particularly if you manipulate it at the right times. 

Why Cortisol Is Necessary for Fat Loss

Cortisol has a well-defined circadian rhythm which, in healthy individuals, exhibits a rise upon waking and slowly declines over the following hours of the morning. Data shows that decreases in the amount of early morning cortisol spikes lead to significantly less vascular differences for nonesterified fatty acids (NEFAs) and glycerol. In non-nerd lingo, less of a cortisol rise in the morning means less potential for fat loss.

It appears this results from a decrease of hormone-sensitive lipase (HSL) activity in fat tissue. The same study also shows there’s a significant decrease in adipose lipoprotein lipase (LPL) activity when morning cortisol is blunted. These two enzymes work to liberate fatty acids for energy use.

The study concluded that cortisol concentration shows synergy with exercise, fasting, and growth hormone to shed unwanted body fat. In this regard, blunting natural, morning cortisol spikes is counterproductive, especially if your goal is fat loss.

Morning cortisol regulation

There are two things that can disrupt morning cortisol rhythms and make this period less optimal for fat loss. The first is a rise in insulin or insulin resistance, and the second is excessive stress. Stress brings with it some “bad” cortisol. 

Bad cortisol results from glucocorticoid excess, which raises blood pressure, induces insulin resistance, increases protein catabolism, and elevates blood glucose. Primasurge helps regulate cortisol rhythmsPRIMA SURGE bottle

Bad cortisol needs to be controlled by stress reduction and entrainment of the circadian rhythm. Usage of timed supplementation may also help. You can find a research-backed dose of ashwagandha, and other cortisol-modulating ingredients, in Androsurge and Primasurge

So what about cortisol release at other times of the day?

Cortisol Release and Time of Day

Concern over cortisol release, which dissipates within a short time frame, for the most part, fails to take into consideration the contents of this article. However, if a cortisol spike is disruptive to sleep or disrupts circadian rhythms, then it’s unhealthy. Since resistance training can elevate cortisol levels by nearly 600% over baseline values, usage of ashwagandha and other supplements may prove useful for afternoon and evening training sessions. You’ll find it much easier to rest without interruption if you reduce the late-night spike in cortisol that arises from training.

With that in mind, training at night tends to be more effective for muscle-building purposes. Morning training tends to be superior to losing fat. People training late at night should avoid stimulants as they can drastically mess with normal cortisol circadian rhythms.

Preserving Muscle Mass While Doing Fasted Cardio

For people doing morning fasted cardio, limiting cortisol release is not important, as you are simply blunting your body’s inherent nature to burn fat for fuel. The problem, however, is that doing large amounts of fasted cardio can lead to loss of muscle tissue. Remember, cortisol is a double-edged sword in that it helps stimulate fat loss, but also breaks down lean tissue. 

If you’re concerned about muscle loss from fasted cardio, an L-citrulline supplement can help. 

ALTIUS pre-workout powder

L-citrulline found in Altius is a protein-sparing amino acid that will not interfere with fat loss significantly. In fact, you’ll be preserving muscle tissue while doing cardio, leading to better body composition in the long run.

Moreover, Burn XT and Lean XT can help control cortisol and increase fat loss from cardio.

We highly recommend taking these upon waking before you hit the gym for fasted cardio.

Wrap-up

Overall, cortisol is much like a seesaw in terms of its benefits and issues. If you don’t allow cortisol to rise naturally in the morning, you effectively blunt its fat loss properties. If you allow cortisol to elevate too much throughout the day, however, you inhibit your potential to build muscle tissue.

For the most part, allowing cortisol to rise in the morning is beneficial, especially if you’re doing fasted cardio; regulating production in the latter parts of the day is also wise for body composition purposes. It is likewise safe to say that fasted cardio is not ideal for retaining muscle. 

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About the author

Elliot Reimers, CNC, M.S. Candidate

I'm a NASM Certified Nutrition Coach (CNC) and M.S. candidate in the Molecular Pharmacology program at Michigan State University. My undergraduate studies in the biological sciences led me to take a keen interest in research pertaining to nutritional science, exercise physiology, and dietary supplements. As such, I've been writing about these topics for the better half of the last decade. My main ambition is to educate readers and help them make sense of these evolving, yet increasingly hard to navigate, fields of study.

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