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

March 3, 2018 Published by
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The fitness realm goes back and forth on the efficacy 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 the main glucocorticoid 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 and substrates for energy production.

Nevertheless, cortisol is essential for 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 concentrations by using amino acids in lieu 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, physiologically. However, cortisol is far from bad, and can, in fact, be quite useful, particularly if you manipulate it at the right times. 

Circadian Rhythms: 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. One study measured lipolysis in the subcutaneous adipose tissue of the anterior abdominal wall (i.e. belly region) under normal conditions and when cortisol levels were suppressed.

Data shows that decreases in the magnitude 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 adipose 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.

Note that both HSL and LPL are part of rate-limiting reactions for the liberation of fatty acids in adipose tissue. These enzymes hydrolyze triglycerides outside cells, in lipoproteins (such as cholesterol), and triglycerides within adipocytes (fat cells).

The study concluded that “Plasma cortisol concentration plays an important role in the regulation of lipolysis in adipose tissue in normal healthy adults,” and actually exhibits synergy with exercise, fasting, and growth hormone – which are all inducers of lipolysis – 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. One is a rise in insulin (or insulin resistance) and two 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 rhythms.

Bad cortisol (prolonged excessive cortisol secretion) 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? Read on for further answers.

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 adaptogens may prove useful for afternoon/evening training sessions. You’ll find it much easier to rest without interruption if you attenuate a 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 for 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, inhibiting cortisol release is not prudent, as you are simply blunting your body’s inherent propensity to burn fat for fuel. The conundrum, 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. 

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.


Overall, cortisol is much like a seesaw in terms of its benefits and ramifications. 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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Elliot Reimers

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