Why Fasted Cardio Is Garbage

Updated: May 2020

Fasted cardio, testosterone boosters, and a green tea detox diet are all things that exist and are supposed to help in fat loss. Except fasted cardio doesn’t work and I’ll show you why.

Well, if all those things worked, why would performance enhancing drugs (steroids) exist then? If all I had to do was skip breakfast and run on the treadmill for a bit to lose some weight, why do calorie restriction diets work?

Fasted Cardio Debunked

Next to green smoothies and eating açai berries by the handful, fasted cardio is one of those new fads that are going around (never mind those elastic waistbands that shrink your stomach).

Now to set the precedent, fasted cardio does have some benefits so it’s not all complete bullshit. It’s still a good form of exercise, and you’re doing cardio so you’re already better than 90% of the population.

Here’s the story: the idea is that fasted cardio burns more fat than if you had a meal or weren’t in a fasted state.

What is fasted cardio?

As an example, you’d have your last meal a few hours before heading to bed, then eat your next meal after fasted cardio. Another option is simply skipping breakfast and jumping right into cardio upon waking. It’s simply fasting (abstaining from food) and doing some form of aerobic exercise.

fasted cardio schedule clock

Wait a second. You’re skipping a meal and performing exercise? Isn’t that just calorie restriction? Well, yeah!

To expand on that point, let’s follow the USDA 2000 calorie average male diet. Over the course of the day, 2000 calories worth of food will be consumed.

Now in comes this magical fasted cardio and you’re getting results! Almost overnight you’re dropping weight and you’re seeing more muscle tone in the mirror – except you’re skipping a meal, and saving a few hundred calories. That’s just plain old calories in, calories out.


What about that weight loss and more muscle definition?

So now you’re eating less and jumping on the treadmill(you may not have been before!) and every day you get just a little more definition so you really think it’s working.

There’s a caveat though. Cortisol, a hormone that is responsible for the fight or flight response, prevents substances that cause inflammation and controls your blood pressure. Your cortisol levels are the highest in the morning.

Cortisol can also increase your blood sugar levels, since it uses your body’s glucose store from your liver. Overtime if you have high cortisol levels, your body will be constantly producing glucose, inducing increased blood sugar levels.

fasted cardio hormone results

Cortisol is produced by the adrenal glands, which are located above the kidneys.

For reference, your adrenal glands also produce other steroids (no, not the kind that you’re thinking of) such as aldosterone and adrenaline. Aldosterone is responsible for maintaining a proper electrolyte balance in your body by excreting potassium into your urine and keep sodium for bodily functions. By losing potassium and retaining sodium, you will hold on to water.

High Cortisol and Fasted Cardio

You do not want high cortisol. Consistently high cortisol can lead to weight gain, high blood pressure and chronic fatigue. Unmanaged or fluctuating blood sugar levels can contribute to high cortisol levels, as well as processed food, and not consuming enough healthy dietary fat and protein. Fasting can lead to fluctuating blood sugar levels, especially when you’re starting out. Your blood sugar levels will fluctuate, since it’s such a long time between meals.  

So now we know why fasting may not work. But what about the cardio aspect? Your body has to use something as energy, either it be fat stores or even your muscle. According to this study done in the UK, protein stores are used first for energy, as opposed to using fat stores. That is exactly what we don’t want.

Having said that, you’re not going to lose chunks of muscle every time you skip a meal and jump on the elliptical machine. But there’s a better way of exercising without breaking down muscle as energy, why wouldn’t you do that?

Can you use fasted cardio without losing muscle?

Yes you can. It especially helps if you’re on performance enhancing drugs (these are the steroids I’m talking about). As I said before, you’re not going to lose chunks of muscle but there will be some muscle loss.

What can you do?

Keep your protein intake high (I usually go between 0.7-1.2 grams per pound of bodyweight). Protein plays a huge importance in building muscle. You need adequate protein intake to build muscle, as well as to maintain muscle.You don’t want to be in a position where you’re “skinny fat”. Being skinny fat is the result of low muscle mass, low protein intake and overall bad diet.

This study exemplifies that a high protein diet can increase lean body mass, with minimal fat gain.

Fasted cardio not working: the binge

For most people, exercising increases hunger. So what do you have after lifting weights? You eat food. Now, add into this equation that you haven’t eaten for most likely 8+ hours and you feel dreadful. Your stomach is empty, you may have low blood sugar from not eating for so long, and you’re most likely cranky.

What do you do? Start eating. And you eat and you eat and you eat, and you don’t stop until you feel terrible and hate yourself (that’s everytime I go to Taco Bell). The answer to avoiding binging is smarter meal prep. 

Fasted cardio making you feel sick

In high school we had this yearly fitness test called the “wild mile”. It was running back and forth in the gymnasium until you couldn’t take it anymore. It was pure death and everyone in the class hated it. I can remember it was the first class of the day and they placed huge trash bins at each end of the gym for when, not if, kids puked.

Looking back, most kids threw up because they didn’t eat breakfast, or just pushed themselves too hard.

feel sick from fasted cardio

The exact same thing can happen with fasted cardio. Low blood sugar can make you feel nauseous, dizzy and make you puke. You’re probably not going to complete your workout if you feel like death. It’s not a viable long term strategy either if it’s making you feel that way.

Having bashed fasted cardio, there are a few benefits. Fasting in general lowers your insulin levels, which is great for fat loss. If you consistently have high insulin levels as a result of a poor diet, you’re just going to be spinning your wheels trying to burn fat.

Weight loss can be easier to maintain with fasting, since you’re restricting yourself to certain eating windows.

Fasted cardio works for some, but not all. The best way to see if it does is try it out for yourself. Start small with just cardio first thing in the morning then work in fasting into your lifestyle.

Lifestyle changes can be more permanent if you ease into them, rather than try to do everything at once.