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blog Pros and Cons of Fasted Cardio

Pros and Cons of Fasted Cardio


It’s a word that strikes fear, dread, and dismay into many a gym rat.

Typically, when we hear the word “cardio” we inevitably think of mindlessly slogging away on the treadmill for hours on end trying to burn off every last bit of unwanted body fat, nary a drop of food in our system.

For decades, this method of training on an empty stomach was seen as the best way to burn body fat.

But in recent years, another more intense form of cardio has garnered the public’s fancy. One that accomplishes the same benefits of traditional cardio in a fraction of the time.

It’s called high-intensity interval training. HIIT for short.

What are the benefits of each, and is one form of cardio better than the other?

You’re about to find out!

But first, let’s clear up exactly what it means to train “fasted”.

What is Fasted Cardio?

Many people think that training fasted simply refers to not having any food in their stomach when they exercise. But, that’s not quite correct.

You see, while you may not feel as though you have anything in your stomach, your body may still be digesting the meal you ate up to 3 hours prior. During this time insulin levels are elevated, which means the body is more in a nutrient storage mode and less of a “fat burning” one.

So, in this sense, training “fasted” would mean that insulin levels are at baseline and your body is longer digesting or absorbing nutrients.

The body enters this fasted state approximately three to six hours after eating, depending on the size (food volume and total calories) and macronutrient composition of the meal.

The greater amounts of protein, fiber, and/or fat a meal contains, the longer your body spends breaking it down and absorbing it.

Since most people won’t be able to tell when their body is completely finished digesting a meal and that their insulin levels are returned to resting levels, those who want to get the benefits of fasted training should aim to train immediately upon waking in the morning.

Now, that we’ve cleared up what fasted training actually is, let’s discuss why someone would want to train fasted anyway.

Pros of Fasted Cardio

Increased Fat Burning
Scientific research has clearly shown that fasted cardio burns considerably more body fat than performing the same cardio session in a fed state. In fact, a 2016 systematic review collecting data from 27 different studies concluded:[1]

“… aerobic exercise performed in the fasted state induces higher fat oxidation than exercise performed in the fed state.”

The reason fat burning is increased when you fast is that the body will burn the nutrients that are readily available first. This means that if you have recently eaten a meal, your body will use the nutrients from that meal to power your body during exercise. Once it’s “burned” through those calories, it will then begin tapping into its energy stores (i.e. body fat) to satisfy its energy needs.[2]

So, if you want to “force” your body to burn fat for fuel during training you want to train fasted and keep the intensity relatively low to moderate as the higher intensity you train, the more your body shifts from burning fat to glycogen (stored glucose) for energy.

May Help Lose “Stubborn” Fat

Everyone, no matter how genetically gifted they may be, struggles with stubborn body fat in one way or another. What makes certain body fat more “stubborn” than other fat is two-fold:

  • The ratio of alpha:beta receptors the fat cells have
  • The amount of blood flow it receives.

To keep things simple, fat cells have two types of receptors -- alpha receptors and beta receptors.[3] [4]

  • Alpha receptors -- reduce fat burning
  • Beta receptors -- encourage fat burning

The more alpha receptors an adipocyte (fat) cell has, the more resistant it is to releasing its stored fatty acids to be burned for energy. Conversely,the more beta receptors it has, the “easier” it is to access and burn.

Areas of stubborn body fat also receive less blood flow that regions of the body where fat seems to be lost more easily. This is important because in order for the beta receptors to be stimulated to release their fatty acids, they have to interact with fat-burning catecholamines like norepinephrine.

If an area receives less blood flow, there’s less opportunity for the fat burning hormones to reach the fat cells and unlock them.

Combine reduced blood flow with a higher proportion of alpha receptors and you have the perfect recipe for stubborn fat.

As it turns out, fasted cardio has been noted to increase blood flow to the abdominal region, which is the area of the body where most people struggle with stubborn fat.[5]

If you combine fasted cardio with a proven fat burning supplement such as yohimbine (which is found in Lipo-6 Black UC), you greatly increase your chances of burning stored body fat.

The reason for this is that yohimbine is an alpha-receptor antagonist. What this means is that yohimbine binds to alpha receptors on fat cells, “blocking” the port, and allowing more of the fat-burning catecholamines to bind to the beta receptors.

Avoid Upset Stomach

Some people just do not feel well training on a full stomach. Training fasted allows them to avoid discomfort, indigestion, and potential nausea that can accompany training soon after eating. Furthermore, some individuals who may not tolerate carbohydrates all that well actually feel slightly lethargic and “heavy” after eating, which reduces training intensity. Performing a fasted workout may allow them to perform better and avoid barfing mid-jog, which we consider a good thing.

Workout Completed

The reason many people struggle to even get in a workout some days is simply due to a lack of time as a result of previously unforeseen circumstances “popping” up at the last minute.

We’ve all had those days where we leave the office intent on heading to the gym only to be called away for some other task or emergency. Inevitably, once the proverbial “fire” is put out, you’re either out of free time to train, or completely distracted and unmotivated.

By starting your day immediately with a workout, you’ve eliminated the possibility of something “coming up” later in the day and you’ll get an extra boost in fat burning. Plus, exercise also creates a flood of mood and brain-boosting chemicals in the body, which helps increase motivation and productivity that will translate to a better day at the office.

Cons of Fasted Cardio

Decreased Training Intensity
While training fasted might be a good fit for some individuals, for many others, the lack of food in their system actually leads to a decrease in training intensity, meaning a less “productive” training session.

When the body doesn’t have a ready supply of glucose in the bloodstream (such as it would proceeding a pre-workout meal), it has to pull from the glucose stored in your muscle (glycogen).

If these stores are low, it then breaks down body fat to provide energy. However, your body cannot oxidize fat as quickly as it can carbohydrates, which will lead to decreased performance.

Now, this may not be an issue for the average person hopping on the treadmill to perform 30 minutes of cardio, but for the high-performing athlete, training fasted usually coincides with an inability to hit their “fifth gear.”

Basically, if you want to train optimally at a high intensity, you don’t want to train on a fasted stomach.

Increased Risk to “Bonking”

Food is fuel. There are no two ways about it. Yes, we do eat for pleasure and enjoyment, but first and foremost, food serves to provide our cells with energy to perform.

Training fasted means that you’re relying on your body’s energy stores, and if you’re currently dieting, or on a low-carb kick, your glycogen stores may not be completely full.[6] This increases your chances of experiencing low blood sugar and “bonking” during training.

If you are someone who tends to feel slightly nauseous or “heavy” when training shortly after eating, try consuming something very light on the stomach and fast-digesting. Even something as simple as a small banana or our whey protein isolate product, Isofit, can do wonders for keeping energy levels high and performance top notch.

Higher Cortisol Levels

Cortisol is the body’s primary stress hormone released in times when we perceive a threat. Exercise is one such “threat” to the body.

Performing exercise generates an increase in cortisol. Fasting, one of the most popular diet fads currently also creates an increase in cortisol levels.

Chronically training fasted may lead to prolonged elevations of cortisol in the body which can actually stunt fat burning and promote fat storage.[as does fasting. Do both of these actions frequently enough (i.e. fasted training) and you may start to develop chronically elevated cortisol levels, which promotes fat storage and reduce fat burning.[7] [8]

No Greater Weight Loss

While training fasted does lead to a greater amount of fat burned during exercise, you might actually be interested to know that it doesn’t necessarily translate to greater weight loss at the end of the day.

In fact, some research by “the hypertrophy doc” Brad Schoenfeld, PhD, has documented that there is no difference in body composition changes or fat loss when comparing fasted vs fed training.[9]

The reason for this is that the body has various regulatory mechanisms that govern substrate utilization in the body. For instance, if you burn higher amounts of fat during one part of the day, the body compensates by burning less fat the other portions of the day and burning a higher amount of carbohydrates.[10]

It’s important to remember that calories burned during exercise make up a very small fraction of the total calories burned during a day. This is why exercise should not be the primary means an individual uses to accomplish weight loss.

Exercise can be a part of any well-structured weight loss plan, but the main driver of fat loss should be a calorie deficit -- consuming fewer calories than your body requires.

In other words, whether you train fasted or fed won’t have nearly as big of an impact on your fat loss results or body composition as will your ability to maintain an energy deficit.

The Bottom Line on Fasted Cardio

As with most things in life, fasted cardio has both good and bad traits. If it helps you accomplish your workout and avoid pre-workout meal-induced lethargy or nausea, then train fasted.

If, however, you’re training fasted because you’ve been told it’s the best way to burn body fat, then you’ve been misled. Training in a fed state ensures blood sugar levels are adequate and energy reserves are full, enabling you to perform at higher intensities for longer periods, ultimately burning more calories and creating a larger caloric deficit.

At the end of the day, fasted cardio is a tool to use, but only if its convenient and bodes well for your physiology. If you find that you don’t perform particularly well fasted, then don’t worry. You can still lose just as much fat, and as an added bonus, not feel insatiably hungry through your whole workout.


1. Vieira, A. F., Costa, R. R., Macedo, R. C. O., Coconcelli, L., & Kruel, L. F. M. (2016). Effects of aerobic exercise performed in fasted v. fed state on fat and carbohydrate metabolism in adults: a systematic review and meta-analysis. The British Journal of Nutrition, 116(7), 1153–1164.
2. Burke, L. M., Kiens, B., & Ivy, J. L. (2004). Carbohydrates and fat for training and recovery. Journal of Sports Sciences, 22(1), 15–30.
3. Lefkowitz, R. J. (1979). Direct binding studies of adrenergic receptors: biochemical, physiologic, and clinical implications. Annals of Internal Medicine, 91(3), 450–458.
4. Strosberg AD. Structure, function, and regulation of adrenergic receptors. Protein Science : A Publication of the Protein Society. 1993;2(8):1198-1209.
5. Manolopoulos, K. N., Karpe, F., & Frayn, K. N. (2012). Marked resistance of femoral adipose tissue blood flow and lipolysis to adrenaline in vivo. Diabetologia, 55(11), 3029–3037.
6. Gjedsted, J., Gormsen, L. C., Nielsen, S., Schmitz, O., Djurhuus, C. B., Keiding, S., … Moller, N. (2007). Effects of a 3-day fast on regional lipid and glucose metabolism in human skeletal muscle and adipose tissue. Acta Physiologica (Oxford, England), 191(3), 205–216.
7. Hill, E. E., Zack, E., Battaglini, C., Viru, M., Viru, A., & Hackney, A. C. (2008). Exercise and circulating cortisol levels: the intensity threshold effect. Journal of Endocrinological Investigation, 31(7), 587–591.
8. Moyer, A. E., Rodin, J., Grilo, C. M., Cummings, N., Larson, L. M., & Rebuffe-Scrive, M. (1994). Stress-induced cortisol response and fat distribution in women. Obesity Research, 2(3), 255–262.
9. Schoenfeld, B. J., Aragon, A. A., Wilborn, C. D., Krieger, J. W., & Sonmez, G. T. (2014). Body composition changes associated with fasted versus non-fasted aerobic exercise. Journal of the International Society of Sports Nutrition, 11(1), 54.
10. Paoli, A., Marcolin, G., Zonin, F., Neri, M., Sivieri, A., & Pacelli, Q. F. (2011). Exercising fasting or fed to enhance fat loss? Influence of food intake on respiratory ratio and excess postexercise oxygen consumption after a bout of endurance training. International Journal of Sport Nutrition and Exercise Metabolism, 21(1), 48–54.