Fasted cardio for fat loss and muscle gain – Is it worth it?

October 13, 2023
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Fasted cardio is a huge thing for anyone looking for fat loss and weight loss. You see people grinding away on the elliptical or treadmill at 5 am hoping to sweat off weight and burn up fat stores with little fuel in the tank.

One of the most common fat burning strategies employed by everyone from elite athletes to bodybuilders and casual gym-goers looking to lose fat and tone up is hitting the gym to do cardio early in the morning on an empty stomach. This strategy was first popularized by Bill Phillips who claimed that just 20 minutes of intense aerobic exercise after an overnight fast can produce greater benefits for fat loss than an hour of cardio in the fed state [1]. His rationale for this theory was that low glycogen levels from overnight fasting cause your body to shift energy use away from carbs and towards mobilizing stored fat for fuel. However, while there may be some truth to the matter, how effective is fasted cardio for losing weight and melting away fat?

I’m diving into the research behind fasted training and if it’s beneficial for boosting fat loss and building muscle.

What you need to know about fasting, exercise, and fat loss

The idea that cardio in a fasted state burns more fat than cardio in a post-prandial state seems like a probable idea, but when you look at the science behind fat burn, things become a bit more complicated. As much as we’d love to say that when you have no food in your stomach, your body turns to fat stores, it’s not that simple.

The human body is incredibly complex and dynamic, and continually adjusts its use of fat for fuel depending on substrate availability. Substrate utilization is regulated by several different factors, including hormonal secretions, enzyme activity, transcription factors, and more, which can change at the drop of a hat [1]. As a general rule of thumb, however, the more carbohydrates you burn during physical activity, the more fat you’re going to burn in the post-workout period and vice versa.

It’s well-known that compared to moderate-intensity steady-state cardio, high-intensity exercise is substantially more beneficial for fat loss in the long run. Despite lower levels of fat oxidation during high-intensity training, research consistently shows greater levels of fat loss in people who perform HIIT as opposed to steady-state cardio where you’re in the infamous “fat-burning zone.”

Several studies have shown that carb consumption prior to aerobic activity (<60% VO2max) can reduce fat oxidation due to insulin-mediated attenuation of adipose tissue lipolysis, increased glycolytic flux, and decreased expression of genes involved in fatty acid transport and oxidation [1]. However, training status and exercise intensity can mitigate the effects of pre-exercise food on fat oxidation.

A 1997 study by Horowitz et al. looked at the fat-burning response of six moderately trained individuals in a fed state versus a fasted state during various training intensities [2]. Participants cycled for 2 hours at different intensities on 4 separate occasions. During 2 trials, subjected consumed a high-glycemic carb meal at 30, 60, and 90 minutes of training, once at low intensity (25% VO2max) and once at a moderate intensity (68% VO2max). Subjects remained in a fasted state for 12-14 hours in the other 2 trials for the entire training duration. Results for the low-intensity trials showed suppression of lipolysis by 22% in the fed state compared with the fasted state, but fat oxidation was similar between groups until 80–90 minutes of cycling. However, fat oxidation increased after 90 minutes only in fasted subjects. On the other hand,  fat oxidation remained the same in moderate-intensity exercise despite a 20–25% drop in lipolysis and plasma free fatty acid (FFA) concentration.

Other studies show similar results, suggesting moderate-high intensity cardiovascular exercise in a fasted state results in substantially more fat breakdown than the body can use for fuel [1]. FFAs that aren’t oxidized for fuel are re-esterified in adipose tissue, which essentially negates the benefits afforded by pre-exercise fasting.

The other factor you have to consider with fasted training is proteolysis or protein degradation. One study found that loss of nitrogen was more than doubled when training in a fasted state compared to a glycogen-loaded state [3]. These conditions resulted in a protein loss of roughly 10.4% of the total caloric cost of exercise after one hour of cycling at 61% VO2max. Based on this, for anyone looking to maximize muscle mass, fasted cardio may not be the most ideal.

What science says: Fasted cardio and fat burn

Several studies show that aerobic exercise in the fed state results in a decrease in the entry of long-chain fatty acids into the mitochondria, which results in a corresponding decrease in fat oxidation [4]. However, there is evidence suggesting that consistent exercise in a fasted state can lead to chronic molecular adaptations favourable to fat oxidation. As an example, six weeks of aerobic training in a fasted state can increase intramuscular fatty acid-binding protein and uncoupling-protein-3 content to a greater extent than training in a fed state [5]. Fasted training has also been shown to promote improvements in whole-body glucose tolerance and insulin sensitivity, along with upregulating specific lipolytic enzymes compared to exercising in a fed state [6, 7].

Here’s proof.

A 2011 study sought to verify differences in fat metabolism during fasted training versus fed training [8]. Researchers compared the effect on oxygen consumption (VO2) and substrate utilization, which was estimated by the respiratory exchange ratio (RER) in 8 healthy young men performing moderate-intensity exercise–36 minutes of cardiovascular training on a treadmill at 65% maximum heart rate in the morning in either a fasted state or a fed state. In both cases, food consumption over a 24 hour period was equal. Those training in a fed state saw significant increases in both VO2 and RER. Twelve hours after training, VO2 was still higher in the fed group, whereas RER was significantly lower, suggesting greater lipid utilization. Researchers concluded that fasting before moderate endurance exercise does not enhance lipid utilization.

There are several other studies on the effects of fasted versus fed training for fat loss, but long story short, fasted cardio may increase fat oxidation, but it doesn’t necessarily increase body fat loss when compared to training in a fed state.

What science says: Fasted cardio and muscle gain

When it comes to bro science, you’ll hear across the board that you need to load up on carbs and protein before lifting to maximize muscle growth. And while fasted cardio may enhance fat oxidation, what does it do for muscle growth?

Carbs are a major factor in strength and if you’ve ever lifted early morning in a fasted state, you’ve probably noticed you’re weaker than you are mid-afternoon after a few good meals. For most people looking to lose fat, they fall into the trap of making mistakes that actually compromise muscle mass, like doing fasted cardio. Yes, they may lose weight, but they’re not actually putting on any muscle mass.

Research is sparse where fasted cardio and muscle catabolism is concerned, but there is some evidence to suggest that training in a fasted state can result in muscle protein breakdown rather than buildup. Your muscles require sufficient nutrients to grow and if those nutrients aren’t being consumed through diet, they are obtained from elsewhere, namely existing muscle tissue. In the study I mentioned before, researchers found that fasted cardio doubled nitrogen loss, which resulted in significant increases in protein loss compared to training in a fed state [3]. The authors concluded that for people looking for muscle growth, fasted cardio isn’t ideal to protect lean muscle tissue and increase gains.

Taken together, unless you’re using some form of supplement that’s going to protect you against muscle loss in a fasted state, fasted cardio probably isn’t the best idea for anyone looking to maximize muscle growth. Instead, opt for a light meal that will supply enough glycogen to maintain cardiovascular performance and protein that will mitigate any risk of muscle protein breakdown.

Should you do fasted cardio?

While it may seem like a good idea if you want to lose fat, the research on fasted cardio for fat loss is heavily mixed, which means there is the potential for adverse outcomes. In any case, there are a couple of things you’ll want to consider:

  1. Will fasted training have negative impacts on my performance?

When you look at the research around fasted training and performance, there’s little evidence suggesting that fasting will negatively impact training sessions lasting less than 60 minutes [9]. However, when you’re going beyond the 60-minute mark, studies do suggest that fueling up before training can improve performance.

A 2013 study on the effects of fed versus fasted training during Ramadan found that both groups maintained training volume and experienced no changes in rates of perceived exertion during sessions [10]. This makes sense when you understand the physiology. During intense training, your muscles rely on glycogen for fuel, and even after an overnight fast, glycogen stores still are not completely depleted. That means that when you’re training in the morning, your body is still running primarily on glucose before it switches over to fatty acids, thus we don’t see performance decrements.

Studies have even found that when muscle glycogen stores are low, 1RM strength, training volume, and other performance metrics are maintained [11, 12]. On top of that, research finds that carb intake before and during training fails to make significant improvements in performance [13, 14, 15].

On the other hand, studies have shown improvements in performance following carbohydrate ingestion, but most evidence points to individual variation. Some people do better with carbs, whereas others don’t.

  1. Will I lose muscle mass?

This one is a huge point where fasted cardio is concerned because there’s been a long-standing notion that training fasted will ruin your gains. While there is a bit of merit to this logic, most research suggests that fasted cardio won’t deteriorate your muscle mass to any great extent.

The study I mentioned above with Ramadan found that even though participants were lifting after a 15-16 hour fast, they didn’t lose muscle mass. Another study of people doing cardio during Ramadan found similar results with no loss of muscle mass by the end of the study period [16].

There are, however, a couple of caveats to this.

  1. Muscle loss is only a concern when your glycogen stores are depleted [17, 18]
  2. Your net protein balance will dictate whether you gain, maintain, or lose muscle mass

If you want to lose fat and build muscle, do this

With all of that said, fasted cardio likely isn’t going to hurt you, but long-duration steady-state activity isn’t the best option if you’re looking to lose fat and build muscle.

What will do that is something called metabolic resistance training, or MRT for short.

MRT is a style of training that combines short-duration, maximum intensity work combined with short periods of rest that’s typical of what we see in interval training. But for this, we kick it up a notch when we add in weights or any other form of resistance, hence the name metabolic resistance training. We’re combining one of the most effective fat-burning workouts with classic strength moves for a killer workout that will hit even the deepest muscles to burn fat and build mass and strength.

But perhaps one of the best things about MRT is that even after you’ve stopped working, your body continues to, making MRT easily the simplest (although I could argue that MRT workouts aren’t easy) and most effective way to build muscle and torch fat and calories.

There are 3 reasons why MRT is a killer.

#1 Revs up fat-burning

If fat loss is your goal, then MRT needs to be in your workout books. The intense fat-burning effects of MRT are primarily the result of what’s called excess post-exercise oxygen consumption, or EPOC. High-intensity, short-duration activity has been shown to burn more calories and more fat than any other style of training–that includes both traditional strength training and classic cardio exercises. In fact, some research suggests that metabolic style workouts can burn up to 25-30% more calories than other forms of exercise [19].

Here’s why.

If you’ve never heard of the EPOC effect before, the analogy of your body like your car is the simplest way to describe it. When you take your car out for a spin, the engine takes time to cool down once you park it back in the driveway. Your body functions much the same after an intense MRT workout. It requires time to come back down to balance and return to its pre-exercise state. However, during that period, your body is still burning calories and fat.

Simply put, the EPOC effect is the amount of energy needed for your body to return to its normal resting state after exercise.

#2 Builds muscle faster

When muscle mass is your goal, steady-state cardio isn’t going to cut it. Conventional thought led us to believe that cardio actually destroys muscle, and while that’s not entirely true, it also doesn’t build it. So, if you want to pack on muscle, you need to boost concentrations of two hormones: testosterone and growth hormone (GH).

High-intensity training is one of the most powerful and effective ways to increase testosterone and growth hormones to bulk up and build muscle fast. That’s because HIIT and MRT workouts produce a larger adaptive response by virtue of recruiting more muscle fibres, as well as providing a larger cardiorespiratory signal for the body to adapt [20]. When stress adaptation increases, it typically leads to better muscle growth.

#3 Boosts cardiovascular fitness

And the last reason why MRT workouts are significantly more effective than steady-state care is that they work your body on every single level of your fitness capabilities to improve strength, power, speed, endurance, stamina, and virtually everything else you can think of. It puts the single-pace performance to shame to produce bigger and better increases in VO2max, along with improving several markers of cardiometabolic health in a fraction of the time as steady-state cardio. It offers many of the same benefits as conventional cardio with none of the time commitments.

Long story short, whatever way you slice it, MRT is substantially more effective for burning fat and building muscle than any other form of training. It will challenge your body in ways you never thought possible so you’re constantly forcing your body to adapt and grow.

MRT exercises are a mix of athleticism, muscle building, and strength endurance that fire up every single muscle in your body to kick your metabolism into high gear and build bigger and stronger muscles. It is explosive full-body movements, isolated heavy lifts, and compound strength training movements to boost testosterone and growth hormone levels, ignite your metabolism, and give you the results you’re looking for.

While it may not be easy in the midst of your workout, the pain is well worth it.