You’ve done it. Even I’ve done it. It seems like almost everyone in the bodybuilding/fitness community has tried it at least once at some point. Some love it, some hate it, and others have mixed feelings about it. Fasted cardio. We do it because we have this “logical” belief that because we’re exercising on an empty stomach after our overnight fast, and depleted glycogen, we must be utilizing fat stores to fuel our cardio, which means more fat loss. Yes, that is in fact true. Ish.
What The Studies Show
Multiple studies have shown that fatty acids are preferentially used in a fasted state of exercise under certain circumstances. One study demonstrated that subjects who performed mild-moderate endurance training after a 12-hour fast while below a 40% VO2 maxthreshold, experienced an elevated level of fat oxidation. Interestingly though, within the same study, the highest amounts of fat burning occurred at 40% VO2 max, in both in the fed and fasted subjects. Intensities above this VO2 max lowered rates of fat oxidation.
So, this shows that there’s a marginal difference in substrate use for cardio, and that above a 59% VO2 max, carbs will be used for fuel regardless of being fasted or not. Okay, so do your cardio at a very low intensity to capitalize on fatty acid oxidation.
What’s The Problem Then?
Time – Do you have two hours to kill daily to increase the utilization of fat stores with the hope that you’ll burn more fat overall? Remember, no promises. As the aforementioned study shows, you need to be moving pretty slowly. Any exercise that gets too intense will blunt this effect.
Fatigue – Some may be able to wake up and just start exercising, but others struggle due to low blood sugar levels and energy. If you feel light-headed, or that you’re going to faint, it’s a sign that you shouldn’t be exercising on an empty stomach. Everyone has different rates of adrenal hormone output to raise blood sugar levels, but if you suffer from some form of hypoglycemia, stay away from fasted exercise.
Muscle Loss – No one wants that because that means your metabolism goes way down. Bioenergetics in the body is never quite consistent. It relies on multiple fuel sources at any given time. Carb availability is quite limited when you do low intensity cardio fasted. Fats might be used mostly, but it’s not the only source of making energy. Doing cardio fasted raises the risk of muscle loss because of a hormone known as cortisol. The hormone is responsible for mobilizing fatty acids, amino acids and other non-carbohydrate substrates into the cell’s energy production system.
The Effect Of Cortisol
When you wake up in the morning, cortisol is at its highest level for the day and tapers as the day progresses. Morning cardio in a fasted state might not be the worst thing in the world, but in today’s society everyone’s cortisol levels are out of whack. We are all increasingly becoming more confronted by a myriad of daily stresses like too much caffeine, emotional anxiety and anger, too much physical trauma, occupational pressure, etc. If stress rules your life, your adrenal glands, which secrete cortisol, can burn out to the point where they’re severely limited in cortisol secretion. This condition, known as adrenal fatigue, is your key to a spectrum of health compromises like hypothyroidism.
In general, too much stress perpetuates high cortisol levels acutely, which makes you more prone to wasting muscle and holding on to fat for dear life, fasted or not.
What Does The Evidence Really Say?
Over the past 10 years, different studies with varying subject inclusion criteria and total energy states, have explored fasted versus fed state cardio, but the results seem to point in the same general direction. For example, one study involving 28 men who were in a +30% caloric surplus above baseline calories, showed that the fed state group saw a 3lb weight increase, while the fasted group saw a 1.7lb increase, suggesting that fat gain is mitigated with 60 minutes of fasted cardio. Therefore, performing fasted cardio to prevent weight gain while in a calorie surplus (or weight gaining phase) might be justifiable.
Another study looked at 16 overweight women performing low volume interval training over a six week period. The data revealed that despite the fact that both fasted and fed state groups lost less than 1% of body fat, no changes in body mass were found and more importantly, there were no differences in between-group comparisons.
Perhaps the most relevant evidence to the reader regarding fasted cardio comes from a 2014 study. Twenty college females performed one hour of low intensity cardio at 70% of maximal heart rate for four weeks. Both groups maintained very similar macronutrient ratios while in a 500 calorie deficit. The data showed that overall body composition changes between the two groups were quite identical. The fasted group lost 1.1 kg of fat mass, while the fed group lost 0.7 lbs. However, there was 1.1 lbs. of fat-free mass lost in the fasted group versus 0.4 lbs in the fed group, which could signify muscle. Despite both groups benefiting in weight loss, between-group comparison statistics were not significant, alluding to the fact that there’s no overwhelming advantage to fasted cardio.
So, what we see is that despite more fatty acids being used during fasted cardio, fat loss isn’t greater compared to fed state cardio. When you look at athletes who sprint, they aren’t ever really burning fat because their intensity is so high, yet they are very lean. Greater workout intensity results in greater body composition changes. You can’t put in that much effort in a fasted state.
Total energy balance over a 24 hour period, or really over many days, is the ultimate determining factor for weight loss.
Conclusion – To Train Fasted, Or Not?
If you feel so inclined to train fasted, go for it, but realize that the research shows that there’s no additional benefit over doing it in a fed state. Not to mention, muscle protein breakdown and protein turnover is increased because branched chain amino acids (BCAAs) are preferentially utilized as a source of energy. Therefore, if you’re going to train fasted, I would be remiss to not at least recommend taking THIS BCAA product to prevent some muscle breakdown.
However, my strong recommendation is to eat a carb dense meal before heavy resistance training, or high intensity interval training, because it’s more time efficient, and results in a greater after burn effect which allows as much as a 7% increase in metabolic rate. This also ensures you should maintain your muscle, and that is always a good thing.
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