Two Truths and a Lie: Fasted Cardio

I'm Trent Napp, a student in the Doctor of Physical Therapy Program at the University of Wisconsin-La Crosse. Formerly, I received my Exercise Sports Science Degree from UWL along with a Biology Minor in 2017. I'm excited to bring content and a new clinical perspective to this page. Let's get started!  

Introduction: 

Fasted cardio can be seen everywhere: the local YMCA, Instagram models and bodybuilders, the college rec center, etc. The idea that waking up early in the morning and headed straight to the gym without even looking at food in order to utilize fats for energy while slugging along on a treadmill, bike, or cardio equipment of choice at a low to moderate intensity for an hour or two. Why do people do this? If you ask someone at the gym, which I did, they may claim that it “helps lose fat because that’s the only energy I have to burn because I haven’t eaten any carbs yet.” Well, let’s get into how that statement shouldn’t be the reason for your fasted cardio workout.   

Forewarning: this blog doesn’t get into the pro’s and con’s of fasted cardio/morning workouts on 24 hr diet and caloric intake (for that see Dr. Jagim’s article review).

Lie: Fasted cardio will result directly in increased fat loss.

Those who are avid fasted cardio-ers explain this concept as diminished blood glucose levels reduce the body’s ability to utilize glucose aka carbohydrates as fuel and therefore will utilize fats as a more efficient fuel source. Former research had shown that consuming carbs prior to a 60% intensity aerobic exercise bout would decrease fat breakdown by reducing the ability of fatty acids to enter the mitochondria [and therefore be utilized for energy]¹. This research is what many bodybuilders during their ‘cut’ hang their hat on. On the surface this seems valid, right? Who wouldn’t want to burn fat at a low effort?

Truth 1: 

There’s much more to the body’s energy system than just “I have less of this (Carbs), so I’ll use this (fats)”. However, we know that carbs are utilized much quicker and as a result if we are only utilizing fats during this type of exercise, the fatty acids can "build up” in the tissue and be redirected. Brad Schoenfeld (an expert in the field of exercise physiology and strength training) states in a study:

“during moderate-to-high intensity cardiovascular exercise in a fasted state-and for endurance-trained individuals regardless of training intensity-significantly more fat is broken down than that the body can use for fuel. Free fatty acids that are not oxidized ultimately become re-esterified in adipose tissue, nullifying any lipolytic benefits afforded by pre-exercise fasting.”²

In other words, no matter the intensity of training, if excessive fats are broken down the body still won't be able to utilize them and will just store them again. 

Truth 2: 

In addition to less lipolytic effects than one may think, we see decreases in the excess post-exercise oxygen consumption (EPOC) aka what your body burns in a period of time after exercise- in fasted groups compared to non-fasted³. This same study also adds that high intensity is better at fat oxidation than low-moderate intensity, killing two birds with one stone. If your goal is to increase muscle mass, low-intensity fasted cardio may not be your best option, either (unless you’re on PEDs). It has been found that when training in a fasted state, nitrogen losses were doubled vs. a non-fasted state mostly thought to be due to a low glycogen storage level. As we know, nitrogen loss is a measurable of protein loss⁴. So concluding Truth #2, less calories burned in EPOC and increased protein breakdown when you're body is in an overnight fasted state. 

Conclusion and alternatives:

Although it seems like a simple solution; low carbohydrate levels result in more fat utilization, the human biochemistry is much more complex than that. Keep in mind that all of these studies are done in an experimental setting, with certain protocol and parameters. These specific studies fail to examine the effect early-morning exercise has on caloric intake the rest of the day (Dr. Jagim has expanded on this in the blog noted below). Personally, I workout early in the morning, like many, with limited food (something quick like a bagel with peanut butter, granola bars, or a shake) because it works best with my schedule.

In my opinion, it is more efficient to do a high intensity interval training (HIIT) workout that takes half the time and also increases the total calories burned in EPOC. HIIT has also shown to increase the total fat burned throughout the day. If you choose to do a HIIT workout, I’d recommend a pre-workout snack/shake, it doesn’t have to be a full meal, because the HIIT is taxing on your energy systems.²

With all this said, do what works best for you, but keep in mind what is actually happening (or not happening) physiologically, how your body responds to different exercise types, and the goals that you are trying to achieve.

See Dr. Jagim’s article review here, which goes into depth about how fasted cardio may improve the rest of the day for a healthy lifestyle.

References

1.      Spriet LL and Watt MJ. Regulatory mechanisms in the interaction between carbohydrate and lipid oxidation during exercise. Acta Physiol Scand 178: 443-452, 2003.

2.     Schoenfeld B. Does cardio after an overnight fast maximize fat loss? Strength Cond J. 33: 23-25, 2011. doi: 10.1519/SSC.0b013e31820396ec

3. Lee YS, Ha MS, and Lee YJ. The effects of various intensities and durations of exercise with and without glucose in milk ingestion on postexercise oxygen consumption. J Sports Med Physical Fitness 39: 341-347, 1999

4.     Lemon PW and Mullin JP. Effect of initial muscle glycogen levels on protein catabolism during exercise. J Appl Physiol 48: 624-629, 1980.