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Is Fasted Cardio Best?

By Dr. Jeremy Girmann

This month’s question comes from Performance Press reader, Andrew:

“I heard that fasted cardio will help me to burn more fat compared to cardio that is done after I eat. Is this true?”

Ah…the cardio conundrum. Before answering this question directly, let me start with a few general thoughts on cardio (with the assumption that ‘cardio’ refers to moderately prolonged aerobic activity such as running, biking, swimming, etc).

Most simply, I like it…

There are those who choose not to include cardio in their exercise routines, opting instead to focus solely on weight training. I happen to be okay with this, particularly if the weight training is done at an appropriate tempo and with sufficient intensity because lifting weights in this way can significantly elevate one’s heart rate and serve as aerobic activity, essentially killing two birds with one stone.

I do, however, find that the inclusion of dedicated aerobic exercise provides several benefits, which I’ll describe from the perspective of personal experience.

Perhaps most intuitively, it strengthens the heart and the lungs. Without getting too technical, it’s a use it or lose it sort of thing. We were made to move, elevate our heart rate, and breath heavily for periods of time. The more consistently you do it, the easier it becomes. We shouldn’t be gasping for air after climbing a single flight of stairs.  

Second, it can stimulate a healthy appetite. If gaining/maintaining muscle while staying lean is the goal, cardio can heighten your hunger for the good stuff. While this is anecdotal and perhaps not true for all, I find that cardio often stimulates an appetite for healthy foods. The body is smart. When treated properly, it demands what it needs most – nutrient dense foods.  

Third, it makes you smarter. Really? Maybe…I can’t remember. While we jog on a treadmill or spin on a bike, thousands of incredible reactions are taking place within our brains.

It’s like a concert of chemicals and the symphony of synapses allows for some awesome things to happen. Exercise boosts brain derived neurotrophic factor (BDNF), for example, which acts to encourage neuronal protection, growth, and integration.  If I’m ever able to figure out how to cure cancer or how to peacefully end war (both on my ‘to do’ list), I’m pretty sure that I’ll accomplish it while on a treadmill – when I’m able to think sinfully clear. While it is true that the majority of us are visual learners, I also think that a majority of us need to move in order to think most optimally.  

Fourth, it facilitates better sleep. In my line of work, I spend the majority of the day living inside my head. My job demands lots of thinking and not much moving, and I doubt that I’m alone on this one. Historically, when the sun was up, we were moving.  There were no plush desk chairs, no files to sort, no computers. When the sun went down, we would stop moving and the brain would know that it’s time to sleep. These days, with no peripheral input from our bodies during the day, our sleep/wake cycles have become severely disrupted. The brain is left thinking, “I’ll stay awake a bit longer. Maybe then he’ll actually start moving.”  

brandie-girmann-running-cardio

On and on it goes. Cardio can enhance the immune system, boost your mood, encourage healthy digestion, and so forth. There’s something to be said for the ol’ “it gets the blood moving” mantra.

Okay, now on to Andrew’s original question. There has been, and continues to be, ongoing debate about the potential benefits of fasted cardio as it applies to body composition. The idea behind fasted cardio is this: When you wake in the morning (or after any period of prolonged fasting for that matter), insulin is low and the body is relatively deficient in glycogen (the storage form of carbohydrates).   Carbohydrates fuel the initial stages of aerobic activity and if they have been depleted (as they are during sleep), the body will turn to fat for a primary source of energy. Thus, performing cardio first thing in the morning would seem to cause the body to tap into fat stores more quickly, ultimately burning more body fat. The question becomes, does this actually occur? Maybe…probably…at least acutely.  

Several studies have demonstrated that beta-oxidation – the process of breaking down fats – is increased when cardio is performed in a fasted state. In fact, one study published in the Journal of Applied Physiology demonstrated that 24-hour fat oxidation was higher in a group that performed fasted morning cardio sessions compared to those that did the cardio after eating breakfast (Iwayama, 2014). End of story, right?  Well, not exactly. While studies like this provide us with valuable insight, we cannot be sure that the findings translate directly into real, meaningful outcomes. In other words, we should not assume that the increase in beta-oxidation observed with fasted cardio would necessarily equate to greater long-term reduction of body fat and improved body composition. In fact, the studies on this have been rather paradoxical.

In 2014, Brad Shoenfeld and colleagues conducted a study of 20 healthy women (average age = 22) on hypocaloric diets, to test the hypothesis that performing aerobic exercise after an overnight fast accelerates the loss of body fat. In the study design, 10 women performed an hour of cardio after an overnight fast while the other 10 completed cardio after first eating a meal. The exercise was done 3 days per week for a total of 4 weeks. The results? Both groups lost weight and fat but neither demonstrated more significant losses.  

What the? Let’s consider further…

As the authors of this analysis rightly suggest, this study is likely to be most meaningful to you if you’re a young, active female on a hypocaloric diet since this profile fits those of their study subjects. If this does not, however, sound like you, the results may or may not be applicable.  

In the process of figuring out what might work best for you, it is necessary to consider all of the variables associated with your unique situation.

Have you been exceptionally stressed lately? Stress can elevate cortisol, which in excess, can presumably tip the balance and result in a greater breakdown of muscle tissue when doing cardio in a fasted state. (*Side note: I almost hate to mention this because so many people, especially guys, are terrified of doing cardio for fear of losing muscle and the very thought is enough to raise their cortisol levels. If the cardio is done strategically, this just won’t happen. You won’t get on a treadmill at 210 lbs. and step off with 10 lbs. less muscle. Relax…)

How do you feel while doing the cardio? I personally feel better when I do cardio after eating a meal. If I choose to skip fasted cardio because it doesn’t make me feel great, it goes without saying that I can’t burn fat from cardio that I didn’t perform. I remind myself of this concept routinely in medical practice – I can develop the best treatment plan in the world but if my patients aren’t going to follow it, it’s all for not. Compliance is key.

How many calories are you eating?  I sweat more during cardio when I’ve eaten a meal. This provides evidence of a very important consideration – the thermogenic effect of food. Some of the food that is eaten prior to exercise is used to produce heat.   This can provide a sort of metabolic “boost” resulting in a greater calorie burn. Some studies have in fact indicated that this might, given the right conditions, be very relevant to maintaining a lean physique and a healthy metabolism. We must, however, be careful. If I were to eat 500 calories before doing cardio and only burn 200 calories during the session while being in caloric excess for the remainder of the day, we can reason what the net effect might be on my efforts to lose fat despite getting a small metabolic boost during the activity.

• Do you use caffeine?

• Do you follow a high carbohydrate diet? How about a high fat diet?

• Do you weight train later in
the day?

• How much lean body mass do
you have?

• How long before or after the cardio do you eat?

• At what intensity do you do
the cardio?

I’m complicating the picture on purpose and doing so for the following reasons:

One – When someone claims to have the definitive answer on this sort of thing, know that there is no definitive answer. While “it depends” represents a pretty unfulfilling response, it’s the truth.  

Two – In accordance with the above and as in most things related to health and fitness, it’s not a one-size-fits-all proposition. You must understand that each person represents a unique case. I would recommend this – try each method for a few weeks and monitor your progress. Use Parrillo’s 9-point body stat system in order to determine your exact body composition so that you know what affect each approach is having and which will provide the greatest benefits for you.

2019-05-17T16:52:27-04:00 May 17th, 2019|Dr Jeremy Girmann, The Press|

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