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To Fast or Not Fast Before Cardio, That is the Question

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To Fast or Not Fast Before Cardio, That is the Question

Many of you have heard of the best-selling fitness book from 1999 entitled "Body-for-Life" by Bill Phillips. In a chapter about cardio activity he throws out a theory about doing your aerobic (pro-longed period) exercise first thing in the AM without eating anything to maximize fat loss. Many, many, many people took this to heart and have done this for years now. Even I'm guilty of it on occasion. The reasoning behind his belief?  A prolonged absence of food brings about a reduction in circulating blood sugar, causing glycogen (stored carbohydrate) levels to fall. That leaves your body no choice but to rely more on fat, rather than glucose, to fuel workouts.

Moreover, the low insulin levels associated with fasting are conducive to fat breakdown, increasing the availability of fatty acids to be used as energy during the exercise session. Seems legit scientifically speaking though right? Who doesn't want to burn more fat while doing the same amount of effort they normally would put into their cardio workout? But (there's always a but right), this is not what actually happens in your body. Here's why this doesn't go as we wish it would go. 

First and foremost, it's shortsighted to simply look at the number of fat calories burned during an exercise session. Your metabolism doesn't operate in a vacuum.

Rather, the body continually adjusts its use of fat and carbohydrate for fuel depending on a variety of factors.

As a general rule, if you burn more carbohydrate while exercising, you'll ultimately burn more fat in the post-workout period and vice versa.

So, why does it matter that you burn only a few more fat calories while exercising, if an hour later, it shifts back to carbohydrate burn?

In the end, it doesn't make a bit of difference. You need to evaluate fat burning over the course of days—not hour to hour basis—to gain a meaningful perspective on its impact on body comp.

Let's say you're a skeptic, though, and figure it's better to burn more fat now rather than later. A bird in the hand is better than two in the bush, right? Well, not in this case.

True, the research does show that fasted cardio can increase fat utilization during exercise compared to performing cardio in the fed state. Except this only occurs at very low levels of training intensity.

During moderate-to-high intensity levels, the body continues to break down significantly more fat when fasted compared to after you've eaten.

So far, so good. Unfortunately, the rate of breakdown exceeds your body's ability to use the extra fatty acids for fuel. In other words, you have a lot of extra fatty acids floating around in the blood that can't be used by working muscles.

Ultimately, these fatty acids are repackaged into triglycerides post-workout, and then shuttled back into fat cells. So you've gone to excessive lengths…only to wind up at the same place.

Ok, so you've read this far and you think "Well I just do fasted low-intensity cardio to burn off those extra fat calories. that will trick my body." Good in theory, but (there's that but again), your training status has an effect on this strategy as well.  What's training status you ask? It's your exercise schedule, and most of you reading this probably are regular exercisers so that whole strategy does not work for you. If you exercise on the regular the benefits of fasted cardio on fat utilization have neglibile effects at even low levels of intensity. To get the metabolic shift to a more favorable fat burn you'll have to do 90 minutes or more. I don't know about you, but I can't do 90 minutes of low-intensity exercise before breakfast and not die of starvation along the way. Just saying.

Fasted cardio makes even less sense when you take into account the impact of excess post-exercise oxygen consumption. EPOC, commonly referred to as the "afterburn," represents the number of calories expended after training. Guess what? Eating before exercise promotes substantial increases in EPOC. And guess where the vast majority of calories expended in the post-exercise period come from? You got it, fat!

So what do you need to think about here? More EPOC equals more fat burned. This favors eating prior to performing cardio.

There's also the intensity factor to consider. Research indicates that high-intensity interval training is more effective than steady-state cardio for fat loss (Bill Phillips actually recommended HIIT as the preferred type of cardio in his book).

Ever try to engage in HIIT session on an empty stomach? Bet you hit the wall pretty quick. In order to perform at a high level, your body needs a ready source of glycogen; deplete those stores and say goodbye to elevated training intensity. The net result is that fewer calories are burned both during and after exercise, thereby diminishing total fat loss.

On top of everything, fasted cardio can have a catabolic effect on muscle. Studies show that training in a glycogen-depleted state substantially increases the amount of tissue proteins burned for energy during exercise. Protein losses can exceed 10 percent of the total calories burned over the course of a one-hour cardio session—more than double that of training in the fed state.                                                                                                                                                                                                                                   Summing up the post here, the strategy of fasted cardio is misguided for fat loss. Even at the 90 minute mark, your body composition effect won't be any better than if you had breakfast prior to working out. And the worst thing is you can lose some of that hard earned muscle instead of burning fat. So the question remains, should you eat before doing cardio or even a metabolic training workout? The answer depends on several factors, including the duration and intensity of training, the timing of previous meals before the cardio session, and individual genetics.

A good rule of thumb is to consume approximately 1/4 gram of carbohydrate and 1/8 gram of protein per pound of your ideal bodyweight (which may differ from your actual weight). Just be smart and listen to your body when you are working out. Some people are morning eaters and some are not. Then there's the few that aren't but need to be so they can get more from their workouts. When in doubt, eat some fruit or have a small shake to jump start the burn.                                                                                                                                                                                                                                                                                                                                                                                                                                                                                   Have more questions? Be sure to reach out to us here.

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