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AM Cardio on an Empty Stomach


Saturday, December 10, 2005. I’m in my second year of college at Oakland University. It’s 6am… I am trying to sleep in after a long week of exams, work, and studying for Mike Stack’s 400 level quizzes in his 100 level class (yes that’s right! I had him as a professor). In my apartment, the wall of my room shared a wall with that of our living room, which proved to have no advantages whatsoever. I was in that perfect sleep position… you know the one! My feet were tightly wrapped under the end of the blanket, a fortress of pillows with just the right amount of fluff surrounded my head. It was shaping up to be a nice relaxing Saturday for me, when all of the sudden…Beep…….Beeeeeeeeep… Beep. Beep. Beep. I didn’t set an alarm clock the night before so I knew it wasn’t that, plus it was coming from the living room! All at once my slumber that previously consisted of “rapid eye movement” transformed into an angry shuffle of rapid foot movement! I charged toward the living room and discovered my roommate chugging away on her exercise bike, punching in her workout numbers on the electronic display-Beep…Beep..Beep. “What are you doing?!” I shouted. She was un-phased and responded, “Didn’t you know you burn more fat doing cardio in the morning?” The rest of the story is really not important, but let’s just say I don’t feel bad for pulling the plug on her exercise bike that morning…and I’ll sleep easier tonight knowing I pulled the plug on this myth for my clients.

Exercising in the morning after an overnight fast is a common strategy employed by individuals of all fitness levels hoping to maximize fat loss. The rationale behind this theory is that upon waking, muscle glycogen (the sugar we store in our muscles and use to perform exercise) is at a reduced level and will force the body into using fat stores as a fuel source. While this sounds like a valid theory and makes sense at first glance it is not supported by any scientific literature.

Issue #1
Studies on performing low intensity exercise (equivalent to walking at 3mph on a treadmill) in a fasted state show more energy is expended from fat after approximately 80-90 minutes of activity. The problem is you have to exercise for almost 1.5 hours to achieve this effect. While you may burn a little more energy from fat as a percentage of total calories burned, the absolute amount of fat you burn is significantly less than if you were to perform moderate intensity exercise. In addition, a percentage of this small increase in fat use comes from fat inside our muscles which has no negative effect on health or appearance.

At moderate and high intensities there is no greater use of fat as a fuel source for exercise in fasted vs. fed state. While it is true that at moderate and high intensities, more fat is readied for use (in a fasted state) it is never actually broken down and used for energy production and therefore is re-stored as body fat. This restoring of fat nullifies any potential benefit from fasted exercise. In addition to any potential benefit of exercising in a fasted state being nullified by the restoring of fat, benefits from consuming calories prior to exercise are not gained. Consuming calories prior to exercise not only increases energy levels (which results in more intense exercise and more calories burned) but there is also an increased thermic effect from exercise when calories are consumed prior to exercise. This increased thermic effect caused by consuming calories pre-exercise would nullify any advantage gained from even low intensity exercise in a fasted state.

Issue #2
Exercising in a fasted state can have a negative impact on the quality of an exercises session. I am sure most clients have seen us handing out glucose tablets to clients during morning or evening sessions. One of the possible reasons for this is because their glycogen depletes quickly, and blood sugar drops, from being in a fasted state (from sleep or from no food since lunch) and they become hypoglycemic. It can take between 5-30 minutes to recover from hypoglycemia and is most situations workouts cannot be finished. Even if you do not experience hypoglycemia your workout intensity is inherently reduced because the main fuel source for any moderate to high intensity training (like WLS, FS, or GRST) is carbohydrate in the form of muscle glycogen. Reduced intensities = less calorie expenditure = less fat loss.

Issue #3
Another important issue that should be noted is the effect of fasted exercise on the breakdown of muscle tissue. Research has noted that muscle tissue breakdown was doubled when training in a glycogen depleted state vs. glycogen loaded state. Over an hour long exercise session at moderate intensities (jog on the treadmill at 4-6 mph) this could account for approximately 10% of calorie loss. For individuals looking to maintain or gain lean mass this would be a major contraindication to fasted training.

Looking at the fuel source used during exercise is shortsighted when trying to quantify the effectiveness of an exercise session with regards to body composition. Most people exercise 4-6 hours during a 168 hour week! The amount of fat burnt during exercise pales into comparison to the amount of fat you burn throughout the day. In order to get an accurate measure of fat loss you must take into account at least an entire 24 hour period. As a general rule, when all else is equal, if you burn more calories from carbohydrate during a workout you will use more fat during the day and vice versa. At the end of the day/week your calorie deficit will be made up for by using fat stores in the body.

Example: 1 week of exercise
Energy expenditure from workouts:
• 3000cals from FAT/ 500cals from CARBS = 3500cals= 1lb fat
• 3000cals from CARBS/ 500cals from FAT=3500 cals= 1lb fat
• No matter how you achieve your deficit your body will use stored fat to make up for it!!!

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