Most bodybuilders, fitness enthusiasts, and personal trainers, will tell you that the best time to do cardio is early in the morning, on an empty stomach. For some reason, which most of them don't know the answer to, they will tell you this is the perfect time to facilitate fat burning.
I'm here to say they are off base in this approach to losing body fat, and I will tell you why. As bodybuilders, most of us endeavor to eat 5-6 meals a day, spaced out over three hour intervals. This helps to keep blood sugar levels stable, keeps us in an anabolic state by maintaining a positive nitrogen balance, aids in better digestion and assimilation, and also helps to keep the waistline smaller, as most of the meals should not be big meals, per se'!
After sleeping all night for 6-8 hours, or whatever amount is required for you personally, the body, upon arising in the morning, is in a very catabolic state. This means a state of negative nitrogen balance, something no bodybuilder wants.
To perform cardio at this time, on an empty stomach, creates a further state of catabolism, thus resulting in muscle breakdown, and muscle tissue loss. The exact opposite of what bodybuilding is all about! It's true that blood sugar levels are low in the morning, but so are amino acids, the building blocks of protein.
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Author, Tim Wescott
If you must do cardio in the morning... no big deal, just make sure to have a protein drink, or a light protein meal, 45-60 minutes before the aerobic session. Your first priority in the morning, after fasting all night during sleep, should be to feed the body protein and carbs, thus restoring the body to an anabolic state once again. You always want to remain in this anabolic state, if possible, that's why we eat every three hours or so.
In my opinion, the best time to do cardio is after you workout with the iron. Performing your cardio workout after training with the weights, is the most ideal time to burn fat. The reasoning behind this thinking is that during your weight training workout you are using glucose, and stored glycogen, as fuel for energy.
Glucose and glycogen are sugars, and sugars are carbs. You want to use carbs as fuel for your training ideally, to sustain you throughout the workout.
After you are done training, your glucose and glycogen levels are obviously depleted, thus making this the perfect time to perform your cardio, and to use stored fat as fuel. At this time, if you are on a high protein diet, as most bodybuilders are, or should be, the body will burn fat as fuel, to help get you through the cardio session.
Type Of Cardio
Keep the cardio intense and brief, or the body will once again go into a catabolic state and use protein for fuel literally eating your hard earned muscle tissue alive. Doing too much cardio is detrimental. The type of cardio that you perform is up to you, but I would recommend doing 15-20 minutes of "High Intensity Interval Training." Hereafter known as HIIT.
HIIT cardio involves an all out burst of effort for about a minute, followed by a cool down pace, also for a minute or so, or until the heart rate slows down substantially. As I stated above, 15-20 minutes of HIIT is great and usually enough for most people. I personally wouldn't go over 30 minutes tops.
One of the advantages of performing HIIT cardio is that it accomplishes more in a shorter time period. Another more important benefit, is that it raises the heart rate and the metabolism. Your metabolism stays elevated long after your cardio session is done. This enables the body to keep on burning more fat just by going through your normal daily routine! I call this effect the "After burn."
I use an Elliptical Trainer most of the time, and I find by using this particular apparatus, I burn over twice as many calories using HIIT, over more traditional methods I have employed in the past. The Elliptical Trainer also has virtually no impact on the feet, hips, or knees.
So there it is, my take on doing AM cardio, and why I think it should not be done on an empty stomach. Try it out and lose some of that excess body fat, while retaining your hard earned muscle, you'll be glad you did!!