Any time of day that suits your schedule is a good time for cardio. The important thing is that you just do it. However, many bodybuilders and fitness models believe that early morning fasted cardio burns more body fat. Although this is still controversial, the evidence is strong and there are many reasons to consider doing cardio first thing in the morning on an empty stomach. The argument in favor of fasted early morning cardio goes something like this:
1. After an overnight 8-12 hour fast, your body's stores of glycogen are depleted and you burn more fat when glycogen is low.
2. Eating causes a release of insulin. Insulin interferes with the mobilization of body fat. Less insulin is present in the morning; so more body fat is burned when cardio is done in the morning.
3. There is less carbohydrate (glucose) in the bloodstream when you wake up after an overnight fast. With less glucose available, you burn more fat.
4. If you eat immediately before a workout, you have to burn off what you just ate first before tapping into stored body fat (and insulin is elevated after a meal.)
5. When you do cardio in the morning, your metabolism stays elevated for a period of time after the workout is over. If you do cardio in the evening, you burn calories during the session, but you fail to take advantage of the "afterburn" effect because your metabolic rate drops dramatically as soon as you go to sleep.
6. Morning cardio gives you a feeling of accomplishment and makes you feel great all day by releasing mood-enhancing endorphins.
7. Morning cardio "energizes" you and "wakes you up."
8. Morning cardio may help regulate your appetite for the rest of the day.
9. Your body's circadian rhythm adjusts to your morning routine, making it easier to wake up at the same time every day.
10. You'll be less likely to "blow off" your workout when it's out of the way early (like when you're exhausted after work or when friends ask you to join them at the pub for happy hour).
11. You can always "make time" for exercise by setting your alarm earlier in the morning.
A common concern about doing cardio in the fasted state, especially if it's done with high intensity, is the possibility of losing muscle. After an overnight fast, glycogen, blood glucose and insulin are all low. This is an optimum environment for burning fat. Unfortunately, it may also be an optimum environment for burning muscle because carbohydrate fuel sources are low and levels of the catabolic stress hormone cortisol are high. It sounds like morning cardio might be a double-edged sword, but there are ways to avert muscle loss.
All aerobic exercise will have some effect on building muscle, but as long as you don't overdo it, you shouldn't worry about losing muscle. It's a fact that muscle proteins are broken down and used for energy during aerobic exercise. But you are constantly breaking down and re-building muscle tissue anyway. This process is called "protein turnover" and it's a daily fact of life. Your goal is to tip the scales slightly in favor of increasing the anabolic side and reducing the catabolic side with nutrition just enough so you stay anabolic and you maintain muscle.
How do you build up more muscle than you break down? First, avoid excessive cardio. If your lean body mass has dropped, try limiting your cardio on an empty stomach to 30 minutes, and then it would be highly unlikely that amino acids will be burned as fuel. Bodybuilding nutritionist Chris Aceto suggests, "A strong cup of coffee should facilitate a shifting to burn more fat and less glycogen. If you can spare glycogen, you'll ultimately spare protein too."
Second, give your body the proper nutritional support. Losing muscle probably has more to do with inadequate nutrition than with excessive aerobics.
Third, keep training with heavy weights, even during a fat loss phase. Using light weights and higher reps thinking that it will help you get more "cut" is a mistake: What put the muscle on in the first place is likely to help you keep it there.
Morning cardio is still controversial in academic circles, but in my book, it's a sure-fire way to double or even triple the fat burning effects of your cardio. Ask any highlevel bodybuilder or fitness model when they do their cardio and the chances are good that they'll tell you they're working up a sweat before breakfast early every morning.
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