Cardio: Morning (Fasting) or Evening?

Fat loss community advocates morning as the most effective time for fat burning. But the scientific community does not unanimously support any particular time of the day to be more effective over other for burning fat. They say that as long as you burn more calories than you consume during a particular period of time, you shall lose weight (energy balance hypothesis). It is also not important, they believe, where the calorie comes from: carbs or fat. This hypothesis looks very natural and acceptable even to laymen. The important thing is to do cardio at a convenient time of the day and to follow a moderately calorie-restricted diet. Maintaining regularity of a fat-loss program is much more important than the time.

Many athletes and body -builders recommend doing cardio first thing in the morning before eating their first meal. There is ample scientific literature which supports the early morning fasting cardio in fasting state. It is believed that this method will mobilize stored body fat and will be utilized as fuel for the cardio and also that the morning cardio will cause rise in the basal metabolic rate (BMR) for the whole day.
The arguments in the favor of doing fasted early morning cardio are:

1. After waking up in the morning, your glycogen stores get depleted due to 8-10 hour fasting overnight. The body will try to mobilize stored body fat for energy for the cardio.
2. When you eat, insulin is released from the pancreas to dispose of the blood glucose. But the same insulin will also inhibit break down of stored fat. Hence doing cardio in the morning fasting state, when there is less insulin, helps breaking the stored body fat.
3. Doing cardio in the morning fasting state, when there is less glucose in the blood stream, more fat will be burnt instead of glucose as fuel.
4. If you do cardio just after eating, you have to first burn out what you have just eaten before mobilizing the stored body fat. Also there will be some rise in insulin in the blood just after eating, which will inhibit fat burning.
5. The fasting cardio has the added benefit of elevated BMR throughout the day, whereas if you go to sleep after a workout, the BMR goes down. When you do a cardio workout early in the morning, you not only burn fat during the workout, but you also continue to burn fat at a higher rate after the workout. This happens due to the elevated basal metabolic rate (BMR) after the workout.
6. Losing weight by burning more fat is not the only reason you should choose the early morning time for cardio. There are other mental benefits too.  Because cardio exercise causes a release of “feel-good” chemicals (endorphins) in your brain, you feel high throughout the day. Endorphins are opiate-like hormones many times more powerful than morphine. Endorphins improve the mood, reduce stress, increase blood circulation and relieve pain. A 30 minute cardio early in the morning is your brain tonic for the entire day.

Research in support of this theory.

 Kansas State University had performed a study and published in Medicine and Science in Sports and Exercise. The study showed that it takes lesser time to burn a kilogram of fat when exercise is done in the fasting state in the morning than when the same amount of exercise is done later in the day. How the researchers got these results? They measured caloric expenditure, respiratory gas exchange, and fatty acid metabolism for the study. They found that, after a 12 hour fast, the amount of fat burnt was 67% of the total energy expenditure whereas it was 50% when the same amount of exercise was done later in the day (or after eating).
Another study by The Journal of Applied Physiology investigated the effects of aerobic exercise on lipid oxidation in fasted versus fed states. The study concluded that, “endurance training enhances lipid oxidation in men after a 12 hour overnight fast.” A scientific paper entitled “Optimizing Exercise for Fat Loss” supports the hypothesis that “The ability of exercise to selectively promote fat oxidation should be optimized if exercise is done during morning fasted metabolism.”