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:
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.”