Research published in the British Journal of Nutrition suggests that it is possible to burn up to 20% more body fat by exercising in the morning on an empty stomach.
They also examined fat metabolism and the consequential calorie intake for exercising in a fasted state. It showed that subjects did not consume additional calories or experience increased appetite following their morning bout of exercise.
In order to lose body fat we need to utilise more fat as energy. Exercise will increase the total amount of energy we expend and a greater proportion of this comes from existing fat if the exercise is performed after an overnight fast.
Gonzalez JT, Veasey RC, Rumbold PLS, Stevenson EJ, Breakfast and Exercise Contingently Affect Postprandial Metabolism and Energy Balance in Physically Active Males, British Journal of Nutrition, 2013, 1; DOI: 10.1017/S0007114512005582.