To investigate the impact of when you exercise , a team of sadistic, Belgian researchers put a group of athletes on a horrible, high-fat diet (probably not much different than the standard American holiday diet) and had them not workout, workout before breakfast, or workout after breakfast.
Naturally, the non-exercising group packed on the pounds (those lazy bastards!). But there was a significant difference in the weight gain between the other two groups.
The after-breakfast exercise group, of course, put on less weight than the non-exercise group, but they still gained. However, the pre-breakfast exercisers hardly gained any weight at all. Their blood chemistry was better too, showing better insulin response.