Over the years, I've been told two things about maximizing fat loss with cardio: (1) Exercise first thing in the morning, before breakfast, which will cause your body to use more fat for fuel. (2) Exercise when you feel best, regardless of whether you've eaten, because your output will be significantly greater and you'll burn more calories.
At times, each argument has made sense to me, and I've tried both techniques while getting ready for contests, as well as during off-season. Recently, I saw a very interesting study that suggested cardio first thing in the morning is, in fact, best for losing fat. But then I read the details.
The study included only 10 men, a very limited sample—not large enough to be convincing. After reading this disappointing study, I started to hunt for better studies but had a pretty difficult time finding much conclusive evidence. I then searched PubMed for studies on exercising after eating.
After reading about a couple studies and doing a Google search, I ended up reading an interesting article on bodybuilding.com that claims to debunk the exercise-before-breakfast "myth." The article was convincing, and after looking at the sources referenced in the article, I returned to the conclusion that I've been making more and more lately: The best time to exercise is the time at which you're most likely to do it, and the best kind of cardio is the kind you're most likely to enjoy. If you exercise when you feel your best, you're more likely to push harder, and if you do something you like, you won't be as focused on the grind.
Now, that's something I'd like to see studied: the output of a person enjoying himself or herself versus a person slogging away on the treadmill at 5:00 a.m. What is your favorite time to work out?