The logic seems sound: If you've not eaten at all and then go do an aerobic activity like running, your body will have to use up more fat and lose more weight right? While yes, you tend to use more fat during the activity, over the long-term fasted cardio alone has no additional effects on weight loss.
Image by See-Ming Lee.
The guys over at Examine.com have pored over a dozen of studies that looked at the effects of training in a fasted or fed state, and concluded:
Fasted training, independent of caloric intake, may not confer any additional fat burning or weight loss effects over a period of time despite increase percentage of fatty acids being used as fuel.
This isn't saying that fasted cardio is dumb or unnecessary.
Running or doing any form of aerobic exercise with no more than black coffee or water is simply a matter of preference and convenience for many folks (who wants to wake up an extra two hours early for those quick morning runs, anyway?).
As with any health- and weight loss-related matter, personal preference trumps any headache-inducing "optimised protocol," since what you like to do or eat (within reason) will help you stick with healthier habits for the long haul. Want to go run butt-crack early in the morning but don't want to upset your stomach by eating first? Go wild. Feel like eating a delicious sandwich before your run? Sounds good. Either way, it apparently has a negligible impact on your weight loss, so just do either because you want to.
Head over to the link below for more, and a peek at the studies the Examine team have analysed.
Is It Better to Do Aerobic Exercise Fasted? [Examine.com]