It's human nature to look for efficient ways get something done. We live in a fast-paced world and we want to make the most out of our time and efforts... especially in the gym. Call it a hunch, but I'm guessing that's why you're here.
So let me get right into this and not waste any of your time - there's a theory going around that you can optimize your fat-loss efforts by doing cardio in a fasted state. Some people (e.g., physique competitors, fitness magazine writers, personal trainers, and "coaches"), swear by it. Me? I'm of the opinion it doesn't matter if you're fasted or fed, when doing steady-state cardio or interval training (switching between periods of low and high intensity work).
As usual, my opinion is based on research and practical experience. From a research standpoint, there have been a couple of studies to show that fasted cardio was not superior to cardio in a fed state.
In one study, 20 young women were split into two groups. Both groups were given customized, calorie-restricted diets and performed an hour of cardio in the morning three times a week. The cardio sessions included a 5-minute warm up and cool down at 50% maximal heart rate (MHR). During the other 50 minutes, they did steady-state cardio at 70% MHR. The difference between groups, as I'm sure you've already guessed, was that one group did cardio on an empty stomach while other did so after eating a meal. The result? After four weeks, both groups lost weight and body fat, but there was no significant difference between them.[1]
In the other study, a research team divided 16 overweight women into two groups. Each group perform 10 intervals on an ergometer (think stationary bike). They did 10 intervals starting with 60s sprints at 90% MHR (so they were cycling like an angry dog was chasing them - a very large angry dog). This was followed by 60s of rest (complete rest or slow pedalling) before starting the next interval. They did this 3 times a week for 6 weeks.[2]
The difference between groups, just like with the first study, was that the women trained only in a fed or fasted state (no crossover). The outcome ended up being similar as well. Both groups lost body fat with no significant difference between the two.
As you can see, the research doesn't show a benefit from doing cardio with an empty stomach or food in your belly. To add to that, I've never seen a benefit from doing one or the other in myself or in any client I've worked with.
Therefore, it doesn't look like there's a better way to do your cardio. The key, in my opinion, is that you do it (assuming it's appropriately programmed). Whether that's in a fed or fasted state depends on you and your preference.
And now you know:
For more information:
1. Schoenfeld, BJ. et al. Body composition changes associated with fasted versus non-fasted aerobic exercise. J Int Soc Sports Nutr. 2012. 11: 54.
2. Gillen, JB. et al. Interval training in the fed or fasted state improves body composition and muscle oxidative capacity in overweight women. Obesity (Silver Spring). 2013. Nov;21(11):2249-55.