My friend, whom we'll called Aaron (because that's his name), recently posed a question regarding food intake and training. To quote indirectly: "what are your opinions on training in a fasted state"? Aaron is asking a good question that merits some thought. First, let's explain the question so we're all on the same page.
Aaron isn't talking about extended periods of fasting (i.e., he's not talking about working out during an extended multi-day fast). He also isn't (we hope) talking about running a marathon while in a fasted state. That would be insane. Rather, he's talking about timing his workout so that it coincides with a time in which he hasn't eaten anything beforehand for several hours (at least). For most people, this means working out first thing in the morning. Why would someone do this?
Typically, the idea of fasted training comes from the group of people that subscribe to a paleolithic type of diet. For those of you trying to remember where "paleo" falls in the evolutionary chain, don't fret. We're not getting into history here (too much). It's a way of eating that focuses on eating like a paleolithic human might have, in the idea that our bodies have not changed much since that point. I'll spare you the details (which you can find here if you're really interested), but suffice to say it's very low on any kind of grain-based carb.
In the Paleo way of thinking, a moderate to high level of carbohydrate intake (yes, even those "complex" ones) is believed to decrease the body's insulin sensitivity, opening the door for many potential health problems.
When you exercise, you deplete to some level the store of glycogen in your muscles. This immediately increases the body's sensitivity to insulin - which is a good thing.
The proposed benefit behind training in a fasted state is that you allow your body to empty itself more fully of its glyogen stores, increasing even more your insulin sensitivity and thus, your "health" and resistance to many potential diseases. In this sense, fasted training is a good thing and can be used occasionally.
However, there are pitfalls.
First, make sure to remember that there are vital parts of your body that depend on glucose (and glucose alone) for fuel. Like your brain. Now, that's not to say you're going to die if you burn through all your glucose in a fit of burpee glory. Your body has this wonderful process called gluconeogenesis, in which it will actually convert fat and protein stores into glucose for vital organ function. (This is also what happens when one embarks on an extended period of fasting, by the way.) This keeps you alive and well long after you've burned through that bagel you had for breakfast. However, when this process occurs, it can often lead to the release of the hormone cortisol (a stressor). If your cortisol levels are already increased (and many people's are, if they drink a lot of coffee, have a stressful job, don't sleep enough, etc), this additional hormone surge can lead to impotence, bad blood sugar levels and ironically, increased levels of fat.
In a nutshell, we can answer Aaron's question by saying that fasted training does have benefits, when paired with the proper diet. However, in terms of health, it's likely more important to control cortisol levels than it is to squeeze that last ounce of glucose from your muscle stores, and thus it's not something we recommend as a daily routine, especially if your workouts are intense and/or last more than 30-45 minutes. We already experience an increased insulin sensitivity with our regular workouts; use fasted training as a tool when necessary, but not as a regular training plan.
Plus, I hate working out in the morning. :)