Dr. Bente Klarlund Pederse of the University of Copenhagen, Denmark, believes that we should spend some of our time training in a glycogen-depleted state in order to maximize our training benefits to increase production of the immune system agent Interleukin-6.
In other words, do some workouts underfueled and make yourself bonk in training.
Pedersen provided some validation for this hypothesis through a clever study in which she had subjects exercise one leg once daily and the other leg twice every other day. The total amount of training was equal for both legs, but the leg that was trained twice every other day was forced to train in a glycogen-depleted state in that afternoon workout. After several weeks of this, subjects engaged in an endurance test with both legs. Pedersen found that the leg trained twice every other day increased its endurance 90 percent more than the other leg.
How do you make yourself bonk, you ask? Running a second workout the afternoon after an early workout will not give your muscles to replace spent glycogen, and being sure not to help your recovery along with sports drinks and gels is a quick way to deplete yourself of fuel.
I can see how this type of training could provide some benefits, outside of the immune system boost. Getting used to the sensations and training your body to better deal with the stress of running on too little fuel while you are training could certainly be worthwhile.
If one were to decide to try doing a workout plan that implemented this training, however, then it should certainly be used sparingly and not every day. Doing it often will negate any benefits of being able to train longer and faster because you are properly fueled, and I can see how it could quickly lead to injury. I would tend to think, although it is not mentioned in the article, that after your second workout you would have to be very careful to refuel properly.
The article also makes a point that your body should always be properly fueled for any competition.
I may try mixing in a few workouts like this to my training plan this summer. If I do, it certainly would not be more than once a week or (more likely) once every other week. I will have to see if I can find any more research into it first.