Should you bonk on purpose?

Although some training in a glycogen-depleted state may be helpful, in competition you want to give your muscles every possible advantage.  Credit: Elisenda Roig/Bongarts/<br>Getty Images
Bonking is, of course, slang for running out of energy during exercise. It usually happens when the working muscles run low on glycogen, which is the body's limiting fuel source for sustained activity.

Bonking is something you'd never want to do on purpose. Or would you?

Believe it or not, one highly respected exercise scientist has suggested that it may be beneficial to bonk regularly in training. Her name is Bente Klarlund Pedersen, Ph.D., and she's a researcher at the University of Copenhagen, Denmark.

Klarlund recently explained her rationale for "intentional bonking" in a lecture entitled "Signaling the Muscles to Adapt: Train Low, Compete High?" which was delivered at the 53rd Annual Meeting of the American College of Sports Medicine.

Benefits to under-fueling workouts

In this provocative lecture, Pedersen made the case that athletes -- and especially endurance athletes -- stand to gain greater fitness by performing some of their workouts in a glycogen-depleted state than by trying to perform all of their workouts in a glycogen-replete state.

In practical terms, she said, they should do some workouts within hours of having completed their last workout, such that there's not enough time to replenish muscle glycogen stores between workouts, and they should also leave their sports drinks and gels at home for some workouts (that is, intentionally under-fuel their muscles during training).

The rationale for this approach centers on something called interleukin-6 (IL-6). Interleukin-6 is an important cytokine, or immune system agent, that plays a variety of crucial roles in coordinating the body's response to tissue trauma and stress. Large amounts of IL-6 are released into the bloodstream by the muscles during exercise and travel to organs throughout the body. IL-6 is believed to facilitate many of the body's adaptations to exercise training, ranging from increased fat burning to greater resistance to muscle damage to improved cognitive function.

The primary trigger for IL-6 release during exercise is glycogen depletion, or bonking. So it follows that training in a glycogen-depleted state will tend to produce stronger training adaptations (of certain kinds, anyway) than training in a glycogen-replete state.

Put to the test
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.

Pedersen's research has clear implications for exercise nutrition. The muscles produce much less IL-6 when carbohydrate is consumed during exercise. Therefore it may be beneficial for athletes to intentionally under-fuel themselves in some workouts so that they produce more IL-6 and stimulate greater training adaptations. It's also known that athletes burn more fat when they don't consume carbohydrate during training, and in the long run become more efficient fat burners, thus increasing their raw endurance.

Finding balance

So then, why not under-fuel yourself in every workout? Because there are disadvantages associated with consistently training in a glycogen-depleted state, and there are advantages to training in a glycogen-replete state. Studies have shown that athletes who consistently consume carbohydrate during exercise are able to handle higher training loads than those who don't, due to enhanced recovery.

Also, athletes are able to perform at a higher level when they consume carbohydrate during training. Going five percent faster or farther in carbohydrate-fueled workouts will itself trigger greater training adaptations of certain kinds than going five percent slower or shorter in under-fueled workouts.

Consequently, the best training recipe is probably a mixture of fully-fueled and under-fueled workouts. What constitutes the optimal balance between these two workout types is still unknown. You may need to experiment to find out what works best for you.

One thing is certain, though: You should always compete in a glycogen-replete state and consume plenty of carbohydrate during longer races. This is what "compete high" refers to in the title of Pedersen's lecture. In workouts it pays to often make things tough on your body by "training low" -- that is, in a low-glycogen state. But in competition you want to give your muscles every possible advantage. Bonking in a race is always bad!

Matt Fitzgerald is the author of several books on triathlon and running, including Runner's World Performance Nutrition for Runners (Rodale, 2005).

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