Should You Intentionally Under-Fuel For A Workout?

Sometimes it pays to run out of gas.

There was a time when coaches withheld water from athletes during training to “toughen them up.” Now we know better. Allowing athletes to become dehydrated in training does nothing except harm their performance. There is, however, new evidence that the old-school logic of toughening up athletes through deprivation may be valid when applied to carbohydrate intake in training.

Recent studies indicate that while consuming carbohydrate during exercise does enhance immediate performance, it can also inhibit some of the body’s beneficial adaptations to training. By contrast, exercising in a carbohydrate-depleted state, which makes workouts tougher for the body, stimulates a stronger fitness-boosting physiological response.

The Evidence

In a 2010 study, researchers from McMaster University showed that, compared to exercise in a state of normal carbohydrate fueling, exercise in a carbohydrate-depleted state might increase the capacity of the muscle cells to burn fuels aerobically by boosting the production of mitochondria. The mitochondria are tiny organelles within muscle cells where aerobic metabolism occurs. Increased mitochondrial density is one of the most important performance-enhancing effects of aerobic exercise.

As part of this new study, 10 men performed a workout consisting of 4 x 5 minutes at high intensity on stationary bikes. They completed this workout twice on the same day with three hours of recovery between sessions. One week later, this double-workout protocol was repeated. In the first trial, half the men were given a high carbohydrate drink after the morning workout, while the others were given a zero-calorie placebo drink that ensured the men were in a carbohydrate-depleted state for the second workout. In the second trial, the drinks were switched.

The authors of the study found increased levels of a compound known as p38 MAPK that is involved in mitochondrial biogenesis following the afternoon workouts that were performed in a carbohydrate-depleted state. This finding suggests that p38 MAPK is a nutrient-sensitive signaling molecule that is most active when exercise is performed in a carbohydrate-depleted state.

Further Evidence

Other research has shown that exercise in a carbohydrate-depleted state is beneficial in another way. Interleukin-6 (IL-6) is an immune system agent and large amounts of it are released into the bloodstream by the muscles and the brain during exercise.  IL-6 is believed to facilitate many of the body’s post-workout fitness gains, ranging from increased fat burning capacity to greater resistance to muscle damage.

The primary trigger for IL-6 release during exercise is glycogen (i.e. muscle carbohydrate) depletion. So it follows that training in a glycogen-depleted state will tend to produce stronger training adaptations than training in a glycogen-replete state. Studies have shown that the muscles produce much less IL-6 when carbohydrate is consumed during exercise.

Intentional Under-Fueling and You

Despite the findings of these studies, it would be overreaching to conclude that athletes should consistently avoid consuming carbohydrate during and immediately after workouts. While intentional under-fueling may have the benefit of boosting the body’s fitness response to workouts, there are other benefits that come from consuming carbohydrate during and after workouts—namely, better performance and faster recovery. So, until more research is performed to determine the optimal balance of normally fueled and carbohydrate-depleted workouts, it is probably best only to pick certain workouts to perform in a carbohydrate-depleted state, either by not taking in carbohydrate during them or by not taking in carbohydrate immediately afterward, or both.