Ingesting sport drinks during exercise: does this interfer with training adaptations?

Endurance exercise training has profound effects on the structural and functional characteristics of skeletal muscle. Scientists refer to muscle's remarkable abilities to adapt to chronic exercise as plasticity. Muscle plasticity is evident by two important outcomes from endurance training, improved endurance performance and increased use of fat. The muscle's ability to use more fat is beneficial to prolonged exercise performance because it slows the rate at which muscle glycogen (a finite source or energy) is used, thus allowing the athlete more power output before fatigue. Among the specific adaptations in the muscle that allow greater use of fat are increased fatty acid transporters located in the muscle cell membrane and increased enzymes that promote fat oxidation.

We know what happens to muscle with training, but what isn't yet totally clear is how these changes are initiated. Current evidence suggests that with every bout of exercise, changes in gene expression are taking place. In this particular scenario, this means an increase in protein expression for certain proteins such as fat transporters and oxidative enzymes. Exercise provokes changes in messenger RNA which in turn increases the muscle cell's ability to produce proteins that are needed for exercise. Why an increase in fat transporters? Perhaps it is a survivial mechanism. Exercise heightens the energy state of the muscle cell, which means more energy sources and oxygen are required. From a survival point of view, if you are going to submit your muscles to this demand, the muscle will have to adapt in order to sustain such activity over and over again. It makes sense from a Darwinian perspective.

Consider the question, "What if you keep feeding the muscles glucose during exercise and by doing so suppress the use of fat?" When glucose is ingested, the muscle will prefer glucose over fat. This results in improved performance because the muscle, 1) can produce more power from glucose over fat, and 2) it's being provided another source of glucose and thus, blood glucose levels are more easily maintained. But, what if glucose ingestion during each bout of exercise interfers with gene expression that favors fat oxidation? Will this stunt muscle adaptations that ultimately lead to improved performance?

In steps Thorbjorn and colleagues from the University of Copenhagen, Denmark. They hypothesized that glucose ingestion during exercise would alter the gene expression of fatty acid transporters and enzymes. There results are published in the July 2009 issue of the Journal of Applied Physiology. To test their hypothesis, they asked 9 men to participant in a unique 10-week training program. To make each participant his own control, training consisted of 1-legged cycling. One leg was assigned the glucose trials and one leg was assigned the placebo trials. They trained one leg at a time on alternate days, 5 days per week. This meant each leg received on average 2.5 training sessions per week. They were given a 6% glucose solution (such as Gatorade) at a rate of 0.7 g/kg/hr during the glucose training sessions. Each participant performed a fatiguing test and performance was measured as length of time. The investigators also measured fat metabolism.

What they found:

• Training improved maximal power and time to fatigue, no difference between glucose and placebo
• Training increased fat oxidation during exercise, no difference between glucose and placebo

The investigator's hypothesis that glucose ingestion during training exercise would alter fat adaptations that normally occur with training was not supported by these results. Noteworthy is that training sessions did not differ between glucose and placebo, meaning each leg received the same training stimuli. This is good to know because it is possible that not ingesting glucose under certain circumstances would reduce training power output, thus reducing the training stimulus. Theoretically, because of this, not ingesting glucose could negatively affect training adaptations. But because there was no difference between trials in terms of training stimulus, this confounding effect seems to not be an issue here.

Bottomline: Keep drinking your Gatorade, your muscles will adapt just as well. And for athletes who train several hours daily, not consuming glucose during exercise will likely have a greater negative impact. Despite carbohydrate ingestion, athlete's muscles are trained to use more fat, which is one reason for their awesome endurance.

Reference: Thorbjorn et al. Glucose ingestion during endurance training does not alter adaptation. J. appl. Physiol. 106, 2009.