Carbohydrates are the premier fuel source for athletic activity providing energy at a much higher rate than fat. However, unlike fat our body has a limited supply of carbohydrates and when this supply is eliminated we “bonk” or “hit the wall”. During exercise your body relies on two sources for carbohydrate: stored carbohydrate in the form of glycogen and blood glucose which can be augmented with ingestion of carbohydrate. Sports drinks, energy bars and gels are extremely popular with endurance athletes. They provide carbohydrates, which are converted to blood glucose while exercising. A glycogen replacement formula consumed immediately after exercise and eating a diet high in complex carbohydrates maximizes glycogen storage. If you want to perform at your potential it is important to make sure that you are mindful of both these factors. But, which source is more important to athletic performance?
The hype generated by sports drink and gel manufacturers may lead one to believe that sucking these products down while exercising are the best means to fuel your body. However, this simply isn’t the case. Your level of glycogen in the muscles before the start of exercise is the most important fuel determinant of performance. In other words if you don’t have an adequate supply of stored glycogen before you start exercise, drinking all the sports drinks in the world will not allow you to perform at your potential. You can not fully compensate for sub-optimal levels of muscle glycogen by taking carbohydrates on the run. In fact, studies have shown that an athlete will perform better if they posses a higher level of muscle glycogen and drink nothing but water than when their glycogen stores are low and they consume carbohydrates throughout an event.
The first factor that limits the effectiveness of sports drinks deals with a bit of biochemistry. When carbohydrates are consumed during exercise they are absorbed through the gut and enter the blood as glucose. Unfortunately, glucose has to undergo a process called phosphorylation before it can be utilized for energy. The enzyme responsible for this process has a relatively low level of activity and can only process a limited supply of glucose for energy at one time. However, glycogen stored in the muscles can be used directly for energy. Muscle glycogen can be used more quickly and efficiently than any other fuel source, faster than glucose and much faster than fat.
A second factor is the limited rate of absorption of carbohydrates through the intestines. Hydration is even more important than consumption of carbohydrates during exercise. Dehydration is more detrimental to your performance than running out of carbohydrates. Therefore, it is imperative to consume liquids throughout training and events. If a sports drink solution is greater than 10% carbohydrate the rate of absorption through the gut plummets and interferes with hydration. Drinking a quart of 10% solution an hour (about as much as you can hope to consume) yields around 70 grams of carbohydrate, a substantial amount of energy, but not enough in itself to fuel your performance. Also, don’t consume carbohydrates in solid form during exercise. Not only can this lead to gastric distress but it also depletes your water supplies.
My point is not to undermine the value of carbohydrate consumption during exercise or take away from the use of sports drinks and gels. On the contrary, these supplements serve an important role in providing energy, reducing the rate of glycogen depletion, and increasing the time until exhaustion. However, I do believe the hype surrounding sports drinks has overshadowed the importance of maximizing glycogen stores. The only way to optimize glycogen stores is to consistently eat a diet rich in complex carbohydrates and quickly replenish lost glycogen after a workout. So, to perform at your potential and get the most from your training; be mindful of your glycogen supply, your most important fuel source during exercise.