Much has been written on nutrition for athletes. One of the more controversial topics has been the need for endurance athletes to incorporate carbohydrates in the form of gels or sports drinks during exercise.
As exercise continues and blood volume is lost through sweating, breathing, and gastrointestinal water usage, available circulating blood diminishes (there is less overall water in the blood, thus it is “thicker”) an endurance athlete will feel the drop in blood volume as “muscle fatigue”.
According to an article by Stacy Sims, PhD., the fructose and maltodextrin solutions in popular gels and sports drinks used by endurance athletes today can lead to a blood volume REDUCTION, not improvement, during exercise. Endurance athletes should implement nutritional strategies to promote blood volume maintenance in order to prevent a reduction in muscular power during exercise.
Look to avoid nutritional products which pull water OUT of the blood, as this perpetuates the drop in blood volume.
Gels and sports drinks contain fructose and maltodextrin which has the effect of slowing the absorption of nutrients in the intestine and can also cause also cause GI issues. In the article, Stacy says "with incomplete and slow absorption, fructose produces a hyperosmolar environment in the intestines. What this means is that there is more solute than water, causing an increased pressure, signaling fluid to be drawn into the intestines, producing the known feelings of bloating, gas, diarrhea, and general GI discomfort.