From Wikipedia, the free encyclopedia:
“GU is an energy gel designed to be quickly and easily digestible so it can be eaten during endurance events, especially long distance running.”
From the GU website: “We at GU agree that a product solely based on simple sugars is not ideal. But we also know that simple sugars, like fructose, have an important place in carbohydrate replacement during exercise. That is why Electrolyte Brew, GU, Roctane, and Chomps are made of a mix of maltodextrin and fructose. Generalization that all simple sugars are evil is misleading. Maltodextrin and galactose are transported from the gut to the blood by a protein transporter. This transporter is the Na+/glucose co-transporter 1 or SGLT1 (Shirazi-Beechey, 1990) and like all transport proteins, can become saturated at high concentrations of glucose, becoming a bottleneck for energy absorption. Net takeaway: There is a limit to how much and how fast glucose can be absorbed, and therefore utilized by working muscles using only one type of carbohydrate transporter.
Fructose, a simple sugar, is transported using a totally different transporter called GLUT5 (Davidson, 1992). The benefit here is that now you can add a simple sugar to a drink containing maltodextrin (glucose), and not be limited by only one carbohydrate transporter. Using more than one carbohydrate transporter allows a greater rate of carbohydrate uptake compared to only one type of carbohydrate at a time. Studies have been done adding fructose to a maltodextrin drink, and found the addition of fructose increased total carbohydrate transport and, ultimately oxidation (Jentjiens, 2004, Wallis, 2005). Peak rates of carbohydrate oxidation using maltodextrin alone were approximately 1.1 g/min, which increased to 1.5 g/min with the addition of fructose (Wallis, 2005). More carbohydrate oxidation using fructose + maltodextrin (36% in this case) means better sparing of glycogen stores. It also allows an athlete to exercise at a higher percent of VO2max once those precious glycogen stores have been depleted and they are reliant on exogenous carbohydrate (Smith, 2010).
Net takeaway: Varying the types of sugar (and thus carbohydrate transporters) improves the rate at which the carbohydrates are used by working muscles.
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