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The importance of carbohydrate for endurance athletes dates back almost 100 years when marathoners were shown to have low blood glucose levels at the end of the 1924 Boston marathon.
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The following year carbohydrate feeding was shown to prevent the decline in blood glucose and improve performance. Subsequent research led to the understanding of the importance of muscle glycogen, the storage form of carbohydrate in muscles, as a fuel for active muscle during prolonged exercise and made the connection between glycogen depletion and fatigue. Researchers discovered that manipulating an athlete's training combined with a high carbohydrate diet several days prior to exercise ( carbohydrate loading) significantly increased muscle glycogen levels and delayed fatigue. Subsequent work throughout the 1970s and 80s continued to show that carbohydrate feeding could enhance endurance performance. Whether or not protein is important for endurance athletes remains less clear, but recent evidence shows it may be important. Researchers examined whether a carbohydrate beverage with added protein (carb-protein) would have any performance benefit compared to a matched carbohydrate beverage (carb), a matched carbohydrate beverage with additional carbs to match the calories provided by protein ( carb-carb), and a placebo (water). Trained cyclists performed four exercise tests where they were asked to ride to exhaustion using one of the 4 beverages. They cycled the longest with the carb-protein beverage, with the next best performance in the carb-carb trial followed by the carb and water trials. Interestingly, the carb-protein trial also had lower levels of muscle damage after exercise and increased muscle strength 24 hours after exercise. These findings support the idea that adding protein to carbohydrate is important for endurance athletes to increase performance and enhance recovery.

Valentine RJ, Saunders MJ, Todd MK, St Laurent TG. Influence of carbohydrate-protein beverage on cycling endurance and indices of muscle disruption. Int J Sport Nutr Exerc Metab. 2008 Aug;18(4):363-78.