Distance runners and other endurance athletes often suffer from impaired immunity from overtraining, which results in an increased incidence of colds and flu.  A British study from Bangor University showed that consuming carbohydrates after intense endurance exercise prevented the breakdown of neutrophil degranulation, which is important for the destruction of bacteria that can cause illness.  The researchers speculated that carbohydrate feeding improved immune function by increasing insulin, which plays a role in immunity.  (International Journal Sports Nutrition and Exercise Metabolism, 21: 205-213, 2011)