Carbohydrate Recommendations Before Exercise
Numerous research studies have supported the performance benefits of consuming adequate preexercise carbohydrate (Ormsbee, Bach, and Baur 2014). Nutrition strategies that focus on optimizing glycogen stores, or “topping off” the gas tanks, ensure an adequate fuel supply to working muscle. Total body glycogen stores can typically be normalized within a 24-hour period (Burke et al. 2011), although overnight, liver glycogen stores can be depleted by up to 80 percent. Much of the overnight depletion comes from liver glycogen, with muscle glycogen stores experiencing minimal impact (Ormsbee, Bach, and Baur 2014). The period leading up to exercise that is, up to 4 hours can be vital to ensure adequate muscle and liver glycogen.
It is recommended that athletes engaging in endurance or intense exercise, or high-volume resistance exercise, consume 1 to 4 g of carbohydrate per kilogram of body weight (g/kg) 1 to 4 hours before exercise commences. That is, athletes should consume 1 g/kg/body weight 1 hour before exercise, 2 g/kg/body weight 2 hours before exercise, 3 g/kg/body weight 3 hours before exercise, or 4 g/kg/body weight 4 hours before exercise. Research does not show differences in performance when carbohydrate is consumed at different points within this window (4 hours before, for example, versus 1 hour before) as long as the appropriate amount is consumed according to the time before exercise. So, practicalities of eating as well as the ability to tolerate food before exercise may dictate when is best to eat. Some athletes can consume a moderate meal containing carbohydrate an hour before exercise; most individuals, however, find it best to give themselves 2 to 4 hours for their pre-event meal to allow for digestion to occur. As long as they are consuming adequate amounts of carbohydrate (1 to 4 g) within 1 to 4 hours, they should have sufficient glycogen stores.
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Consuming carbohydrates causes an increase in blood glucose, which, in turn, causes the pancreas to release insulin into the blood. One to four hours is probably sufficient time for blood glucose and insulin levels to return to normal; however, less than 1 hour may not be. Eating less than an hour prior to exercise can be problematic for some individuals. In addition to GI distress, some athletes can have adverse drops in blood glucose related to the increase in insulin levels resulting from the recent consumption of carbohydrate-containing foods. Exercise also initiates an insulin-independent mechanism of clearing glucose out of the blood and into the cell by increasing GLUT4 translocation. The net effect of increased insulin levels and exercise-induced GLUT4 translocation is that glucose can be cleared out of the blood too quickly and can result in exercise-induced hypoglycemia. While these perturbations in blood glucose levels are transient and return to normal within about 20 minutes, athletes can feel shaky, queasy, and light-headed in the meantime. To avoid this negative reaction it is important to allow 1 hour or more for digestion of carbohydrate-containing foods prior to exercise in individuals experiencing these symptoms (Ormsbee, Bach, and Baur 2014).
While glycemic index may have limited utility for athletes overall, one exception is that it may help diminish the likelihood of exercise-induced hypoglycemia. The glycemic index measures the blood glucose response to carbohydrate-containing foods and consuming low glycemic index carbohydrates may result in an attenuated blood insulin and glucose response. Because blood glucose levels do not rapidly increase after eating a low-glycemic index food, less insulin is released and exercise-induced hypoglycemia is less likely. Ingesting CHO prior to exercise has also been shown to suppress fatty acid oxidation, but some research suggests that choosing low-glycemic index foods limits the effect. However, carbohydrate that is consumed during exercise will impact blood glucose levels as well as fuel substrate utilization, thus minimizing the effects of the glycemic characteristics of the pre-event food (Burke et al. 2011). This indicates that selecting low-glycemic index foods prior to exercise in efforts to affect metabolism and substrate utilization during exercise is only effective if carbohydrate is not consumed during exercise. Research on this topic is equivocal in that some studies report an endurance performance benefit of selecting low-glycemic index foods, but most do not. Based on the available research, strategies that focus on using the glycemic index to consume low-glycemic index foods prior to exercise may best suited for individuals who do not consume carbohydrate during an event or those who are more susceptible to exercise-induced hypoglycemia, since at the very least there does not appear to be a disadvantage of consuming low glycemic index foods (Burke 2010; Ormsbee, Bach, and Baur 2014). More research is needed to provide generalized recommendations regarding glycemic index.
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