The metabolic stress resulting from a training bout or competition is closely associated with the intensity, volume, and type of exercise; the training and nutritional status of the athlete; and environmental factors (37). In terms of metabolic stress, the glycogenolytic effects of exercise are of particular interest. Muscle glycogen stores can be significantly affected by acute bouts of endurance exercise, intermittent exercise, and resistance exercise (31, 54, 164). When dietary carbohydrate intake is inadequate and frequent training is undertaken, muscle glycogen stores will not be replenished, which will result in glycogen depletion (33, 54). Muscle glycogen depletion will be accompanied by a progressive decrease in performance (21, 54). A reduction in muscle glycogen also can alter cell signaling, which can negatively influence cellular growth and adaptations (39). If chronic glycogen depletion occurs, the athlete will experience overtraining attributable to an inability to meet the energy demands of training (142).
Several nutritional strategies can be used to restore muscle glycogen stores and enhance muscular adaptations. Ivy and Portman (76) presented a model of nutrient timing that is designed to optimize performance and muscular adaptations. In this model, the athlete follows a combination of dietary interventions before, during, and after exercise to maximize muscle glycogen storage.
Preexercise Supplementation The first dietary supplementation occurs before exercise. The pretraining meal or supplement will increase muscle glycogen stores if they are not fully restored, increase liver glycogen content, ensure that the athlete is well hydrated particularly when liquid sources are used, and prevent hunger (22). Burke and Deakin (22) suggested that athletes consume 1 to 4 g CHO × kg-1 × body weight-1 1 to 4 hr before exercise begins, especially if the exercise bout is prolonged.
Supplementation During Exercise Another strategy suggested by Ivy and Portman (76) is to consume a carbohydrate and protein beverage within 30 min of initiating exercise and then periodically throughout the exercise bout. This supplementation regime has been suggested to increase the postexercise recovery rate as a result of an increased anabolic hormone response during both resistance and endurance exercise (7, 25, 98, 109). This type of supplementation regime also has been demonstrated to result in a greater postexercise insulin and growth hormone response (25, 153), a decrease in muscle protein breakdown in conjunction with an increased postexercise muscle protein synthesis rate (122, 134), and a reduction in postexercise muscle damage and soreness (7). Additionally, this supplementation has been reported to increase exercise capacity, thus potentially enhancing the adaptive stimulus of the exercise bout (77). Ivy and Portman (76) suggested that this beverage should contain a 4:1 ratio of carbohydrate to protein. Therefore, if the athlete consumed 25 g of carbohydrate, he would concomitantly consume about 6 g of protein.
Postexercise Supplementation The focus of postexercise supplementation is to promote glycogen resynthesis and stimulation of protein synthesis. Two important aspects of postexercise dietary supplementation are the dietary content and timing of supplementation (21, 74).
The amount of carbohydrate consumed after exercise is directly related to the amount of muscle glycogen synthesized (13, 35, 75). Approximately 1.0 to 1.85 g CHO × kg-1 × body weight-1 × h-1 consumed immediately after exercise appears to maximize muscle glycogen synthesis (21, 78). If less carbohydrate is consumed (0.8 g × kg-1 × hr-1), then the addition of 0.4 g of whey protein hydrolysate plus free leucine and phenylalanine per kilogram of body weight per hour can stimulate greater glycogen synthesis (161). Conversely, the addition of amino acids and proteins does not appear to increase the rate of glycogen synthesis when high amounts of carbohydrate (>1.2 g × kg-1 × hr-1) are consumed (78). The addition of protein does offer some benefits by stimulating an increase in circulating insulin levels (160). Increases in insulin levels have been associated with an increase in the uptake of amino acids, a stimulation of muscle protein synthesis, a reduction in muscle protein breakdown, and an increase in protein balance (9, 10). Thus, it can be recommended that athletes consume a carbohydrate and protein beverage to stimulate muscle glycogen and increase protein synthesis rates.