Coggan A R, Coyle E F
Exerc Sport Sci Rev. 1991;19:1-40.
It is well recognized that energy from CHO oxidation is required to perform prolonged strenuous (greater than 60% VO2 max) exercise. During the past 25 years, the concept has developed that muscle glycogen is the predominant source of CHO energy for strenuous exercise; as a result, the potential energy contribution of blood glucose has been somewhat overlooked. Although during the first hour of exercise at 70-75% VO2max, most of the CHO energy is derived from muscle glycogen, it is clear that the contribution of muscle glycogen decreases over time as muscle glycogen stores become depleted, and that blood glucose uptake and oxidation increase progressively to maintain CHO oxidation (Fig. 1.7). We theorize that over the course of several hours of strenuous exercise (i.e., 3-4 h), blood glucose and muscle glycogen contribute equal amounts of CHO energy, making blood glucose at least as important as muscle glycogen as a CHO source. During the latter stages of exercise, blood glucose can potentially provide all of the CHO energy needed to support exercise at 70-75% VO2max if blood glucose availability is maintained. During prolonged exercise in the fasted state, however, blood glucose concentration often decreases owing to depletion of liver glycogen stores. This relative hypoglycemia, although only occasionally severe enough to result in fatigue from neuroglucopenia, causes fatigue by limiting blood glucose (and therefore total CHO) oxidation. The primary purpose of CHO ingestion during continuous strenuous exercise is to maintain blood glucose concentration and thus CHO oxidation and exercise tolerance during the latter stages of prolonged exercise. CHO feeding throughout continuous exercise does not alter muscle glycogen use. It appears that blood glucose must be supplemented at a rate of approximately 1 g/min late in exercise. Feeding sufficient amounts of CHO 30 minutes before fatigue is as effective as ingesting CHO throughout exercise in maintaining blood glucose availability and CHO oxidation late in exercise. Most persons should not wait, however, until they are fatigued before ingesting CHO, because it appears that glucose entry into the blood does not occur rapidly enough at this time. It also may be advantageous to ingest CHO throughout intermittent or low-intensity exercise rather than toward the end of exercise because of the potential for glycogen synthesis in resting muscle fibers. Finally, CHO ingestion during prolonged strenuous exercise delays by approximately 45 minutes but does not prevent fatigue, suggesting that factors other than CHO availability eventually cause fatigue.
人们普遍认识到，进行长时间剧烈运动（大于最大摄氧量的60%）需要来自碳水化合物氧化的能量。在过去25年里，形成了这样一种观念，即肌肉糖原是剧烈运动中碳水化合物能量的主要来源；因此，血糖的潜在能量贡献在一定程度上被忽视了。虽然在以70 - 75%最大摄氧量进行运动的最初一小时内，大部分碳水化合物能量来自肌肉糖原，但很明显，随着肌肉糖原储备的消耗，肌肉糖原的贡献会随着时间的推移而减少，而血糖的摄取和氧化会逐渐增加以维持碳水化合物的氧化（图1.7）。我们推测，在数小时的剧烈运动过程中（即3 - 4小时），血糖和肌肉糖原提供等量的碳水化合物能量，这使得血糖作为碳水化合物来源至少与肌肉糖原一样重要。在运动后期，如果血糖供应得以维持，血糖有可能提供支持以70 - 75%最大摄氧量进行运动所需的所有碳水化合物能量。然而，在空腹状态下进行长时间运动时，由于肝糖原储备的消耗，血糖浓度常常会降低。这种相对低血糖，虽然只是偶尔严重到足以因神经低血糖症导致疲劳，但通过限制血糖（进而限制总碳水化合物）的氧化而导致疲劳。在持续剧烈运动期间摄入碳水化合物的主要目的是维持血糖浓度，从而在长时间运动后期维持碳水化合物的氧化和运动耐力。在整个持续运动过程中摄入碳水化合物不会改变肌肉糖原的利用情况。似乎在运动后期必须以大约每分钟1克的速度补充血糖。在疲劳前30分钟摄入足够量的碳水化合物与在整个运动过程中摄入碳水化合物在维持运动后期血糖供应和碳水化合物氧化方面效果相同。然而，大多数人不应等到疲劳才摄入碳水化合物，因为此时葡萄糖进入血液的速度似乎不够快。在间歇或低强度运动过程中全程摄入碳水化合物也可能是有利的，而不是在运动接近尾声时摄入，因为静止的肌纤维有糖原合成的潜力。最后，在长时间剧烈运动期间摄入碳水化合物会使疲劳延迟约45分钟，但并不能防止疲劳，这表明除了碳水化合物供应之外的其他因素最终会导致疲劳。
