Sports Fitness Training

Pre-Exercise Carbohydrate Intake

Adequate carbohydrate intake is especially important before endurance training or competition. Given that many endurance events last 2 hours or longer, and that glycogen stores are sufficient for approximately 2 hours of intensive exercise, pre-exercise carbohydrate feedings carry utmost significance. When properly ingested before exercise, carbohydrates have been shown to improve performance especially when combined with during ­exercise feedings. Additionally, ingesting carbohydrates during these two critical time periods also induces a glycogen-sparing effect which prolongs intensive exercise.

The goals of pre-training nutrition are to maximize glycogen stores, avoid a significant increase in plasma insulin concentrations (resulting in a hypoglycemic state), and avoid gastrointestinal upset during exercise. The pre-exercise period can be defined as the 4- hour period before an activity and can be further delineated into two distinct phases: 2 to 4 hours before and 30 to 60 minutes prior to exercise.

The 2- to 4-hour period preceding exercise takes on special significance in cases in which glycogen concentrations may be depleted as a result of intense exercise or insufficient carbohydrate ingestion in the days before an event. In these instances, the intake of a sizable amount of carbohydrates, approximately 300 g, 4 hours before exercise has been shown to increase work output and performance. Similar findings, along the same lines of enhanced endurance and work output, have also been reported when greater than 200 g CHO were consumed 3 to 4 hours before endurance-type exercise Therefore, research indicates that athletes should ingest 200 to 300 g CHO in the 2 to 4 hours preceding exercise. The case can even be made for a greater intake in special circumstances (before significantly long endurance events, following a fasted state, and following successive days of intense endurance exercise).

To promote sustained carbohydrate availability throughout an exercise session, the 30 to 60 minutes before an event is significant not only in that a sufficient intake must be achieved, but the proper type of carbohydrate must also be consumed. In light of this importance, properly manipulating the glycemic index of this meal may optimize carbohydrate availability for ensuing exercise. Because a potential disadvantage of pre-exercise carbohydrate intake is an exaggerated insulin response, which suppresses fat metabolism and curtails available glucose for subsequent exercise, the glycemic index becomes an indispensable tool. For example, Foster et al. reported that feeding 75 g of glucose (a high-glycemic carbohydrate) 30 minutes before exercise impaired the resulting cycle time to exhaustion.

In contrast, Thomas and colleagues noted that consuming 1 g CHO/kg body weight of a low-GI food (lentils) 1 hour before endurance exercise prolonged work capacity when compared with the ingestion of an equal amount of a high-glycemic food (potatoes). The authors suggested that this performance increase was attributable to glycogen sparing following the consumption of low-glycemic carbohydrates. Thus, according to this study, low-GI meals appear to elicit a sustained source of energy throughout exercise and recovery However, other studies have expressed no differences in performance following ingestion of low- or high-glycemic foods, although the investigators did find that high-GI meals caused a decline in blood glucose concentration before exercise. Perhaps these differences in glucose concentration during the early phases of exercise are short-lived and do not negatively affect the performance of most athletes. Further research is needed in this area, but it is clear that low-GI foods definitely promote a more favorable metabolic environment for exercise.

Regardless of glycemic index, it is evident that adequate glycogen stores before exercise is the predominant nutritional factor in subsequent performance. In the 30­to 60-minute window preceding exercise, most studies suggest that 60 to 75 g of carbohydrates consumed in this period are sufficient to top off glycogen stores and improve performance One should make sure that these feedings do not cause excessive fullness or gastrointestinal upset, however, which can actually impair exercise performance.

Carbohydrate Intake During Exercise

The primary goals of carbohydrate fee dings during exercise are to maintain glucose levels to sustain energy and work output, and spare liver glycogen. When ingested in sufficient quantity, elevated carbohydrate availability during endurance exercise can improve work capacity in events lasting more than 1 hour Also, the manner in which carbohydrates are ingested during exercise, specifically in terms of feeding schedule and composition (solid or liquid), is an important performance-related variable.

With regard to quantity, research indicates that the maximal oxidation rate for exogenous carbohydrate is approximately 60 g/hr It is this finding that gives rise to the common belief that carbohydrates ingested at a rate of 45 to 75 g/hr are sufficient to improve exercise performance. Accordingly, various types of carbohydrates, including glucose, sucrose, and maltodextrins, are all effective choices to make to enhance training and competition. In fact, the glycemic index does not appear to elicit any physiologically important differences. Ingesting fructose, however, because it is slowly absorbed, is not advised during exercise because of the increased risk of gastrointestinal distress.

Although the effect of feeding frequency does not seem to influence subsequent carbohydrate oxidation during endurance exercise, withholding carbohydrates until late in an exercise bout will impair performance Similarly, this points to the necessity of ingesting carbohydrates throughout exercise not only to sustain energy, but to avoid possible gastrointestinal disturbance. Correspondingly, consuming carbohydrates in liquid or solid form elicits similar effects, although liquid solutions provide the added benefit of maintaining hydration. To coincide with the 45 to 75 g/hr recommendation, this would typically involve ingesting approximately 1 liter of sports drink (carbohydrate-electrolyte solution) per hour. Fortunately, these solutions playa dual role of providing both adequate energy and hydration without causing excessive fullness.

Post-Exercise Carbohydrate Intake

The primary objective of ingesting carbohydrates following exercise is to replenish glycogen stores for subsequent exercise bouts. In doing so, overtraining is prevented and adequate energy is again available for forthcoming exercise. It was previously thought that full restoration of muscle glycogen stores takes approximately 48 hours. If performed properly, however, research now indicates that this process can be accomplished in approximately 20 hours.

Research also confirms that glycogen restoration occurs most rapidly when carbohydrates are consumed immediately post-exercise In fact, when carbohydrate feedings ensue immediately following exercise, the rate of muscle glycogen storage is almost twice that of when they are consumed after a 2-hour delay, However, because full glycogen restoration can still be achieved via adequate intake (regardless of timing), these results indicate the importance of immediate feedings only when further exercise is imminent. Therefore, this strategy is most important when only 4 to 8 hours of recovery pass between exercise sessions. As the time for recovery increases (longer than 24 hours), the importance of carbohydrate quantity takes precedence over feeding frequency or timing. Interestingly, the frequency of feedings does not significantly alter muscle glycogen restoration rates as long as the daily carbohydrate needs of endurance athletes (7-10 g/kg body weight) are met.

The optimal rate of glycogen storage appears to be achieved when at least 0.7-1.0 g CHOlkg body weight is consumed every 2 hours in the early stages of recovery, leading to a daily total of 7 to 10 g/kg body weight. Although this can be achieved with solid or liquid forms of carbohydrate, particular glycemic types seem to have differing effects on subsequent glycogen restoration. Glucose and sucrose fee dings have been shown to induce similar rates of glycogen recovery, whereas fructose produces a tower rate of storage. These findings have sparked suggestions that post-exercise carbohydrate intake should consist mainly of moderate- to high-glycemic foods. Given their more rapid introduction to the bloodstream, it appears that these types of carbohydrates are more beneficial in achieving swift glycogen restoration. Indeed, studies have indicated that high-glycemic carbohydrate foods consumed post-exercise promote greater glycogen storage than do an equal amount of low-GI foods. Specifically, it has been suggested that foods with a low glycemic index should not make up more than cine-third of recovery meals.