Among studies on the various aspects of nutrient timing, postexercise investigations constitute an overwhelming majority of the scientific literature. Collective findings from these studies have provided and continue to provide insight into how nutritional strategies can optimize specific aspects of the recovery process. Throughout this chapter and others, the importance of maintaining maximal glycogen levels is evident. Also, much research interest exists in determining the extent to which providing nutrients in the postexercise period will affect muscle protein balance. Finally, several studies have used prolonged resistance training programs with various nutrient timing strategies to determine the changes in resistance training adaptations such as improved strength and power as well as body composition parameters.
The recovery and maintenance of optimal levels of muscle glycogen are a key consideration for almost any type of athlete. An extremely consistent finding in the literature is that athletes who ingest 1.5 g carbohydrate per kilogram body weight within 30 minutes after exercise experience greater muscle glycogen resynthesis than when carbohydrate is delayed by 2 hours (Ivy 1998). While many studies continue to explore the mechanisms associated with these increases, research has revealed that exercise leads to an increase in sensitivity to the hormone insulin, which increases markedly after carbohydrate ingestion (Ivy 1998). Multiple studies have agreed that solid or liquid forms of carbohydrate yield a similar result (Keizer, Kuipers, and Van Kranenburg 1987; Reed et al. 1989; Tarnopolsky et al. 2005). High levels of fructose ingestion are not advised because this form of carbohydrate is associated with lower levels of glycogen resynthesis than other forms of simple carbohydrate (Conlee, Lawler, and Ross 1987). An important consideration, which is highlighted in figure 9.5, is that delaying carbohydrate ingestion by as little as 2 hours can reduce muscle glycogen resynthesis by 50% (Ivy 1998).
If glycogen depletion occurs, which typically results from prolonged-duration (>90 minutes) moderate-intensity exercise (65% to 85% V.O2max) but can also result from shorter durations of higher intensity or situations in which the athlete began the workout with less than maximal muscle glycogen levels, an aggressive regimen of carbohydrate administration is necessary. A carbohydrate intake of 0.6 to 1.0 g/kg body weight per hour during the first 30 minutes, and again every 2 hours for the next 4 to 6 hours, can adequately replace glycogen stores (Jentjens and Jeukendrup 2003; Jentjens et al. 2001). A 165-pound (75-kg) athlete, for example, should ingest 45 to 75 g carbohydrate within 30 minutes of completing exercise and an identical dose every 2 hours after exercise for the next 4 to 6 hours.
Additional strategies have been investigated, and a slightly more aggressive approach has shown that maximal glycogen resynthesis rates can be achieved if 1.2 g carbohydrate per kilogram body weight per hour is consumed every 30 minutes over a period of 3.5 hours (Jentjens and Jeukendrup 2003; Van Loon et al. 2000). Consequently, the recommendation is that athletes have frequent feedings of carbohydrate in high amounts over the 4 to 6 hours after exercise to ensure recovery of muscle and liver glycogen (Jeukendrup, Jentjens, and Moseley 2005; Tarnopolsky et al. 2005).
An important consideration connected with these studies, however, is the practicality associated with the athlete’s immediate need to recover. For example, if an athlete is participating in a sporting activity that requires a follow-up performance within this 2- to 4-hour time period (e.g., track and field and swimming athletes participating in multiple events that often have preliminary heats and semifinal and final heats), then findings from these studies are of the utmost importance. If, however, the athlete does not need to recover in less than 4 hours, other studies illustrate that eating high-carbohydrate meals and snacks at regular intervals can also result in maximal muscle glycogen levels. Research has shown that maximal glycogen levels are restored within 24 hours if optimal levels of dietary carbohydrate are available (typically around 8 g carbohydrate per kilogram body weight per day) and the degree of glycogen depletion is not too severe (Keizer, Kuipers, and Van Kranenburg 1987). Another study suggested a carbohydrate intake of 9 to 10 g/kg body weight per day for athletes completing intense exercise bouts on consecutive days (Nicholas, Green, and Hawkins 1997). Also, it is possible that providing energy in the form of carbohydrate may help to alter inflammatory or proteolytic (breakdown of protein) cascades or other untoward events that will ultimately delay optimal recovery in an exercising athlete.
The addition of protein to carbohydrate has evolved into a dynamic area of research; studies suggest that this combination may help promote even greater recovery of muscle glycogen as well as attenuate muscle damage. Ivy and colleagues (2002) instructed cyclists to complete a 2.5-hour bout of intense cycling before ingesting (1) a low-carbohydrate + protein + fat (80 g carbohydrate + 28 g protein + 6 g fat), (2) a low-carbohydrate + fat (80 g carbohydrate + 6 g fat), or (3) a high-carbohydrate + fat (108 g carbohydrate + 6 g fat) supplement immediately after exercise and 2 hours postexercise. The aim was to determine if the carbohydrate + protein + fat combination promoted greater restoration of muscle glycogen. Glycogen levels were similar between the two carbohydrate + fat conditions (low carbohydrate and high carbohydrate), but muscle glycogen levels were significantly greater in the carbohydrate + protein + fat treatment. The authors concluded that a carbohydrate + protein + fat supplement may be more effective due to a greater insulin response (Ivy et al. 2002; Jentjens et al. 2001; Zawadzki, Yaspelkis, and Ivy 1992), but this guideline has yet to be universally accepted.
Separate studies by Berardi and colleagues (Berardi, Noreen, and Lemon 2008; Berardi et al. 2006) and Tarnopolsky and colleagues (1997) had cyclists complete exercise bouts of 60 and 90 minutes, respectively, on separate occasions before ingesting either carbohydrate + protein or carbohydrate only. Both research teams concluded that carbohydrate ingestion increased muscle glycogen compared to placebo (Berardi et al. 2006; Tarnopolsky et al. 1997). Berardi and colleagues, however, reported greater glycogen levels (Berardi et al. 2006) in addition to increased performance and work output (Berardi, Noreen, and Lemon 2008) when the carbohydrate + protein combination was consumed postexercise. Furthermore, increasing the availability of the essential amino acids, possibly the branched-chain amino acids in particular, may influence the recovery process by optimizing protein synthesis and glycogen synthesis after exercise (Borsheim et al. 2002; Ivy 1998; Ivy et al. 2002; Tarnopolsky et al. 1997; Tipton et al. 1999a; Zawadzki, Yaspelkis, and Ivy 1992). A major consideration for an athlete or coach relative to promoting optimal glycogen levels should be the time available before a subsequent training session or competition.
In summary, clear evidence exists that ingestion of carbohydrate as a single meal (1.5 g carbohydrate per kilogram body weight within 30 minutes after exercise) or as frequent feedings (0.6 to 1.2 g carbohydrate per kilogram body weight per hour every 30 to 60 minutes for up to 3 to 6 hours) can result in rapid restoration of muscle glycogen levels. Furthermore, the addition of protein to carbohydrate has been shown to result in greater glycogen resynthesis (and also greater protein synthesis), but overall the absolute amount of carbohydrate ingested is the primary factor that facilitates recovery of muscle glycogen.