Carbohydrate Recommendations After Exercise
Many athletes realize the value of protein consumption postworkout; but often underappreciated, however, is the importance of consuming adequate carbohydrate at this time as well. Short-duration, intense exercise can significantly lower muscle glycogen stores, and exhaustive endurance exercise can result in near depletion (Beelen et al. 2010). Because of the potential performance impairment of exercising with low glycogen stores, as well as the associated fatigue, it is important that athletes replete their stores to optimize their training capacity.
There are two distinct phases of muscle glycogen synthesis postexercise. The first phase is characterized by rapid glycogen synthesis within the first 30 to 60 minutes, and operates independent of insulin. As described previously, exercise stimulates the translocation of the glucose transporter GLUT4 to the cell membrane and causes blood glucose to be taken up into the cell. This GLUT4 translocation, as well as the large concentration gradient of high glucose outside of the cell to low glucose inside of the cell, is responsible for the increased rate of glycogen synthesis in this first phase. This initial, rapid phase of glycogen synthesis only occurs in the presence of very low glycogen stores (such as the result of exhaustive exercise), when stores are 25 percent or lower than baseline levels (Price et al. 1994). While the rate of synthesis can be very high in this phase, if glucose is not available in sufficient quantities in the blood, resynthesis will quickly stall out in an effort to maintain blood glucose levels.
The second phase of muscle glycogen synthesis is insulin-dependent. The rate of synthesis is not as rapid as in the initial phase, though the second phase is still an important stimulator of muscle glycogen synthesis and is not reliant upon severe depletion of glycogen stores. Exercise increases the cell’s sensitivity to insulin, and so postexercise glucose uptake and glycogen synthesis are increased (Beelen et al. 2010). Thus, nutrition intervention that enhances the effects of insulin, such as eating carbohydrate-containing foods, is essential during this second phase of muscle glycogen synthesis.
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Both phases of glycogen synthesis are reliant upon provision of adequate dietary carbohydrate postexercise. Various amounts of carbohydrate have been studied, and research indicates that consuming 1.2 g of carbohydrate per kilogram of body weight per hour (1.2 g/kg/hr) up to several hours after exercise seems to be the optimal amount to maximize muscle glycogen synthesis (Poole et al. 2010). Total synthesis can be optimized when carbohydrate is provided over several small-to-moderate feedings at regular intervals, versus consuming one large carbohydrate meal in the same time frame. For example, an athlete can consume a carbohydrate snack every 15 to 30 minutes for up to 4 hours that amounts to 1.2 g/kg/hr for optimal glycogen synthesis.
However, consuming this amount is not always feasible for athletes due to logistic or practical constraints or both. Coingestion of protein with carbohydrate has been shown to be beneficial for muscle glycogen resynthesis and may decrease the amount of carbohydrate needed. Protein ingestion stimulates insulin release and research indicates that when carbohydrate intake is suboptimal postexercise (<1.0 g/kg/hr), coingestion of protein and amino acids (~0.4 g/kg/hr) can support maximal glycogen synthesis; however, no additional benefit was seen when carbohydrate intake was optimal (1.2 g/kg/hr). While additional functions of protein and protein resynthesis will be discussed in the next section, protein does help athletes maximize muscle glycogen resynthesis when postexercise carbohydrate intake is less than optimal. Additionally, there are advantages for the athlete looking to lose or maintain body weight by consuming slightly less carbohydrate in the presence of adequate protein, which will be discussed in Chapter 8.
Timing of carbohydrate consumption is an important consideration. The aforementioned recommendations are provided for maximal muscle glycogen synthesis at a rapid rate. This maximal glycogen synthesis rate is especially important in situations when athletes have less than 8 hours before their next workout, such as in the case of two-a-day practices or tournament-style competitions. However, the timing of carbohydrate intake appears to be less important when there are 24 hours to recover before the next exercise bout. In these situations, it is more about the total carbohydrate intake over the course of 24 hours versus the intake immediately after and up to 4 hours after exercise. Knowing an athlete’s exercise and competition schedule is essential for determining optimal recovery nutrition strategies. Similar recommendations apply to the type of carbohydrate intake postexercise; if there are less than 8 hours between exercise bouts, the type of carbohydrate may be important. It appears that high glycemic index foods may promote faster glycogen synthesis rates (Poole et al. 2010) than low glycemic index foods. However, when athletes have more than 8 hours to recover, the type of carbohydrate may be less important. It also appears that liquid form versus solid food form does not affect muscle glycogen synthesis, though there are other practical considerations that will be discussed at the end of the section in “Putting it all together.”
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