The need for carbohydrate is a foremost thought among athletes. Pasta dinners the night before games and other high-carbohydrate meals help maximize muscle glycogen stores while gels and sport drink during training or competition help fuel working muscle as well as maintain blood glucose levels (11,14). After training or competition, carbohydrate is invaluable to recover glycogen stores (39,47,60,72), maximize muscle net protein balance (75), and help fuel other repair mechanisms. Glucose is the primary carbohydrate fuel for working muscle, and its contribution to total energy increases relative to exercise intensity and decreases with prolonged duration secondary to waning muscle glycogen (62,63).
Many other factors will influence how much carbohydrate is used during exercise including an athlete's general diet (46), timing, and composition of the most recent meal (13,25,28,68) and the consumption of a sport beverage during exercise (8,12,54,56). In addition to pre- and post-exercise considerations, interest in the role of the timing and type of carbohydrate in the post-workout recovery period is growing as a means of refueling and supporting recovery efforts and maximizing skeletal muscle protein synthesis potential (39,47,58,60,71,72,75). This review will provide an overview of carbohydrate level, timing, and type as well as discussing exercise metabolism and performance with special attention to making recommendations for athletes to optimize performance.
MUSCLE GLYCOGEN AND BLOOD GLUCOSE
Muscle tissues (biopsies) taken before and after exhaustive exercise show that muscle glycogen content is reduced (4,27,36). Meanwhile, the difference between the level of glucose in the arteries supplying skeletal muscles and in the veins that drain them is greater when the muscles are engaged in physical activity (65,80). Therefore, glucose uptake complements glycogen breakdown during exercise. Furthermore, both hypoglycemia and muscle glycogen depletion have proved to be independently involved in reducing athletic performance and promoting fatigue (10,12,64).
Kavouras et al. (46) reported that endurance athletes consuming a high-carbohydrate diet (>600 g/d) while reducing training had mixed muscle glycogen levels of 104.5 ± 9.4 mmol/kg wet wt. Muscle glycogen levels tend to be 10 to 25% higher in type II vs type I muscle fibers (31,32,69), and muscle glycogen levels can be increased with moderate- to high-intensity training (3,49,50). For instance, MacDougall et al. (50) reported a 66% increase in muscle glycogen content following 5 months of weight training and that increases were demonstrated in both type I and II by 39 and 31%, respectively (51). The total amount of muscle glycogen is approximately 300 to 500 g depending on an athlete's gender, size, and training status, while liver glycogen stores range between 60 and 120 g. Meanwhile, a blood glucose concentration of 70 to 110 mg/100 mL would approximate 4 to 6 g of glucose for a total blood volume of 5.5 L or the equivalent of 16 to 24 kcal.
The forms of carbohydrate commonly found in natural foods including grains, fruits, vegetables, and dairy are starches and sugars such as glucose, fructose, lactose, and sucrose. Manufactured sport bars, shakes, gels, etc, use glucose, fructose, maltodextrin, corn syrup, high-fructose corn syrup, starches, fruit pastes, and purees (Table) with the choice based on sensory and functional properties and cost. In addition, the type of carbohydrate chosen can have different properties in the digestive tract as well as varied glycemic and insulinemic effects. Furthermore, the combination of other nutrients, particularly amino acids, peptides, and protein, brings about additional considerations for recovery, net muscle protein synthesis, and soreness.
CARBOHYDRATE METABOLISM DURING EXERCISE
It has long been known that during exercise, the utilization of carbohydrate by skeletal muscle increases (4,36). However, because free glucose is found in relatively low levels in the blood and within muscle cells, increased glucose demands during exercise must be met by glycogen breakdown (glycogenolysis) (4,36) and by an increased uptake of circulating glucose via the translocation of GluT4 transporters to the sarcolemma in an insulin-independent manner (16,29,30,59,83). Glucose is metabolized via glycolysis yielding pyruvate, which can be converted to lactic acid (lactate) or enter mitochondria for aerobic metabolism (Figure 1). Increased exercise intensity leads to the recruitment of more type II muscle fibers that have lower O2 availability and less mitochondria than type I fibers and thus are more carbohydrate dependent and produce more lactic acid.
The increased reliance on carbohydrate as an energy source when exercise intensity increases is demonstrated by an elevation in the respiratory exchange ratio and the level of lactic acid in the blood during high-intensity exercise (37). At roughly 50 to 65% o2peak, the percentage contribution made by fatty acids to energy expenditure tends to peak and as intensity increases further the reliance on carbohydrate does as well, especially muscle glycogen (62). Romijn et al. (62) reported that after 30 minutes of cycling at 25% o2peak, carbohydrate made minor contribution to ATP provision; but at 85% o2peak, carbohydrate, primarily glycogen, accounted for roughly two-thirds of the energy expended (Figure 2).
Elsewhere, Gaitanos et al. (23) reported that 6 seconds of all-out cycling reduced the glycogen content of vastus lateralis muscle by 14%. Meanwhile, a 30-second cycling sprint was reported to reduce muscle glycogen content by as much as 27% (17), and a 75-second cycling sprint to fatigue reduced muscle glycogen content by approximately 20% (35). Spriet et al. (70) reported that during three 30-second maximal cycling sprints separated by 4 minutes of rest, glycogen made a progressively lower contribution to energy expenditure during the later sprints, which led to a decreased work output. Multi-exercise resistance workouts to fatigue have been noted to reduce muscle glycogen content by up to 25 to 40% in the active muscle (61,73).
During prolonged exercise, as muscle glycogen stores wane, there is an increasing reliance on plasma glucose as a carbohydrate resource (62,63). This transition typically takes an athlete closer to fatigue. Furthermore, a strong relationship exists between the depletion of muscle glycogen stores and/or hypoglycemia and the onset of fatigue during endurance exercise bouts in the range of 60 to 85% o2peak (68). As blood glucose levels approach 45 mg/100 mL, one may experience lightheadedness, lethargy, and nausea (neuroglycopenia) (68). For highly trained individuals, exercise at this intensity could be endured for more than 2 to 3 hours (72). This is a typical time frame for sports such as endurance cycling (e.g., tour stages and cycling centuries), marathons (42 km or 26.2 miles), triathlons (sprint or Olympic distance), and Nordic skiing.
CARBOHYDRATE INGESTION BEFORE EXERCISE
General daily recommendations for athletes are 8 to 10 g of carbohydrate per kilogram of body mass daily or ≥60% energy (1). Many athletes engage in carbohydrate loading (or glycogen supercompensation) to maximize glycogen stores, a practice that involves very high-carbohydrate intake (>70% daily caloric intake from carbohydrate), reduced training, and/or cessation in the days leading to an endurance event or game (6,46,66). While the classic method of glycogen loading involved glycogen depletion with 1 or more strenuous exercise bouts and a very low-carbohydrate intake for a couple of days, more recent studies suggest a similar degree of glycogen optimization by simply tapering and ceasing exercise and consuming a high-carbohydrate diet (6,26,46,66). For instance, Bussau et al. (6) provided participants a high-glycemic carbohydrate diet at 10 g·kg−1·d−1 and significantly increased muscle glycogen levels from 95 ± 5 mmol/kg wet wt (baseline) to 180 ± 15 mmol/kg wet wt after 1 day.
In a classic study by Bergstrom and Hultman (4), it was determined that an initial muscle glycogen content of 3.31, 1.75, or 0.63 g/100 g of wet muscle grams allowed their participants to tolerate a standard endurance workload for 167, 114, and 57 minutes prior to fatiguing. Thus, one potential benefit of ingesting carbohydrate in the hours prior to exercise would be to raise muscle glycogen levels prior to the onset of exercise. Along this line of thinking, Coyle et al. (13) reported that eating a high-carbohydrate meal 4 hours before 105 minutes of exercise at 70% o2peak increased muscle glycogen levels by 42% at the onset of exercise and led to greater reliance on muscle glycogen and total carbohydrate during the exercise bout. Meanwhile, research efforts on the ingestion of carbohydrate at or within 60 minutes of exercise have yielded equivocal results with regard to performance (7,15,20,22,25,28,34,55,67,74).
For instance, Sherman et al. (67) fed either 75 or 150 g of glucose polymer 60 minutes prior to 90-minute cycling at 70% o2peak followed by a time trial and reported that the cyclists were 13% faster. Other studies have also reported a positive impact of carbohydrate ingestion within an hour of endurance exercise (25,28,55,74). On the other hand, Foster et al. (22) reported decreased performance in cycling to exhaustion at 80% o2peak when participants were provided 70 g of glucose 30 minutes prior. Meanwhile, several related studies of ingesting carbohydrate within an hour of exercise have resulted in neither a positive nor negative effect on performance (7,15,20,34). Furthermore, research on the type of carbohydrate (high or low glycemic index) within 1 hour prior to training has also led to equivocal findings (19,74). Regarding carbohydrate type, the ingestion of either a high or low glycemic meal or water 45 minutes prior to cycling for 135 minutes did not alter glycogen utilization or performance (20). In a similar study, performance was not different during 2½ hours of cycling at 70% o2peak after ingesting either a high or low glycemic meal 30 minutes before the onset of exercise (19).
Goodpaster et al. (28) had cyclists ingest 1 g/kg of glucose, resistant starch (70% amylase/30% amylopectin), 100% amylopectin (waxy maize), or a placebo in a 18.7% solution 30 minutes before a 90-minute ride at 66% o2peak, which was followed by a 30-minute isokinetic performance trial. Serum glucose and insulin levels were significantly higher 15 minutes after the cyclists ingested glucose compared with the other treatments. Likewise, after 30 minutes and just prior to exercise onset, serum insulin levels were still higher for the glucose treatment trial. When compared with placebo, carbohydrate oxidation was greater in the glucose, resistant starch, and amylopectin trials, and performance was greater in the glucose and amylopectin trials. Carbohydrate oxidation and performance did not differ among the different carbohydrate types (28).
Carbohydrate ingestion immediately prior (≤5 minutes) to exercise can result in improvements, especially if an individual has not eaten for extended periods (e.g., overnight fast). For instance, Neufer et al. (52) reported that when cyclists ingested 45 g of carbohydrate 5 minutes prior to cycling for 45 minutes at 80% of their o2peak followed by a time trial, they were able to generate a 10% greater average of work output. It is logical to think that any improvements in performance related to ingesting carbohydrate immediately before exercise would basically be the same as ingesting carbohydrate early in the session or distributed throughout the exercise bout (2). This supports the notion that during higher intensity exercise, which would last a shorter time (<1 hour), performance can be enhanced by ingesting carbohydrate just few minutes prior to the onset of exercise. However, added benefits are not likely to result from consuming additional carbohydrate during the exercise bout (24).
CARBOHYDRATE DURING EXERCISE
Carbohydrate consumption during exercise increases the availability of carbohydrate to working muscle fibers, which can have a positive influence on endurance (8,12,51) performance as well as intermittent high-intensity performance (54,56), the latter of which could be applicable to sports such as football, ice hockey, and soccer. Carbohydrate type is an important consideration as glucose, maltose, sucrose, amylopectin, and maltodextrins are oxidized at higher rates than fructose, amylose, and galactose (44).
Whether or not carbohydrate ingestion can slow the rate of muscle glycogen, breakdown remains debated. Some research involving cyclists suggests no effect while studies involving treadmill running have suggested that carbohydrate ingestion may actually slow the rate of glycogen breakdown, especially in type I fibers (12,33,76,77). Regardless, carbohydrate ingestion during endurance exercise can extend performance time prior to fatigue (12,51). For instance, McConell et al. (51) provided highly trained cyclists with an 8% carbohydrate solution before and every 15 minutes during a time trial performed at 70% o2peak and reported an average increase in time to reach volitional exhaustion of approximately 30% (47 minutes) over placebo. Elsewhere, in a study design whereby cyclists completed 4 distinct 70-km self-paced time trials while intermittently receiving 116 ± 6 g of a carbohydrate beverage or placebo and starting with high or low pre-exercise, glycogen content led to higher measures of power output and pace in the final 14% of the time trial (84). Other studies have also observed that carbohydrate ingestion during prolonged endurance or intermittent exercise followed by a time trial can enhance measures of performance (18,21,56).
The ergogenic effects of glucose administration during exercise result from maintaining euglycemia and supplying muscle with an energy source as glycogen stores become depleted (11,14). However, glucose uptake and oxidation may peak around 1.0 to 1.2 g/min late in exercise (44,72). Even when higher amounts of glucose are consumed during exercise, glucose oxidation still plateaus (79). However, some studies involving a blend of carbohydrate types have reported higher oxidation rates. For instance, Wallis et al. (82) reported that providing maltodextrin and fructose can lead to oxidation rates of 1.5 g/min throughout 150 minutes of cycling at 55% maximum power output (64.2 ± 3.5% o2peak). Jentjens et al. (42) reported a 21% increase in carbohydrate oxidation to 1.2 g of carbohydrate per minute after ingesting a mixture of glucose and sucrose at 2.4 g/min while cycling at 63 ± 2% o2peak.
CARBOHYDRATE AFTER EXERCISE
Ingesting carbohydrate, either in liquid or solid form shortly after training or competition, is crucial to maximizing muscle glycogen recovery (39,47,60,72). Timing is critical. If carbohydrate is ingested within 30 minutes or so after exercise, enhanced glucose uptake occurs as a result of the increased GluT4 translocation during exercise. On the contrary, if carbohydrate delivery is delayed by 2 hours, the rate of glycogen recovery is slowed by 50% (39). Irrespective of post-exercise timing, maximal glycogen resynthesis is realized if 1.2 g of carbohydrate per kilogram per hour is consumed every 15 to 30 minutes (41,78) for up to 5 hours, while maximal glycogen levels are restored within 24 hours if dietary carbohydrate intake levels of 8 g of carbohydrate per kilogram per day (47) are achieved. A carbohydrate intake of 9 to 10 g of carbohydrate per kilogram per day is suggested for athletes who are completing intense exercise bouts on consecutive days (53).
The glycemic and insulinemic effect of different carbohydrates is an important consideration. For instance, Conlee et al. (9) reported that fructose is not an effective promoter of muscle glycogen recovery after glycogen-reducing exercise. This is attributable to its relatively low insulinemic effect and subsequent low glucose availability and uptake in skeletal muscle. Meanwhile, Wallis et al. (81) reported that when fructose is mixed with glucose in a 1:2 ratio, the rate of glycogen recovery is not significantly different during the 4 hours after glycogen-depleting exhaustion exercise compared with an equivalent amount of glucose.
Similarly, post-exercise ingestion of varying forms of starch has been shown to impact resynthesis of muscle glycogen but not resulting performance. In this regard, Jozsi et al. (45) reported decreased muscle glycogen resynthesis when cyclists ingested a 3,000-kcal diet over a 24-hour period consisting of 100% amylose when compared with dietary ingestion of glucose, maltodextrin, or a waxy starch (100% amylopectin). Twenty-four hours after a glycogen-depleting exercise bout and ingesting each respective isocaloric and isocarbohydrate diet, muscle glycogen levels were increased by a statistically similar level after glucose (+197.7 ± 31.6 mmol·kg−1·dry−1 muscle weight), waxy starch/amylopectin (+171.8 ± 37.1 mmol·kg−1·dry−1 muscle weight), and maltodextrin (+136.7 ± 24.5 mmol·kg−1·dry−1 muscle weight), which was greater compared with ingestion of 100% amylose (+90.8 ± 12.8 mmol·kg−1·dry−1 muscle weight). However, subsequent performance was not dependent on which type of carbohydrate was ingested (45).
High-molecular weight carbohydrates have become popular in more recent years based on purported faster absorption and more potent insulinemic effect than simpler sugars such as glucose. For instance, when well-trained men endured glycogen-depleting exercise and then consumed 75 g of a commercial processed high-molecular weight, low osmolality carbohydrate with fast digestive kinetics (Vitargo; SweCarb, Skeppsbron, Sweden) or a glucose/maltodextrin solution (low-molecular weight, high osmolality) immediately after and at 30, 60, and 90 minutes (58) post exercise, it was reported that the high-molecular weight, processed carbohydrate solution, led to a 68% faster glycogen recovery within the first 2 hours of recovery. Meanwhile, Stephens et al. (71) provided 100 g of the same high-molecular weight carbohydrate (58) or a glucose/maltodextrin solution to 8 healthy men after cycling to exhaustion at approximately 73% o2peak. Participants then rested for 2 hours before completing a 15-minute time trial. During the resting period, both blood glucose and insulin were significantly higher in the high-molecular weight carbohydrate group as compared with the glucose/maltodextrin trial. Additionally, work output for the high-molecular weight carbohydrate group was significantly greater (average 10% higher) during a 15-minute time trial that began 2 hours after completion of the initial exhaustive exercise bout.
Higher glycemic index foods may allow for a more rapid glycogen recovery versus lower glycemic index foods. Meanwhile, the coingestion of carbohydrate (4 g/kg body mass) with caffeine (8 mg/kg body mass) has been reported to result in a greater accumulation of glycogen during recovery from exhaustive exercise (57). The addition of protein to carbohydrate has also been reported to enhance muscle glycogen recovery. For instance, researchers depleted leg muscle glycogen stores of athletes by having them a cycle for 2.5 hours, and after 4 hours, muscle glycogen recovery was greater when a protein supplement was added to carbohydrate (40). However, when a relatively high amount of carbohydrate was provided after glycogen depletion exercise, additional protein did not further enhance glycogen recovery (43). Likewise, Howarth et al. (38) provided cyclists 1.2 g of carbohydrate per kilogram per hour, 1.2 g of carbohydrate + 0.4 g of protein per kilogram per hour, or 1.6 g of carbohydrate per kilogram per hour for 2 hours after completing 2 hours of cycling. They reported no difference in the rate of glycogen recovery between the trials, but the combination of carbohydrate and protein led to higher mixed muscle fractional synthetic rate and whole body net protein balance. In relation to the latter, carbohydrate has been reported to support protein and more specifically essential amino acids in net muscle protein synthesis largely by reducing post-training muscle protein breakdown (75), while carbohydrate by itself does very little to promote muscle protein synthesis but can offset changes in protein breakdown (5). Koopman et al. (48) reported that if, indeed, carbohydrate plays a supporting role in muscle protein synthesis after training, it can be negated by ample protein intake.
- The general recommendation for carbohydrate intake among athletes is 6 to 10 g/kg body weight (1). For endurance athletes training aggressively or competing daily, a carbohydrate intake at the high end is better suited.
- Athletes need to experiment with timing and type of carbohydrate to identify what works best for them.
- General recommendation for carbohydrate intake 3 to 4 hours prior to exercise for an adult is 1 to 2 g/kg or roughly 200 to 350 g. This would be appropriate to raise glycogen stores at the onset of exercise and potentially enhance performance especially if there was an extended fasting period prior (e.g., sleep).
- Many athletes choose foods that they have tolerated well in the past and that have minimal indigestible material (e.g., fiber). This meal should be lower in fat to allow for an optimal rate of emptying from the stomach and should provide fluids to optimize hydration status.
- During endurance exercise bouts, athletes should strive to ingest 30 to 60 g of carbohydrate per hour of performance to maintain blood glucose levels and optimize glucose uptake and oxidation. This can be achieved by drinking 600 to 1,200 mL of a 6 to 8% carbohydrate sport drink per hour.
- Immediately after training or competition, it is recommended that athletes ingest at least 1.5 g of carbohydrate per kilogram of body weight.
- Ingesting 1.2 g of carbohydrate per kilogram of body weight every 30 minutes over a 5-hour period can promote maximal glycogen resynthesis.
- Maximal glycogen levels can be restored within 24 hours at dietary carbohydrate intake levels of 8 g of carbohydrate per kilogram per day.
- Waiting to ingest carbohydrate for a couple of hours after strenuous exercise will dramatically reduce the rate of glycogen recovery.
- Despite lower glycemic and insulinemic responses with fructose versus glucose, at this time, it does not seem that there is a performance benefit to using either if fructose is well tolerated.
- Glycemic index of a pre-exercise food(s) can clearly impact metabolic response; however, the impact on exercise performance is unresolved.
- Waxy maize starch (amylopectin) can offer lower insulinemic responses compared with glucose if used prior to exercise but result in similar carbohydrate oxidation rates and performance during prolonged endurance exercise. Also, waxy maize starch can be used after exercise to facilitate fast muscle glycogen recovery in a manner similar to glucose and maltodextrin if ingested in the post-exercise period.
- Commercial processed high-molecular weight carbohydrates may enhance post-exercise insulin levels and the rate of glycogen resynthesis, which could be beneficial for short-term recovery periods prior to subsequent performance.
- Amylose and resistant starches are not recommended as an exclusive carbohydrate source viable carbohydrate option before, during, or after exercise.
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