Adequate carbohydrate stores (muscle and liver glycogen and blood glucose) are critical for optimum athletic performance. Consuming adequate carbohydrate on a daily basis is necessary to replenish muscle and liver glycogen between daily training sessions or competitive events. Consuming carbohydrate prior to exercise can help performance by "topping off" muscle and liver glycogen stores. Furthermore, consuming carbohydrate during exercise can improve performance by maintaining blood glucose levels and carbohydrate oxidation.
Carbohydrate Availability During Exercise
Muscle glycogen represents the major source of carbohydrate in the body (300-400 g or 1,200 to 1,600 kilocalories), followed by liver glycogen (75 to 100 g or 300 to 400 kilocalories) and, lastly, blood glucose (25 g or 100 kilocalories). These amounts vary widely between individuals, depending on factors such as dietary intake and state of training. Untrained individuals have muscle glycogen stores that are roughly 80 to 90 mmol/kg of wet muscle weight. Endurance athletes have muscle glycogen stores of 130 to 135 mmol/kg of wet muscle weight. Carbohydrate-loading increases muscle glycogen stores to 210 to 230 mmol/kg of wet muscle weight (1).
Exercise energetics dictate that carbohydrate is the preferred fuel for exercise intensities at and above 65% of V02max – the levels at which most athletes train and compete. Fat oxidation cannot supply adenosine triphosphate (ATP) rapidly enough to support such high-intensity exercise. While it is possible to exercise at light to moderate levels (<60% of V02max) with low levels of muscle glycogen and blood glucose, it is impossible to meet the ATP requirements required for heavy exercise when these fuels are depleted. The utilization of muscle glycogen is most rapid during the early stages of exercise and is exponentially related to exercise intensity (1).
There is a strong relationship between the pre-exercise muscle glycogen content and the length of time that exercise can be performed at 70% of V02max. The greater the pre-exercise glycogen content, the greater the endurance potential. Bergstrom and associates (2) compared the exercise time to exhaustion at 75% of VO2max after three days of three diets varying in carbohydrate content. A mixed diet (50% calories from carbohydrate) produced a muscle glycogen content of 106 mmol/kg and enabled the subjects to exercise 115 minutes. A low carbohydrate diet (less than 5% of calories from carbohydrate) produced a muscle glycogen content of 38 mmol/kg and supported only an hour of exercise. However, a high-carbohydrate diet (>82% of calories from carbohydrate) provided 204 mmol/kg of muscle glycogen and enabled the subjects to exercise for 170 minutes.
Liver glycogen stores maintain blood glucose levels both at rest and during exercise. At rest, the brain and central nervous system (CNS) utilize most of the blood glucose and the muscle accounts for less than 20% of blood glucose utilization. During exercise, however, muscle glucose uptake can increase 30-fold, depending on exercise intensity and duration. Initially, the majority of hepatic glucose output comes from glycogenolysis, however, as the exercise duration increases and liver glycogen declines, the contribution of glucose from gluconeogenesis increases (1).
At the beginning of exercise, hepatic glucose output matches the increased muscle glucose uptake so that blood glucose levels remain near resting levels. Although muscle glycogen is the primary source of carbohydrate during exercise intensities above 65% of V02max, blood glucose becomes an increasingly important source of carbohydrate as muscle glycogen stores decline. When hepatic glucose output can no longer keep up with muscle glucose uptake during prolonged exercise, the blood glucose drops. While a few athletes experience central nervous system symptoms typical of hypoglycemia, most athletes note local muscular fatigue and have to reduce their exercise intensity (1).
Liver glycogen stores can be emptied by a fifteen hour fast and can fall from a typical level of 490 mmol on a mixed diet to 60 mmol on a low carbohydrate diet. A high carbohydrate diet can increase liver glycogen content to about 900 mmol (1).
Carbohydrate Recommendations for Training
Building up and maintaining glycogen stores during training requires a carbohydrate-rich diet. When adequate carbohydrate is not consumed on a daily basis between training sessions, the pre-exercise muscle glycogen content gradually declines and training or competitive performance may be impaired. Daily restoration of the body’s carbohydrate reserves should be a priority for athletes involved in intense training.
Costill and colleagues evaluated glycogen synthesis on a 45% carbohydrate diet during three successive days of running 16.1 kilometers at 80% of V02Max (3). Pre-exercise muscle glycogen levels started at 110 mmol/kg and fell to 88 mmol/kg on day two and 66 mmol/kg on day three. Another study out of Costill's lab found that a diet providing 525 to 648 g of carbohydrate promoted glycogen synthesis of 70 to 80 mmol/kg and provided near maximal repletion of muscle glycogen within 24 hours (4).
Fallowfield and Williams also evaluated the importance of a high carbohydrate intake on recovery from prolonged exercise (5). Their subjects ran at 70% of V02Max for 90 minutes, or until volitional fatigue, whichever came first. During the next 22.5 hours, the runners consumed isocaloric diets containing either 5.8 or 8.8 g of carbohydrate/kg. After the rest period, the runners ran at the same intensity to assess endurance capacity. Those that consumed 8.8 g of carbohydrate/kg were able to match their running time of the first race. Even though the two diets were isocaloric, the running time of those who consumed only 5.8 g of carbohydrate/kg decreased by over 15 minutes.
For many athletes, the energy and carbohydrate needs of training are greater than the requirements of competition. Some athletes involuntarily fail to increase caloric intake to meet the energy demands of increased training.
Costill and colleagues studied the effects of 10 days of increased training volume at a high intensity on muscle glycogen and swimming performance (6). Six swimmers self-selected a diet containing 4,700 calories/day and 8.2 g of carbohydrate/kg/day, while four swimmers self-selected a diet containing only 3,700 calories/day and 5.3 g of carbohydrate/ kg/day. These four swimmers could not tolerate the heavier training demands and swam at significantly slower speeds, presumably due to a 20% decline in muscle glycogen.
The feeling of sluggishness associated with muscle glycogen depletion is often referred to as "staleness" and blamed on overtraining. Athletes who train exhaustively on successive days must consume adequate carbohydrate and energy to decrease the threat of fatigue caused by the cumulative depletion of muscle glycogen.
Glycogen depletion associated with training can occur during training in sports which require repeated, near-maximal bursts of effort (such as football, basketball, and soccer) as well as during endurance exercise. A revealing sign of glycogen depletion is when the athlete has difficulty maintaining a normal exercise intensity. A sudden weight loss of several pounds (due to glycogen and water loss) may accompany glycogen depletion.
A review of the literature by Sherman and Wimer questions the assumption that a high carbohydrate diet optimizes training adaptation and athletic performance (7). They report that the relationship between muscle glycogen depletion and exhaustion is strongest at moderate training intensities. However, Sherman and Wimer note that it is well established that low blood glucose, muscle and/or liver glycogen concentrations can contribute to fatigue during certain types of exercise.
Because dietary carbohydrate contributes directly to maintenance of body carbohydrate reserves, Sherman and Wimer recommend continuing to advise athletes to eat a high carbohydrate diet. They also recommend watching for signs of staleness during training and taking note of those athletes whose dietary habits make them more prone to glycogen depletion.
It is suggested that athletes who train heavily consume 7 to 10 g of carbohydrate/kg/day (8). The typical American diet supplies about 4 to 5 g of carbohydrate/kg/day. An intake of 6 to 7 gm of carbohydrate/kg/day is sufficient when the athlete exercises hard (>70% of VO2max) for about an hour per day. An intake of 8 to 10 g of carbohydrate/ kg/day is recommended when the athlete exercises hard for several hours or more per day.
Some athletes may need to reduce fat intake to below 30% of total calories to obtain 8 to 10 g of carbohydrate/kg/day. Sugar intake may be increased to meet the increased carbohydrate requirement, but the majority of the carbohydrate should come from complex carbohydrates. They are more nutrient-dense and, compared to sugary foods, provide more B vitamins necessary for energy metabolism as well as more fiber and iron. Sugar, however, contributes to tooth decay and many products that are high in sugar are also high in fat.
Athletes should consume sufficient calories in addition to carbohydrate. Consumption of a reduced energy diet will impair endurance performance due to muscle and liver glycogen depletion. Adequate carbohydrate intake is also important for athletes in high-power activities (eg, wrestling, gymnastics, and dance) who have lost weight due to negative energy balances (8).
Desire for weight loss and consumption of low-energy diets is prevalent among athletes in high-power activities. Negative energy balance can harm high-power performance due to impaired acid-base balance, reduced glycoloytic enzyme levels, selective atrophy of Type II muscle fibers, and abnormal sarcoplasmic reticulum function. Adequate dietary carbohydrate may ameliorate some of the damaging effects of energy restriction on the muscle (8).
Athletes participating in ultra-endurance events (those lasting over four hours) have the highest carbohydrate requirements. Saris and colleagues studied food intake and energy expenditure during the Tour de France (9). In this demanding 22 day, 2,400 mile race, the cyclists consumed an average of 850 g of carbohydrate per day or 12.3 g/kg/day. About 30% of the total energy consumed was provided by high carbohydrate beverages.
Brouns and associates evaluated the effect of a simulated Tour de France study on food and fluid intake, energy balance, and substrate oxidation.(10,11). Although the cyclists consumed 630 g of carbohydrate (8.6 g/kg/day), they oxidized 850 g of carbohydrate per day (11.6 g/kg/day). In spite of ad libitum intake of conventional foods, the cyclists were unable to ingest sufficient carbohydrate and calories to compensate for their increased energy expenditure. When the diet was supplemented with a 20% carbohydrate beverage, carbohydrate intake increased to 16 g/kg/day and carbohydrate oxidation rose to 13 g/kg/day.
Ultra-endurance athletes who require over 600 g of carbohydrate/day should consider supplementing their dietary intake with high carbohydrate beverages if they cannot eat enough conventional foods to meet their carbohydrate and energy requirements (11). Both Saris and Brouns recommend that ultra-endurance athletes in training or competition consume 12 to 13 g of carbohydrate/kg/day. They also suggest that this range represents the maximum contribution of carbohydrate to energy metabolism during extreme ultra-endurance exercise (9).
The glycemic index indicates how much a food increases the blood glucose level relative to glucose, which has a glycemic index of 100. The glycemic index of a food is based on the total area under the two-hour glucose response curve after the food is eaten, compared to the standard response to the same amount of glucose. In practical terms, the glycemic index is influenced by the form (liquid or solid) in which the food is eaten, its fiber content, the presence of protein and fat, and food processing and preparation methods.
The glycemic index is not simply a function of whether the carbohydrate is in a liquid or solid form. An orange has a glycemic index of 66 which is almost identical to the value of 67 for orange juice. The glycemic index is also not a function of whether the food is a starch (eg, pasta) or simple carbohydrate (eg, table sugar). For example, a baked potato has a glycemic index of 98, which is close to the value of 100 for glucose.
The glycemic index concept has limitations. The numbers that are available are largely based on tests using single foods. High glycemic foods often do not affect the glycemic response when combined with other foods in meals. The glycemic index is based on a percentage change in blood glucose, whereas changes in mg/dL have more clinical utility for disease states such as diabetes. Also, the glycemic index is based on equal grams of carbohydrate (50 g), not average serving sizes.
High Carbohydrate Supplements
Some athletes train so heavily that they have difficulty eating enough food to obtain the amount of carbohydrate needed for optimum performance. Athletes who have this problem can consider a commercial high-carbohydrate supplement (11). Most products are 18 to 24% carbohydrate and contain glucose polymers (maltodextrins) to reduce the solution's osmolality and potential for gastrointestinal distress.
High carbohydrate supplements do not replace regular food, but are designed to supply supplemental calories and carbohydrate when needed. If the athlete has no difficulty eating enough food, these products are unnecessary.
High carbohydrate supplements can be consumed before or after exercise (eg, with meals or in between meals). Though ultra-endurance athletes may also utilize them during exercise, they are too concentrated in carbohydrate to double for use as a fluid replacement beverage.
During endurance exercise lasting 90 to 120 minutes at 70% of V02max (eg, running a marathon), muscle glycogen stores become progressively lower. When they drop to critically low levels (the point of glycogen depletion), high intensity exercise cannot be maintained. In practical terms, the athlete is exhausted and must either stop exercising or drastically reduce the intensity of exercise.
Muscle glycogen depletion is a well-recognized limitation to endurance exercise. Athletes using glycogen supercompensation techniques (carbohydrate loading) can nearly double their muscle glycogen stores.
The carbohydrate loading sequence was originally a week-long regimen starting with an exhaustive training session one week before competition. For the next three days, the athlete consumed a low-carbohydrate diet, yet continued exercising, to lower muscle glycogen stores even further. On the three days prior to competition, the athlete rested and ate a high-carbohydrate diet to promote glycogen supercompensation.
This regimen had many drawbacks. Three days of reduced carbohydrate intake often caused hypoglycemia and ketosis with associated nausea, fatigue, and irritability. The dietary manipulations also proved to be too cumbersome for many athletes.
The revised method of carbohydrate loading proposed by Sherman and colleagues eliminates many of the problems associated with the old regimen (12). During the first three days, the athlete consumes a normal diet providing about 5 g of carbohydrate/kg/day. On the sixth day before the event, the athlete trains at 70% of V02max for 90 minutes. On the fifth and fourth days before the event, the athlete trains for 40 minutes at 70% of V02max. The athlete trains for 20 minutes at 70% of V02max on the third and second day before the event and rests the day before the event. During the last three days, the athlete consumes a high carbohydrate diet providing 10 g of carbohydrate/kg/day.
It is essential that training be reduced prior to competition. The final three days, when the athlete tapers and eats a high-carbohydrate diet, is the real "loading" phase of the regimen. The modified regimen results in muscle glycogen stores equal to those provided by the classic carbohydrate-loading regimen.
In a field study conducted by Karlsson and Saltin, runners participated in a 30 kilometer race after eating a normal diet or high-carbohydrate diet (13). The high-carbohydrate diet provided muscle glycogen levels of 193 mmol/kg, compared to 94 mmol/kg for the normal diet. All runners covered the 30 kilometer distance faster (by about eight minutes) when they began the race with high muscle glycogen stores. Carbohydrate loading enables the athlete to maintain high-intensity exercise longer, but will not affect pace for the first hour of the event.
Endurance training promotes muscle glycogen supercompensation by increasing the activity of glycogen synthase -- the enzyme responsible for glycogen storage. The athlete must be endurance trained or the regimen will not be effective. Since glycogen stores are specific to the muscle groups used, the exercise to deplete the stores must be the same as the athlete's competitive event (1).
A commercial high-carbohydrate supplement can be added if the athlete has difficulty consuming enough carbohydrate through food. Athletes who have diabetes or hypertriglyceridemia may have medical complications if they carbohydrate load and should obtain medical clearance before attempting the regimen.
For each gram of glycogen stored, additional water is stored. While some athletes note a feeling of stiffness and heaviness associated with the increased glycogen storage, these sensations usually dissipate with exercise.
Carbohydrate loading will only help athletes engaged in intense, continuous endurance exercise lasting >90 minutes. Above-normal muscle glycogen stores will not enable the athlete to exercise harder during shorter duration exercise. The stiffness and heaviness associated with the increased glycogen stores may actually hurt performance during shorter events such as 5 and 10 kilometer runs.
Carbohydrate in the Hour Before Exercise
Athletes have been warned not to eat large amounts of carbohydrate prior to exercise. This admonition was based on results of a study conducted in the late 1970s that indicated that consuming 75 g of glucose 30 minutes prior to exercise reduced endurance by causing hypoglycemia (14). The high blood insulin levels induced by the pre-exercise carbohydrate feeding were blamed for this chain of events.
As a result of this early research, some practitioners have advised athletes to either avoid carbohydrates entirely before exercise or to consume low glycemic foods. The rationale is that low glycemic foods such as beans and pasta provide a slow but sustained release of glucose to the blood, without an accompanying insulin surge. By comparison, sugar and high glycemic foods such as bread, potato, and many breakfast cereals rapidly increase blood glucose and blood insulin levels.
A 1987 study by Hargreaves and colleagues contradicted the earlier findings (15). In this study, cyclists consumed 75 g of glucose, 75 g of fructose, or water 45 minutes prior to bicycling to exhaustion. Although the glucose feeding caused high blood insulin and low blood glucose levels, there were no differences in the exercise time to exhaustion between the rides taken with glucose, fructose, or water.
Consuming a high glycemic carbohydrate an hour before exercise may actually improve performance, particularly if the athlete has fasted overnight. Sherman and colleagues compared the ingestion of 1.1 g/kg and 2.2 g/kg of a carbohydrate beverage one hour prior to exercise (16). The subjects cycled at 70% of VO2max for 90 minutes and then underwent a performance trial. Serum insulin was initially elevated at the start of and during exercise and blood glucose initially decreased. Time trial performance was significantly increased 12.5% by the carbohydrate feedings, presumably via increased carbohydrate oxidation.
The hypoglycemia and hyperinsulinemia following pre-exercise carbohydrate feedings are transient and probably will not harm performance unless the athlete is sensitive to a decrease in blood glucose. Athletes should evaluate their responses to high-carbohydrate foods with differing glycemic indexes in training to find what works the best.
A low-glycemic carbohydrate may be an option for athletes who are sensitive to decreases in blood glucose. Thomas and colleagues compared the consumption of 1 g of carbohydrate/kg of lentils (low glycemic), potato (high glycemic), glucose (high glycemic), and water one hour prior to exercise. The subjects cycled to exhaustion at 65 to 70% of VO2max. Compared to the potato, glucose, and water trials, the lentil feeding provided a more gradual rise and fall in blood glucose. The endurance time for the low glycemic lentils was 20 minutes longer than for all other trials, which were not different from each other (17).
Athletes who are sensitive to having their blood glucose decreased can choose from several strategies. These are: 1) consume a low-glycemic carbohydrate before exercise; 2) take in carbohydrate a few minutes before exercise; or 3) wait until exercising to consume carbohydrate. The exercise-induced rise in the hormones epinephrine, norepinephrine, and growth hormone inhibit the release of insulin, and counter insulin's effect in lowering blood glucose.
Consuming sugar immediately before anaerobic exercise such as sprinting or weight lifting will not improve performance because there is already enough ATP, CP, and muscle glycogen stored for these tasks. It will not provide athletes with a quick burst of energy, allowing them to exercise harder. Eating too much sugar before exercise can increase the risk of gastrointestinal distress in the form of cramps, nausea, diarrhea, and bloating.
Carbohydrate Two to Four Hours Before Exercise
Athletes are often advised to eat two to three hours before exercise, to allow adequate time for gastric emptying. The rationale is that if any food remains in the stomach at the start of exercise, the athlete may become nauseated or uncomfortable when blood is diverted from the gastrointestinal tract to the exercising muscles. So, rather than getting up at the crack of dawn to eat, many athletes who train or compete in the morning simply forgo food prior to exercise.
This overnight fast lowers liver glycogen stores and can impair performance, especially if the athlete engages in prolonged endurance exercise that relies heavily on blood glucose.
During exercise, athletes rely primarily on their pre-existing glycogen and fat stores. Although the pre-exercise meal does not contribute immediate energy for exercise, it can provide energy when the athlete exercises hard longer than an hour. It can also prevent athletes from feeling hungry, which in itself may impair performance. The carbohydrate in the meal can elevate blood glucose to provide energy for the exercising muscles.
Consuming carbohydrate two to four hours prior to morning exercise helps to restore suboptimal liver glycogen stores, which will help endurance events that rely heavily on blood glucose. If muscle glycogen levels are also low, consuming carbohydrate several hours before exercise can help to increase them as well. If gastric emptying is a concern, liquid meals may be considered.
Sherman and colleagues evaluated the effect of a 312 g, 156 g, and 45 g liquid carbohydrate feeding four hours prior to exercise (18). The carbohydrate feedings provided 4.5 g/kg, 2 g/kg, and 0.6 g/kg, respectively. Interval cycling was undertaken for 95 minutes, followed by a performance trial after a 5 minute rest. The 312 g carbohydrate feeding improved performance by 15%, despite elevated insulin levels at the start of exercise.
Nuefer and associates have found that endurance performance was also improved when a mixed meal (cereal, bread, milk, and fruit juice) supplying 200 g of carbohydrate was consumed four hours before exercise (19).
The ideal pre-exercise meal is high in carbohydrate, palatable and well tolerated. The research by Sherman and colleagues suggests that the pre-exercise meal contain 1 to 4.5 g of carbohydrate/kg, consumed one to four hours prior to exercise (16, 18). To avoid potential gastrointestinal distress, the carbohydrate and calorie content of the meal should be reduced the closer to exercise the meal is consumed. For example, a carbohydrate feeding of 1 g/kg is appropriate an hour before exercise, whereas 4.5 g/kg can be consumed four hours before exercise.
A number of commercially formulated liquid meals are available to the athlete. Some of these were initially designed for hospital patients (eg, Sustacal and Ensure), while others have been specifically created for and marketed to the athlete (eg, GatorPro, Nutrament, and Exceed Nutritional Beverage).
These products satisfy the requirements for pre-exercise food -- they are high in carbohydrate, palatable, and provide both energy and fluid. Liquid meals can often be consumed closer to competition than regular meals due to their shorter gastric emptying time. This may help to avoid pre-competition nausea for those athletes who are tense and have an associated delay in gastric emptying.
Liquid meals also produce a low stool residue and so help to keep immediate weight gain following the meal to a minimum. This is especially advantageous for wrestlers who need to "make weight.” Liquid meals are also convenient for athletes competing in day-long competitions, tournaments, and multiple events (eg, triathlons).
Liquid meals can also be used for nutritional supplementation during heavy training when caloric requirements are extremely elevated. They supply a significant amount of calories and contribute to satiety.
Carbohydrate Intake During Exercise
Carbohydrate feedings during exercise lasting at least an hour enable athletes to exercise longer and/or sprint harder at the end of exercise. Coyle and colleagues have demonstrated that consuming carbohydrate during cycling exercise at 70% of V02max can delay fatigue by 30 to 60 minutes (20, 21).
Coyle and associates compared the effects of carbohydrate feedings on the onset of fatigue and decrease in work capacity of cyclists (20). The carbohydrate feedings enabled the cyclists to exercise an average of 33 minutes longer (159 minutes compared to 126 minutes) before reaching the point of fatigue. The carbohydrate feedings maintained blood glucose at higher levels, thereby increasing the utilization of blood glucose for energy.
Coyle and colleagues also measured performance during prolonged strenuous bicycling with and without carbohydrate feedings (21). During the ride without carbohydrate, fatigue occurred after three hours and was preceded by a drop in blood glucose. During the ride where the cyclists were fed carbohydrate, blood glucose levels were maintained and the cyclists were able to ride an additional hour before reaching the point of fatigue. Both groups utilized muscle glycogen at the same rate, indicating that endurance was improved by maintaining blood glucose levels, rather than by glycogen sparing.
Carbohydrate feedings maintain blood glucose levels at a time when muscle glycogen stores are diminished. Thus, carbohydrate oxidation (and therefore ATP production) can continue at a high rate and endurance is enhanced.
Running performances with and without carbohydrate feedings have also been evaluated. During a 40 kilometer run in the heat, Millard-Stafford and colleagues found that a carbohydrate feeding (55 g per hour) increased blood glucose levels and enabled runners to finish the last 5 kilometers significantly faster compared to the run without carbohydrate (22). In a treadmill run at 80% of V02max, Wilber and Moffatt (23) found that the run time when fed carbohydrate (35 g per hour) was 23 minutes longer (115 minutes) compared to the run without carbohydrate (92 minutes).
Carbohydrate feedings may also improve performance in sports such as football and basketball that require repeated bouts of high intensity, short duration effort. Davis and colleagues evaluated the effect of carbohydrate feedings on performance during intermittent, high intensity cycling (24). The subjects performed repeated one-minute sprints at 120 to 130% of V02max separated by three minutes of rest until fatigue. Before and every 20 minutes during the exercise, the subjects drank a placebo or 6% carbohydrate-electrolyte drink that provided 47 g of carbohydrate/hr. The average time to fatigue in the carbohydrate trial was 89 minutes (21 sprints) compared to 58 minutes (14 sprints) for the placebo. The results of this study suggest that the benefits of carbohydrate feedings are not limited to prolonged endurance exercise.
The performance benefits of a pre-exercise carbohydrate feeding appears to be additive to those of consuming carbohydrate during exercise. In a study by Wright and colleagues (25), cyclists that received carbohydrate both three hours before and during exercise were able to exercise longer (289 minutes) than when receiving carbohydrate either before exercise (236 minutes) or during exercise (266 minutes).
Combining carbohydrate feedings improved performance more than either feeding alone. However, the improvement in performance with pre-exercise carbohydrate feedings was less than when smaller quantities of carbohydrate were consumed during exercise. If the goal is provide a continuous supply of glucose during exercise, the athlete should consume carbohydrate during exercise.
Carbohydrate's primary role in fluid replacement drinks is to maintain blood glucose concentration and enhance carbohydrate oxidation (26). Carbohydrate feedings enhance performance during exercise lasting an hour or longer, especially when muscle glycogen stores are low. In fact, carbohydrate ingestion and fluid replacement independently improve performance and their beneficial effects are additive.
Below and Coyle evaluated the effects of fluid and carbohydrate ingestion, alone or in combination, during one hour of intense cycling exercise (27). In the four trials, the subjects ingested either: 1) 1,330 ml of water which replaced 79% of sweat loss, 2) 1,330 ml of fluid with 79 g of carbohydrate, 3) 200 ml of water which replaced 13% of sweat losses, or 4) 200 ml of fluid with 79 g of carbohydrate. When a large volume of fluid or 79 g of carbohydrate was ingested individually, each improved performance by about 6% compared to the placebo trial. When both the large volume of fluid and carbohydrate were combined, performance was improved by 12%.
Coyle and Montain suggest that athletes take in 30 to 60 g (120 to 240 kcal) of carbohydrate every hour to improve performance (28). This amount can be obtained through either carbohydrate-rich foods or fluids.
Liquid vs Solid Carbohydrate During Exercise
The benefits of consuming beverages containing carbohydrate during exercise are well established. However, endurance athletes often consume high carbohydrate foods such as sports bars, fig bars, cookies, and fruit. Solid food empties from the stomach more slowly than liquids. Also, the protein and fat found in many high carbohydrate foods can delay gastric emptying. This raises the question of whether solid carbohydrates are as effective as liquid carbohydrates in increasing blood glucose and improving performance.
Manuel Lugo and colleagues evaluated the metabolic effects of consuming liquid carbohydrate, solid carbohydrate, or both during two hours of cycling at 70% of VO2max, followed by a time trial (29). The liquid was a 7% carbohydrate-electrolyte beverage and the solid carbohydrate was a sports bar that provided 76% of calories from carbohydrate, 18% from protein, and 6% from fat. Each feeding provided 0.4 g of carbohydrate/kg (an average of 28 g per feeding and 56 g per hour) and was consumed immediately before and every 30 minutes during the first 120 minutes of exercise.
While the caloric content of the treatments varied, they were isoenergetic with respect to carbohydrate. Carbohydrate availability and time trial performance were similar when equal amounts of carbohydrate were consumed as liquid, solid, or in combination. Regardless of carbohydrate form, there were no differences in blood glucose, insulin, or total carbohydrate oxidized during 120 minutes of cycling at 70% of VO2max.
Robert Robergs and colleagues at the University of New Mexico in Albuquerque compared blood glucose and glucoregulatory hormone (insulin and glucagon) responses to solid and liquid carbohydrate feedings during two hours of cycling at 65% of VO2max, followed by a 30 minute maximal isokinetic ride (30). The liquid was a 7% carbohydrate-electrolyte beverage and the solid carbohydrate was a meal replacement bar that provided 67% of calories from carbohydrate, 10% from protein, and 23% from fat. Each feeding provided 0.6 grams of carbohydrate/kg body weight/hr (an average of 20 g per feeding and 40 g per hour) and was consumed at 0, 30, 60, 90, and 120 minutes of exercise. Two resting glycemic response trials were also conducted. Following consumption of 75 g of either liquid or solid carbohydrate, blood glucose and insulin levels were measured every 20 minutes for two hours.
The resting glycemic response study found that the liquid carbohydrate feeding was associated with greater insulin-dependent glucose disposal than the solid carbohydrate feeding for the same total carbohydrate intake. This was attributed to the combined protein, fat, and fiber in the solid carbohydrate, which are known to delay gastric emptying and so blunt the insulin response to a given amount and type of carbohydrate in the food. However, there were no differences between liquid and solid carbohydrate feedings on blood glucose, glucoregulatory hormones, and exercise performance during prolonged cycling.
Despite the possibility that solid carbohydrate feedings containing protein and fat are emptied more slowly by the stomach at rest and during exercise, solid and liquid carbohydrate feedings are equally effective in increasing blood glucose levels and improving performance.
Each carbohydrate form (liquid versus solid) holds certain advantages for the athlete (31). Sports drinks and other liquids encourage the consumption of water needed to maintain hydration during exercise. Also, carbohydrate must be in a liquid or semi-liquid state before leaving the stomach. However, compared to liquids, high-carbohydrate foods, sports bars, and gels can be easily carried by the athlete during exercise and provide both variety and satiety.
Drinking 5 to 10 ounces (150 to 300 ml) of a sports drink containing 4 to 8% carbohydrate (eg, Gatorade, Allsport, and Powerade) every 15 to 20 minutes can provide the proper amount of carbohydrate. For example, drinking 20 ounces each hour of a sports drink which contains 6% carbohydrate provides 36 g of carbohydrate. Drinking the same quantity each hour of a sports drink containing 8% carbohydrate provides 48 g of carbohydrate. Eating one banana (30 g), one Power Bar (47 g), two gels (about 50 g) or three large graham crackers (66 g) every hour also supplies an adequate amount of carbohydrate.
The American College of Sports Medicine suggests that both fluid and carbohydrate requirements can be met by consuming 600 to 1,200 ml per hour (20 to 40 ounces) of beverages containing 4 to 8% carbohydrate (26).
Fructose During Exercise
Some athletes take fructose during exercise. Since fructose causes a lower blood glucose and insulin response than glucose, athletes may mistakenly believe that fructose is a superior energy source.
Murray and colleagues compared the physiological, sensory, and exercise performance responses to the ingestion of 6% glucose, 6% sucrose, and 6% fructose solutions during cycling exercise (32). Blood insulin levels were lower with fructose as expected. However, fructose was associated with greater gastrointestinal distress, higher perceived exertion ratings, and higher serum cortisol levels (indicating greater physiological stress), than glucose or sucrose. Cycling performance times were also significantly better with sucrose and glucose than fructose.
The lower blood glucose levels associated with fructose ingestion may explain why fructose does not improve performance. Fructose metabolism occurs primarily in the liver, where it is converted to liver glycogen. Fructose probably cannot be converted to glucose and released fast enough to provide adequate energy for the exercising muscles. In contrast, blood glucose is maintained or elevated by feedings of glucose, sucrose, or glucose polymers. These have been shown to enhance performance and are the predominant carbohydrates in sports drinks.
The greater incidence of gastrointestinal distress (bloating, cramping, and diarrhea) often reported with high fructose intakes may be due to the slower intestinal absorption of fructose compared to glucose.
Carbohydrate Feedings After Exercise
The restoration of muscle and liver glycogen stores following strenuous training is important to minimize fatigue associated with repeated days of heavy training. Athletes who consume 7 to 10 g of carbohydrate/day will nearly replace their muscle glycogen stores during consecutive days of hard workouts.
The time period in which carbohydrate is consumed following exercise is also important for glycogen repletion. Ivy and colleagues evaluated glycogen repletion following two hours of hard cycling exercise that depleted muscle glycogen (33). When 2 g of carbohydrate/kg was consumed immediately after exercise, muscle glycogen synthesis was 15 mmol/kg. When the same carbohydrate feeding was delayed for two hours, muscle glycogen synthesis was cut by 66% to 5 mmol/kg. By four hours after exercise, total muscle glycogen synthesis for the delayed feeding was still 45% less than for the feeding given immediately after exercise.
Reed and colleagues evaluated the effect of the carbohydrate form on glycogen repletion following exercise. The researchers provided 3 g of carbohydrate/kg in liquid or solid form following two hours of cycling exercise at 60 to 75% of V02max. The subjects received half of the feeding immediately after exercise and half at two hours following exercise. There was no difference in muscle glycogen storage rates between the liquid and solid feedings at two hours post exercise or at four hours post exercise.
Delaying carbohydrate intake for too long after exercise may reduce muscle glycogen storage and impair recovery. Athletes who are not hungry after exercising can consume a high-carbohydrate drink (eg, sports drink, fruit juice, or a commercial high-carbohydrate beverage). This will also aid in rehydration.
Providing liquid or solid carbohydrate with equal carbohydrate contents after exercise produces similar rates of glycogen repletion. Reed and colleagues provided 3 g of carbohydrate per kg in liquid or solid form to subjects after they had cycled for two hours at 60 to 75% of VO2max (34). The subjects received half of carbohydrate immediately after exercise and half two hours after exercise. There was no difference in muscle glycogen storage between rates between the liquid and solid carbohydrate feeding.
Athletes who exercise hard for >90 minutes daily should consume 1.5 g of carbohydrate/kg immediately after exercise, followed by additional 1.5 g of carbohydrate/kg feeding two hours later (35). The first carbohydrate feeding can be a high-carbohydrate beverage and the following feeding can be a high-carbohydrate meal. Replenishing muscle glycogen stores after exercise is particularly beneficial for athletes who train hard several times a day. This will enable them to get the most out of their second workout.
There are several reasons that glycogen repletion occurs faster following exercise. The blood flow to the muscles is much greater immediately after exercise and the muscle cell is more likely to take up glucose. Also, the muscle cells are more sensitive to the effects of insulin during this time period, which promotes glycogen synthesis.
Glucose and sucrose are twice as effective as fructose in restoring muscle glycogen after exercise (36). Most fructose is converted to liver glycogen, whereas glucose appears to bypass the liver and is stored as muscle glycogen.
The type of carbohydrate consumed (simple versus complex) does not appear to influence glycogen repletion following exercise. Roberts and colleagues compared simple and complex carbohydrate intake during both glycogen depleted and non-depleted states. The researchers found that significant increases in muscle glycogen could be achieved with a diet high in simple or complex carbohydrates (37).
Burke and associates investigated the effect of glycemic index on muscle glycogen repletion following exercise (38). The subjects cycled for two hours at 75% of V02max to deplete muscle glycogen, then consumed foods with either a high glycemic index or a low glycemic index. The total carbohydrate feeding over 24 hours was 10 g of carbohydrate/kg, evenly distributed at meals eaten at 0, 4, 8, and 21 hours after exercise. The increase in muscle glycogen content after 24 hours was greater with the high glycemic diet (106 mmol/kg) than with the low glycemic diet (71.5 mmol/kg). The most rapid increase in muscle glycogen content during the first 24 hours of recovery may be achieved by consuming foods with a high glycemic index.
Athletes may have impaired muscle glycogen synthesis following unaccustomed exercise that results in muscle damage and delayed onset muscle soreness. The muscular responses to such damaging exercise appear to decrease both the rate of muscle glycogen synthesis and the total muscle glycogen content (39). While a diet providing 8 to 10 g of carbohydrate/kg will usually replace muscle glycogen stores within 24 hours, the damaging effects of unaccustomed exercise significantly delays muscle glycogen repletion. Also, Sherman notes that even the normalization of muscle glycogen stores does not guarantee normal muscle function after unaccustomed exercise.
Barry Sears, PhD, author of Enter the Zone and Mastering the Zone claims that high carbohydrate diets impair athletic performance and cause obesity. The Zone books treat carbohydrates and insulin as villains and recommend a complicated, carbohydrate-restricted diet. Sears tells people to eat exactly 40% of calories as carbohydrate, 30% as protein, and 30% as fat at each meal and snack.
Athletes must supposedly follow the Zone diet to reach their maximum athletic performance. The diet allegedly promotes optimal athletic performance by altering the production of eicosanoids so that the body makes more “good” eicosanoids than “bad” ones. Sears claims that eicosanoids are the most powerful hormones and control all physiological functions.
Zone diet proponents recommend limiting carbohydrate to keep the body from producing too much insulin, because high insulin levels allegedly increase the production of “bad” eicosanoids. "Bad" eicosanoids purportedly impair athletic performance by reducing oxygen transfer to the cells, lowering blood glucose levels, and interfering with body fat utilization. According to Sears, insulin also makes people fat by causing carbohydrates to be stored as body fat.
The protein content of the Zone diet supposedly increases glucagon levels and helps to increase the production of “good” eicosanoids by opposing the effect of insulin. “Good eicosanoids” supposedly increase endurance by increasing oxygen transfer to the cells, promoting the utilization of stored fat, and maintaining blood glucose levels.
Athletes be impressed and intimidated by such scientific-sounding information. However, the scientific basis for this diet can be faulted on many fronts. Eicosanoids don't cause disease -- they are the biologically-active, hormone-like compounds known as prostaglandins, thromboxanes, and leukotrienes. Eicosanoids help to regulate inflammation, the blood's tendency to clot, and the immune system. The claim that eicosanoids are all-powerful is ridiculous -- the body's physiology just is not that simple. There is also no evidence that insulin even makes “bad” eicosanoids or that glucagon makes “good” eicosanoids (40).
The metabolic pathways which supposedly connect diet, insulin and glucagon, and eicosanoids do not appear in standard nutrition or biochemistry texts. The idea that this diet (or any diet) completely regulates insulin and glucagon is not supported by endocrinology. Next, the notion that the insulin and glucagon control the production of eicosanoids is not supported by biochemistry. And finally, the belief that eicosanoids control all physiological functions (including athletic performance) is not only unfounded and ridiculous, it is an appalling over-simplification of complex physiological processes (40).
Athletes need carbohydrates to perform at their best. Contrary to what the Zone books claim, eating a high carbohydrate meal one to four hours before exercise improves performance by raising blood glucose and “topping off” glycogen stores (18,19,25). Consuming carbohydrate during exercise lasting an hour or more aids endurance by providing glucose for the muscles when muscle glycogen stores are low (20-25). And, taking in carbohydrate right after hard training enhances muscle glycogen storage (33-35).
In reality, what matters for weight loss is not carbohydrates or insulin, but energy. Body weight depends on how many kilocalories are consumed compared to how many are burned off. There is also no evidence that insulin makes people fat (41).
The Zone diet is merely a low-energy diet. The Zone books attempt to disguise this by having people count fancy protein and carbohydrate blocks instead of kilocalories. Although Sears does not emphasize energy intake, the Zone diet provides only about 1,200 kilocalories (120 g of carbohydrate) for the average woman and 1,700 kilocalories a day (170 g of carbohydrate) for the average man (42). The diet is also inadequate in thiamin, pyridoxine, magnesium, copper, and chromium (42).
The Zone diet does not increase the ability to burn fat during exercise. The Zone diet also does not change the body’s preference for carbohydrate over fat as fuel for exercise. The best way for athletes to increase their fat-burning ability is to keep exercising. And as far gradual loss of body fat, that comes from burning more kilocalories during exercise than are eaten at the table, not from some special dietary ratio (41).
Lastly, athletes cannot train or compete well for very long on this low energy, low-carbohydrate diet. Athletes require adequate calories and carbohydrate to maintain their glycogen stores and muscle tissue. Those who follow the Zone diet will eventually find themselves in a twilight zone of near starvation and impaired performance (41).
Carbohydrate is the preferred fuel for most sports. Since the depletion of endogenous carbohydrate stores (muscle and liver glycogen and blood glucose) impairs athletic performance, athletes should strive to optimize their carbohydrate stores before, during, and after exercise.
Athletes should consume 7 to 10 g of carbohydrate/kg/day to replenish muscle and liver glycogen following training sessions or competitive events. One to four hours prior to exercise, athletes should consume 1 to 4 g of carbohydrate/kg to "top off" muscle and liver glycogen stores. During exercise lasting an hour or longer, athletes should consume 30 to 60 g of carbohydrate per hour to maintain blood glucose levels and carbohydrate oxidation. To optimize glycogen repletion following exercise lasting 90 minutes or longer, athletes should consume 1.5 g of carbohydrate/kg within 30 minutes, followed by an additional 1.5 g of carbohydrate/kg feeding two hours later.
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Table 1 -- Carbohydrate Recommendations for Athletes
Typical US diet supplies 4 to 5 g of carbohydrate/kg/day
Recommended intake for most athletes – 7 to 10 g of carbohydrate/kg/day
6 to 7 g/kg for one hour of training per day
8 g/kg for two hours of training per day
l0 g/kg for three hours of training per day
12 to 13 g/kg for fours hours or
more of training per day
2 – carbohydrate loading guidelines
Day Training (70% of Vo2max) Diet
1 90 minutes 5 g of carbohydrate/kg
2 40 minutes 5 g of carbohydrate/kg
3 40 minutes 5 g of carbohydrate/kg
4 20 minutes 10 g of carbohydrate/kg
5 20 minutes 10 g of carbohydrate/kg
6 Rest 10 g of carbohydrate/kg
TABLE 3 – CONSIDERATIONS IN USING THE GLYCEMIC INDEX
High glycemic carbohydrates are recommended before exercise for athletes who are not sensitive to having their blood glucose lowered.
Low glycemic carbohydrates are recommended before exercise for athletes who are sensitive to having their blood glucose lowered.
High glycemic carbohydrates are recommended during exercise to raise blood glucose and promote carbohydrate oxidation.
High glycemic carbohydrates are recommended following exercise to enhance glycogen repletion.
TABLE 4 – PROS AND CONS OF LIQUID VS SOLID CARBOHYDRATES
Liquid Pros: Liquid Cons:
Replace sweat losses Large volume, difficult to carry
Empty rapidly from stomach Do not provide variety or satiety
Solid Pros: Solid Cons:
Compact, easy to carry Require additional water for digestion
Provide variety and satiety Do not replace sweat losses
Consume 1 to 4 g of carbohydrate/kg, one to four hours before exercise
Consume 30 to 60 g of carbohydrate every hour during exercise
Consume 1.5 g of carbohydrate/kg immediately after exercise followed by an additional 1.5 g of carbohydrate/kg feeding two hours later
TABLE 6 – DANGERS OF THE ZONE DIET FOR ATHLETES
Inadequate kilocalories (approximately 1,700 for men and 1,200 for women)
Inadequate dietary carbohydrate (approximately 170 g for men and 120 g for women)
Inadequate micronutrients (thiamin, pyridoxine, magnesium, copper, and chromium)
False hope that Zone diet will enhance athletic performance