When the mixed diet preceded the race, glycogen stores in the leg muscles were almost completely wiped out at the end of the competition for 6 of the 10 athletes. After the carbohydrate-loading regimen, glycogen levels at the end of the race were still fairly high (above 11 grams per kilogram) for 9 of the 10 runners. All 10 participants achieved better times in the 30K with the carbohydrate-loading diet than with the lower-carbohydrate, mixed diet. The average improvement in running time with carbohydrate loading was 8 minutes—from 143 to 135 minutes. This represents an improvement in race pace of almost 26 seconds per mile (1.6 km).
The idea that the difference in running time between races was directly related to muscle glycogen levels was supported by the finding that if an individual’s glycogen levels were already reasonably high after the mixed diet, the improvement in time with the carbohydrate diet would be small. The biggest improvements in 30K running time were observed in runners who had low glycogen levels after the mixed diet but were able to dramatically increase muscle glycogen with the special carbohydrate-loading regimen.
The mechanism for the improved running time with the high-carbohydrate diet was that increased glycogen concentrations permitted a quality running pace to be sustained longer during the race. Identical paces were maintained by all runners for the low- and high-carbohydrate runs during the first 3.75 kilometers (2.33 mi) of the 30K race. By the 11.25-kilometer (6.99 mi) mark, however, three of the mixed-diet runners had slowed their speeds, and by the end of the 30K all mixed-diet runners had slowed down. In the two races, the first individuals to slow down were those with the lowest muscle glycogen levels, and these tended to be the athletes whose leg-muscle glycogen depots had not been loaded.
This classic study was one of the first scientific investigations to show a direct link between muscle glycogen levels and endurance performance. It demonstrated that the problem associated with limited glycogen concentrations was an inability to maintain pace during prolonged running (i.e., during running lasting longer than one hour).
Carbohydrate Requirements for Runners
Scientific research has addressed the question of how much carbohydrate runners need to ingest in order to maximize muscle glycogen concentrations. An early study found that 150-pound (68 kg) males who consumed 280 grams of carbohydrate during the 24 hours following an exhaustive workout doubled the glycogen content of their leg muscles compared to eating just 100 grams of carbohydrate during the 24-hour period. However, ingesting 500 grams of carbohydrate quadrupled intramuscular glycogen concentration.2
In subsequent research, 10 experienced runners with an average O2max of 58 ml • kg-1 • min-1 completed a highly demanding, glycogen-depleting workout consisting of 16 kilometers (9.94 mi) of running at an intensity of 80 percent of O2max followed by five 1-minute sprints at the lofty power output of 130 percent of O2max.3 This session was completed on four occasions and was followed by the ingestion of 188, 375, 525, or 648 grams of carbohydrate during the 24-hour period after the workout. The greatest extent of glycogen synthesis occurred when the runners consumed 648 grams of carbohydrate. It made no difference whether the carbohydrate was consumed in two big meals or seven smaller snacks—glycogen restoration was the same.
Glycogen storage in a runner’s muscles increases in direct proportion to the amount of carbohydrate consumed up to a high threshold intake beyond which intramuscular glycogen depots are full and no further storage is possible.4 The amount of dietary carbohydrate required to refill glycogen storage areas in the muscles depends on a runner’s body size. The runners in the 648-gram study weighed about 165 pounds (75 kg); less-massive runners would usually need less carbohydrate to fill glycogen depots while heavier competitors would need more.
Follow-up research indicated that about 4 grams of carbohydrate per pound (.45 kg) of body weight are required to restore muscle glycogen maximally following a strenuous, glycogen-depleting workout.5 Thus, a 120-pound (54 kg) runner would need approximately 480 grams of carbohydrate (1,920 calories of carbohydrate) during the 24 hours after a demanding workout, but a 175-pound (79 kg) runner would require 700 grams (2,800 calories). Anecdotal evidence suggests that many runners take in far less than the ideal 4 grams per pound;6 it is likely that their training and race performances suffer as a result.
Simple and Complex Carbohydrates
Runners often wonder whether simple or complex carbohydrates are better for maximizing muscle glycogen concentrations on a day-to-day basis. By definition, simple carbohydrates are sugars while complex carbohydrates are starches (i.e., sugars linked end to end in long molecules). It is tempting to believe that simple carbohydrates might be processed more quickly by the digestive system and reach the muscles more rapidly after meals. Scientific research, however, tells us that while the terms simple and complex do have specified chemical definitions, the two types of carbohydrate can be processed fairly similarly inside a runner’s body. The digestive system quickly breaks down the complex carbohydrates in foods such as white bread, potatoes, and white rice into glucose and speeds the passage of glucose into the bloodstream almost as quickly as it would permit entry for a dose of pure glucose.7
To determine whether simple or complex carbohydrates are more effective in promoting glycogen storage, researchers from the Loughborough University of Technology in the United Kingdom studied 6 female and 9 male runners.8 To deplete their muscle glycogen stores, the 15 athletes ran to exhaustion on treadmills at an intensity of 70 percent of O2max. The athletes were then randomly assigned to either a control, pasta (i.e., complex carbohydrate), or confectionery (i.e., simple carbohydrate) group.
The pasta-group runners doubled their normal carbohydrate intake for the 3 days following the 70 percent workout by eating extra pasta. The confectionery-group athletes also doubled carbohydrate intake but focused on sugary foods such as cake, cookies, candy, and pie. Both groups took in about 540 grams (2,160 calories) of total carbohydrate per day while control subjects consumed just 322 grams (1,288 calories) of carbohydrate daily.
After the 3 days of special eating, the athletes once again ran until exhaustion at 70 percent of O2max. The pasta and confectionery groups upgraded the total distance completed by 26 percent after their carbohydrate loading, but the control group increased distance covered by only 6 percent. The logical conclusions were that an ample intake of carbohydrate (~540 grams per day) enhances recovery from exhausting exercise and improves performance during strenuous, follow-up exercise, and that simple and complex carbohydrates were equally effective from the standpoints of glycogen loading and endurance enhancement. Of course, the nutritional value of cakes and cookies is lower than that of whole-grain foods and fruits and vegetables, so the latter would be preferable from a health standpoint.
Research has also addressed the question of whether the original Swedish depletion-repletion strategy of carbohydrate loading produces higher muscle glycogen concentrations than more conventional eating plans. As mentioned at the beginning of the chapter, the depletion-repletion technique called for runners to exercise to exhaustion, consume a low-carbohydrate diet for 3 days (depletion), exercise to exhaustion again, and then follow a high-carbohydrate diet with as much as 95 percent of total daily calories coming from carbohydrate for 3 more days in order to maximize glycogen storage in the muscles.
While the depletion-repletion approach did produce higher muscle glycogen levels, it was not without problems. The low-carbohydrate component of the strategy tended to make runners irritable—and also reduced their capacity to carry out high-quality training.9 The two exhaustive running sessions also made it more difficult for runners to peak for important races. Fortunately, a study carried out at Ohio State University disclosed that the same level of glycogen supercompensation achieved with the Swedish depletion-repletion plan could be attained with a less counterproductive combination of exercise and diet.9
In this study, eight fit runners tapered by running 90, 40, 40, 20, and then 20 minutes per day over a 5-day per
iod and rested on the sixth day. During this 6-day period, the runners’ food-consumption pattern included 3 days of a 50 percent carbohydrate diet (353 grams of carbohydrate and 3,000 total calories per day) followed by 3 days on a 70 percent carbohydrate diet (542 grams of carbohydrate and 3,000 total calories each day). With this plan, muscle glycogen levels were as high as those achieved with the difficult depletion-repletion strategy.
A separate study verified that the depletion-repletion pattern is not necessary for achieving extra-high muscle glycogen concentrations but suggested that intake of simple carbohydrate might provide an advantage from the standpoint of glycogen synthesis. In research carried out at the University of Western Ontario in London, Ontario, 20 experienced runners who trained about 50 miles (81 km) per week and had completed an average of five marathons were divided into four groups of five subjects.
One group consumed a high-protein, high-fat, low-carbohydrate diet (i.e., the Swedish depletion approach) for 3 days in which only 15 percent of total calories came from carbohydrate, and then ingested a high-carbohydrate diet dominated by simple carbohydrates (e.g., sugared cereal, ripe fruit, candy, sweet breads, pastries) for the subsequent 3 days. A second group also followed the depletion approach for 3 days with the 15 percent carbohydrate diet but then ate lots of complex carbohydrates (e.g., nonripe fruits and vegetables; whole-grain breads, cereals, and pasta) for the final 3 days. The third and fourth groups ingested a more normal diet for the first 3 days, with 50 percent of calories coming from carbohydrate, and then switched to either simple or complex carbohydrate for the last 3 days.
Over the final high-carbohydrate days, all 20 runners took in 70 percent of total calories in the form of carbohydrate. During the 6 days of dietary manipulation, the runners carried out their normal training but ran only 3 miles (4.8 km) on the sixth day to enhance glycogen storage.10
The three days of depletion were not helpful. Those runners who had not used depletion stored just as much glycogen over the 6-day period as those who had used the depletion plan. Interestingly, the greatest upswing in glycogen storage occurred in the group whose members ate fairly normally for 3 days, with 50 percent of calories coming from carbohydrate, and then consumed simple carbohydrate for three days. This pattern roughly doubled the rate of glycogen storage in muscles achieved by the other three plans. This research suggests that the ingestion of simple carbohydrates might boost glycogen storage during periods when either the rapid restoration of glycogen or the maximal stockpiling of glycogen is necessary. It is interesting to note that the ultimate simple carbohydrate, table sugar, often makes up 10 to 15 percent of an elite Kenyan runner’s daily caloric intake.
Protein Requirements for Runners
Scientific research has addressed the question of whether endurance runners have heightened dietary protein requirements. Among endurance competitors, protein demands might be increased because of training-related needs in order to synthesize increased quantities of aerobic enzymes, build new capillaries around the muscles, amplify mitochondrial biogenesis, strengthen connective tissues, and fortify the heart. Protein also furnishes a small but relevant portion of the energy required to perform prolonged running; increased protein synthesis is also required to repair subcellular damage to muscles, tendons, and ligaments incurred during prolonged or intense training.11
The evidence suggests that the recommended daily allowance (U.S. RDA) of protein of about 0.8 grams of protein per kilogram (2.2 lb) of body weight per is inadequate for runners engaged in regular endurance training. In one study, well-trained endurance runners who ran from 7 to 9 miles (11-14 km) daily consumed either 0.86 grams of protein per kilogram of body mass per day (i.e., close to the recommended amount) or a high-protein diet of 1.49 grams per kilogram of body mass while their sweat and urine were collected to determine losses of body nitrogen.12 Since nitrogen is contained in protein, nitrogen losses in sweat and urine can be used to estimate the amount of protein metabolized within the body each day.
The endurance runners consuming approximately the recommended daily allowance of protein actually experienced a net loss of nitrogen (i.e., they lost more protein than they took in) on about 50 percent of their training days while the runners consuming a greater amount of protein never sustained a nitrogen deficit. This suggests that recommended levels of protein intake are inadequate for runners engaged in vigorous endurance training and that about 1.5 grams of protein per kilogram of body mass would be adequate.
Follow-up research has indicated that an intake of just 1.0 gram per kilogram of body mass is adequate for runners engaged in very light training; accomplished runners engaged in high-volume or high-quality training may need 1.5-1.6 grams per kilogram of body weight per day.13 One and one-half grams of protein per kilogram are the same as 0.7 grams per pound of body weight. Most North American and European endurance runners routinely ingest an adequate amount of protein to satisfy their daily needs without supplementation.
Fat Requirements for Runners
Most runners have no problem satisfying their daily fat-intake requirements. With carbohydrate making up about 70 percent of the total energy pie, 15 percent of calories can come from fat and 15 percent from protein. From an energy standpoint, the source of fat makes little difference: Each gram of ingested fat can provide 9 calories of potentially energy for running. From a health standpoint, the story changes. Runners should bias their fat intake toward lipid sources which are rich in omega-3 and monounsaturated fats, including dark-fleshed fish and olive oil. These fats have beneficial effects on cardiovascular and immune-system functioning while simultaneously providing needed energy.
Some runners attempt to restrict fat intake nearly completely, which is a huge mistake. Fat plays a vital role in human and exercise physiology, not only supplying energy but also supporting cell functioning, including the establishment of healthy cell membranes. It promotes the absorption of certain vitamins, protects vital organs, and preserves body temperature.
Proper Diet for Sprinters
Throughout the history of running competitions, athletes have believed that the intake of certain foods could optimize high-speed sprinting. As long ago as the fifth century BC, Charmis, a sprinter from Sparta, thought that eating nothing except figs during training would boost his performances while Dromeus of Stymphalos became highly successful in high-velocity competition while consuming only meat.14
The modern sprinter tends to align himself with Dromeus rather than Charmis, believing that high-protein diets are essential for sprint success.15 However, science reveals that the high-intensity interval training favored by sprinters can deplete glycogen reserves in the muscles to a significant extent, suggesting that high-carbohydrate diets would be preferable to high-protein intakes. One study found that just 30 seconds of all-out sprinting reduced muscle glycogen concentrations by up to 25 percent.16 Another investigation detected a 14 percent drop-off in glycogen after only 6 seconds of sprinting.17
Carbohydrate Needs
Given the impact of high-speed running on muscle glycogen levels, it is not surprising that high-protein, low-carbohydrate diets tend to reduce an athlete’s ability to perform the kind of high-intensity training favored by sprinters.18 Such diets may lower muscle glycogen concentrations to such an extent during periods of vigorous training that it is impossible for leg muscles to sustain high levels of force production. Research reveals that sprint performance declines by 10 to 15 percent when leg-muscle glycogen concentration drops below about 25 millimoles per kilogram wet weight of muscle.19
Individual workout quality can drop precipitously when a low-carbohydrate diet is followed. In one inquiry, just 2 days of low-carbohydrate eating reduced leg-muscle glycogen concentrations by approximately 50 percent compared with a high-carbohydrate diet. This approach significantly reduced the average intensity of training during both 10- and 30-minute interval workouts.20
In a separate study, athletes who followed either a high- or low-carbohydrate eating patte
rn for 36 hours undertook a rigorous interval workout involving repeated 60-second sprints with 3-minute recovery intervals.21 The high-carbohydrate athletes were able to complete an average of 14.3 sprint intervals compared to just 10.4 intervals for the low-carbohydrate runners, and total exercise time increased by 37 percent (from 42 to 57.5 minutes) in the high-carbohydrate group. The first few sprint intervals were similar in power output for the two groups, but intensity fell dramatically for the low-carbohydrate subjects as the workout proceeded. This suggests that low-carbohydrate eating can lead to situations in which performance-hampering levels of muscle glycogen are reached earlier in high-intensity training sessions, decreasing the quality of the subsequent work performed.
A greater intake of carbohydrate may stimulate recovery between high-speed interval sessions on the track. In one study, athletes carried out an interval workout consisting of five sets of 5 maximal sprints with 30 seconds of recovery between sprints and 5 minutes of active recovery between the sets.22 This was followed by one last set of 10 6-second sprints with 30 seconds of recovery. The overall workout was repeated 2 days later. During the 2 days of recovery between the workouts, some of the athletes followed a normal daily carbohydrate intake of 450 grams per day while a second group followed a high-protein, low-carbohydrate plan providing under 100 grams of carbohydrate each day. During the second workout, the normal-carbohydrate athletes were able to increase power during the first 5 sets of sprints compared with results from the first training session, an improvement that the low-carbohydrate athletes were unable to achieve.
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