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31. 7. 2012.

The athlete’s diet

Vegeterian diet

In an effort to eat a healthy diet and increase their carbohydrate intake, many athletes have adopted vegetarianism. Vegans are strict vegetarians who eat only food from plant sources. Lactovegeterians also consume dairy products. Ovovegeterians add eggs to their vegetable diets, and lacto-ovovegeterians eat plant foods, dairy products, and eggs.
Can athletes perform well on a vegetarian diet? Athletes who are strict vegans must be very careful in selecting the plant foods they eat to provide a good balance of the essential amino acids, sufficient calories, and adequate sources of vitamin A, riboflavin, vitamin B12, vitamin D, calcium, zync, and iron. Adequate iron intake is of particular concern in female vegetarian athletes because of the lower bioavailability of iron in plant-based diets and because of women’s greater risk for anemia and low iron stores. Some professional athletes have noted significant deterioration in athletic performance after switching to strict vegetarian diets. The problem usually is traced to unwise selection of foods. Including milk and eggs in the diet decreases the risk of nutritional deficiencies. Anyone contemplating switching to a vegetarian diet should either read authoritative material on the subject written by qualified nutritionists or consult a registered dietitian or sport nutritionist.

Precompetition meal

For years, many athletes have eaten the traditional steak dinner several hours before competition. This practice might have originated from the early belief that muscle consumes itself to fuel its own activity and that steak would provide the necessary protein to counteract this loss. But we know that the steak is probably the worst food an athlete could eat before competing. Steak contains a relatively high percentage of fat, which requires several hours for full digestion. During competition, this would cause the digestive system to compete with the muscles for the available blood supply. Also, nervous tension is typically high before a big competition, so even the choicest steak cannot truly be enjoyed at this time. The steak would be more satisfying and less likely to disturb performance if the athlete were to eat it either the night before or after the competition. But if steak is out, what should the athlete eat before competing?
Although the meal ingested a few hours before competition might contribute little to muscle glycogen stores, it can ensure a normal blood glucose level and prevent hunger. This meal should contain only about 200 to 500 kcal and consist mostly of carbohydrate foods that are easily digested. Foods such as cereal, milk, juice, and toast are digested rather quickly and won’t leave the athlete feeling full during competition. In general, this meal should be consumed at least 2h before competition. The rates at which food is digested and nutrients are absorbed into the body are quite individual, so timing the precompetition meal might depend on prior experience. In one study of endurance cyclists, a prolonged cycling exercise trial to exhaustion at 70% of the subject’s VO2max was performed under two different conditions, with 14 days between trials: 100g of carbohydrate breakfast fed 3h before exercise(Fed) and no feeding before exercise(Fasted). Subjects tested under the Fed condition exercised 136 min before reaching exhaustion compared with 109 min in the fasted trial, indicating the importance of the precompetition meal.
A liquid precompetition meal might be less likely to result in nervous digestion, nausea, vomiting, and abdominal cramps. Such feedings are commercially available and generally have been found useful both before and between events. Finding time for athletes to eat is often difficult when they must perform in multiple preliminary and final events. Under these circumstances, a liquid feeding that is low in fat and high in carbohydrate might be only solution.

Muscle glycogen replacement and loading

Earlier was established that different diets can markedly influence muscle glycogen stores and that endurance performance depends largely on these stores. The theory is that the greater the amount of glycogen stored, the better the potential endurance performance because fatique will be delayed. Thus, an athlete’s goal is to begin an exercise bout or competition with as much stored glycogen as possible.
On the basis of muscle biopsy studies conducted in the mid-1960s, Astrand proposed a plan to help runners store the maximum amount of glycogen. This process is known as glycogen or carbohydrate loading. According to Astrand’s regimen, athletes should prepare for an aerobic endurance competition by completing an exhaustive training bout seven days before the event. For the next three days, they should eat fat and protein almost exclusively to deprive the muscles of carbohydrate, which increases the activity of glycogen synthase, an enzyme responsible for glycogen synthesis and storage. Athletes should then eat a carbohydrate-rich diet for the remaining three days before the event. Because glycogen synthase activity is increased, increased carbohydrate intake results in greater muscle glycogen storage. Training intensity and volume during this six-day period should be markedly reduced to prevent additional muscle glycogen depletion, thus maximizing liver and muscle glycogen reserves. Originally, an additional intense training bout was performed four days prior to competition.
This regimen has been shown to elevate muscle glycogen stores to twice the normal level, but it is somewhat impractical for most highly trained competitors. During the three days of low carbohydrate intake, athletes generally find training difficult. They are also often irritable and unable to perform mental tasks, and they typically show signs of low blood sugar, such as muscle weakness and disorientation. In addition, the exhaustive depletion bouts of exercise performed seven days before the competition have little training value and can impair glycogen storage rather than enhance it. This depletion exercise also exposes athletes to possible injury or overtraining.
Considering these limitations, many proposed that the depletion exercise and the low carbohydrate aspects of Astrand’s regimen be eliminated. Instead, according to Sherman and colleagues, the athlete should simply reduce training intensity a week before competition and eat a normal, mixed diet containing 55% of the calories from the carbohydrate until three days before competition. For these days, training should be reduced to a daily warm-up of 10 to 15 min of activity and accompanied by a carbohydrate-rich diet. Following this plan, as seen in the figure below, glycogen will be elevated to nearly 200 mmol/kg of muscle, the same level attained with Astrand’s regimen, and the athlete will be better rested for competition.

It is possible to increase carbohydrate stores rapidly after even a very short near-maximal-intensity bout of exercise. In a study of seven endurance athletes, scientists found that cycling for 150s at 130% of VO2max followed by 30s of all-out cycling and 24h of high-carbohydrate intake was sufficient to nearly double muscle glycogen stores in just one day.
Diet is also important in preparing the liver for the demands of endurance exercise. Liver glycogen stores decrease rapidly when a person is deprived of carbohydrates for only 24h, even when at rest. With only 1h of strenuous exercise, liver glycogen decreased by 55%. Thus, hard training combined with a low-carbohydrate diet can empty the liver glycogen stores. A single carbohydrate meal, however, quickly restores liver glycogen to normal. Clearly, a carbohydrate-rich diet in the days preceding competition will maximize the liver glycogen reserve and minimize the risk of hypoglycemia during the event.
Water is stored in the body at a rate of about 2.6g of water with each gram of glycogen. Consequently, an increase or decrease in muscle and liver glycogen generally produces a change in the body weight of from 0.5 to 1.4kg(1-3 lb). Some scientists have proposed monitoring of changes in muscle and liver glycogen stores via recording the athlete’s early morning weight immediately after rising – after emptying the bladder but before eating breakfast. A sudden decrease in weight might reflect a failure to replace glycogen, a deficit in body water, or both.
Athletes who must train or compete in exhaustive events on successive days should replace muscle and liver glycogen stores as rapidly as possible. Although liver glycogen can be depleted totally after 2h of exercise at 70% VO2max, it is replenished within a few hours when a carbohydrate-rich meal is consumed. Muscle glycogen resynthesis, on the other hand, is a slower process, taking several days to return to normal after an exhaustive process bout such as the marathon(see figure below). Studies in the late 1980s revealed that muscle glycogen resynthesis was most rapid when individuals were fed at least 50g(about 0.7g/kg body weight) of glucose every 2h after the exercise. Feeding subjects more than this amount did not appear to accelerate the replacement of muscle glycogen. During the first 2h after exercise, the rate of muscle glycogen resynthesis is much higher than later in recovery. Thus, an athlete recovering from an exhaustive endurance event should ingest sufficient carbohydrate as soon after exercise as a practical. Adding protein and amino acids to the carbohydrate ingested during the recovery period enhances muscle glycogen resynthesis above that achieved with carbohydrate alone. 

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