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Why carbohydrates are important for active people

This is an excerpt from Nancy Clark's Sports Nutrition Guidebook by Nancy Clark.

If you are trying to stay away from forms of carbohydrate such as bagels, potatoes, and breads because you mistakenly believe carbohydrate to be fattening, think again. They are not fattening, and you need them to fuel your muscles so that you can enjoy your exercise program.


The average 150-pound (68 kg) man has about 1,800 calories of carbohydrate stored in the liver, muscles, blood, and body fluids in approximately the distribution shown in table 6.2.


Table 6.2


The carbohydrate in the muscles is used during exercise. The carbohydrate in the liver is released into the bloodstream to maintain a normal blood glucose level and feed the brain (as well as the muscles). These limited carbohydrate stores influence how long you can enjoy exercising. When your glycogen stores get too low, you hit the wall—that is, you feel overwhelmingly fatigued and yearn to quit. In a research study, cyclists with depleted muscle glycogen stores were able to exercise only 55 minutes to fatigue (as measured by an inability to maintain a specified pedaling speed on a stationary bicycle), as compared with more than twice as long—about 120 minutes—when they were carbohydrate loaded (Green et al. 2007). Food works!


In comparison to the approximately 1,800 calories of stored carbohydrate, the average lean 150-pound (68 kg) man also has 60,000 to 100,000 calories of stored fat—enough to run hundreds of miles. During low-level exercise such as walking, the muscles burn primarily fat for energy. During light to moderate aerobic exercise, such as jogging, stored fat provides 50 to 60 percent of the fuel. When you exercise hard, as in sprinting, racing, lifting weights, or other intense exercise, you rely primarily on glycogen stores. Unfortunately, for competitive athletes, fat cannot be used exclusively as fuel because the muscles need a certain amount of carbohydrate to function well at high intensities, such as a surge up a hill or a sprint to the finish. Fitness exercisers can get away with a lower carbohydrate intake than elite athletes who push themselves to exhaustion can.


Biochemical changes that occur during training influence the amount of glycogen you can store in your muscles. The figures that follow indicate that well-trained muscles store 20 to 50 percent more glycogen than untrained muscles do (Costill et al. 1981; Sherman et al. 1981). This change enhances endurance capacity and is one reason a novice runner can't just load up on carbohydrate and run a top-quality marathon (table 6.3).


Table 6.3


Because of the unfounded fears that carbohydrate is fattening or the belief that you need a high-protein diet to build muscle or a high-fat diet for endurance, many athletes today are skimping on carbohydrate foods. Some go on the paleo diet or keto diet; others go gluten free. The resulting low-carbohydrate intake can potentially hurt performance; it contrasts sharply with the diet of 2.5 to 4.5 grams of carbohydrate per pound of body weight (5 to 10 g/kg)—or 55 to 65 percent carbohydrate—recommended by most exercise and health professionals for people who train for one to three hours a day.


A case in point is ice hockey, an incredibly intense sport that relies on both muscular strength and power. During a game, carbohydrate is the primary fuel, and muscle carbohydrate (glycogen) stores decline between 38 and 88 percent. Muscle glycogen depletion relates closely to muscular fatigue. A motion analysis of elite ice hockey teams showed that the players with a high-carbohydrate (60 percent) diet skated not only 30 percent more distance but also faster than the players who ate their standard low-carbohydrate (40 percent) diet. In the final period of the game, which often determines whether a team wins or loses, the high-carbohydrate group skated 11 percent more distance than they did in the first period; the low-carbohydrate group skated 14 percent less. The researchers reached the following conclusions (Ackermark et al. 1996):

  • Low muscle glycogen stores at the start of the game can jeopardize performance at the end of the game.
  • Three days between games (with training on two of those days) plus a low-carbohydrate (40 percent) diet does not replace normal muscle glycogen stores (the players in the high-carbohydrate group had 45 percent more glycogen).
  • The differences in performance between the well-fueled players and those who ate inadequate carbohydrate were most evident in the last period of the game.


Whether your sport is ice hockey, soccer, rugby, football, basketball, or any intense sport, remember to eat responsibly. Make carbohydrate the foundation of each meal and protein the accompaniment.


After exercise, it is important to consume carbohydrate to replenish muscle glycogen stores. In a landmark study, exercise physiologist J. Bergstrom and his colleagues (1967) compared the rate at which muscle glycogen was replaced in subjects who exercised to exhaustion and then ate either a high-protein, high-fat diet, or a high-carbohydrate diet. The subjects on the high-protein, high-fat diet (similar to an Atkins or other high-protein, low-carbohydrate diet with abundant eggs, chicken, beef, cheese, and nuts) remained glycogen depleted for five days (figure 6.1). The subjects on the high-carbohydrate diet totally replenished their muscle glycogen in two days. This result shows that protein and fat aren't stored as muscle glycogen and that carbohydrate is important for replacing depleted glycogen stores. Other research suggests that three sets of biceps curls (8 to 10 repetitions per set) reduce muscle glycogen by 35 percent (Martin, Armstrong, and Rodriquez 2005). With repeated days of low carbohydrate and high repetitions, the muscles of bodybuilders and marathon runners can soon become depleted. Hence, every athlete should eat meals in which two-thirds of the plate is dedicated to wholesome carbohydrates (grains, vegetables, fruits) and one-third to protein.