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Carbohydrates – the High Octane Fuel for Exercise Adequate carbohydrate stores (muscle and liver glycogen and blood glucose) are critical for optimum performance during both intermittent high-intensity exercise (e.g. basketball and soccer) and prolonged endurance exercise (e.g. marathon, Olympic distance triathlon). During high-intensity training, athletes should consume enough carbohydrate to meet the energy requirements of their training programs and replenish muscle and liver glycogen between training sessions and competitive events (1). Muscle glycogen represents the major source of carbohydrate in the body (300 to 400 g or 1,200 to 1,600 kcal), followed by liver glycogen (75 to 100 g or 300 to 400 kcal), and, lastly, blood glucose (25 g or 100 kcal). These amounts vary substantially among 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 (2). Carbohydrate is the predominant fuel for most types of exercise (3). Muscle glycogen and blood glucose provide about half of the energy for moderate-intensity exercise (65% of VO2max) and two-thirds of the energy for high-intensity exercise (85% of VO2max). It is impossible to meet the energy requirements for high intensity, high power output exercise when these carbohydrate fuels are depleted (3). The utilization of muscle glycogen is most rapid during the early stages of exercise and is exponentially related to exercise intensity (2,4). Exercise Fuel Use As the intensity of exercise increases, muscle glycogen constitutes a greater portion of the energy source Different Exercise Intensities (Weightlifting, sprinting, etc) 300 (Soccer, dancing, etc) Muscle glycogen 200 Energy Expended (cal/kg/min) 100 0 Muscle triglyceride (Leisurely walking, slow cycling) Plasma FFA Plasma glucose 25 65 85 Maximal Oxygen Consumption, % Abbreviations FFA, free fatty acid. Romijn JA, et al. Am J Physiol. 1993;265(Part 1):E380-E391. 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 decreases, the contribution of glucose from gluconeogenesis increases (2,4). At the beginning of exercise, hepatic glucose output matches the increased muscle glucose uptake so that blood glucose levels remain near resting levels (4). Although muscle glycogen is the primary source of carbohydrate during exercise intensities between 65 and 75% of VO2max, blood glucose becomes an increasingly important source of carbohydrate as muscle glycogen stores decline (3). Hypoglycemia occurs when the hepatic glucose output can no longer keep up with muscle glucose uptake during prolonged exercise (4). Liver glycogen stores can be depleted by a 15-hour fast and can decrease 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 approximately 900 mmol (2). Targets for Carbohydrate Intake Carbohydrate recommendations for athletes range from 3 to 12 grams of carbohydrate/kg per day (1). Athletes with very light training programs (low intensity exercise or skill-based exercise) should consume 3 to 5 g of carbohydrate/kg per day. These targets may be particularly suitable for athletes with large body mass or a need to reduce energy intake to lose weight. Athletes engaged in moderate-intensity training programs for 60 minutes per day should consume 5 to 7 g of carbohydrate/kg per day. During moderate- to high-intensity endurance exercise for one to three hours, athletes should consume 6 to 10 g of carbohydrate/kg/day. Athletes participating in moderate- to high-intensity endurance exercise for 4 to 5 hours per day (e.g. Tour de France) should consume 8 to 12 g of carbohydrate/kg per day. These are general recommendations and should be adjusted with consideration of the athlete’s total energy needs, specific training needs and feedback from their training performance (see Tables 1 and 2 for lists of carbohydrate-containing foods and for examples of diets containing 5 and 12 g carbohydrate/kg body weight/day). Carbohydrate intake should be spread over the day to promote fuel availability for key training sessions – before, during and after exercise (1). Table 1. Carbohydrate Content of Selected Foods Each provides about 25 grams of carbohydrate Grains 2 slices whole wheat bread ½ deli-style bagel 1 2-ounce English muffin 1 cup oatmeal 1 cup ready-to-eat breakfast cereal 1 package snack-type cheese crackers (6 to package) 2 fig cookie bars ½ cup rice 1 cup cooked pasta 5 cups popcorn ½ large soft pretzel 17 mini-pretzels 1 12-in. flour tortilla 1 oz tortilla chips and ½ to 1.5 cup salsa Sports Drinks, Bars and Gels 2 cups sport drink (6%-8% carbohydrate-containing sport drinks) 1 energy bar (average of many energy bars) 1 carbohydrate gel ½-1 bottle of commercial nutrition or weight loss shake Diary Products and Other Beverages Beans and Starchy Vegetables 2 cups milk (low-fat or fat-free) 1 cup low-fat chocolate milk 1 4.5-oz container fruit flavored yogurt 1½ 8-oz cartons sugar-free yogurt 1 cup vanilla-flavored soy milk 1 package instant hot chocolate (made with water) ½ cup black beans ½ cup baked beans ¾ cup kidney beans 1 cup lima beans 1 cup green peas 3/4 cup corn ¾ cup mashed potatoes 1 medium baked potato with skin Mixed Dishes Fruit and Juice 1 slice thin-crust pizza with meat or veggie toppings ½ slice thick crust pizza with meat or veggie toppings 1 small bean and rice burrito ½ cup black beans and rice 1½ cups canned chicken noodle soup ¾ cup tomato soup 1 cup cooked ramen noodles ½ 6-in. sub sandwich ½ cup macaroni and cheese 2 cups fresh strawberries 1 large orange ¾ cup orange juice ½ cup cranberry-apple juice 1 medium apple Table 2. Diets containing 5 g carbohydrate/kg/day (3000 kcal/day) and 12 g/kg/day (5000 kcal). 5 g carbohydrate/kg/day; 3000 kcal Breakfast 1 slice whole wheat toast Morning Snack 1/2 med. bagel 1 tsp. 1 T. cream butter/margarine cheese 6 oz. orange juice 1 cup bran flakes ½ c. cottage cheese (4.5% fat) 1 peach half 1 cup 2% milk Water Lunch Afternoon Snack 1 c. tomato 1.5 oz trail soup mix (made w/water) 2 slices 1 cup 2% whole milk wheat bread 2 T. peanut butter Dinner 1-1/2 c. mashed potatoes -1.5 tsp. butter/margine 1 large apple ¼ c. gravy 1 c. 2% milk 2 c. cantaloupe pieces 1 c. 2% milk -1 oz. American cheese Water 4 oz. roast beef 1 c. green beans Approximate composition: 346 g carbohydrate, 129 g protein, 123 g fat Evening snack Grilled cheese sandwich (ingredients below) - 2 slices whole wheat bread Table 2, continued 12 g carbohydrate/kg/day; 5000 kcal Breakfast Pancakes (four 4-inch) ¼ c. pancake syrup Morning snack 1 bagel Lunch 2 T. peanut butter Turkey sandwich (ingredients below) -2 slices whole wheat bread -2 oz. lean turkey 1 tsp. 1 large butter/margarine banana 12 oz. orange juice 10 oz. sports drink 2 c. tomato soup (made with water) 8 oz. skim milk -1 T. lite mayonnaise 1 oz. Canadian bacon 2 c. apple juice 2 c. mixed green salad 4 T. reduced fat salad dressing ¼ c. raisins (in salad) Afternoon snack 2 c. sports drink ½ c. cottage cheese 4 peach halves 1 chewy granola bar Dinner 2 c. spaghetti and meatballs 1 T. grated Parmesan cheese 1 c. cooked peas and carrots 2 bread sticks (4 inch) 1 tsp. olive oil (drizzle on bread sticks) 2 c. lemonade ½ c. applesauce Evening snack 12 oz. chocolate milk (1%) 2 oz. graham crackers 2 T. hazelnut spread Approximate composition: 839 g carbohydrate, 137 g protein, 128 g fat The absolute quantity of carbohydrate (grams of carbohydrate per kg of body weight), rather than the percentage of energy from carbohydrate, is important for exercise performance. An athlete’s estimated carbohydrate requirements should consider the amount of carbohydrate required for optimal glycogen restoration or the amount expended during training. These estimates should also be provided according to the athlete’s body weight to account for the size of the athlete’s muscle mass. Carbohydrate guidelines based on grams per kg are user-friendly and practical – it is relatively easy for athletes to determine the carbohydrate content of meals and snacks to achieve their daily carbohydrate goals (1). Athletes should consume sufficient energy as well as carbohydrate to optimize muscle and liver glycogen storage. The restrained eating practices of some athletes (often in weight-restricted sports or aesthetically judged sports) can interfere with meeting targets for carbohydrate intake and compromise glycogen storage (1). It is important to choose nutrient-dense carbohydrates (whole grain products, fruits, vegetables, and beans) and to add other foods to recovery meals and snacks to provide a good source of protein and other nutrients (5). These nutrients may assist in other recovery processes, and in the case of protein, may promote additional glycogen recovery when carbohydrate intake is below targets or when frequent snacking is not possible (1). References 1. Burke LM, Hawley JA, Wong S, Jeukendrup AE. Carbohydrates for training and competition. J Sports Sci 2011, in press. 2. Jacobs KA, Sherman WM. The efficacy of carbohydrate supplementation and chronic high carbohydrate diets for improving endurance performance. Int J Sport Nutr. 1999;9:92-115. 3. Coyle EF. Substrate utilization during exercise in active people. Am J Clin Nutr. 61 (suppl): 1995; 968S-979S. 4. Hargreaves M. Exercise physiology and metabolism. In: Clinical Sports Nutrition. 3rd ed., Burke, L. and Deakin, V. Eds. McGraw-Hill, Australia, 2006. 5. Position of the American Dietetic Association, Dietitians of Canada, and the American College of Sports Medicine: Nutrition and athletic performance. Rodriguez NR, Di Marco NM, Langley S; American Dietetic Association; Dietitians of Canada; American College of Sports Medicine. J Am Diet Assoc. 2009; 109:509-27.