Runners and long-duration, high intensity exercisers use glycogen, a form of stored glucose, for energy. Muscle glycogen content before a marathon exercise session can influence the length of time a person can maintain his or her exercise. These glycogen stores can be manipulated by decreasing exercise and increasing carbohydrate intake during the week before the marathon. Glycogen stores only last for 60 to 90 minutes. That leaves untrained runners in deep trouble early on in the marathon.
Trained muscles develop the ability to store 20 to 25 percent more glycogen than untrained muscles, according to research by Dr. David Gostill at Ball State University. Well-trained runners can store enough glycogen to last 15 to 20 miles. Carbohydrate loading only benefits activities involving high-intensity exercise lasting longer than 60 to 80 minutes. Marathon running falls into this category. Other appropriate carbo loading activities include long distance swimming, cross country skiing, 30K runs, soccer, long distance canoe racing, and triathlons.
In contrast, football games, 10K and 5K races, downhill ski races, walking and hiking, or weight lifting do not require carbo loading. Costil (1) demonstrated that the rate of glycogen production in the body was greatest when carbohydrate intake was 525 grams per day or more. Blom (2) showed that glycogen resynthesis was maximal when subjects consumed 25 grams of glucose per hour. Keizer (3) showed no further increase in the rate of glycogen synthesis measured by Blom at an intake of 70 grams of carbohydrates per hour. Based on these studies it appears that 25 grams of carbohydrates per hour (600 grams per day) is sufficient for a maximal rate of glycogen production and high intensity, long duration exercise efficiency.
1. Costill D, Sherman W, Gind C, Maresh C, Witten M, Miller J. The role of dietary carbohydrate in muscle glycogen resynthesis after strenuous exercise. American Journal of Clinical Nutrition 1981 34:1831-1836.
2. Blom P, Hostmark A, Baage O, Kardel K, Machlum S. Effect of different post-exercise sugar diets on the rate of muscle glycogen synthesis. Medicine and Science in Sports and Exercise 1987 19:491-496.
3. Keizer H, Kuipers H, Van Kranenburg G, Geurten P. Influence of liquid and solid meals on muscle glycogen resynthesis, plasma fuel hormone response and maximal physical working capacity. International Journal of Sports Medicine 1987 8: 99-104.