Energy needs for exercise vary considerably depending on the intensity and duration of the activity and on the size of the individual.1 The more strenuous the exercise and the higher the body weight, the more energy is required. For example, the average 60-kg man running at a moderate pace (14.5 km/hr) will expend about 750 kcal/hr, whereas a 90-kg man, running for the same time period, would burn over 1200 kcal.
Both fat and carbohydrate are oxidized to provide energy during exercise. Whether the body uses predominantly fat or carbohydrate depends on the intensity of the activity.1 Fat is particularly important as a source of energy during long, endurance events that last for several hours, such as cycling or distance running. Marathon runners derive over 75% of their energy needs from metabolism of fat. However, as exercise intensity increases, glucose stored in muscle and liver cells as gly-cogen becomes more important than fat as a fuel source. Sprinters burn mostly glucose in muscle during short, high-intensity exercise.
Body stores of glycogen are limited and contain only about 1200 kcal. Used alone, gly-cogen can provide energy for only short periods (about 60 mins). Therefore, most exercise - such as tennis, soccer, and cycling -is fueled by mixtures of fat and carbohydrate.1 Because glucose stores are relatively small (compared with fat stores), during endurance events, glucose stores are depleted long before fat stores. When glucose stores are used
Fig. 5.39: Energy fuels during exercise of different intensity. Contribution of the major substrates for energy after 30 min exercise at 25%, 65%, and 85% of maximal oxygen uptake.
(Adapted from Romijn JA, et al. Am J Physiol. 1993; 265:E380)
up, muscles become fatigued and the athlete begins to feel exhausted. Highly trained athletes can store more glycogen in their muscles and are able to preserve these stores by using more energy from fat during exercise.1
Along with proper training, dietary choices have a strong influence on the amount of glucose stored in muscle. Diets high in carbohydrate stimulate muscles to store more glucose and can increase endurance.2,3 Athletes consuming 60-70% of calories as carbohydrates are better able to build large reserves of muscle glycogen than those consuming 40% of calories as carbohydrate (the normal amount of carbohydrate in the typical diet is about 45 %).2
For endurance athletes, at least two-thirds of total calories should come from carbohydrate. This means eating 500-600 g of carbohydrate each day. Emphasis should be on eating complex carbohydrates because, compared with simple carbohydrates, they contain more of the nutrients needed by athletes (they are richer in B vitamins, minerals, and fiber). Fat intake should be only 20-30% of total calories. Body fat stores, even in very lean athletes, contain much more fat than is needed during training or competition. For example, because each half kilo of fat contains approximately 3500 kcal, a 70-kg athlete with only 15% body fat has over 80000 kcal stored as fat.
For several hours after a strenuous workout, muscle cells are "hungry" for glucose, and depleted glycogen stores can be quickly and efficiently replenished.4 Therefore, athletes should try to drink three to four cups of fruit juice or eat the equivalent of about five slices of bread within 2 hours after exercise. This carbohydrate will be used to replenish gly-cogen stores, and recovery from the exercise will be enhanced.4
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