Energy sources during exercise in the postabsorptive state

During exercise, the energy consumption may be increased by 20-fold. The primary factor determining whether carbohydrates or fat are preferentially used during exercise is the exercise intensity, the proportion of energy derived from carbohydrates growing progressively larger with increasing intensity. At a moderate exercise level of 100 W, demanding an oxygen uptake of around 1.5 L/min, equalling an energy expenditure of 1800 kJ/h, the proportions might typically change to 60% carbohydrates and 40% fat. In this situation, the demand for carbohydrates (65 g glucose/h, i.e. 1080 kJ) is met by glycogenolysis (around 40-45 g/h) and glucose uptake (around 20 g/h), whereas the demand for fat is met by lipolysis in adipose tissue and muscle, supplying 18 g fatty acids (i.e. 720 kJ). Under normal circumstances, protein is not an important metabolic fuel during exercise, and it is considered unlikely that, even during prolonged exercise, protein oxidation can cover more than 10% of the energy demand of the exercising body [29]. In spite of this, activation of protein metabolism is an integral part of the acute metabolic response of the body to exercise [30].

To roughly estimate the RQ during exercise and therefore the relative demand for carbohydrates and fat, Dill and coworkers used an equation, empirically derived from their data:

Respiratory quotient during exercise

Although obviously not generally applicable, the equation can be used here to illustrate the increasing demand for carbohydrates at increasing exercise intensities. When the exercise intensity is increased to 200 W (oxygen uptake = 2.8 L/min), this formula indicates an RQ value of 0.92 (indicating 70% carbohydrate and 30% fat oxidation). This corresponds to a combustion of 134 g carbohydrates and 26 g fatty acids.

Similarly, exercise at 250 W can be calculated to demand combustion of 192 g carbohydrates and 22 g fatty acids. It is therefore evident that fat combustion will level off with increasing exercise intensities, when simultaneously carbohydrate oxidation, as well as hepatic and muscular glycogenolysis and muscle glucose uptake, increases exponentially (above the 'lactate threshold').

When exercise is prolonged, fat combustion increases. This change is most likely secondary to a continuing depletion of the body's carbohydrate stores. During prolonged exercise at 40% of Vo2max in overnight fasted untrained subjects, plasma free fatty acids contributed to 60% of the fuel demand during the fourth hour of exercise compared to only 30% during the first hour [32]. Asmussen and Christensen describe two subjects being able to work at intensities demanding oxygen uptakes of 2.3 and 2.7L/min (165-190 W) for 3 h [31]. During the first hour, the subjects combusted 94 g carbohydrates and 35 g fatty acids; during the third hour the corresponding figures were 65 g carbohydrates and 48 g fatty acids.

Endogenous glycogen is the dominant fuel during the initial period of moderate to severe exercise, and during sustained exercise at work rates corresponding to 60-80% of Vo2max, fatigue coincides with the depletion of muscle glycogen [33,34]. With the continuous depletion of endogenous glycogen, the utilization of plasma-derived glucose increases and has, during prolonged exercise, been reported to cover up to 75-90% of the estimated carbohydrate oxidation by muscle [35]. The increased glucose uptake by skeletal muscle during heavy exercise must be balanced by a glucose release from the liver of the same magnitude. Because there are only limited possibilities for increasing gluconeogenesis in liver (from 3 to 5 g/h [36], mainly secondary to increased availability of glycerol due to the increased lipolysis in adipose tissue), the majority of the increased glucose output from the liver has to be derived from glycogenolysis. With liver glycogenolytic rates of 25 g/h during heavy and 60 g/h during very heavy exercise, the liver glycogen supply of around 75 g will be rapidly depleted. However, fatigue due to hypoglycemia can be postponed by the ingestion of glucose. Although it may be difficult to ingest large amounts of glucose during exercise, it has been reported that as much as 60 g/h of ingested glucose may be taken up by the body [37] during heavy exer cise of long duration. It has been observed that the total amount of carbohydrate used during a marathon race was higher than could be accounted for by the endogenous glycogen stores in the working muscles and the liver. From this, it was concluded that glycogen reserves in inactive muscle and other tissues must also have been mobilized [38]. Glycogenolysis with net lactate release from inactive muscle has been demonstrated during exercise [39].

Relatively little is known about endogenous triglycerides as a potential source of energy for the contracting muscle, but it seems likely to be an important fuel during exercise. During 1.5 h of cycle ergometer exercise to exhaustion, it was found that the decrease in thigh muscle triglyceride concentration averaged 25%

[40]. The authors calculated that 70% of total oxidized fatty acids originated from endogenous triglycerides, whereas 30% came from plasma-derived free fatty acids, and that the energy contribution of endogenous triglycerides was 70% of that of glycogen. During the Swedish 7-h Wasa ski race, it was calculated that the decrease in muscle triglycerides corresponded to twice as much energy as the decrease in muscle glycogen

[41]. See also the data by Hurley et al. [42], which are discussed in the section on training below.

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