BODY FUEL STORES
In medical science it is traditional to consider that the average or normal person is a 70-kg (154-lb) healthy male. The following discussion will use this average subject as a reference point.
Exercises 3.2 lists the weights of the different organs of the body in a 70-kg (154-lb) man. The largest organ is muscle, which comprises about 26 kg (about 57 lb) of total body weight, followed by fat tissue (10.5 kg or 23 lb) and bone (10 kg or 22 lb). The amount of fat tissue in different individuals varies greatly; for our average healthy male we will assume that the value is about 15% of total body weight.
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Exercises 3.2 shows that water is the major constituent of the human body, comprising as much as 64% of body weight. But the most important point shown in the Exercises is that fat is the largest energy store in the body. By comparison, the carbohydrate stores are quite trivial. To emphasize the importance of this difference, let us consider how long our sample subject could run at world-class marathon pace if burning exclusively carbohydrates or exclusively fats.
The oxygen cost of running at world-record marathon pace (19.8 km/hr) for an athlete of average efficiency is 66.5 ml/kg/min. The total oxygen consumption by a 70-kg runner running at 19.8 km/hr would therefore be 70×66.5 ml/min, that is, 4.66 L/min. We know that each liter of oxygen utilized produces approximately 20 kJ; thus our runner will bum 93.1 kJ/min (1 calorie = 4.2 kJ). Exercises 3.2 shows that at this rate of energy consumption, the total-body carbohydrate store in this particular athlete would last only 126 minutes (2:06:00), whereas the body fat stores would last about 59:49:00.
But our empirical observation is that no world-class marathon runner could ever hope to run at 19.8 km/hr for close to 60 hours. Clearly, fats cannot provide all the energy required at world-class marathon pace. We will consider this topic in greater detail subsequently.
It therefore seems fair to conclude that female distance runners have greater fat stores than do male distance runners but that both have considerably less fat stores than the average healthy untrained person.