If an individual is overweight or obese, then clearly that individual must have been through a period when his or her intake of energy was consistently greater than his or her energy expenditure. (Note that this is true for everyone during the period of growth.) It does not necessarily follow that this is true now; an obese subject may be in energy balance, with a stable weight. Then we can ask the question: if energy intake was greater than energy expenditure, did this arise through (1) an elevated rate of energy intake, compared with people of normal and steady body weight; or (2) a diminished rate of energy expenditure (again, compared with people of normal and steady body weight)? The answer may not, of course, be the same for all obese subjects. This question is of interest because if the answer is (2) - i.e. diminished energy expenditure - then it implies that the individual will also have a particularly hard job losing excess calories, because he or she has a 'biologically' low metabolic rate; it also implies that we might look, in metabolic terms, for the cause of this oddity of metabolism.
This is a deceptively simple question that has taken many years to answer. The reason for the difficulty in answering it lies in the very precision of energy balance discussed earlier. In most people, as we have seen, energy intake and energy expenditure match each other to within a fraction of one percent over a reasonably long period. On the other hand, from day to day they may differ considerably. We must ask: what sort of a mismatch between energy intake and energy expenditure are we looking for?
Many people who are overweight have become so over a period of many years of gradual accumulation. Take as an example a person of height 1.8 m and body weight 100 kg, i.e. body mass index of 30.9 kg/m2; and suppose that he or she increased in weight from 70 kg (body mass index 21.6 kg/m2) over a period of 10 years. The gain is 30 kg of adipose tissue with an energy density of about 30 MJ/kg (as earlier), i.e. a total 'integrated mismatch' between energy intake and expenditure of 30 x 30 = 900 MJ, or 90 MJ per year, or about 250 kJ per day. A 'throughput' of energy is about 10 MJ/day (i.e. the amount we eat and expend), so this is an imbalance of about 2.5%. The task is therefore to measure both energy intake and energy expenditure to a degree of precision which will enable us to say whether one or the other is 2.5% outside normal values, which in themselves vary from person to person and which both vary enormously from day to day. This is an almost impossible experimental task. What is more, we have to do this not when the person comes to the clinic complaining of weighing 100 kg, but before that, during the phase of weight gain.
In fact, the question has been answered quite clearly in a slightly indirect way. The energy expenditure of obese subjects has been measured and compared with that of normal subjects (Table 11.2). On average, it is clear that obese subjects have higher rates of energy expenditure than subjects of normal weight. Now we see why this is an indirect answer: it is not even one of the results we were expecting. At first sight, an increased rate of energy expenditure should result in thinness rather than fatness. But remember that fat itself - i.e. the triacylglycerol in adipose tissue and other tissues - is not 'metabolically active'; energy expenditure occurs in the other components of the body, the fat-free or lean body mass. Fat-free mass is also increased in obese people; it represents the non-fat components of adipose tissue and other supporting tissues. The rate of energy expenditure is, in fact, closely related to the fat-free or lean body mass in people of all body weights, lean and obese (Fig. 11.5). Thus, we see that obese people have a high rate of energy expenditure because they
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