Most people manage to balance their food intake with energy expenditure remarkably precisely. Indeed, even people who are overweight or obese are in energy balance and their weight is more or less constant. As discussed in section 1.3.2, leptin is central to the control of both food intake and energy expenditure, and there are a number of mechanisms involved in short-term control of food intake, with regulation of both hunger and satiety.
Very rarely, people are overweight or obese as a result of a physical defect of the appetite control centres in the brain — for example, some tumours can cause damage to the satiety centre, so that the patient feels hunger, but not the sensation of satiety, and has no physiological cue to stop eating.
More commonly, obesity can be attributed to a psychological failure of appetite control. At its simplest, this can be blamed on the variety of attractive foods available. People can easily be tempted to eat more than they need, and it may take quite an effort of will-power to refuse a choice morsel. Even when hunger has been satisfied, the appearance of a different dish can stimulate the appetite. Experimental animals, which normally do not become obese, can be persuaded to overeat and become obese by providing them with a 'cafeteria' array of attractive foods.
A number of studies comparing severely obese people with lean people have shown that obese people do not respond to the normal cues for hunger and satiety. Rather, in many cases, it is the sight of food that prompts them to eat, regardless of whether they are 'hungry' or not. If no food is visible, they will not feel hungry; conversely, if food is still visible they will not feel satiety. There have been no similar studies involving more moderately overweight people, so it is not known whether the apparent failure of appetite regulation is a general problem or whether it affects only severely obese people with body mass index > 40.
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