Energy Intake and Body Weight
SUSAN A. JEBB
Energy intake is an especially critical element in the regulation of body weight because the flexibility in energy intake is very much greater than that in energy expenditure. Figure 7.1 illustrates that it is relatively easy to double energy intake on a single day but extremely difficult to make a similar increase in energy expenditure. It is possible for an individual to starve or consume little energy on some days, yet the energy expenditure of a sedentary individual cannot be reduced by more than about 30%. Day-to-day variability in energy expenditure is estimated to be about 8%, compared to 25% for energy intake. Changes in energy intake thus have considerable potential to influence body weight.
The relative maintenance of body weight over prolonged periods of time has been cited as evidence that energy intake is regulated to match energy needs (see Chapter 8). Advances in basic science have revealed some components of this metabolic control system. The picture is complex, involving a network of gastrointestinal, metabolic, and hormonal signals that are integrated in the brain and trigger a coordinated cascade of neuropeptides that either stimulate or inhibit consumption (see Chapters 1, 2, and 6). These signals work across various time frames to bring an eating episode to a conclusion, to trigger the next meal, or to influence overall energy intake in the longer term over days, weeks, or even months.
Although most research on the control of energy intake has been conducted in small animals, there is good evidence that many of the pathways act in a similar manner in humans. However, the complexity of the system in humans is enhanced by a variety of additional cognitive factors that can in many circumstances override the innate physiological control of food intake. The nature of these cognitive factors is a product of various environmental factors, social and emotional influences, and learned experiences. Humans have the capacity to eat when they are not actually hungry or in need of food, perhaps