Weight gain, obesity and psychological stress are pervasive elements of modern developed societies. The rise in obesity seems to reflect a strong drive to consume food, especially energy-dense fatty or sugary foods, that continues to be present even when we are full. In parallel with increased obesity, we also see increased levels of stress. Could it be that stress has an impact on eating behaviour and that a stressful environment favours the ingestion of foods rich in sugar and fat?
The consequences of obesity and their threat to long-term health have been disseminated widely. But in contrast to the prevalence of tobacco smoking, for example, which has been dropping steadily in the UK since the 1970s, obesity levels are high and rising, especially in children. The rise in obesity reflects a strong drive to consume food, especially energy-dense fatty or sugary foods, that continues to be present even when we are full. It seems that our stressful, modern environment has the capacity to inundate intrinsic neural mechanisms that homeostatically regulate appetite and food intake. Some have called this our ‘obesogenic’ environment: an environment that somehow promotes an increase in food intake, primarily via the ready availability of high-energy foods, and simultaneously a decrease in energy expenditure.
It seems that our stressful, modern environment has the capacity to inundate intrinsic neural mechanisms that homeostatically regulate appetite and food intake. It is clear that forces other than homeostatic energy balance and food availability must drive food intake in the face of a surplus energy store. For those of us who live in the developed world at least, food supply is abundant, safe, varied and inexpensive. With virtually no counteracting drive to rein in our drive to eat (except perhaps social or health pressures at a cognitive level), an increase in obesity may be inevitable. In NeuroFAST we are interested in another important drive: the effect of stress on feeding behaviour and the rewarding properties of consuming energy-dense food.
Stress is generally defined as an external challenge to an organism’s physiological equilibrium. A stressful stimulus requires the organism to adapt its physiology or behaviour to counter the stressor’s harmful effects. During acute stress, neural mechanisms control the release of a key stress hormone, called cortisol (or corticosterone in rodents). Cortisol acts to enhance metabolism and promote attentiveness and preparedness in the organism. Given that the objective of the stress response is to preserve energy balance in the short term, it may be counterintuitive to suppose that stress could increase food intake. During stress, time spent searching for, consuming and digesting food may be better spent defending against the stressor itself. Indeed, many studies in rats have demonstrated that chronic stressors decrease food intake and that this is dependent on the duration and intensity of the stress. However, palatable food intake can be driven by corticosterone as well as by stressful stimuli.
A consequence of chronic social stress is to increase food intake in a way that favours the intake of high energy foods.Like physical stressors, other types of stress have a variety of effects on appetite including an increase in food intake and body weight. Social stress in humans is considered to be commonplace in modern society, and a commonly used model of social stress in rats is the ‘visible burrow model’. In this model, a group of male rats is housed together, and a hierarchy naturally develops where one male rat becomes dominant and the others become subordinate. During this process all of the rats suffer stress and have a reduced food intake.
However, once the hierarchy is established, subordinate animals become further stressed, displaying a higher basal level of corticosterone than the dominant animal. Subordinate animals’ food intake remains reduced, and they lose body weight, whereas the dominant rat restores its food intake and tends to maintain body weight. Upon termination of the chronic social stress, both dominant and subordinate animals increase their body weight but only the subordinate animals are hyperphagic; thus, after recovery from social stress, subordinate rats have a larger increase in adipose tissue than the dominant animal.
This effect is amplified when rats are fed a high-fat diet, suggesting that a consequence of chronic social stress is to increase food intake in a way that favours the intake of high energy foods, and that this can lead to obesity.