Effect of Stress on Food Intake

Under a variety of stressful conditions, food intake is altered in both animals and humans. However, different stressors seem to have different effects, with some resulting in an increase and some in a decrease in food intake. In human studies, some subjects appear to be nonresponders to stress, whereas of the responders, some markedly reduce their intake (stress fasters) while others increase their intake (stress eaters).

Stress-induced eating is of particular interest because of the role it may play in contributing to obesity. Much overeating behavior is reported to be stress related, and some obese subjects report a decrease in anxiety after eating.

In everyday life there are a number of specific stressors that have an impact on food intake, particularly in women. Since the 1960s, Western society has placed increasing demands on the population, particularly women, to be slim and to aspire to an ideal of thinness. There has been an associated increase in body image dissatisfaction, particularly among young women.

This has also coincided with a dramatic increase in the prevalence of dieting and of eating disorders such as anorexia and bulimia nervosa. The prevalence of eating disorders seems to be in direct proportion to the prevalence of dieting behavior in a given community, and longitudinal studies of young adults suggest that the risk of developing an eating disorder is significantly higher among dieters than nondieters. Indeed, most of those suffering from an eating disorder report that their eating problem started as normal dieting that then got out of control.

Low self-esteem is an additional factor associated with dieting, body image dissatisfaction, and alterations in eating pattern, although it is difficult to know whether this is a cause or an effect. Subjectively, low mood and low self-esteem are often reported to be contributing factors to unwelcome weight gain.

Experimentally induced stress or dysphoria in volunteers causes an increased food intake in some individuals. This effect occurs particularly in those subjects who are dieting or are restrained eaters, but not in those who are not restricting their food intake. Interestingly, subjects who binge eat often report that this occurs in response to stress or dysphoria and, at least initially, binges seem to relieve anxiety.

Dieting and dietary restraint are intimately bound up with mood and self-esteem. Subjects who restrict their food intake to lose weight record ratings of happiness that are directly related to weight loss or weight gain. In contrast, subjects who are not restricting intake report that weight change has little impact on ratings of happiness.

Feedback to subjects about weight change will therefore alter mood, which in turn can affect food intake and weight. In a study where subjects were weighed regularly but the results were fed back to them as consistently heavier than their true weight, restrained eaters reported more mood changes than nonrestrained eaters in response to the feedback, including anxiety, depression, and low self-esteem. Interestingly, these restrained eaters, who experienced lowered mood, then ate more in a subsequent test meal.

Thus, there is growing evidence that stress can alter food intake. However, there is debate about how this effect is mediated. Stressful events can alter a number of different biological variables, which in turn could affect food intake. It has been hypothesized that stress-induced increases in food intake are mediated by endogenous opiates. Stress increases the release of ß-endorphin, a naturally occurring opiate that increases food intake, but also reduces the perception of pain and the anxiety associated with stress.

An alternative is that these effects are mediated by the neurotransmitter 5-hydroxytryptamine (5-HT). 5-HT has attracted great interest because its function seems to be altered in disorders such as depression, which are known to be closely associated with the occurrence of stressful life events, and in eating disorders such as bulimia nervosa. In addition, altering 5-HT function (e.g., by using medication) has a profound effect on eating behavior. In human and animal studies, interventions that increase 5-HT function reduce food intake; conversely, reducing 5-HT function increases food intake.

Could levels of 5-HT be altered by stress? Stress increases the level of a naturally occurring hormone, cortisol. When this occurs, 5-HT function is reduced. Reduced 5-HT function would be expected to lead to increased food intake, and experimental work suggests that this increase would occur particularly in carbohydrate intake, rather than protein. Interestingly, this alteration in the balance of macronutrients could itself affect 5-HT function. This is discussed in more detail in the next section.