Food Deprivation

There is evidence to suggest psychological effects of undernutrition. In severe cases, such as anorexia nervosa, neuropsychological function is impaired primarily as a result of structural changes in brain anatomy resulting from starvation. Evidence that undernutrition is associated with psychological problems in those not suffering from eating disorders was first hinted at in the Minnesota Study of Semi-Starvation in the 1950s. Volunteers who were kept on a half-calorie-intake diet for a period of months reported mood swings, increased irritability, poorer memory, and an inability to concentrate. These self-reported effects were not supported by objective testing, but the lack of a non-deprived control group means that there may have been an affect that could have been masked by a practice effect.

Dieting to lose weight is one of the most common food-choice-related behaviors in the Developed World, and it has been consistently associated with negative psychological consequences such as preoccupation with body shape and depression. In addition, a number of investigators have found that dieting to lose weight is associated with impairments in cognitive function, with dieters performing more poorly than non-dieters on measures of reaction time, immediate memory, and the ability to sustain attention. This is unlikely to be due to pre-existing differences between individuals who happen to be dieting and those who are not dieting at the time of testing since, within individuals, performance is poorer when dieting than when not dieting. It is unlikely that these effects are due to the gross physical effects of food deprivation since experimentally induced food deprivation of varying lengths fails to produce a comparable impairment in task performance; in addition, poorer task performance is found amongst dieters who claim not to have lost any weight over the course of the diet.

Rather than being a function of food deprivation per se, the poorer task performance amongst current dieters appears to be a function of the preoccupying concerns with hunger and body shape that are characteristic of dieters. Indeed, the impairments in task performance amongst dieters appear to be comparable in both structure and magnitude to those that result from the preoccupying concerns that are characteristic of clinical depression and anxiety disorders. Specifically, the primary deficit appears to be a reduction in the amount of available working-memory capacity, working memory being the primary cognitive system that allocates processing capacity to ongoing cognitive operations. A threshold hypothesis has been formulated to account for this phenomenon. Non-dieting highly restrained eaters are characterized by an enduring trait concern with body shape, which consumes a certain amount of working-memory capacity (explaining why non-dieting restrained eaters perform at a level intermediate between the levels of current dieters and unrestrained eaters). When they decide to diet, they then experience preoccupations with food and an increased desire to eat; this extra drain on working-memory capacity reaches a point where insufficient capacity is available to maintain task performance. Support for this hypothesis can be seen in the results of a study in which highly restrained non-dieters were instructed to imagine eating their favorite food or to imagine their favorite holiday whilst performing a reaction-time task. When imagining their favorite food, but not their favorite holiday, restrained non-dieters performed as poorly as current dieters on the reaction-time task.

Although evidence seems to be mounting that the poor cognitive function of current dieters is due to psychological and not biological factors, work continues to examine some of the more subtle possible biological mechanisms that may underlie the effects. One possible mechanism is that a low dietary intake of the amino-acid tryptophan (the precursor for 5-HT) leads dieters to have impaired serotonergic function. However, analysis of the urine of dieters for the 5-HT metabolite 5-hydroxyindoleacetic acid found no evidence for this. Another possibility (not yet investigated) is that, by avoiding red meat, dieters experience mild iron deficiency, with deleterious consequences for hemoglobin status, brain oxygen supply, and neurotransmitter function.

The types of dieter studied so far are those who attempt to lose weight in an unsupported and unsu-pervised manner. Comparisons between this type of dieter and those who attempt to lose weight in the context of an organized weight-loss group reveal dramatic differences. Those who diet as part of a group do not show the impairments in task performance typical of unsupported dieters. In addition, unsupervised dieters display an elevated stress response after 1 week of attempted weight loss (as measured by salivary cortisol levels), whereas supported dieters do not. It would appear, therefore, that the poor performance characteristic of unsuper-vised dieting is a result of the stress associated with this type of weight-loss attempt and that the psychological manifestation of this stress is the preoccupying thoughts outlined above.

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