Neuroimaging in Eating Disorders and Obesity: Implications for Research

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Medicine and psychiatry have benefited from developments in investigational techniques. Neuroimaging is one such domain that has technically progressed enormously in recent years, resulting in, for example, higher temporal and spatial resolution. Neuroimaging techniques have been widely used in a range of psychiatric disorders, providing new insights into neural brain circuits and neuroreceptor functions in vivo. These imaging techniques allow researchers to study not only the configuration of brain structures but also aspects of normal and anomalous human behavior more accurately.

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Biology of eating behavior

The central control of eating behavior is held by two interdependent systems, a homeostatic and a hedonic system (Fig. 1).2 The homeostatic component or “nutrostat” links somatic and metabolic signals with autonomic nervous activity and the regulation of food intake. The hypothalamus is deemed to be the central key factor of this system. The discovery of leptin as the missing link in the homeostatic control of eating contributed substantially to the unfolding of the cascade of central

Neuroimaging

Imaging techniques are classically divided into structural (eg, computed tomography or CT; magnetic resonance imaging or MRI) and functional imaging techniques (eg, single photon emission [computed] tomography or SPECT/SPET; positron emission tomography or PET; functional magnetic resonance imaging or fMRI). However, newly developed methods such as magnetic resonance spectroscopy (MRS) and diffusion tensor imaging (DTI) combine both modalities.

General

Since Phineas Gage's case there has been a growing interest in brain lesion studies, because they can provide key information about the localization of brain areas involved in certain aspects of human behavior.5 Anomalies, either congenital or posttraumatic, in the prefrontal, temporal, mesiotemporal cortices and the thalamus, predominantly on the right-hand side, are associated with pathologic eating patterns, including abnormal food intake.6

In addition, there is ample evidence that changes in

Magnetic resonance spectroscopy in eating disorders

Neuronal degeneration as a result of starvation may explain the brain shrinkage observed in AN patients, and this may be associated with changes in the metabolic profile, which can be observed with MRS. Metabolic changes in white matter have been reported in the thalamus and the parieto-occipital lobe30 and in frontal lobe areas.31, 32 Some investigators found no evidence for neuronal degeneration,30 whereas others observed abnormal metabolite level signals in frontal gray matter,33, 34

Structural imaging and MRS in obesity

The literature on structural abnormalities associated with excess body fat is limited and inconsistent, though alterations of brain morphology have been described in overweight and obese young adults.35, 36 Both gray and white matter changes appear to be associated with increased adiposity. Pannacciulli and colleagues35 demonstrated focal gray matter volume reductions in several brain areas (post-central gyrus, frontal operculum, putamen, and middle frontal gyrus), accompanied by enlarged

Functional imaging

Functional neuroimaging methods provide information on the integration and interaction of brain regions in spatially distinct neural networks during cognitive or behavioral challenges or in response to physiologic stimuli.38, 39 The development of ligands relating to neurotransmitter/neuroreceptor systems has increased the specificity of SPECT and PET over that obtained by the measurement of blood flow alone. Meanwhile, fMRI has emerged as an advanced means to examine cerebral functioning, with

Globally reduced metabolism

Since the first PET data obtained from eating disorders,41 the field has expanded considerably. Overall, underweight AN patients were found to have a reduced global and regional cerebral glucose metabolism (rCMRGlu) in the acute state, and this increases with recovery. Most studies found the decrease in global metabolism to be related to weight loss42, 43, 44 and BMI and glucose values.42, 43

Regional metabolism and blood flow abnormalities

A consistent finding in AN is a hypometabolism in frontal and parietal cortices.42, 43, 45, 46 Specific

Stimulus provocation

Several technical issues need to be considered before interpreting the response of the brain to salient eating disorder cues. The key areas relating to the hedonic response to food may be subject to susceptibility artifacts (eg, the orbitofrontal cortex might be difficult to visualize due to the proximity of the sinuses). Further, the baseline physiology, hunger, and blood glucose level, and so on, may vary between groups and act as significant confounders. Several studies have examined the

Dopamine

It has been speculated that dopamine-related disturbance of reward mechanisms might contribute to altered hedonics of feeding behavior and the anhedonic temperament in AN. Frank and colleagues81 demonstrated higher dopamine D2/D3 receptor binding in recovered AN subjects in the anteroventral striatum, suggesting either decreased intrasynaptic dopamine concentration or increased D2/D3 receptor density or affinity to be present in AN. Moreover, in the AN group, dopamine binding potential in the

Obesity

Functional neuroimaging provides an increasingly important tool for investigating how different regions of the brain work in concert to orchestrate normal eating behaviors and how they conspire to produce obesity and other eating disorders.97, 98

Summary

The body of literature on neuroimaging studies in eating disorders has grown exponentially over the last 2 decades and has contributed a better insight into the clinical eating disorders. In summary, global and regional reduced resting state perfusion and metabolism have been found in eating disorders, associated with the nutritional state of patients. Furthermore, eating disorder patients have been shown to respond differently from control subjects when presented with disease-related cues such

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      Finally, Beato-Fernández et al. (2009) concluded that functional brain abnormalities in patients with AN might be related to the storage of a distorted prototypical image of the body in the left parietal lobe, and activation of the right temporal area after exposure to images of patients’ own bodies might be consistent with this adverse response. There is also an evident attentional bias towards food and body shape in AN patients, and this bias could be associated with increased activation in disturbed neural networks involved in self-regulation and hedonic motivation (Kaye et al., 2009; Van den Eynde and Treasure, 2009). Therefore, in addition to body-image disturbances, functional neuroimaging studies have also considered neuropsychological impairments or deficits in AN patients.

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