The Use of Functional Magnetic Resonance Imaging in the Study of Appetite and Obesity

The brain regulates food intake in order to maintain energy homeostasis, which is required for survival (Woods, 2009). Information about the immediate and long-term state of energy balance in the body comes from three main sources: peptide hormones from the gut and adipose tissue, circulating nutrients such as glucose and lipids, and vagal afferents. These signals are integrated in the nucleus of the solitary tract (NTS) in the medulla, which is the major afferent target of the vagus nerve, and in the hypothalamus (Morton et al., 2014). They modulate eating behavior by communicating with the brain centers involved in learning, motivation, and decision making (Begg and Woods, 2013). However, food choices not only are under the influence of current energy balance but are also guided by the presence of food cues such as the sight, the smell, or the taste of food, which signal availability; by habits and social factors (e.g., eating at specific times); and by cognitive factors, such as a desire for health or the cost of food (Zheng et al., 2009).Here, we will review how the function of these systems can be studied in humans by functional magnetic resonance imaging (fMRI). fMRI relies on a measure called blood oxygenation level dependent (BOLD) contrast, which signals the activity-dependent change in the ratio of deoxyhemoglobin to oxyhemoglobin concentration due to increased neural activity (Kwong et al., 1992). It essentially measures energy usage by active neurons. The most common approach to study appetite with fMRI has been using food cues that have gained predictive value through previous experiences (Dagher, 2012). BOLD subtraction analyses are generally employed to investigate food-specific brain activations in different states (i.e., fasted vs. fed; lean vs. obese) and their modulation by peripheral signals of energy, personality traits (e.g., eating style, impulsivity), and outside influences such as stress or health information. In addition, the functional connectivity between brain areas can be assessed both during food tasks and rest. Connectivity analyses give information on how interactions between brain areas may underlie abnormalities in eating behavior (Carnell et al., 2012).