Blaming the Brain for Obesity: Integration of Hedonic and Homeostatic Mechanisms

Abstract

The brain plays a key role in the controls of energy intake and expenditure, and many genes associated with obesity are expressed in the central nervous system. Technological and conceptual advances in both basic and clinical neurosciences have expanded the traditional view of homeostatic regulation of body weight by mainly the hypothalamus to include hedonic controls of appetite by cortical and subcortical brain areas processing external sensory information, reward, cognition, and executive functions. Hedonic controls interact with homeostatic controls to regulate body weight in a flexible and adaptive manner that takes environmental conditions into account. This new conceptual framework has several important implications for the treatment of obesity. Because much of this interactive neural processing is outside awareness, cognitive restraint in a world of plenty is made difficult and prevention and treatment of obesity should be more rationally directed to the complex and often redundant mechanisms underlying this interaction.

Keywords: Appetite; Cognition; Cortex; Hypothalamus; Limbic System; Physical Activity; Reward; Self-Control.

Fig. 1

The brain uses nutritional information from both outside and inside the body before, during, and after ingestive behavior. During the initiation phase, attention may be shifted to ingestive behavior because of hunger or an opportunity to consume a highly rewarding food item in the absence of metabolic hunger. During the procurement phase, unconditioned and conditioned stimuli from food and food cues interact through the external senses with the cognitive, emotional, and executive brain. During the consumatory phase, interoceptive signals from taste and the gastrointestinal tract reaching the ‘metabolic’ brain guide the relatively stereotypical processes of digestion, transport, and absorption. Anticipatory reflexes shaped by prior experience thereby help keeping regulated parameters within optimal limits. Finally, during the postingestive or metabolic phase, signals from the gut, and organs that can store and/or metabolize energy such as adipose tissue and liver inform the brain about the metabolic consequences of the meal, providing reward from satisfaction and generating episodic memorial representations of the entire meal for future reference. By integrating external and internal information, the brain can regulate long-term body weight flexibly and adaptively, to accommodate special circumstances (allostasis).

Fig. 2

Schematic diagram showing the three heavily interconnected major brain areas constituting the core processor for the control of ingestive behavior and its relation to the gastrointestinal tract and other peripheral organs involved in energy storage and utilization. The hindbrain is mainly concerned with meal size control, as it possesses all the elements to detect sensory information mediated by vagal afferents and circulating factors, and generate motor output associated with the ingestion, digestion, and absorption of food. The cortico-limbic system, consisting of large cortical areas, basal ganglia, hippocampus, and amygdala, is intimately connected to the hypothalamus and brainstem and provides the emotional, cognitive, and executive support for ingestive behavior. The hypothalamus and its connections with the other areas is central for the drive to eat and can potently modulate peripheral organs by autonomic and endocrine outflow.

Fig. 3

Schematic diagram showing the neural basis for the selection of specific foods. Before ingestion, available foods and their cues act through external sensory channels to retrieve representations with prior experience (‘food memories’) that have access to reward-based decision making circuits. If there is a specific need state for that food (nutrient), it is sampled. If the taste experience confirms the stored information retrieved from memory, ingestion follows, eliciting interoceptive signals such as metabolites and hormones which feed back to the hindbrain and hypothalamus to stop eating when satiated and update the memory. If tasting reveals any problem with the food (spoiled food or otherwise aversive), the food item is not ingested and the memory updated. Abbreviations: Amy, amygdala; Hipp, hippocampal complex; Nac, nucleus accumbens; OFC, orbitofrontal cortex; PFC, prefrontal cortex; VTA, ventral tegmental area.

Fig. 4

Basomedial hypothalamic neurons as master integrators of internal and external nutritional information to achieve allostasis. AGRP/NPY hunger neurons translate hunger signals from both the internal milieu and hedonic feedback into sustained and complete appetitive and consumatory ingestive behavior by programming a downstream neural network that increases the incentive or reward value of particular foods. While AGRP/NPY neurons could be thought of as the gas pedal, POMC/CART neurons represent the brake pedal, acting through parallel downstream pathways.

Fig. 5

Schematic diagram showing interactions between metabolic and hedonic consequences of food and their relationship to obesity. Obesity can be caused by overstimulation of hedonic processes leading to hedonic overeating or by impaired metabolic processes leading to lack of compensation following overfeeding. Both processes are subject to an interaction with genetic and epigenetic predisposition. The obese state can also lead to a secondary impairment of both hedonic and metabolic processes, leading to a vicious circle and further progression of obesity, as well as causing metabolic disturbances in offspring.