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NutrInsight • Satiety: News Insights
5 HoW DoES NEuRoIMAgINg HELP uS uNDERSTAND MECHANISMS oF HuNgER AND SATIETy?
Pr Hans-Rudolf Berthoud, Head of the Neurobiology of Nutrition Laboratory, Pennington Biomedical Research Center, U.S.A.
Basic processes of food intake control such as hunger, appetite, cravings, fullness, and satiety are organized in a distributed neural network involving most parts of the brain. Modern neuroimaging with functional Magnetic Resonance Imaging (fMRI) and Positron Emission Tomography (PET) are presently the most powerful and the only non-invasive tools to study the activity of these distributed circuits.
5.1 Fundamentals of energy balance regulation
Figure 1 illustrates how the brain receives and integrates various signals from the periphery of the organism, the “internal milieu”. Following food intake, nutrients sensing mechanisms, neural or hormonal, send information to the brain about the presence of nutrients in the gastro-intestinal tract. In addition to signals originating from the gut and the adipose tissue, the brain integrates signals reflecting energy input and energy expenditures and acts as a homeostatic regulator, adjusting intake to needs in order to maintain the body energy balance. The hypothalamus and the brainstem are key structures of the brain homeostatic regulator [Berthoud 2002; 2004].
The end-result of the activity of the brain homeostatic regulator is the maintenance of a constant body weight. A period of starvation induces weight loss. As soon as access to food is restored, food intake increases and energy expenditures decrease until body weight is restored to pre-starvation levels. Such a mechanism suggests that there is a “defended” level of body adiposity. Conversely, a period of over-feeding leads to an increase in body weight. At the cessation of over-feeding, food intake spontaneously decreases while energy expenditures increase and a decrease in body weight is observed.
In addition to these homeostatic processes, numerous factors of the environment and lifestyle influence food intake. Such factors include food cues, availability of foods, stress, social habits and physical activity level. These influences modify the activity of brain structures involved in cognitive and emotional responses such as cortico-limbic systems.
It is important to note that all components of both the homeostatic and hedonic systems are subject to genetic predisposition. Recent evidence has also demonstrated the potent role of epigenetic mechanisms and early life programming in brain responses.
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