Regulation of food intake
Adult individuals are usually characterized by a balance between energy intake and energy expenditure. Food intake is controlled by various feed back loops.
Short-term and long-term regulatory systems can be differentiated. The feed back signals are integrated in the central nervous system (CNS) and translated into an appropriate feeding response. Short-term regulation of food intake mainly involves feed back signals originating in the gastrointestinal (GI) tract. They may be positive (e.g. from the oral cavity) or negative in nature. Distension of the stomach (or the forestomachs in ruminants) leads to the activation of vagal afferents projecting to the nucleus of the solitary tract (NTS) in the hindbrain being connected with the hypothalamus. The presence of nutrients in the small intestine (or rumen in ruminants) is sensed and the information transmitted via vagal afferents or via the release of GI hormones, e.g. cholecystokinin (CCK). Besides CCK, other hormones (e.g. gastrin-releasing peptide and the pancreatic hormones amylin, glucagon and insulin) also play an important role in the regulation of food intake. These satiety hormones act partly via receptors on vagal or splanchnic afferent nerves (CCK) whereas amylin and insulin appear to act directly in the CNS. Feed back signals from the hepatoportal area are postabsorptive in nature. The availability of glucose (propionate in ruminants), the oxidation of fatty acids and the energy status in general appear to be sensed in the hepatoportal area with the signals being transmitted mainly via hepatic vagal afferents. The lipostatic theory of the long term regulation of food intake and body weight postulates the presence of humoral factors whose concentration depends on the size of the adipose tissue. The most important signals are leptin and insulin whose plasma concentrations increase with the degree of obesity. Both constitute negative feed back signals acting directly on the brain, their main target organ being the hypothalamus, the main integrating center for the control of food intake. Within the CNS, a plethora of substances interact in a complex system to control food intake. Among them are the neurotransmitters serotonine, histamine, norepinephrine and dopamine and the neuropeptides corticotropin releasing factor (CRF)/melanocortin (MC) and neuropeptide Y (NPY). The anorectic effects of leptin and insulin, e.g., also appear to be mediated via the NPY and CRF/MC systems.