[Physiology of food intake and regulation of body weight]

Feeding is often influenced by competitive needs or by environmental factors. Nevertheless, in physiological conditions hunger is elicited by depletion of macronutrients. The first question is what specific signals cause food intake? Recent data suggest that the signal comes not from exclusive depletion of carbohydrates or of anyone type of the major macronutrients but from overall decreased cellular power production (hypoischymetric signal). Locomotion free metabolic rate has been shown to decrease preceding hunger and to increase in order to determine satiety. Satiation, which occurs largely before ingested nutrients can cross the intestinal barrier and replete the inner milieu, obeys a specific mechanism. Following the stimulation by ingestion of the oro-gastro-intestinal receptors metabolic hormones such as glucagon and insulin are released and enhance metabolism of endogenous reserves. The so-induced endogenous "meal" brings about an anticipatory hypermetabolism which inhibits further ingestion. Post-absorptive satiety subsequently will replace the pre-absorptive state of satiation. The long term regulation of body weight is more subtle. When the actual body weight exceeds its defended value, responsiveness towards signals of hunger diminishes. This hyporesponsiveness seems to be a consequent of an enlargement of the microscopic fat depot at the level of hypothalamic receptive structures which parallels the peripheral increase of the macroscopic fat depot. Thus, hunger promoting signals become less effective and feeding decreases. A symmetrical process takes place in case of body weight loss which results in central hyperresponsiveness to hunger related stimuli and thus facilitates feeding and body weight recovery.