Cephalic Insulin Release Research Articles

Overfeeding, autonomic regulation and metabolic consequences.

The autonomic nervous system plays an important role in the regulation of body processes in health and disease. Overfeeding and obesity (a disproportional increase of the fat mass of the body) are often accompanied by alterations in both sympathetic and parasympathetic autonomic functions. The overfeeding-induced changes in autonomic outflow occur with typical symptoms such as adiposity and hyperinsulinemia. There might be a causal relationship between autonomic disturbances and the consequences of overfeeding and obesity. Therefore studies were designed to investigate autonomic functioning in experimentally and genetically hyperphagic rats. Special emphasis was given to the processes that are involved in the regulation of peripheral energy substrate homeostasis. The data revealed that overfeeding is accompanied by increased parasympathetic outflow. Typical indices of vagal activity (such as the cephalic insulin release during food ingestion) were increased in all our rat models for hyperphagia. Overfeeding was also accompanied by increased sympathetic tone, reflected by enhanced baseline plasma norepinephrine (NE) levels in both VMH-lesioned animals and rats rendered obese by hyperalimentation. Plasma levels of NE during exercise were, however, reduced in these two groups of animals. This diminished increase in the exercise-induced NE outflow could be normalized by prior food deprivation. It was concluded from these experiments that overfeeding is associated with increased parasympathetic and sympathetic tone. In models for hyperphagia that display a continuously elevated nutrient intake such as the VMH-lesioned and the overfed rat, this increased sympathetic tone was accompanied by a diminished NE response to exercise. This attenuated outflow of NE was directly related to the size of the fat reserves, indicating that the feedback mechanism from the periphery to the central nervous system is altered in the overfed state.

Thermogenic and Hormonal Responses to Palatable Protein and Carbohydrate Rich Food

Simultaneous variations of oxygen consumption, and plasma insulin and norepinephrine were measured during the postprandial cephalic and gastrointestinal phases of feeding in six human subjects following the ingestion of various nutrients. On alternative days the subjects were given foods (1280 kjoules) either rich in carbohydrates (sugar pie) or in proteins (fish). Both nutrients produced an initial (0-40 min) enhanced thermogenesis and an early (2 min) cephalic insulin release. During that period, elevations of plasma norepinephrine were also observed with pie feeding at 10 and 30 min and at 10 min with fish. Palatability ratings indicated that both food items were equally tasting. During the gastrointestinal phase (40 to 120 min) the variations of these same parameters including glucagon seem to be explained by the content in carbohydrates and proteins in the food rather than by its palatability. Indeed during that period the protein meal was more thermogenic and the carbohydrate meal induced the expected insulin secretion. These results suggest that the palatability of the food is responsible for the early cephalic increase in postprandial thermogenesis, and for the insulin and norepinephrine release. During the subsequent gastrointestinal phase the increased thermogenesis is related to the composition of the food which exerts its action by the biochemical processes involved in the disposal of the absorbed nutrients.

Cephalic insulin release in anorexic women.

Cephalic phase insulin release (CPIR) was examined in women with anorexia nervosa and in lean, age matched controls. Following an overnight fast, a palatable food was presented to subjects and plasma insulin was measured at baseline and every minute for a 10 minute period following the presentation of the food. Elevation in plasma insulin occurred five minutes after food presentation in the anorexic group only. The finding of a significant CPIR in anorexics but not controls was unexpected and tends to rule out a deficient cephalic insulin response as a contributor to the self-starvation observed in anorexia nervosa.

Vagotomy Abolishes the Differential Palatability of Food

The magnitude of preabsorptive insulin release varies as a function of palatability. This suggests that the palatability-dependent rise in plasma insulin could accentuate the systemic stimulus to eat, which in turn could modulate the size of the meal. The present experiment was designed to test this hypothesis by investigating the ingestive responses of vagotomized rats, which no longer exhibit preabsorptive insulin release, when they are offered nutritionally identical diets differing only in their sensory characteristics. The presented foods were three forms of the rat's regular diet: unaltered, sweetened with sodium cyclamate or adulterated with quinine hydrochloride. It was shown that vagotomized rats ate exactly the same amount of the three diets in three different test-meals: without cephalic insulin release, meal size no longer seems to be modulated by oral cues.

Ontogeny of cephalic insulin release by the rat

Developmental aspects of cephalic insulin release in rats were evaluated by examining insulin secretion in response to the sweet taste of saccharin at different ages. A cephalic insulin response was present at the time of normal weaning (21–22 days) and by 34 days of age its magnitude was indistinguishable from that of adults. The early appearance of cephalic insulin release prompted a second study examining whether this response is present at birth; i.e., prior to any suckling experience. In newborn rat pups, no increase in plasma insulin was seen as a result of introduction of saccharin into the oral cavity. It was concluded that maturational and/or learning changes responsible for cephalic insulin release occur during the suckling period. The discussion concerned whether these changes involve maturation of the afferent loop, the efferent loop and/or learning as a consequence of the suckling experience.