Nutritional implications of the cephalic-phase reflexes: Endocrine responses

Cephalic phase hormonal release occurs through activation of vagal-efferent fibers in response to food-related sensory stimuli. Thus, tasting, chewing and expectorating food elicits hormonal release prior to nutrient absorption. Differential sensitivity of cell types within the islet to neural activation determines the profile and magnitude of hormonal release. While the magnitude of cephalic phase insulin release is relatively small (25% above baseline), pancreatic polypeptide, a hormone almost exclusively under vagal control increases 100% above baseline when individuals taste, chew and expectorate food. Thus, the cephalic phase pancreatic polypeptide response is a sensitive indicator of vagal activation to food stimuli. The physiological significance of the cephalic phase hormonal responses is demonstrated by experimental manipulations which inhibit or bypass cephalic phase insulin release. Under these circumstances hyperglycemia and hyperinsulinemia are evident. Conversely, supplementation of insulin during the preabsorptive time period improves glucose tolerance in certain clinical populations. These data suggest that cephalic phase insulin release plays a role in glucose homeostasis.