Preabsorptive insulin release in bulimic women and chronic dieters

The elusive cephalic phase insulin response: triggers, mechanisms, and functions

The cephalic phase insulin response (CPIR) is classically defined as a head receptor-induced early release of insulin during eating that precedes a postabsorptive rise in blood glucose. Here we discuss, first, the various stimuli that elicit the CPIR and the sensory signaling pathways (sensory limb) involved; second, the efferent pathways that control the various endocrine events associated with eating (motor limb); and third, what is known about the central integrative processes linking the sensory and motor limbs. Fourth, in doing so, we identify open questions and problems with respect to the CPIR in general. Specifically, we consider test conditions that allow, or may not allow, the stimulus to reach the potentially relevant taste receptors and to trigger a CPIR. The possible significance of sweetness and palatability as crucial stimulus features and whether conditioning plays a role in the CPIR are also discussed. Moreover, we ponder the utility of the strict classical CPIR definition based on what is known about the effects of vagal motor neuron activation and thereby acetylcholine on the β-cells, together with the difficulties of the accurate assessment of insulin release. Finally, we weigh the evidence of the physiological and clinical relevance of the cephalic contribution to the release of insulin that occurs during and after a meal. These points are critical for the interpretation of the existing data, and they support a sharper focus on the role of head receptors in the overall insulin response to eating rather than relying solely on the classical CPIR definition.

Use of c-peptide as a measure of cephalic phase insulin release in humans

Cephalic phase insulin release (CPIR) is a rapid pulse of insulin secreted within minutes of food-related sensory stimulation. Understanding the mechanisms underlying CPIR in humans has been hindered by its small observed effect size and high variability within and between studies. One contributing factor to these limitations may be the use of peripherally measured insulin as an indicator of secreted insulin, since a substantial portion of insulin is metabolized by the liver before delivery to peripheral circulation. Here, we investigated the use of c-peptide, which is co-secreted in equimolar amounts to insulin from pancreatic beta cells, as a proxy for insulin secretion during the cephalic phase period. Changes in insulin and c-peptide were monitored in 18 adults over two repeated sessions following oral stimulation with a sucrose-containing gelatin stimulus. We found that, on average, insulin and c-peptide release followed a similar time course over the cephalic phase period, but that c-peptide showed a greater effect size. Importantly, when insulin and c-peptide concentrations were compared across sessions, we found that changes in c-peptide were significantly correlated at the 2 min (r = 0.50, p = 0.03) and 4 min (r = 0.65, p = 0.003) time points, as well as when participants' highest c-peptide concentrations were considered (r = 0.64, p = 0.004). In contrast, no significant correlations were observed for changes in insulin measured from the sessions (r = -0.06-0.35, p > 0.05). Herein, we detail the individual variability of insulin and c-peptide concentrations measured during the cephalic phase period, and identify c-peptide as a valuable metric for insulin secretion alongside insulin concentrations when investigating CPIR.

Cephalic phase insulin release: A review of its mechanistic basis and variability in humans

Cephalic phase insulin release (CPIR) is a transient pulse of insulin that occurs within minutes of stimulation from foods or food-related stimuli. Despite decades of research on CPIR in humans, the body of literature surrounding this phenomenon is controversial due in part to contradictory findings . This has slowed progress towards understanding the sensory and neural basis of CPIR, as well as its overall relevance to health. This review examines up-to-date knowledge in CPIR research and identifies sources of CPIR variability in humans in an effort to guide future research. The review starts by defining CPIR and discussing its presumed functional roles in glucose homeostasis and feeding behavior. Next, the types of stimuli that have been reported to elicit CPIR, as well as the sensory and neural mechanisms underlying the response in rodents and humans are discussed, and areas where knowledge is limited are identified. Finally, factors that may contribute to the observed variability of CPIR in humans are examined, including experimental design, test procedure, and individual characteristics. Overall, oral stimulation appears to be important for eliciting CPIR, especially when combined with other sensory modalities (vision, olfaction, somatosensation). While differences in experimental design and testing procedure likely explain some of the observed inter- and intra-study variability, individual differences also appear to play an important role. Understanding sources of these individual differences in CPIR will be key for establishing its health relevance.

Tasting fat: Cephalic phase hormonal responses and food intake in restrained and unrestrained eaters

Restrained eaters exhibit strict cognitive control over their food intake, primarily by limiting intake of high-fat foods. Earlier studies indicate a relationship between dietary restraint and cephalic phase insulin release, which is hypothesized to influence hunger and food intake. To compare cephalic phase hormonal responses to high- and low-fat stimuli and determine if the sensory experience of tasting fat alters hormonal responses and influences subsequent food intake in restrained and unrestrained eaters, normal weight women classified as unrestrained (n=11) or restrained (n=11) eaters were tested under 3 conditions: (1) fasting, (2) sham-feeding a non-fat cake, and (3) sham-feeding a high-fat cake. Following an overnight fast, arterialized venous blood was drawn prior to and for 30 min immediately following a 3-min sham feed. Plasma samples were analyzed for insulin, glucose, glucagon and pancreatic polypeptide (PP). Subjects were subsequently given a selection of high-fat and low-fat foods and allowed to select what they wished to eat. Cephalic phase PP was significantly greater following oral sensory stimulation by the high-fat food (205.4+/-83.6) compared to the fasting control (11.1+/-38.8, p=0.04). No significant differences in hormonal responses to the food stimuli were found between restrained and unrestrained eaters but the restrained eaters consumed more food after the high-fat condition (p<0.05) relative to the fasted condition and compared to the unrestrained group (p<0.05). In conclusion, the sensory experience of tasting fat increases food intake in restrained eaters and increases vagal efferent activity compared to a non-fat food in both populations.

Cephalic phase responses, craving and food intake in normal subjects

Cephalic phase responses (CPRs) are elicited during exposure to food cues. They gear up the body to optimize digestion or they compensate for unwanted changes during a meal. The cue reactivity model of binge eating predicts that CPRs are experienced as craving for food, thereby increasing food intake and playing a role in abnormal eating behaviour. The present experiment was designed to measure CPRs in normal women and to examine its relationship with craving, food intake and restraint. Results show that normal subjects do react to food exposure with changes in heart rate, heart rate variability (HRV), salivation, blood pressure, skin conductance and gastric activity. These CPRs presumably gear up the body and presumably do not reflect compensatory responses. Significant correlations between restraint and blood pressure, between blood pressure and craving, and between craving and food intake were also found. These results are in line with the cue reactivity model and suggest that research into physiological CPRs and craving in the field of eating disorders is valuable.

Classically conditioned responses following repeated insulin and glucose administration in humans

This paper describes the neural basis and the role of Pavlovian conditioning in the modification of blood glucose and related endocrine parameters after repeated insulin and glucose administration. Pavlovian conditioning requires that conditioned stimulus (CS) and unconditioned stimulus (US) are both detected in the central nervous system (CNS), where the CS-US association takes place. We will therefore elucidate the detectability of insulin and glucose in the CNS. Since current data focus almost exclusively on animals, we conducted a placebo-controlled insulin conditioning experiment in humans (Experiment 1). Compared with the control group with CS-placebo pairings throughout, the experimental group with previous CS-insulin pairings in the acquisition phase showed a conditioned decrease in blood glucose and a trend for a conditioned baseline insulin increase, and an increase in cortisol levels relative to baseline and cumulative number of neuroglycopenic symptoms in the CS-placebo test session. The conditionability of glucose administration also had to be examined; experiments using an arbitrary CS and glucose are extremely rare, even in animals. Glucose is the natural stimulus for endogenous insulin secretion, so studies on cephalic-phase insulin release (CPIR) will be reviewed in this paper. We implemented a placebo-controlled three-group design (Experiment 2): Subjects received either CS-insulin, CS-glucose or CS-placebo pairings during the acquisition. Together, our results demonstrate the conditionability mainly of insulin, but also of glucose effects in healthy humans. The clinical relevance and future research perspectives are outlined with an emphasis on insulin in the brain and its role in learning and memory.

Palatability and dietary restraint: effect on cephalic phase insulin release in women

The palatability of food has been shown to influence the cephalic phase reflexes. To determine if food palatability affects the magnitude of cephalic phase insulin release (CPIR) in humans, normal-weight women were asked to list foods that they found palatable and unpalatable. Subjects then stayed overnight in the hospital on two separate days. On each morning following an overnight fast, an intravenous line was inserted and arterialized venous blood drawn for the measurement of plasma insulin and glucose. Blood samples were taken prior to and following a modified sham-feed. Subjects sham-fed the palatable or unpalatable foods (that they had previously identified) for a 2-min period. Foods were administered in a counterbalanced order. During the protocol, hunger and food palatability were monitored. The Three Factor Eating Questionnaire was administered to assess eating attitudes. No significant difference in the magnitude of cephalic phase insulin release was found between the two treatments. However, a statistically significant correlation (r = 0.61, p < 0.05) was found between an individual's degree of dietary restraint as measured by the Three Factor Eating Questionnaire and the magnitude of CPIR. These data suggest that the sensory attributes of food may play less of a role in modulating CPIR than an individual's psychological attitude towards food.

Cephalic phase insulin release in bulimia

Cephalic phase secretions are associated with the sight, smell, and taste of food, as opposed to its postingestional consequences. These secretions are thought to influence metabolism and eating behavior. Cephalic phase insulin release (CPIR), in particular, might be related to hunger and overeating. It was hypothesized that bulimics, who often show endocrine abnormalities, may have an altered CPIR that, in turn, might be related to the precipitation and maintenance of binges. This study investigated whether (1) the profile or magnitude of the CPIR in bulimics differs from that of non-eating disordered controls, (2) food ingestion alters subsequent CPIR, and (3) mood and desire to binge are related to CPIR. Findings indicated little abnormality in bulimics' profile of insulin secretion. Although biological variables were not related to hunger or desire to binge, for bulimics, dysphoric moods were. The results may suggest more complex determinants of binge eating than physiological state alone.

Cephalic phase insulin release in normal weight males: verification and reliability

The existence and reliability of cephalic phase insulin release (CPIR) were tested in 20 normal weight males. Each subject was challenged three times with the same food stimulus over a 5-day period. Four baseline blood samples were taken at 5-min intervals before food ingestion and then every 2 min for 16 min postingestion. Significant increases in plasma insulin were found at 4 min postingestion on each trial day. CPIR was found to be highly reproducible between trials (r = 0.83; P less than 0.001). Fifty percent of the subjects exhibited a significant increase of plasma insulin above their own baseline mean on the first trial, whereas 75 and 72% exhibited increases on trials 2 and 3, respectively. Only two subjects (10%) did not demonstrate a response on any trial. A significant decline in plasma glucose was observed at 4 min postingestion on trials 2 and 3. No significant changes in plasma glucagon were found during any trial day. This study confirms a reliable CPIR in normal weight males.

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.