The cephalic phase of digestion refers to the reflexes that occur in response to the thought, smell or taste of food. In general these reflexes are elicited largely by receptors in the oropharynx or head and mediated by neuronal pathways connecting the central nervous system to the digestive tract. Sensory contact with food is known to elicit secretion of saliva, gastric acid, insulin, pancreatic juice, glucagon, digestive enzymes and to increase gastric motility. These changes modulate parameters such as blood sugar, respiratory quotient, plasma fatty acid levels and bile secretion. Non-digestive and metabolic changes elicited upon sensory contact with food include changes to the pupillary size and general systemic arousal. The magnitude of these changes can be correlated with the perceived palatability of the foods and as such, not all foods will elicit the same response. Differences between the cephalic response to a particular food can also vary between individuals. Reviewing such changes is therefore an important aspect of understanding responses to food.
The cephalic response is elicited by specific effector regions within the gastrointestinal tract. However it is important to differentiate between the cephalic response and the later gastric, intestinal and postabsorptive phases of digestion because these phases can elicit different responses in the same effector regions. For example, the cephalic response to saccharin ingestion results in the release of insulin that begins within one minute of ingestion and lasts for around ten minutes, with a peak of around five minutes. This response is correlated with the palatability and the concentration of the saccharin solution ingested and results from taste receptors in the mouth. This early insulin response is quite different from the postabsorptive insulin response, that is caused by the same effector systems, but initiated not by taste receptors, but by glucose receptors in the stomach and the small intestine. The cephalic phase insulin response is just one example of a number of responses that use the same effector systems as later more pronounced responses.
The term cephalic response can be further subdivided into primary cephalic responses and secondary cephalic responses. In this regard the primary cephalic response is the initial physiological reflex caused by the receptor activation and the secondary response is any subsequent response caused by the primary response. In the case of the saccharin induced cephalic phase insulin receptor activation by the taste buds of the mouth, the primary response could be said to be the subsequent release of insulin. However, a large number of modulatory substances that affect insulin release have been identified including neurotransmitters neurohormones, hormones and prostaglandins. As such these modulatory factors can be thought of as the secondary cephalic response, with cholinergic and β-adrenergic transmitters, gastric inhibitory polypeptide (GIP), glucose and amino acids being the prominent stimulatory factors, and somatostatin and α-adrenergic hormones being the prominent inhibitory factors.
While a general understanding of terminology is interesting the real question of interest relates to the function of the cephalic phase as a whole. In this regard we can state that the purpose of the initial reflex response to the sight, smell, taste, touch or thought of food is to prepare for the arrival of food. While this may be regarded as the de facto text book answer, in reality the specific purpose of the cephalic phase in more complex. As more is understood about its complexity, speculation has widened our understanding of the cephalic phase. In this regard, the cephalic phase may serve to provide specific information about food before it is ingested, such as the amount and composition. Alternatively the cephalic phase may provide a feed forward function to prepare for anabolic condition. The cephalic phase has also been proposed to provide a protective function to prevent changes in homeostasis caused by large intakes of nutrients. Another possibly is that it may simply be a conditioned response related to the unconditioned response of the gut.
Of course it might be argued that all of the above definitions are valid and that the true purpose of the cephalic phase lies somewhere in the middle of all of these ideas. However, from a philosophical point of view the cephalic phase is also interesting to ponder. The control of actual physiological processes by the mind is ingrained in ancient Chinese and Ayurvedic medicine, and yet such concepts are relatively new to Western allopathic medicine. The cephalic phase of digestion provides a glimpse of the power of thought to override, modulate, control and influence cellular processes. Of course, science provides no explanation of where a thought originates from or how that thought is transferred from the non-material to the physical world where it appears as an electrical impulse on a neurone in the brain. And while such philosophical musings may appear to stretch the bounds of nutritional science, the questions they raise about the influence of food on health are important because they go beyond the physical observable realm.
RdB
